DRILLING PROJECT LEG 86, HOLE 576A1 The Marine ...Regional sea-floor morphology and near-surface seismic profile char-acteristics have been mapped and discussed by Damuth et al. (1983)

34. GEOTECHNICAL PROPERTIES OF NORTHWEST PACIFIC PELAGIC CLAYS: DEEP SEADRILLING PROJECT LEG 86, HOLE 576A1

The Marine Geotechnical Consortium2

ABSTRACT

Geotechnical property results from Deep Sea Drilling Project Hole 576A sediments represent the most complete da-ta set ever compiled for a continuously depositional, sedimentary section that spans 65 m.y. Geotechnical analyses in-cluded index-property and sediment characterization, and consolidation, permeability, and triaxial testing. Excellentagreement exists between test data generated in this multilaboratory effort.

Sediment characterization and index-property data delineate three well-defined units that conform with major litho-logic divisions. Within Subunit IA these data appear inconsistent with accepted geotechnical relationships, showing wa-ter content, plasticity, and grain size increasing over similar intervals. These unusual relationships are attributed, inpart, to silt-sized bonded clay aggregates. Within Subunit IB geotechnical properties become more uniform, and amarked increase in X-ray amorphous material occurs below 28 m sub-bottom. A pronounced change in the geotechnicalbehavior of Hole 576A sediments occurs in Unit II and corresponds to the presence of calcareous oozes.

Consolidation test results show overconsolidated sediments in the upper 28 m and normally consolidated sedimentsbetween 28 and 55 m sub-bottom. Mechanisms to explain the state of overconsolidation that are consistent with the geo-logical history of the area include cementation and interparticle bonding, the release of high hydrostatic pressures dur-ing core recovery, and artifacts resulting from the consolidation test process. Additionally, the relevance of the precon-solidation stress determined using consolidation test results may be limited.

Variations in the shear strength gradient with depth are consistent with the consolidation test results. The gradientfor the overconsolidated interval (0 and 28 m sub-bottom) is significantly higher than between 28 and 55 m sub-bottom,where the sediments become normally consolidated.

INTRODUCTION

The constraints of standard coring techniques havelimited most studies of the geotechnical properties of ma-rine sediments to samples recovered from less than 15 msub-bottom. Many of these studies have tried to relatevariations in geotechnical properties to variations in ge-ological processes. In many instances, however, the rela-tive geological complexity of the area or an insufficientgeotechnical data base have precluded the developmentof such relationships. At Deep Sea Drilling Project(DSDP) Site 576, we were able to evaluate these relation-ships for a fairly simple, continuously depositional, pe-lagic clay environment whose geological history is wellsubstantiated over the last 65 m.y.

Site 576 was triple cored (Holes 576, 576A, 576B) toget enough sediment samples for detailed sedimentolog-ical, stratigraphic, geochemical, and geotechnical analy-ses. At the request of the Marine Geological Consorti-um, Hole 576A was continuously cored and preserved inthe full-round for detailed shore-based geotechnical pro-perty analyses. These analyses were intended to evaluate

1 Heath, G. R., Burckle, L. H., et al., Init. Repts. DSDP, 86: Washington (U.S. Govt.Printing Office).

^ The Marine Geotechnical Consortium for Deep Sea Drilling Project Leg 86 consistedof the University of Colorado (CU), Boulder, Colorado; Humboldt State University (HSU),Arcata, California; Institute of Oceanographic Sciences (IOS), Wormley, Godalming, UnitedKingdom; Oregon State University (OSU), Corvallis, Oregon; San Diego State University(SDSU), San Diego, California; Sandia National Laboratories (SNL), Albuquerque, NewMexico; Texas A&M University (TAMU), College Station, Texas; University of Rhode Island(URI), Narragansett, Rhode Island; and United States Geological Survey (USGS), Denver,Colorado. The following persons contributed to this study. CU: Robert Schiffman, AndrewDean; HSU: Ronald Chaney, J. Suehanek, B. Vega, T. McFarland; IOS: Peter J. Schultheiss,David Gunn; OSU: George Keller; SDSU: Iraj Noorany, M. Faghihi; SNL: Les Shephard,Joel Lipkin, Jean Kinker; TAMU: William Bryant, Elliot Taylor, Mark Johns; URI: ArmandSilva, William Levy, Ricci Siciliano, Kathleen Dadey; USGS: Harold Olsen.

the interrelationships between the geotechnical proper-ties and the geological processes of an abyssal hill envi-ronment, specifically as they pertain to variations in thesediment stress history, consolidation-permeability rela-tionships, and shear strength.

GEOLOGIC SETTINGSite 576 is located east of Shatsky Rise in the western

North Pacific red clay province (Fig. 1) very near mag-netic Anomaly Ml3, which indicates a basement age of129 m.y. (Site 576 chapter, this volume). Regional sea-floor morphology and near-surface seismic profile char-acteristics have been mapped and discussed by Damuthet al. (1983). The location of Site 576 is characterized bya smooth to gently rolling seafloor. Seafloor bathymetrygradually deepens from southwest to northeast at a gra-dient of approximately 1:1000, reflecting an abyssal hilloceanic basement covered by pelagic sediments. Waterdepth at Site 576 is 6127 m.

The sedimentary section of Hole 576A can be dividedinto three general lithologic units on the basis of macro-scopic descriptions and smear-slide analyses (Site 576chapter, this volume; Fig. 2). Unit I consists of pelagic,brown clay extending to 55 m sub-bottom that can be di-vided into two subunits. Subunit IA (0 to 28 m sub-bot-tom) is a pelagic, yellowish brown, burrowed clay. Theabundance of burrows and the silt-sized fraction gener-ally decrease down through Subunit IA. Subunit IB (28to 55 m sub-bottom) is an unburrowed, homogeneous,dark brown, pelagic clay generally containing less than20% silt. The contact between these subunits is grada-tional over several meters. Unit II is composed of inter-bedded carbonate oozes and brown pelagic clays (55 to

723

MARINE GEOTECHNICAL CONSORTIUM

50° N

^

140° E 160° 180° 160°W

Figure 1. Location of Site 576 in the northwest Pacific. Areas shallower than 4 km stippled, 5-km contourplain, 6-km contour hachured.

66 m sub-bottom). Evidence of graded bedding and cross-bedding in some of the carbonate ooze layers and sharpbasal contacts between the carbonates and interbeddedpelagic clays suggest a turbidite origin for at least someof the oozes. The base of Unit II was penetrated only inHole 576B, at a sub-bottom depth of 76 m. Unit III, re-covered only at the base of Hole 576B, consists of a chertor silicified clay unit.

Sedimentation rates (uncorrected for consolidation)decrease downcore in Unit I from 10 m/m.y. at the sur-face to less than 3 m/m.y. at the base of the MatuyamaEpoch ( - 2 0 m sub-bottom; Heath, Rea, and Levi, thisvolume). This downcore decrease in sedimentation ratereflects the change from primarily eolian material in Sub-unit IA to an increased authigenic sediment componentin Subunit IB. The change in dominant sediment com-ponents results from changes in depositional patterns asthe location of Site 576 moved north and west in andout of various wind belts in response to seafloor spread-ing (Leinen, this volume).

GEOTECHNICAL CORE PROCESSING

Hole 576A cores were obtained by the Glomar Chal-lenger on Leg 86 using the hydraulic piston corer. Imme-diately after recovery each full-round core was cut in-to 1.5-m-long sections, capped, taped, annotated, andstored upright at 4°C in the ship's cold room. Upon ar-rival in Japan the cores were air-freighted to the DSDPrepository at Scripps Institution of Oceanography (SIO),where they were again stored upright at 4°C prior togeotechnical sampling in September 1982.

Members of the consortium institutions and labora-tories were present for all geotechnical sampling. Priorto sub-sampling, each core section was examined visu-ally through the core liner to identify areas of obviousdisturbance and to note major changes in lithology. Theselithologic changes were correlated with shipboard core

Core Units

20

40

60

1

2

3

4

5

6

7

SUBUNIT IA 0 to 28 m (contact transition extends overseveral meters). Yellowish brown to brown pelagic clay ofPliocene and Quaternary age, with an estimated sedimenta-tion rate of 10 m/m.y. to 3 m/m.y. Based on previousstudies and the abundance of silt-sized quartz, the ship-board scientists estimated that this unit is largely of eolianorigin.

SUBUNIT IB 28 to 55 m. Dark brown pelagic clay, veryfine grained, partly zeolitic. Initial shipboard estimatesof sedimentation rate: 2 m/m.y. to 0.2 m/m.y. (uncor-rected for consolidation).

UNIT I IThis unit extends from a depth of 55 m to the bot-

tom of Hole 576A and consists of interbedded dark brownpelagic clay similar to Subunit IB and pale brown nanno-fossil ooze.

80 L

Figure 2. Generalized description of the lithologic section recovered atHole 576A.

descriptions for Holes 576 and 576B to develop a core-processing plan that would adequately sample each ma-jor lithologic unit.

The core-processing procedure involved cutting the1.5-m full-round sections into shorter, full-round sections(15 to 40 cm) that were capped, taped, and waxed forsubsequent detailed geotechnical analyses. Consortiummembers hand-carried these sections from SIO to theirrespective laboratories, where they were radiographed, de-scribed, and stored in a cold room prior to being tested.

724

GEOTECHNICAL PROPERTIES, HOLE 576A

Analyses performed on the ends of the shorter, full-round sections before capping and sealing at SIO includ-ed visual core descriptions and measurements of vaneshear strength, water content, bulk density, and com-pressional velocity. Compressional-velocity measurementswere performed parallel to bedding; the vane shear shaftwas oriented normal to bedding. In addition, clay min-eralogy and fabric samples were obtained from the coresection ends for laboratory analyses. All full-round coresections not selected for laboratory analyses at that timewere cut into 25-cm full-round sections and stored verti-cally in the DSDP repository cold room for possible fu-ture sample requests.

Additional core processing and sampling were perform-ed by the consortium in June 1983. At that time, a num-ber of remaining full-round sections were distributed tointerested consortium laboratories before longitudinallysplitting and describing the remaining full-round sections.Geotechnical measurements were performed on the splitsections in the manner described above, except that in thevane shear measurements the vane shaft was oriented pa-rallel to bedding.

TEST PROCEDURES AND METHODS

Sediment Characterization and Index Properties

Table 1 summarizes the tests performed during thisstudy. For most of the standard geotechnical tests (e.g.,index properties, sediment classification), the appropri-ate American Society for Testing and Materials (ASTM)Annual Book of Standards is referenced (Table 1). Fornonstandardized tests (e.g., consolidation, triaxial tests),suitable references in the geotechnical literature are cit-ed. Variations from standard procedures (e.g., salt cor-rections) are also noted. Lambe (1951) defines the stan-dard geotechnical terminology (e.g., plastic limit, activi-ty) used throughout this chapter.

All samples hand-carried to the individual laborato-ries were X-rayed prior to subsampling to assess the de-gree of core disturbance or flow-in. If any visible signsof disturbance or flow-in were observed, the sample wasnot tested. Tabulated results include data obtained dur-ing both sample processing sessions at SIO and furtheranalyses at the individual consortium laboratories.

Consolidation and Permeability Tests

One-dimensional consolidation tests were performedusing three different laboratory procedures. The standardconsolidation test involves the incremental loading of arelatively thin, laterally confined sediment sample. Axi-al strain-time relationships are obtained by measuringthe change in sample height during the test. A completedescription of this technique can be found in Lambe(1951). A variation of this method involves the applica-tion of sufficient back pressure to redissolve gas bubblesand completely saturate the consolidation sample. Loweet al. (1964) describe the benefits of this method. A seri-ous drawback to both of the above methods is that morethan 15 days are required to complete a test. Another testused was the constant rate of deformation (CRD) test,which only requires a few hours per test. The CRD test

imposes a uniaxial deformation at a prescribed constantrate and provides a continuous record of the changes ofconsolidation parameters with respect to the applied ef-fective stress. Gorman et al. (1978), Znidarcic (1982), andZnidarcic and Schiffman (1983) describe the CRD meth-od in detail.

Two fundamental properties that contribute directlyto the consolidation process are compressibility and per-meability. Compressibility is defined as the amount ofsediment volume compression per unit increment of stress.The compression index (Cc) is calculated from the slopeof the void ratio-effective stress (e versus log σ') curveobtained from consolidation test results as follows:

C c =-Ae

( logσ')

where Ae is the void ratio change over a correspondinglog increment of the effective stress (σ') (Lambe, 1951).The compression index is reserved for a loading sequence.If an unloading sequence is being analyzed, the com-panion term is the expansion index, Ce.

Permeability is defined as the capacity of a sedimentto transmit fluid and is calculated by the ratio of thevolumetric rate of flow to the gradient that drives fluidthrough a cross-sectional area of sediment. Three meth-ods available to measure permeability directly include:(1) constant head, (2) falling head, and (3) flow pump.The constant-head method measures the flow rate underan applied pressure gradient. The falling-head method,which is the conventional method used for fine-grainedsediments, relates the flow rate to a constantly changinghydraulic gradient. These conventional tests are describedin Lambe (1951). The flow-pump method measures thepressure gradient while maintaining a controlled flow rate(Olsen, 1966). A critical assessment of each of these me-thods (Pane et al., 1983) has shown that permeabilityresults for soft clays may be biased by seepage-inducedconsolidation unless very small pressure gradients areused.

In addition to measuring permeability directly, two in-direct methods were used to deduce the coefficient ofpermeability from the theoretical consolidation relation-ships (Znidarcic and Schiffman, 1983). The first meth-od uses the results from the standard consolidation testand is based on linear, infinitesimal strain consolidationtheory. The second method uses CRD test results and non-linear, finite strain, consolidation theory to calculate thecoefficient of permeability.

Strength Tests

Several different types of strength tests were perform-ed on Hole 576A sediments. In addition to the labora-tory vane shear tests performed during the core process-ing, three other types of strength tests were completed:(1) unconsolidated-undrained triaxial compression (UU);(2) isotopically consolidated, undrained triaxial com-pression (CIU); and (3) anistropically consolidated-un-drained triaxial compression (CAU; Table 1). Samplesfor these tests were prepared by cutting a length of core(typically 100 mm) contained in a plastic core-liner tube

725


Table 1. Summary of referenced procedures used in the geotechnical property analysis of Hole 576A sed-iments.

TestReference fortest procedure

Variations fromstandard procedure

Sediment characterizationIndex properties

Clay mineralogyWater contentLiquid limitPlastic limitSpecific gravity

Wet-bulk density

Grain size (pipette method)Compressional wave velocity

Consolidation

Consolidation (standard)

Consolidation (back-pressured)Consolidation (constant rate of strain)

Permeability

Constant headFalling headFlow pump

Strength Tests

Vane shear

Uncons^üdated-undrainedConsolidated-undrained

isotopicallyanistropically (KQ)

Heath and Pisias (1979)ASTM D2216ASTM D423ASTM D424ASTM D854

ASTM D2937

Folk (1974)Hamilton (1965)

ASTM D2435

Lowe et al. (1964)Znidarcic and Schiffman (1983)

Olsen (1966)Silva et al. (1981)Pane et al. (1983)

ASTM D2850

Bishop and Henkel (1962)Silva et al. (1983)

Talc internal standardSalt corrected, Noorany (1984)Salt corrected, Noorany (1984)Salt corrected, Noorany (1984)Some tests done using air-compari-

son pycnometersSmall rings (1-in. diam. and less)

and piston subsamples used incore

DesalinatedMeasurements made directly in the

sediment, not through the coreliner

Load increment ratio of 0.5 used insome tests

Rotation rate = 60°/min.Vane diameter and height 12.7 mm

Note: Variations from the referenced standard procedures are also noted in the text.

with either a hand saw or a high-speed saw. After cut-ting, the sample was extruded directly from the core lin-er or the liner was split longitudinally and a wire sawwas then used to separate and remove the sample fromthe core-liner halves. The sample was then trimmed to afinal diameter of 35 mm and a height of 70 to 85 mmusing a vertical sample lathe and a wire saw.

SEDIMENT CHARACTERIZATION AND INDEXPROPERTIES

Clay Mineralogy

Quantitative clay mineralogy results on sediments ob-tained with piston cores close to Site 576 are consistentwith known clay mineral distribution patterns in the NorthPacific (Leinen and King, in press; Heath and Pisias,1979; Griffin et al., 1968). Leinen and King reported da-ta from samples located at the Brunhes/Matuyamaboundary (ranging in sub-bottom depth from 3.84 to5.92 m) within a lOMcm2 area around Site 576 andfound virtually no variation in the <20 µm opal-freemineral assemblage. Illite is the dominant clay mineralpresent (-39%), with lesser amounts of chlorite (~ 19%)and minor amounts of kaolinite and smectite (~ 10%).Detailed downcore clay mineral analysis on a piston corelocated approximately 10 km from Site 576 shows a slight

increase in smectite (5-6% to 12-15%) and decrease inchlorite (-38% to 20-25%) with depth to 12 m sub-bottom. Leinen and King (1985) indicate that these sub-tle changes suggest either a change in the weathering re-gime in the continental source areas for the eolian mate-rial or a change in the dominant wind patterns thattransport the mineral aerosol to the northwest Pacific.

Preliminary clay mineral analyses on Hole 576A sedi-ments have emphasized sediments in Subunit IB below30 m (Table 2, M. Leinen, pers, comm., 1984). Resultsfrom one near-surface sample obtained within SubunitIA are, very similar to the piston core mineralogical re-sults summarized above, except for a slightly higher il-lite component (50% as opposed to 39%) and a lowerkaolinite component (trace amounts as opposed to— 10%). Using a talc internal standard, clay mineral re-sults from Subunit IB indicate the single most importantfactor influencing the mineralogy is the pronounced down-hole increase in the ratio of amorphous to crystallinematerial. This increase is particularly evident below 45 msub-bottom (Table 2) and is attributed to a probable in-crease in the amount of iron oxides, manganese oxides,and very fine-grained material with insufficient crystalsize to produce good diffractions. Another possible causecontributing to this marked increase in X-ray amorphousmaterial may be related to the effects of grain size on the

726


Table 2. Preliminary mineralogy results from Hole 576A sediment.a

Core-Section

1-14-24-54-65-56-16-36-4

Sub-bottomdepth(m)

0.2330.2533.9135.8144.5947.6150.6553.99

Smectite(%)

89

136

14372

IlliteW

5023314630

8117

Kaolinite(%)

1563

10321

Chlorite(%)

22664

10242

Plagioclase(%)

33

2

Quartz(%)

1794 '53455

Total(%)

10055616568263020

Percentamorphous

045393532747080

Smectite:illiteratio

0.160.390.420.130.470.380.640.29

Note: Relative percentages of each component were determined using a talc internal standard and weighting factors developed forNorth Pacific sediments by Heath and Pisias (1979).

a M. Leinen, unpublished observations.

standardization procedure (M. Leinen, pers, comm.,1984).

Mineralogical analyses on Hole 576A sediments usedfor consolidation testing (E. Taylor, pers, comm., 1984)reveal a downhole increase in the relative percentage ofsmectite over illite in the crystalline clay-sized fraction.However, the much larger increase in the amount of X-ray amorphous material present in the clay-sized frac-tion (up to 80%, Table 2) over the same intervals wouldbe the controlling mineralogical factor influencing thegeotechnical behavior of Hole 576A sediments.

Electron Microscopy

Scanning and transmission electron microscopy (SEMand TEM) and energy dispersion X-ray spectrometry(EDS) were performed on select Hole 576A samples toevaluate particle-to-particle relationships, orientations,and arrangements. These relationships may prove criti-cal to enhancing our understanding of the geotechnicalbehavior of these sediments.

SEM micrographs within Unit I (Fig. 3) show an open,flocculated structure consisting of a series of linked clayaggregates. Intergrown clay particle domains evident aslocally continuous particle arrangements are apparent inseveral micrographs. The origin of these intergrown do-mains is uncertain, although they may result from theincipient growth of authigenic clay minerals, silica com-pounds, or iron oxides. Indications of cementation be-tween clay-particle contacts are evident in Figure 3D.EDS analyses on the contacts between a silt grain andother particles show an enrichment of potassium and ironthat suggests cementation (Fig. 4) although, because ofthe limited resolution of the EDS system, no definite con-clusions can be reached.

Transmission electron micrographs from Subunit IA(22.17 m sub-bottom) and Subunit IB (46.02 m sub-bot-tom) show a very complex clay microstructure consist-ing of fine-grained, high void ratio clays occurring asclay floccules or as domains oriented face-to-face (Fig. 5).Authigenic minerals and larger opaque particles are of-ten present.

Water Content

Water contents are quite variable within all of the sed-iments in Subunit IA (Fig. 6 and Table 3). At the sedi-

ment surface (< 50 cm sub-bottom) water content val-ues range from 130 to 164%. In general, water contentthen increases with depth to approximately 10 m sub-bottom where the values range from 156 to 223%. Be-low 10 m sub-bottom both the absolute values and therange of water-content values decrease down to andthrough the transition zone (~ 20 to 28 m sub-bottom)as the sedimentary section gradually changes to SubunitIB. Values range from 102 to 142% over this interval. At28 m, water content is significantly higher than in theimmediately overlying transition zone.

Throughout Subunit IB (28-55 m sub-bottom) watercontents generally decrease (Fig. 6 ). Distinct intervals(~ 5 m) of increasing and decreasing water-content val-ues probably reflect subtle lithologic changes within Sub-unit IB and are not a result of variations in interlabora-tory test procedures. These cyclic variations are also evi-dent in many of the other properties discussed below.

A pronounced change in water content occurs at thecontact between the overlying pelagic clay section (UnitI) and the interbedded carbonates and pelagic clays ofUnit II (Fig. 6). Values at the base of Unit I (55 m sub-bottom) range from 110 to 120%; whereas within Unit IIbelow 55 m, water content values decrease to near 50%within the carbonate oozes. Water content within Unit IIdecreases slightly with depth. The occasional high wa-ter-content values observed in Unit II are associated withthe interbedded pelagic clay layers in this unit. Thesehigh values are similar to those reported at the base ofSubunit IB.

Atterberg Limits

Small, local downcore variability in both liquid andplastic limit values (Fig. 7 and Table 3) probably resultsfrom minor procedural differences between laboratories.This variability does not, however, obscure the impor-tant downcore trends. Liquid limit exhibits a general in-creasing trend from 131% very near the seafloor to 186%at approximately 16 m sub-bottom, while the plastic limitfluctuates about an average value of 54% over the sameinterval. Liquid limits above 20 m sub-bottom are gen-erally less than the natural water contents resulting in li-quidity indices (/L) of greater than 1.0 (Fig. 8). Below20 m sub-bottom, 7L values decrease gradually through-out the remainder of Subunit IA to a minimum of ap-

727


Figure 3. Scanning electron micrographs of sediments located at 11.6 m (A), 15.6 m (B), and 43.3 m (C) sub-bottom showing intergrown clay parti-cle domains (A, B, C), a flocculent microstructure (B), and authigenic material (C). Arrows on micrograph D (11.6 m sub-bottom) show possibleindications of interparticle cementation. Bar scale on micrographs A and D equals 1 µm and on B and C it equals 2 µm.

proximately 0.4. Plastic limits remain relatively constantabout an average value of 54%, whereas the liquid lim-its decrease to a low of 140% at 23 m sub-bottom andthen begin to increase again through the transitional zoneto Subunit IB.

Liquid and plastic limit as well as plasticity index (7P)values are consistently higher in Subunit IB than in ei-ther Subunit IA above or Unit II below (Fig. 7). This isalso evident on the standard plasticity chart (Fig. 9; Ca-sagrande, 1948) which shows that the changes in Atter-berg limits correspond with the major lithologic divi-sions. As in Subunit I A, increases and decreases in liq-uid and plastic limit values in Subunit IB correspondwith zones of increasing and decreasing water content.Liquid and plastic limit values average 205 and 82%, re-spectively.

Liquidity indices (Fig. 8) are quite uniform within Sub-unit IB, averaging 0.32. This region of uniform 7L valuescoincides almost exactly with the depth interval repre-sented by Subunit IB (28 to 55 m sub-bottom), suggest-ing that Subunit IB can be clearly distinguished usingthe liquidity index.

Atterberg limit data clearly define the contact betweenUnit I and the underlying carbonate ooze of Unit II.Liquid and plastic limit values for the oozes, averaging54 and 30%, respectively, are lower in Unit II than inUnit I. Unit II carbonate sediments occur in the low-plasticity region of the standard plasticity chart (Fig. 9),which reflects the difference in the geotechnical behav-ior between the lower plasticity carbonate oozes and thehigher plasticity pelagic clays. One Atterberg limit teston an interbedded pelagic clay interval at 64 m indicates

728


Energy

Energy

Figure 4. Electron dispersive X-ray spectrometry (EDS) results. (A)EDS results obtained on individual particles present at 7.25 m(solid line) and at the particle contacts (dots) which suggest inter-particle cementation. (B) EDS results from intergrown domainspresent at 15.56 m sub-bottom showing iron enrichment which al-so suggests possible interparticle cementation.

that these interbedded clays are very similar to the pe-lagic clays tested in Subunit IB. Liquid limits, for themost part, do not exceed the natural water content, re-sulting in higher liquidity indices for Unit II sedimentsthan for the overlying Subunit IB sediments.

Grain Size and ActivityGrain-size data vary ± 10% around the mean for Hole

576A sediments (Fig. 10). This variability is attributedto natural variations in the sediment components accu-mulating in this area and slight differences in interlabo-ratory test procedures. From the seafloor to approximate-ly 12 m sub-bottom, the silt content increases from about34 to 45%; whereas sand percentages vary between 0 and7%. This general downward coarsening trend to 12 msub-bottom appears anomalous because water contentand liquid limit increase over this same interval. Thenormal trend generally shows water content increasingas grain size decreases. This coarsening trend, plus in-creasing values of activity over this interval (Fig. 11,~0.9 to 2.7), suggest that the mineralogy may vary sig-nificantly from that reported in the preliminary mineral-ogical analyses. An alternative explanation may be that

Figure 5. Transmission electron micrographs of sediments located at22.17 m (A) and 46.02 m (B and C) sub-bottom showing a complexclay microstructure of particle domains, floccules, and zeolitic ma-terial. Large semispherical feature at the top of B is microcrystal-line zeolitic material.

the clay-sized fraction was not completely dispersed pri-or to obtaining aliquots for size analysis. In either case,these unusual trends are consistent with consolidationand shear-strength results presented later in this chapter.

From approximately 12 m to the base of Subunit IAat 28 m sub-bottom (Fig. 10), the clay-sized fraction grad-ually increases as the silt-sized fraction decreases. Thegradual increase in the clay-sized fraction probably re-flects a decrease in eolian input to this area and marksthe transition from Subunit IA to Subunit IB (Site 576chapter, this volume). Activity becomes less variablethrough this transition interval with values approachingthose evident near the seafloor (~ 1.1). These values aremore typical of the relationship one normally expectsbetween grain size and plasticity.

729


130 180Water content (%)

230 280

Figure 6. Water content-depth profile for Hole 576A sediments. Solidcircles (•) below 55 m represent values obtained in carbonate ooze(% dry weight; salt corrected for 359/o).

From the transitional material of Subunit IA (20 to28 m sub-bottom) to the pelagic clay in Subunit IB, theamount of clay-sized material reaches a maximum andthen remains relatively constant (Fig. 10; - 8 5 % clay-sized fraction). Although some scatter in the grain-sizedata exist, within certain intervals of Subunit IB the ano-malous correlation between grain size, water content, andplasticity identified in Subunit IA is also evident. This ismost apparent between 40 and 50 m sub-bottom. As thelikelihood of a large silt- and sand-sized fraction is min-imal within these authigenic clays, the most likely expla-nation for these relationships is that the coarse fractionconsists of bonded or cemented clay aggregates and thatthe dispersion technique used in the size analyses (Folk,1974) is not" adequate for these aggregates. As mentionedearlier, clay-fabric analyses show indications of cemen-tation in these sediments.

A large increase in the silt-sized fraction occurs at thecontact between Subunit IB and Unit II. Below this con-tact, the grain-size profile in the carbonate sedimentsbecomes more uniform with an average of 54% silt. In-terbedded pelagic clay layers contain approximately 75%clay, with the remainder being silt. Activity values clear-ly define the carbonates and clays within Unit II (Fig. 11).In general, the carbonate activity values are the lowestobserved in Hole 576A sediments.

Bulk Density

Increasing water content normally corresponds withdecreasing bulk density in sediments having constant spe-cific gravity. However, no clear decreasing trend is evi-

dent in the upper 12 m of Subunit IA (Fig. 12 and Ta-ble 3). This may result from variable mineralogy or spe-cific gravity related to the increasing grain size throughthe interval. From 12 m sub-bottom depth to the base ofSubunit I A, increases in bulk density correspond to de-creases in water content. This general trend continuesdown through Subunit IB where bulk density increasesto a maximum of 1.50 Mg/m3.

A large break in the bulk-density profile marks thetop of Unit II at 55 m sub-bottom (Fig. 12). Values inthis unit are variable but are the highest observed inHole 576A, averaging 1.71 Mg/m3. No trend with depthis readily apparent in Unit II, possibly because of thedifficulty in obtaining an undisturbed sample in calcare-ous sediments. The higher bulk-density values occur inthe carbonate oozes.

The measured bulk-density values were used to calcu-late the effective overburden stress-depth profile forHole 576A sediments, assuming hydrostatic conditions(Fig. 13; Shephard et al., 1978). This profile was usedwith consolidation test results to determine the state ofconsolidation.

Compressional Velocity

Compressional velocity data display a large degree ofscatter throughout the recovered sedimentary section (Fig.14 and Table 3). Values average 1489 m/s (σ = ±5.5 m/s)in Subunit IA and 1496 m/s (σ = ±4.0 m/s) in SubunitIB. These velocities approximate the velocity of seawa-ter. Velocity data in Unit II show a pronounced increaseto 1536 m/s (σ = ±15.50 m/s), corresponding to thechange in lithology from the overlying pelagic clays to theunderlying carbonate oozes.

Summary

Sediment characterization and index-property data de-lineate three well-defined units that appear to be consis-tent with the major lithologic divisions delineated in theSite 576 chapter (this volume). Sediments from SubunitIA (0-28 m sub-bottom) have water contents that in-crease from the seafloor to approximately 12 m sub-bot-tom and then gradually decrease to 28 m sub-bottom.Over this same interval (0-28 m) grain size and activi-ty display similar trends, resulting in anomalous index-property relationships. Although inconsistent with ac-cepted geotechnical relationships, these results are sup-ported by the consolidation and shear strength datapresented later. Liquidity indices are generally greaterthan 1.0 in the upper 20 m, indicating possible overcon-solidation to at least that depth. Limited mineralogyresults indicate that illite is the dominant clay mineralpresent.

Results from Subunit IB (28-55 m sub-bottom) dis-play a marked change from the overlying sediments inSubunit IA. Water contents gradually decrease with depthand grain size appears much more uniform. Changes inbulk density are well correlated with the changes in wa-ter content. Atterberg limit results show much higher plas-ticity indices and lower liquidity indices than in the over-lying sediments in Subunit I A. Mineralogy results showthe amount of X-ray amorphous material in Subunit IBincreases markedly over that present in Subunit I A.

730


Table 3. Summary of sediment characterization and index-property analyses performed on Hole576A sediments.

Core-Section

Sub-bottom Water Bulk Compress.depth content density velocity

(m) (%) (Mg/m3) (m/s)

Shear Liquid Plasticstrength limit limit Liquidity

(kPa) (%) (%) index

1-11-11-11-11-11-11-11-11-11-11-11-11-11-11-11-11-11-21-21-21-21-21-21-21-21-21-21-21-21-21-21-21-21-21-21-21-21-21-21-31-31-31-31-31-31-31-31-31-31-41-41-41-41-41-41-41-41-41-41-41-41-41-41-41-41-41-41-41-41-41-51-51-51-51-5

0.140.170.230.240.260.330.420.450.470.490.550.580.650.740.890.931.341.641.651.671.701.751.801.841.891.941.982.082.092.142.162.202.252.352.392.442.452.592.843.003.243.343.583.603.843.863.934.024.144.524.664.714.734.854.884.944.985.245.255.506.016.056.156.186.246.306.496.526.746.997.037.057.157.217.24

163164152

130

130140

155159153147

225

211

211

263

196162

151175

147168

156153146146149

146

140149

175

166

161175

156

170183199

197181

202211147

122162

157200

173

1.33

1.38

1.331.34

1.25

1.24

1.361.34

1.371.341.371.36

1.36

1.37

1.29

1.34

1.34

1.32

1.29

1.36

1.411.34

1.33

1482

1496

1483

1487

1502

1491

1484

14821485

1486

1484

14761478

1498

1485

1483

1489

1496

1488

1.961.181.86

6.37

4.5

8.13

5.685.295.88

5.29

3.63

4.21

2.65

6.9610

7.74

14.418.43

5.29

9.6

10.397.74

6.37

10.68

9.3113.62

12.25

8.23

11.179.02

7.15

14.41

6.379.6

15.688.53

9.31

131

127

150

135

121

148

74

47

49

67

1.39

1.3

0.93

1.37

44

75

51

1.46

1.7

1.9

731


Table 3. (Continued).

Core-Section

1-5-5

1-5-5-5-5-5-5-5-5-5-5-5

1-51-51-51-52-12-12-12-12-12-12-12-12-12-12-12-12-12-22-22-22-22-22-22-22-22-22-32-32-32-32-32-32-32-32-32-32-32-42-42-42-42-42-42-42-42-52-52-52-52-52-52-52-62-62-62-62-62-62-62-62-63-13-1

Sub-bottomdepth

(m)

7.277.287.367.547.677.707.827.857.958.018.048.078.108.208.258.298.499.259.359.409.449.479.499.559.659.679.699.94

10.0010.0910.2210.2610.7810.8910.9211.0911.1111.4411.6511.8611.9611.9712.0612.1612.1912.2412.4212.6312.8012.8313.3013.3513.4013.4913.5213.6813.7314.0014.8014.9014.9414.9915.5315.5415.6116.2216.2316.4216.4416.8017.3917.4217.6917.7318.2218.25

Watercontent

(%)

167

163175

170183176176

214

179171

161

205192

198209

186177

156189186204

223198184195203205157195153193153

183216195163

171144178178179162

141157

173

185199174149170

148161146

143

Bulkdensity

(Mg/m3)

1.35

1.37

1.27

1.35

1.31.35

1.361.3

1.28

1.311.371.281.291.34

1.4

1.29

1.31.33

1.31

1.34

1.361.411.35

Compress.velocity(m/s)

1482

1486

1486

1483

1490

1496

14881494

1497

1487

1478

1488

1493

1488

Shearstrength

(kPa)

5.8816.27

5.88

13.8218.62

8.2321.854.317.45

8.82

16.569.9

9.19.02

14.4111.17

12.849.6

8.53

10.09

12.258.825.88

49.29

29.310.09

38.0228.22

11.76

19.5

13.6211.96

15.97

Liquidlimit(<%)

135

118

154

176

167

170

186

162

173

Plasticlimit(%)

39

42

43

36

56

87

43

44

62

Liquidityindex

1.33

1.62

1.13

0.98

0.83

0.99

1.07

1.1

732



Core-Section

3-13-3-3-3-3-3-3-3-3-3-3-13-13-13-13-13-13-23-23-23-23-23-23-23-23-23-33-33-33-33-33-33-33-33-43-43-43-43-43-43-43-43-43-43-53-53-53-53-53-53-53-53-53-63-63-63-63-63-63-63-63-63-63-63-63-63-63-63-64-14-14-14-14-14-14-1

Sub-bottomdepth

(m)

18.3518.3618.4418.6918.9418.9719.0019.0319.1319.1419.1919.3019.3419.4019.4419.5219.5320.6120.6420.6720.6920.7320.7420.7620.9320.9721.8022.1422.2422.3422.3522.3922.4222.5922.7222.7422.8022.8222.8422.8822.8922.9623.7723.8924.2524.2724.3124.3424.6424.8025.4225.4425.4925.7425.7525.9225.9625.9926.0226.2326.3326.5726.7926.8526.8926.9927.0027.0427.1927.9428.0028.1528.1928.3528.3928.44

Watercontent

(%)

144

135143151147

146147

135143

148137

150116

123

118

119123

121132135135

130

127

126

102103

111117121116125

122132142119

123122111

115

113121

119119114

117116

119123138

141134135

138

Bulkdensity

(Mg/m3)

1.381.361.351.42

1.37

1.36

1.41

1.48

1.42

1.37

1.39

1.44

1.461.4

1.41.42

1.39

1.411.43

1.51.42

1.41.411.34

1.36

1.36

Compressvelocity(m/s)

1505

14791490

1489

1490

1484

14921478

1491

1481

1490

1486

1485

1483

1487

1485

1490

1486

1488

1488

14881492

1494


(kPa) (%) (%) index

18.62

11.9611.96

20.2923.42

16.2719.99

20.6811.76

18.03

13.03

14.921.07

21.36

14.11

15.68

24.89

23.6215.48

14.919.99

15.19

22.8316.2726.66

28.03

15.48

16.56

25.2824.7

16.5617.353.23

13.039.6

17.64

10.39

145 47 1.05

162

163153

93

8098

0.37

0.510.4

140 42 0.7

149 50 0.7

168167

7564

0.490.54

733



Core-Section

4-14-14-14-14-14-24-24-24-24-24-24-24-24-24-24-24-24-24-34-34-34-34-34-34-34-34-34-44-44-44-44-44-44-44-44-44-44-44-44-44-44-44-54-54-54-54-54-54-54-54-54-54-54-54-54-54-54-54-64-64-64-64-64-64-64-64-64-64-64-65-15-15-15-15-15-1

Sub-bottomdepth(m)

28.4728.5028.6928.9429.0929.3729.3929.4629.4729.5329.7530.0930.2530.3430.3830.4030.4230.6530.7331.2231.4631.5131.5231.5431.6331.6731.8232.2432.3132.3932.4632.4732.7532.8732.9633.0133.0733.1933.2133.2633.3133.5533.7733.8133.9233.9534.0334.0734.1734.2434.2934.3934.4334.4734.5034.5434.5634.6235.2935.4135.4735.5435.8235.9736.2136.2336.2936.3436.3636.4237.2937.3037.3237.4437.8637.95

Watercontent

(%)

131

140

144

139138146143

137139149145150153

154

153144146

151

156161

134130

148136

102

180152

155

148146

159

135136131124

130127

125124120

119

102

139

139

Bulkdensity

(Mg/m3)

1.37

1.35

1.41.36

1.331.31

1.351.35

1.341.35

1.38

1.33

1.4

1.34

1.36

1.42

1.391.431.4

1.41

1.35


1490

1492

1494

1499

1498

1497

1495

1495

1494

1502

14921494

1497

1500

1493

1496

1495

1492

Shearstrength

(kPa)

10.6811.969.02

18.62

10.39

11.96

18.62

24.7915.6815.48

34.01

18.62

33.3220.48

32.8335.28

22.34

35.9739.69

22.34

36.2621.85

33.3254

38.61

60

21.8537

24.3

17.35

283324.89

Liquidlimit(%)

182

185

186

220

213

236209

217211

Plasticlimit(%)

71

48

77

112

101

8273

10581

Liquidityindex

0.7

0.7

0.56

0.29

0.3

0.340.5

0.230.38

734



Core-Section

5-5-5-5-5-5-5-5-5-25-25-25-25-25-25-25-25-25-25-25-35-35-35-35-35-35-35-45-45-45-45-45-45-45-45-45-55-55-55-55-55-55-55-55-55-55-55-55-55-55-55-65-65-65-65-65-65-65-65-65-65-65-75-75-75-76-16-16-16-16-16-16-16-16-16-26-2

Sub-bottomdepth(m)

37.9738.0838.1538.1738.2838.3438.5938.7338.9439.1039.1239.2039.2339.2439.3739.7440.0040.0940.1940.2840.3640.7441.0241.0741.2941.5941.7341.9442.0242.2542.6542.7342.7842.8942.9143.2343.2543.2943.3743.3843.3943.5243.5443.5943.7944.0044.4344.5744.5944.6144.8044.8644.9444.9745.5945.6045.7745.9946.0946.1346.1946.3546.4446.4746.6946.9446.9847.2047.3747.4247.5247.5947.6047.6747.7047.84

Watercontent

(<%)

136127

130

130130

130

136141

142129139

130

142

127114

116

115

117110

104

101

106105

104104

111

106113111

10912394

123128123132

120130123125118142144145137144

132138

Bulkdensity

(Mg/m3)

1.46

1.371.37

1.37

1.37

1.381.38

1.37

1.421.48

1.45

1.46

1.5

1.46

.44

.46

.44

.43

.3

.41

.471.4

1.41

1.41.441.411.29

1.371.37

1.37


1482

1494

1493

1493

1485

1501

14871504

1498

1490

1496

1495

1493

1492

1491

1495

Shearstrength

(kPa)

22.93

442930.673425.09

36

3023.72

40.19

54

4949

34.69

38

37.3446

81

36.65

17

86

9439.3

81

26.66

3622

26.66

33

31.7565

Liquidlimit(o/o)

224

229

210

208205

166

170

184197

184

202

218

246

Plasticlimit(<%)

96

96

74

8385

64

58

6173

74

79

88

91

Liquidityindex

0.36

0.35

0.39

0.260.25

0.45

0.43

0.380.31

0.35

0.36

0.27

0.35

735



Core-Section

Sub-bottom Water Bulk Compress.depth content density velocity(m) (%) (Mg/m3) (m/s)


(kPa) (%) (%) index

6-26-26-26-26-26-26-26-26-26-26-26-26-36-36-36-36-36-36-36-36-36-36-36-36-36-36-36-46-46-46-46-46-46-46-56-56-56-56-56-56-56-56-56-56-56-56-56-66-66-66-66-66-66-66-66-66-66-76-76-76-76-77-7-7-7-7-7-7-7-7-17-27-27-27-27-2

48.2148.4448.4748.5548.6148.6948.8048.9549.4049.4849.5949.6549.7849.8749.9349.9450.0050.0750.2650.3150.3450.7050.7850.8650.9550.9951.0951.2451.3351.3652.2352.2952.4252.4952.7352.7652.8453.0553.0953.1053.3853.6553.6853.6953.8253.8753.9954.2254.3054.3254.3954.5854.6255.5755.6055.6455.6955.9455.9756.0556.1256.1756.2156.4456.6556.6956.9757.0857.1057.3257.5957.7457.8657.9458.7259.02

132130131139

116

121

112

127

119

121

112120

110122

114

118109121111120113116

109113

110122130

120102461024850

47121

6662

84119118

54

12793

6557

555356

565152

1.421.381.43

1.42

1.43

1.4

1.43

1.441.431.47

1.43

1.43

1.48

1.451.78

1.77

1.7

1.531.431.48

1.391.49

1.53

1.781.74

1.741.741.79

1497

1497

1503

1493

1493

1490

1490

1506

1499

1564

1502

1504

15151543

1506

1524

81

30.97

89

35.48

54

29.69

5226.95

4424.01

26

19.312635.9765

46.35

10544

108

226 97 0.27

227 71 0.3

207 83 0.31

206 96 0.22

194 86 0.35

186 84 0.35

55.96

36.2659 29 0.63

2.13

1510

1525

1515

45.96

22.15

42.83

1311.374.8

5.598.53

4.0254 31 0.87

736



Core-Section

7-27-27-27-37-37-37-37-37-37-37-37-37-37-37-37-47-47-47-47-47-47-47-47-47-47-57-57-57-57-57-57-57-57-57-57-57-57-57-57-57-57-57-57-67-67-67-67-67-67-67-67-67-77-77-77-77-7

Sub-bottomdepth

(m)

59.0459.1359.1959.3059.3259.3959.5759.6159.6559.7959.9860.0360.0960.5460.6060.7061.1561.1761.2061.2461.4061.5361.5961.7661.9962.2362.4362.5062.5562.6362.7062.7862.8362.8463.0263.0863.1763.1863.4163.4763.5363.5963.6163.8763.9168.9563.9964.2264.2864.8464.8765.0965.2565.3465.3965.7565.89

Watercontent

(%)

53

4949

5252

51537172

567451

53

5353

54

121

54

5253

52

46

48515150

49113

51534649

47

51

Bulkdensity

(Mg/m3)

1.76

1.731.78

1.76

1.711.84

1.73

1.71

1.74

1.751.721.8

1.8

1.761.781.79

1.79

1.66

Compress.velocity

(m/s)

1546

1553

15181538

1528

1546

1539

1518

1546

1522

1544

1520

1546

1516

Shearstrength

(kPa)

5.29

7.4519.5

5.29

11.175.59

14.7

8.82

1786

9.02

5.88

17.64

9.31

17.84

7.74

4.02

8.2311.96

6.3770.66

129

Liquidlimit(%)

53

56

49

40

45

240

Plasticlimit(%)

31

28

30

32

88

Liquidityindex

0.99

1

1.15

2.31

1.51

0.13

Results from Unit II (55-64 m sub-bottom) reflect thelithologic change from predominantly pelagic clays tocarbonate ooze interbedded with pelagic clays. Water con-tent and activity values for the carbonates are the lowestobserved in Hole 576A; whereas bulk density and com-pressional velocity values are the highest observed. At-terberg limit data show low plasticity and liquidity indi-ces in the carbonates.

CONSOLIDATION AND PERMEABILITYCHARACTERISTICS

Skempton (1970) defined consolidation as the resultof all processes causing the progressive transformation

of an argillaceous sediment from a soft clay to a shale.These processes include: (1) interparticle bonding, (2) des-sication, (3) cementation, and (4) the squeezing out ofpore water under the increasing weight of overburden.The effect of each process on consolidation varies throughtime.

Consolidation tests are used to evaluate the sedimentresponse to an applied load. These results provide in-sight to the relative degree of consolidation the sedimenthas experienced in situ under the imposed load of thesedimentary column. The state of consolidation is deter-mined using the ratio of preconsolidation stress (σc0 tothe present effective overburden stress (σó). Preconsoli-

737


y X '–

*

*

x—X —

>—m

> r K

^ " • *

3f s5

MMini

^ j^iC

*‰ *

*

-~*

**l £*

X

Y'

-a

* * *

X

*

*

-x * 4

*

i i i i

*

*

-

X

K

-

i i I i i i i

30 80 130 180Water content (%)

230 280

Figure 7. Atterberg limit and water-content results obtained from Hole576A sediments. Liquid and plastic limit values fall on the rightand left side of the profile, respectively. Solid circles (•) below 55 mrepresent values obtained in carbonate ooze.

dation stress, defined as the maximum effective stressthe sediment has experienced, was calculated using thegraphical reconstruction technique of Casagrande (1936).The value of σ is calculated assuming hydrostatic condi-tions. The ratio of the preconsolidation stress to the ef-fective overburden stress yields the overconsolidation ra-tio (OCR = σi/σ ).

A sediment is considered normally consolidated if thepresent effective overburden stress is the greatest ever im-posed. Thus, for a normally consolidated sediment thepreconsolidation stress equals or closely approximates theeffective overburden stress (σc' = σó). A sediment is over-consolidated if it has been consolidated under a stressthat exceeds the present effective overburden stress (σc'> σó). Sediment that has not fully consolidated underthe present overburden stress is underconsolidated. Forunderconsolidated sediments the pore-water pressure ex-ceeds the hydrostatic pressure and the preconsolidationstress will be less than the effective overburden stress (σc'< σó). Thus, sediments with overconsolidation ratios ofapproximately 1.0 are considered normally consolidated,greater than 1.0 are overconsolidated, and less than 1.0are underconsolidated.

Consolidation Test Results

Consolidation test results from Hole 576A sedimentsprobably represent the most extensive suite of consoli-dation data for any marine sedimentary section (80 con-

10

20

30

40

50

60

70

-I—i—i—i—i—r

' i i I i _! I I I L0.5 1.0 1.5

Liquidity index2.0 2.5

Figure 8. Liquidity index-depth profile for Hole 576A sediments. Val-ues greater than 1.0 indicate material with natural water contentsgreater than the liquid limit. Solid circles (•) below 55 m representvalues obtained in carbonate ooze.

solidation tests performed on 63.70 m of sediment; 1test every 0.80 m). This extensive testing program wasinitiated because the geological history of the area nearSite 576 is well-understood and the principles of consol-idation theory suggested that such data may be used todefine a model of the consolidation process for "a per-fect marine clay section" that has been continuously de-positional over the last 65 m.y. (Heath, Rea and Levi,this volume). All geological and geotechnical informa-tion available prior to beginning the testing program in-dicated that these sediments would be normally consoli-dated below the upper few meters. However, test resultsindicate that the sediments at Hole 576A are overcon-solidated in the upper 28 m, normally consolidated inthe interval between 28 and 39 m sub-bottom, and nor-mally to underconsolidated in the interval between 39and 55 m sub-bottom (Fig. 15; Tables 4 and 5). The car-bonate-ooze consolidation test results are included in thedata compilation for comparison with the pelagic clayresults even though these results are not emphasized inthe following discussion because of their differences ingeotechnical behavior and mode of deposition.

Typical e versus log σ' curves for tests performed inboth Subunit IA (0-28 m sub-bottom) and Subunit IB(28-55 m sub-bottom) reveal significant differences ininitial void ratios and the sediment response to both anapplied load and an unloading sequence (Fig. 16). Thesedifferences are also reflected in the average values of se-veral consolidation parameters and the permeability val-

738


200

150

£ 100

50

© o o

o

Θ

01

×o

o

o

oo

oX

*× X

×^ X X

V X

x X

50 100 150 200 250Liquid limit

Figure 9. Standard plasticity chart used for classifying fine-grained sediments (Casagrande, 1948). Casa-gande's A line is shown. Subunit IB sediments (X) are generally much more plastic than Subunit IAsediments (0) or the carbonate oozes (•).

100

Figure 10. Grain size-depth profile for Hole 576A sediments. Solidcircles (•) below 55 m represent values obtained in carbonate ooze.

Activity

Figure 11. Activity-depth profile for Hole 576A sediments. Solid cir-cles (•) below 55 m represent values obtained in carbonate ooze.

ues reported for each of the major lithologic units (Ta-bles 4 and 5). A compilation of the consolidation testresults is illustrated in Figure 17, which shows the voidratios at applied loads of 50, 100, 300, and 1000 kPaplotted versus sub-bottom depth.

The overconsolidated surficial sediments from Hole576A were not totally unexpected. Consolidation test re-sults from many different marine geological environmentsindicate that most near-surface sediments are in a stateof "apparent overconsolidation' (Hamilton, 1964; She-

739


10

20 -

~ 30

-9 40 -

50 -

60 -

70

* x

X

XX

X

x x

×

× × ^xx5>

X

XX

-

•

5?

*>

X

×

× X

<

× ×

i>

X

x x

1

1 1

X

X

X

X

8 XK x

X

x<

Xt X

*< X

•

1 1

1 1 1

-

-

•j

1 1 1

1.2 1.4 1.6 1.8

Bulk density (Mg/m3)

2.0

Figure 12. Bulk density-depth profile for Hole 576A sediments. Solidcircles (•) below 55 m represent values obtained in carbonate ooze.

100 200Effective overburden stress (kPa)

300

Figure 13. Effective overburden stress-depth profile computed usingbulk-density values for Hole 576A sediments. Effective overbur-den stress was calculated assuming hydrostatic pore-water pressureconditions using the equation of Shephard et al. (1978).

1425 1500 1525 1550Compressional velocity (m/s)

1575

Figure 14. Compressional velocity-depth profile for Hole 576A sedi-ments. Solid circles (•) below 55 m represent values obtained incarbonate ooze.

phard et al., 1978; Silva et al., 1980). Apparent overcon-solidation is generally attributed to the cohesion thatresults from the molecular interaction between solid par-ticles (Terzaghi, 1956). For clay sediments, Terzaghi sug-gested these cohesive forces are at least 5 kPa. In near-surface sediments these forces are large relative to thesmall overburden stress, resulting in a calculated precon-solidation stress value that is much larger than the effec-tive overburden stress and, consequently, an overconsol-idation ratio that is significantly greater than unity.

A number of mechanisms or processes can influencethe state of consolidation (Brumund et al., 1976). Themost probable mechanisms responsible for the overcon-solidation of the Subunit IA sediments are (1) changesin soil structure due to chemical alterations in the formof cementing agents and interparticle bonds, (2) the ef-fects of high hydrostatic pressure release during core re-covery, and (3) artifacts arising from the consolidationtest process. Dadey (1983) assumed that the effects ofcementation and interparticle bonding on the sedimentmicrostructure would be relatively constant with depth;thus, its significance would be gradually diminished withincreasing overburden. Consequently, as is evident inFigure 15, the overconsolidation ratio will decrease withdepth, eventually reaching a value that fluctuates aroundone. This occurs at approximately 25 to 30 m sub-bot-tom in Hole 576A sediments.

Brumund and Callender (1975) concluded that the ap-parent overconsolidation present in near-surface sedi-ments was related to the release of high hydrostatic pres-sures during sampling. Lee (1984) however, suggests that

740


0.16 0.25 0.5 1.00 3.00 5.00Overconsolidation ratio (OCR)

7.00 9.00

Figure 15. Overconsolidation ratio-depth profile computed using stat-ic-load consolidation test results from Hole 576A sediments. Over-consolidation scale reciprocal below 1.00, linear above 1.00.

clays exposed to hydrostatic pressures of 75 to 100 MPawould have an induced effective stress of only 2 to 5 kPa.This is not of sufficient magnitude to affect significantlythe preconsolidation stress values determined using theCasagrande method for Hole 576A sediments.

Scully et al. (1984) have shown that completely re-molded, normally consolidated sediments develop a fic-titious preconsolidation stress, which they attribute toseating of the porous stones and loading platens duringinitial testing and possibly to wall fraction developed dur-ing the consolidation test. This artificial preconsolida-tion stress is of uncertain magnitude, but may contrib-ute to the apparent overconsolidation observed in Hole576A sediments.

The sediments between 28 and 39 m sub-bottom ap-pear normally consolidated while those between 38 and55 m sub-bottom appear normally to underconsolidat-ed. In fact, OCR results indicate that in some instancesthe sediments between 39 and 55 m sub-bottom are asunderconsolidated as the sediments between the seafloorand 28 m sub-bottom are overconsolidated (Fig. 15). Themechanisms generally responsible for underconsolidationinclude high rates of sediment accumulation, chemicalalterations, and coring disturbance (Sangrey, 1977). Sed-iment accumulation rates at Hole 576A are generally muchless than 10 m/m.y. This rate is sufficiently slow to per-mit the dissipation of any excess pore water pressures

that may have developed during the sedimentation pro-cess. Available geochemical data for Site 576A (McDuff,this volume) suggest that any chemical alteration effectsshould be prevalent within all of Subunit IB and notlimited to only select intervals. Although coring and sam-pling disturbance may have contributed to the observedstate of underconsolidation, this effect is very difficultto quantify.

The discrepancy between the expected normal stateof consolidation and the consolidation test results strong-ly indicates that the technique employed for determiningthe preconsolidation stress (Casagrande, 1936) should becarefully evaluated. Various investigators, including Cool-ing and Skempton (1942), Bishop et al. (1965), and Sch-mertmann (1955), have concluded that the Casagrandemethod is inadequate to define the preconsolidation stressunder certain circumstances. Other methods used to de-termine the preconsolidation stress (Burmister, 1951;Schmertmann, 1955) require a rebound and reloading cy-cle.

Examination of the e versus log σ' curves (Fig. 16) andthe profile of expansion index versus sub-bottom depth(Fig. 18) shows that the amount of sediment expansionor rebound that occurs during unloading increases withdepth below the seafloor. As the expansion index in-creases, the error in the determination of the preconsoli-dation stress, as determined by the Casagrande method,also appears to increase. An examination of the unload-ing and reloading portions of the e versus log σ' curveat 32.47 m sub-bottom (Fig. 16) shows that the unload-ing portion of the hysteresis loop in this example startedat 400 kPa. Thus, the actual preconsolidation stress ofthe hysteresis-loop curve is 400 kPa. However, when thepreconsolidation stress is determined using the Casa-grande method on the hysteresis loop the preconsolida-tion stress is underestimated by 140 kPa (35%).

These results and the subsequent analysis of 16 e ver-sus log σ' curves containing a hysteresis loop have beenused to develop a relationship between the expansion in-dex and the preconsolidation stress as determined by theCasagrande method (Fig. 19). This relationship was usedto adjust the values of the preconsolidation stress deter-mined using the Casagrande method. The adjusted val-ues are listed in Table 6 as σc4.

Another method to determine the preconsolidationstress is based on the characteristics of the rebound curveand the assumption that these characteristics are con-stant for a given sediment sample (Fig. 20). An examina-tion of all the Site 576A e versus log σ' curves with re-bound portions substantiates this assumption. The re-bound method for determining the preconsolidation stressutilizes a point on the constant or maximum void ratioline that occurs prior to the maximum point of curva-ture on the e versus log σ' curve. This point (point B inFig. 20) should be located at the maximum effective stressoccurring on the line of constant void ratio. If that pointappears ambiguous, a point one complete log cycle be-fore the point of maximum curvature on the e versus logσ' curve can be used. From that point, a line parallel tothe rebound portion of the e versus log σ' curve is ex-tended to intersect the virgin curve. That intersection is

741


Table 4. Summary of incrementally loaded consolidation test results performed on Hole 576A sediments.

Sub-bottomdepth

(m)

1.402.452.553.004.525.005.607.407.549.72

10.2311.1111.6011.6612.2113.1513.5215.5016.1516.2219.5420.7622.4322.6522.8624.3425.6526.02

Initialvoid ratio

(e0)

6.123.773.784.704.344.194.464.564.624.775.645.165.274.915.745.044.774.584.614.484.222.973.813.652.753.663.103.24

Average values (Unit IA)

30.0930.4030.6032.1532.4734.4035.2939.3740.3641.0441.6542.0243.3044.6144.6544.9745.6047.2048.1548.2149.6049.6551.3653.65

3.244.334.134.114.323.663.564.003.843.323.223.182.802.672.883.263.313.743.623.143.763.403.203.09

Average values (Unit IB)

54.2258.7260.6563.61

1.361.372.841.37

Average values (Unit II)

Void ratioat σc'

5.833.253.364.343.943.794.054.174.084.184.814.524.364.414.974.353.823.774.073.713.812.683.393.282.233.122.862.89

2.803.893.483.653.663.083.253.703.523.002.692.882.502.512.532.962.923.223.192.853.34

2.772.64

1.201.132.401.04

Overburdenpressure σ0'

(kPa)

4.487.527.869.44

14.6716.4018.1023.9024.2730.8132.3934.7436.2236.3938.1240.9642.1248.8050.6550.8563.0067.8774.6475.5376.4082.7088.0489.63

106.13107.65108.02113.51114.62121.58125.09141.52145.35147.86150.46152.05157.78163.66163.83165.23168.50174.47177.93178.15183.95184.21191.45201.40

204.70232.28246.22267.50

Preconsolidationpressure σc'

(kPa)

25765234495788676989

1061218474

10615214094

13513596

1956173

113120218

96

908888

14514014015511370

12013012090

10915592

14075

18313013067

110280

385400320400

Overconsolidationratio

(OCR)

5.5810.116.623.603.343.484.862.792.842.893.273.482.322.032.783.713.231.932.672.651.522.870.820.971.481.452.481.07

3.10

0.850.820.811.281.221.151.240.800.480.810.860.790.570.660.950.560.830.431.030.730.710.360.571.39

0.82

1.881.721.301.50

1.60

Compressionindex

(cy

2.001.251.372.201.991.632.302.252.342.302.753.04

2.673.013.082.702.502.582.452.191.671.571.450.971.532.101.25

2.12

1.231.701.561.861.841.951.621.691.481.331.431.330.911.161.401.401.391.401.891.521.601.191.361.73

1.49

0.450.301.350.35

0.61

Expansionindex(Ce)

0.060.070.06

0.09

0.090.15

0.110.15

0.10

0.140.120.150.160.200.140.130.220.13

0.12

0.13

0.13

0.12

0.200.170.220.260.23

0.230.25

0.250.160.160.280.280.270.34

0.33

0.33

0.22

0.030.030.150.01

0.06

Permeability(indirect)(cm/sec)

6.48E-55.40E-6

6.30E-61.68E-6

1.93E-68.70E-71.04E-69.40E-7

1.69E-61.52E-6

3.82E-73.25E-7

6.10E-73.01E-72.60E-77.30E-8

3.25E-76.40E-8

1.15E-7

4.66E-6

2.70E-8

1.24E-7

5.2OE-84.45E-86.20E-92.90E-83.85E-83.OOE-8

4.10E-83.90E-84.25E-8

2.09E-81.92E-82.20E-8

3.1OE-8

1.47E-8

3.13E-8

1.80E-63.90E-72.81E-72.50E-7

6.80E-7

Permeability(direct)

(cm/sec)

1.3OE-5

1.49E-5

2.40E-5

3.40E-6

7.80E-75.33E-7

9.70E-7

3.3OE-7

1.32E-72.35E-7

1.15E-7

7.42E-6

9.60E-8

2.20E-7

6.60E-81.09E-71.42E-71.11E-7

1.41E-7

7.80E-8

7.OOE-8

1.08E-7

6.70E-8

1.10E-7

1.30E-62.40E-6

2.30E-6

2.00E-6

determined to be the preconsolidation stress. Values de-rived by this method are listed in Table 6 as σc5.

The interpretation of the consolidation test results canvary significantly depending on the technique employedfor determining the preconsolidation stress (Table 6). Re-sults using the Casagrande technique (σc'i) indicate thatSubunit IA sediments are overconsolidated and SubunitIB sediments are normally to underconsolidated. The se-cond technique used for determining the preconsolida-

tion stress (θç4) results in almost all Hole 576A sedimentsbeing overconsolidated. The third technique, which usespoint B (σc'5), results in the upper sediment (0-28 m sub-bottom) being, in essence, less overconsolidated than theresults using σcl, and the lower sediment (28 to 55 msub-bottom) normally consolidated. This third techniqueappears to reflect best the anticipated state of consolida-tion of Subunit IB based on the geological history of thisarea.

742


Table 5. Summary of constant rate of deformation consolidation tests performed on Hole 576A sediments.

Sub-bottomdepth

(m)

4.607.60

11.2013.6016.3020.8524.4026.1030.2032.5535.2539.4540.4441.0442.1043.7045.1045.7049.3052.4054.5058.8061.7063.70

Initialvoid ratio

(e 0 )

3.584.054.744.404.493.013.693.383.183.993.323.214.193.072.922.572.973.102.972.991.611.271.161.08

Void ratioat σc'

4.123.584.023.623.552.442.732.492.833.553.072.813.732.622.612.262.812.882.592.701.430.860.800.75

Overburdenpressure σ 0 '

(kPa)

14.9324.4635.0142.2651.1368.6082.9289.97

107.28114.91124.92141.83145.63147.90152.40159.89165.84168.81182.67203.28206.65232.87253.89270.35

Preconsolidationpressure σ c '

(kPa)

69100105120160179172215

8811890

10048

1201151156588

166118223675700700

Overconsolidationratio

(OCR)

4.624.093.002.843.132.612.072.400.821.030.720.700.330.810.750.720.390.520.910.581.082.902.762.59

Compressionindex(Cc)

1.712.022.422.042.702.112.122.101.972.562.452.231.731.151.861.76

. 1.832.162.252.091.18

Expansionindex

(Ce)

0.150.140.260.070.02

0.40

Permeability(indirect)(cm/sec)

5.19E-84.85E-83.15E-85.35E-85.65E-84.50E-83.21E-8

3.95E-85.40E-83.55E-84.60E-84.5OE-82.36E-82.55E-81.55E-71.83E-63.05E-76.30E-7

Permeability(direct)

(cm/sec)

1.70E-75.10E-76.70E-74.45E-71.65E-7

7.20E-8

The predicted field consolidation curve for Hole 576Apelagic sediments (Fig. 21) was constructed using the voidratio at the preconsolidation stress and the calculated insitu vertical effective stress. This curve can be used to in-terpolate in situ porosity-permeability-depth relation-ships for any portion of the sedimentary section. Thequadratic line of best fit defines an e versus log σ' curvethat has a compression index of 2.2 as compared to anaverage Cc of 2.12 for Subunit IA sediments (Table 4)and an initial void ratio of 5 to 5.4.

Permeability

As indicated earlier, several means of measuring orcalculating the coefficient of permeability were employed.All direct measurements were performed during the courseof the consolidation test. The indirect determinations werecalculated using either conventional consolidation theoryor CRD test results. All reported permeability values weredetermined at the preconsolidation stress (Fig. 22). Thepermeability profile is characteristic of other results re-ported for fine-grained marine sediments and shows de-creasing permeability with depth.

Summary

Detailed analysis of Hole 576A consolidation test re-sults indicate that the accepted methodology for inter-preting the geological significance of consolidation datais deficient. Sediments in the upper 28 m are overconsol-idated regardless of how the preconsolidation stress is de-termined. Sediments between 28 and 39 m sub-bottomare normally consolidated; whereas sediments between39 and 55 m sub-bottom appear normally to undercon-solidated. A number of mechanisms have been proposedto explain the variation in consolidation behavior, al-though many of these mechanisms do not appear con-sistent with the well-substantiated geological history of

this area. Much of the apparent discrepancy between theexpected consolidation state of Hole 576A sedimentsbased on available geological and geotechnical informa-tion and the actual consolidation test results may resultfrom the Casagrande technique employed to determinethe preconsolidation stress. An alternative method forcalculating σc', based on the characteristics of the re-bound portion of the e versus log σ' curve, yields σc'val-ues for Subunit IB that are more consistent with theknown geology in the area.

SHEAR STRENGTH

Shear strength tests were performed on Hole 576A sedi-ments to determine the characteristics of the undrainedshear strength profile with depth and to evaluate the sig-nificance of this profile relative to the observed consoli-dation state of Hole 576A sediments. The shear strengthtests performed included: (1) laboratory vane shear; (2)unconsolidated, undrained triaxial compression; (3) iso-tropically consolidated, undrained triaxial compression;and (4) anistropically consolidated, undrained triaxialcompression (Table 7). Results of these tests and theirrelationships to the observed geotechnical behavior ofHole 576A sediments are summarized in the followingsections.

Undrained Shear Strength

The undrained shear strength of Hole 576A sedimentswas determined using the laboratory vane shear and theresults from unconsolidated-undrained triaxial (UU) com-pression tests. Results of the vane shear tests are sum-marized in Figure 23 and Table 3. Table 8 summarizesthe results of the UU triaxial tests. These data have beennormalized using the ratio of the undrained shear strengthto the effective overburden stress calculated for the sub-bottom depth at which the strength measurement was

743


10 100 1000Vertical effective stress (kPa)

10000 10 100 1000Vertical effective stress (kPa)

10000

10 100 1000

Vertical effective stress (kPa)10000 10 100 1000

Vertical effective stress (kPa)10000

Figure 16. Void ratio versus log of effective stress curves (e versus log σ') obtained from Hole 576A sediments located in SubunitIA (A, 4.20 and B, 13.52 m sub-bottom) and Subunit IB (C, 32.47 and D, 45.60 m sub-bottom).

made (su/σó; Fig. 24) for comparison to a computed insitu profile presented later. The values of σó were ob-tained from Figure 13.

Shear Strength Parameters

Effective stress and undrained shear strength parame-ters were determined from consolidated-undrained tri-axial tests for both isotropic and anistropic loading con-ditions. Table 7 lists the samples tested and the test con-ditions employed for each specimen. Examples of thestress paths and the effective stress parameters for both

the isotropic and anisotropic condition are presented inFigures 25 and 26 with the corresponding shear-strengthparameter values derived from the tests summarized inTables 9 through 11.

Calculated Undrained Shear Strength Profiles

The shear strength parameters measured on Hole 576Asediments provide a basis for calculating an in situ un-drained shear strength profile. The calculations involvethe use of the SHANSEP method (Ladd and Foott, 1976;Ladd et al., 1977), which relates the in situ normalized

744


1.0 1.5 2.0 2.5

Void ratio3.0 3.5

60

Figure 17. Summary of consolidation test results for Hole 576A sediments. Sediment response to increasing applied stressfor each sample tested is displayed as changing void ratio at the same depth. A pronounced change in compressibilitywith depth at four applied stress levels occurs between Subunits IA and IB.

undrained shear strength to the measured shear strengthparameters as follows:

where su is undrained shear strength, σó is effective over-burden stress, NC represents the normally consolidated

condition, OCR is the overconsolidated ratio, and m isa constant that depends on the type of sediment. On thebasis of the results of CIU triaxial tests on overconsoli-dated samples (Fig. 27 and Table 10), a value of m =0.5 was computed. This value is lower than the typicalm value of 0.8 suggested by Ladd et al. (1977) based onCAU simple shear tests. The discrepancy between m val-

745


OrrrI ' ' ' • ' • ' ' ' I

10

20

30

40

50

60

70k. ,.

i i I i i i i i i i i

0.0 0.1 0.2 0.3Expansion index (Cp

0.4 0.5

Figure 18. Expansion index-depth profile determined using the re-bound portion of the e versus log σ' curve generated from consoli-dation-test results completed on Hole 576A sediments.

ues is due partly to the differences between CIU andCAU tests and partly to the differences between triaxialand simple shear tests.

In order to apply the SHANSEP method to the sedi-ment conditions at Hole 576A, it is first necessary toconvert the su/σó values from the CIU tests to the in situcoefficient of earth pressure at rest condition, Ko, as de-termined by the CAU test. A study of the data in Tables 9and 11 indicates that the su/σó values from the CIU testsshould be multiplied by 0.89 to obtain the estimated val-ues for the CAU condition. The adjusted values are sum-marized in Table 12.

The values of su/σó reported for the upper 28 m ofsediment in Table 12 must be further adjusted to accountfor the apparent in situ overconsolidated condition de-termined on the basis of consolidation test results. Thiswas completed using the SHANSEP equation and a val-ue of m = 0.5 (Table 13) together with the OCR valuesdetermined from consolidation test results in Figure 13.The computed values of normalized in situ strength, su/σó, and the in situ strength, su, computed using SHAN-SEP are plotted versus depth in Figures 28 and 29,respectively.

Interpretation of Shear Strength Profiles

Interpretation of the normalized shear strength pro-files presented in Figures 24 and 28 requires considerationof two factors: (1) the effects of sample disturbance and(2) the in situ state of consolidation.

250

200

=* 150

S 100

£ 50

-50 r0.0 0.1 0.2

Expansion index (C.)0.3 0.4

Figure 19. Relationship between the expansion index and the differ-ence between a known preconsolidated stress determined using ahysteresis loop and the preconsolidation stress determined graphi-cally using the Casagrande technique (see test for explanation). Asthe expansion index increases the difference between the knownand calculated preconsolidation stress values also increase. Aster-isks(*) represent regression analysis results (σ = 0.83).

The effects of disturbance are evident from a com-parison of the normalized strength-depth profiles in Fig-ures 24 and 28. At all depths, the undrained strengthsobtained by both the vane shear and UU triaxial testsare considerably lower than those computed for the insitu condition. This decrease in shear strength is consis-tent with the known effects of sample disturbance onsediment shear strength.

The influence of the in situ sediment consolidationstate on the undrained shear strength at Hole 576A isclearly demonstrated by the data presented in Figures 24and 28. Results of consolidation tests summarized in Fig-ure 15 indicate that the sediments are overconsolidatedto a depth of 28 m sub-bottom. The normalized shearstrength values (Fig. 28), as well as the computed in situvalues (Fig. 29) are consistent with the consolidation testresults from the upper 28 m. The shear strength resultsshow a much higher gradient over this interval than thegradient that exists in the normally consolidated sedimentsbetween 28 and 55 m sub-bottom.

An examination of the computed undrained strengthprofile (Fig. 29) reveals the difference between the sedi-ment strength characteristics of Subunits IA and IB. Italso demonstrates the usefulness of the SHANSEP meth-

746


Table 6. Summary of preconsolidation stress values calcu-lated using Casagrande's technique and the modifiedtechniques incorporating the sediment expansion char-acteristics described in the text.

Sub-bottomdepth(m)

1.402.452.553.004.525.005.607.407.549.7210.2311.1111.3011.6011.6612.2113.1513.5215.5016.1516.2219.5420.7622.4322.6522.8624.3425.6526.0230.0930.4030.6032.1532.4734.4032.2939.3740.3641.0441.6542.0243.3044.1644.6544.9745.6047.2048.1548.2149.6049.6551.3653.6554.2258.7260.6563.61

σcl

flcPa)

25765234495788676989106121

847410615214094135135961956173113120218969088881451401401551137012013012090109155921407518313013067110280385400320400

OCR

5.5810.116.623.603.343.384.862.792.842.893.273.48

2.322.032.783.713.231.932.672.651.522.870.820.971.481.452.481.070.850.820.811.281.221.151.240.800.480.81

0.780.570.660.950.560.830.431.030.730.710.360.571.391.881.721.301.50

σc4

(kPa)

257652

49

679689106136

74

17014012117120711420815182

120

10590

160190230281254

169237

237126145299236275273

319

299

OCR

5.5810.116.62

3.34

3.952.893.273.91

2.03

4.153.232.483.374.071.803.062.021.08

1.45

1.170.85

1.481.672.002.312.03

1.161.60

1.560.790.881.821.431.631.56

1.79

1.56

σc5

(kPa)

70

4554

64

8511010568

85751109783190115

74

8580

61125

18021011078180

180140180340200200185

250250

170330360150240

OCR

15.6

3.06

2.84

2.623.16

1.87

2.011.532.171.901.312.791.54

0.89

0.950.75

0.561.10

1.481.670.770.54

1.180.881.102.071.211.181.06

1.401.36

0.881.641.760.640.97

Note: The overconsolidation ratios reported are those calculatedusing the adjusted preconsolidation stress. Blanks indicate nodata available.

od for estimation of the in situ strength from the resultsof laboratory tests on partially disturbed samples. Thisprofile also represents our best estimate of the in situ un-drained strength of Subunits IA and IB in Hole 576A.

The sediment cores obtained from the depths of 55 to64 m sub-bottom were highly disturbuted and did notprovide enough data for an adequate SHANSEP analy-sis of the carbonate-ooze sediments in Unit II.

σC4, σC5

σC2

σC3

Log effective stress

Figure 20. Hypothetical void ratio-log effective stress (e versus log σ')curve showing how the calculated preconsolidation stress varies de-pending on the technique used (see explanation in text). σcl is thepreconsolidation stress calculated using the Casagrande (1936) tech-nique. σc2 is the preconsolidation stress calculated using the Casa-grande technique on the hysteresis loop on the e log a' curve. σc3 isthe actual preconsolidation stress for the hysteresis loop. σc4 is thepreconsolidation pressure determined using the rebound character-istics (Fig. 19) and the hysteresis loop. σc5 is the preconsolidationpressure determined using point B and the slope of the reboundcurve.

DISCUSSION OF RESULTS

The geotechnical property results for Hole 5 76A sedi-ments represent the most complete geotechnical data setever compiled for a continuously depositional sedimen-tary section that spans 65 m.y. Excellent agreement ex-ists between index property, consolidation, permeability,and shear strength test data generated during the initialsampling at SIO and at the various geotechnical labora-tories participating in the analyses. These results are al-so valuable because they demonstrate the degree of in-terlaboratory variation one can expect from each typeof test performed provided that similar techniques areadopted.

Predicted geotechnical property results (i.e., based onthe geological and geotechnical information available pri-or to drilling) and actual geotechnical property measure-ments show significant differences. Several importantfactors or combination of factors have influenced thegeotechnical property results observed in Hole 576A sedi-ments. These primary factors include: (1) core distur-bance, (2) time, (3) cementation and interparticle bond-ing, (4) mineralogy, and (5) grain size.

The term core disturbance includes a number of pro-cesses. Any sampling method will result in stress reliefand expansion. In addition, mechanical alteration of thesediment fabric results from the coring, handling, and

747


6

10 100

Vertical effective stress (kPa)

1000

Figure 21. Void ratio-log effective stress curve determined using thevoid ratios calculated at the preconsolidation stress and the effec-tive overburden stress calculated for each consolidation sampledepth. The curve is representative of in situ field conditions. Aster-isks(*) represent a quadratic fit to the field data (σ = 0.74).

initial testing procedures. Such effects disturb the sedi-ment fabric and generally reduce both the shear strengthand the preconsolidation stress as determined in the lab-oratory for a particular sample. Although difficult toquantify, a comparison of the su/σó ratios determinedusing the results of laboratory tests and results correctedfor in situ conditions using SHANSEP clearly demon-strates the effects of disturbance (Figs. 24 and 28).

Time has a significant effect on the geotechnical be-havior of fine-grained sediments. Sedimentation rates atSite 576 were generally less than 10 m/m.y. over the last65 m.y. These rates are sufficiently slow to allow com-plete dissipation of any excess pore pressures resultingfrom the sedimentation process. Thus, the time-depen-dent physical processes, (i.e., primary and secondary con-solidation) have been completed or are continuing at sucha slow rate that the Site 576 sediments should be verynear a state of physical equilibrium.

Temporal variations in chemical processes (i.e., dia-genesis, cementation, and interparticle bonding) couldalso influence the geotechnical behavior of fine-grainedsediments. Dadey (1983) has proposed clay-particle ce-mentation as the primary factor responsible for the over-consolidation evident in Subunit IA sediments. The ce-mentation effect may be more pronounced in high voidratio shallow sediments where the influence of accumu-lated overburden relative to the effects of cementation is

10 10 7 10'6 10"5

Coefficient of permeability (cm/s)

Figure 22. Coefficient of permeability-depth profile for Hole 576Asediments. Permeability values determined by direct measurement(falling head, constant head, and flow pump) are illustrated byconnected dots. Permeability values calculated using consolidationtheory are illustrated by solid circles (•). All permeability valueswere obtained at the preconsolidation stress.

not as significant as in more deeply buried sediments.Similar reasoning suggests that cementation effects shouldbe reflected in the near-surface sediment shear strength.The variations observed in the shear strength gradientevident in the SHANSEP results between Subunits IAand IB may reflect these effects. However, the variabilityin near-surface shear strength values precludes any de-finitive assessment of the cementation effects on shearstrength.

The most common clay minerals present in Hole 576Asediments are illite, smectite, chlorite, and kaolinite. Il-lite comprises approximately 50% of the clay mineralspresent. The quartz content ranges between approximate-ly 3 and 17% (Table 2). The largest mineralogical varia-tion in Hole 576A sediments is the pronounced increasein X-ray amorphous material at depth and the variablequartz content. The higher expansion indices and in-creased plasticity of Subunit IB sediments suggest thatthe geotechnical behavior of the X-ray amorphous com-ponent is similar to smectite.

Examination of the relationships between void ratioat 50 kPa (Fig. 17), silt content (Fig. 30), and the com-pression index (Fig. 31) show very unusual relationshipswith depth, particularly in Subunit IA. As void ratio andcompression index increase over this interval, silt con-tent also increases. Examination of scanning electron mi-crographs obtained from the silt-size fraction revealedthis component was made up of quartz grains and clay

748


Table 7. Shear strength test program completed on Hole576A sediments.

Core-Section

Sub-bottomdepth(m)

Test type

UU (CIU)n (CAU)nc

1-21-31-31-4(2)1-4(1)1-5(1)1-5(2)2-12-42-62-6(1)2-6(2)2-6(3)3-23-53-4(3)3-4(2)3-4(1)3-64-1(1)4-2(2)4-1(2)4-2(3)4-1(3)4-2(4)4-2(1)5-35-3(1)5-45-4(2)5-4(3)5-4(1)5-4(2)5-4(3)5-4(4)5-56-26-2(1)6-2(2)6-36-36-47-6

2.93.03.95.55.66.36.4

10.013.716.817.317.417.521.821.624.724.724.826.729.229.329.529.529.629.629.740.240.540.742.342.442.642.742.943.044.048.348.849.049.749.851.864.3

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

Note: Subscripts nc and oc stand for normally consolidated andoverconsolidated sediments, respectively. The test series num-ber is shown in parentheses in the core-section column.

aggregates. No other silt-size components were identi-fied. At 13 m sub-bottom quartz accounts for only 17%of the total 52% sand and silt-size fraction present. Giventhese relative proportions, the quartz grains are proba-bly dispersed within the sediments and will have little orno effect on the sediments geotechnical behavior. Thebonded clay aggregates comprising 35% of the coarsefraction at 13 m sub-bottom are sufficiently bonded toresist deflocculation. If the clays have not undergone ex-tensive diagenetic alterations with time, then these ag-gregates probably result from clay particle cementation,possibly by iron oxides. In either case, these aggregatesmust form a high void ratio, porous structure that is dis-persed among a clay particle matrix in a manner so asnot to decrease the overall soil compressibility. The con-cept of bonded clay aggregates is not new; however, thisstudy does present indications that clay aggregates do ex-ist and can influence the geotechnical behavior of fine-grained sediments.

In conclusion, the geotechnical property results fromHole 576A sediments, in particular the consolidation testresults, appear inconsistent with the known geologicalhistory of the area. The consolidation test results are sub-stantiated by both the anomalous relationships occurringbetween index properties (i.e., grain size increases as voidratio increases) and the shear-strength results. A numberof possibilities have been presented to explain these ob-servations, including core disturbance, particle cemen-tation, formation of bonded clay aggregates, and inade-quate techniques for determining the preconsolidationstress. Available information precludes definitive conclu-sions as to which of these possibilities is most signifi-cant, although it is likely that each process has influ-enced the results to some degree. These results do showconclusively, however, that conventional geotechnical re-lationships developed largely from terrestrial studiesshould not be applied to the marine environment with-out very careful consideration and analysis.

ACKNOWLEDGMENTS

We are indebted to the DSDP Leg 86 scientific party and opera-tional personnel for providing the time and patience necessary to suc-cessfully complete Hole 576A. This study would not have been possi-ble without the gracious assistance of Amy Altman and the DSDP cu-ratorial staff during the core processing. Sally Frew politely perseveredwhile typing and retyping this report. Phil Valent, Mike Riggins, andBob Prindle kindly agreed to review this manuscript and provided in-numerable helpful comments. Partial financial support was providedby the Department of Energy through contracts with Sandia NationalLaboratories (DOE-AC04-76DP00789) and Argonne National Labo-ratory (DOE-31-109-ENG-38).

REFERENCES

American Society for Testing and Materials, 1984. Annual Book ofASTM Standards, Part 19, Natural Building Stones; Soil andRock: Philadelphia (ASTM).

Bishop, A. W., Webb, D. L., and Lewin, P. I., 1965. Undisturbedsamples of London clay from the Ashford common shaft: Strength-effective stress relationship. Geotechnique, 15:1-31.

Bishop, A. W, and Henkel, D. J., 1962. The Measurement of SoilProperties in The Triaxial Test: London (Edward Arnold, Ltd.).

Brumund, W. E, and Callender, G. W., Jr., 1975. Compressibility ofundisturbed submarine sediments. Proc. Civ. Eng. Oceans/III,Am. Soc. Civ. Eng., 1:363-379.

Brumund, W. E, Jonas, E., and Ladd, C. C , 1976. Estimating in situmaximum past (preconsolidation) pressure of saturated clays fromresults of laboratory consolidometer tests. Transp. Res. Board,Spec. Rept. 163:4-12.

Burmister, B. M., 1951. The application of controlled test methods inconsolidation testing. Consolidation Testing of Soils, ASTM Spec.Tech. Publ. 126:83-91.

Casagrande, A., 1936. The determination of the preconsolidation loadand its practical significance. Proc. Int. Conf. Soil Mech. Found.Eng., 1st, 3:60-64.

, 1948. Classification and identification of soils. Trans. Am.Soc. Civ. Eng., 113:901.

Cooling, L. E, and Skempton, A. W., 1942. A laboratory study ofLondon clay. J. Inst. Civ. Eng., 17:251-256.

Dadey, K. A., 1983. Stress history of unlithified marine sediments inthe northwest Pacific [M.S. thesis]. University of Rhode Island,Kingston.

Damuth, J. E., Jacobi, R. D., and Hayes, D. E., 1983. Sedimentationprocesses in the Northwest Pacific basin revealed by echo-charactermapping studies. Geol. Soc. Am. Bull., 94:381-395.

Folk, R. L., 1974. Petrology of Sedimentary Rocks: Austin (HemphillPubl. Co.).

Gorman, C. T., Hopkins, T. C , Deep, R. C , and Drnevich, V. P.,1978. Constant-rate of strain and controlled gradient consolida-tion testing. Geotech. Test. J., 1:3-15.

749


Griffin, J. J., Windom, H., and Goldberg, E. D., 1968. The distribu-tion of clay minerals in the world ocean. Deep Sea Res., 15:433-459.

Hamilton, E. L., 1964. Consolidation characteristics and related pro-perties of sediments from experimental mohole (Guadalupe site).J. Geophys. Res., 69:4257-4269.

, 1965. Sound speed and related physical properties of sedi-ments of experimental mohole (Guadalupe site). Geophysics, 30:257-261.

Heath, G. R., and Pisias, N. G., 1979. A method for the quantitativeestimation of clay minerals in North Pacific deep-sea sediments.Clays Clay Miner., 27:175-184.

Ladd, C. C , and Foott, R., 1976. New design procedure for stabilityof soft clays. J. Geotech. Eng. Div. Am. Soc. Civ. Eng., pp.763-786.

Ladd, C. C , Foott, R., Ishikara, K., Schlosser, S., and Paulos, H. G.,1977. Stress-deformation and strength characteristics, state of artreport. Int. Conf. Soil Mech. Found. Eng., 9th, ISSMFE, Tokyo.

Lambe, T. W., 1951. So/7 Testing for Engineers. New York (Wiley).Lee, H. J., 1984. State of the art: Laboratory determination of the

strength of marine soils. Presented at ATSM Symp. Lab. In SituDeter. Strength Mar. Soils, San Diego.

Leinen, M., and King, T. W., 1985. Mineralogy of North Pacific sedi-ments from North Pacific candidate sites. In Shephard, L. E. (Ed.),1983 Subseabed Disposal Program Annual Report: Site Assess-ment, Sandia National Laboratories, pp. 113-140.

Lowe, J., III., Zaccheo, P. F., and Feldman, H. S., 1964. Consolida-tion with back pressure. J. Soil Mech. Found. Div., Am. Soc. Civ.Eng., 90:69-86.

Noorany, I., 1984. Phase relations in marine soils. J. Geotech. Eng.Div., Am. Soc. Civ. Eng., 110: 539-543.

Olsen, H. W., 1966. Darcy's Law in saturated kaolinite. Water Resour.Res., 2:287-295.

Pane, V., Croce, P., Znidarcic, D., Ko, H.-Y., Olsen, H. W., and Schiff-man, R. L., 1983. Effects of consolidation on permeability mea-surements for soft clay. Geotechnique, 33:67-72.

Sangrey, D., 1977. Marine geotechnology: State of the art. Mar. Geo-technol., 2:45-80.

Scully, R. W., Schiffman, R. L., Olson, H. W., and Ko, H.-Y., 1984.Validation of consolidation properties of phosphatic clay at veryhigh void ratios. Am. Soc. Civ. Eng. Symp. Consol. Settling, pp.158-181.

Schmertmann, J. H., 1955. The undisturbed consolidation behaviorof clay. Trans. Am. Soc. Civ. Eng., 120:1201-1227.

Shephard, L. E., Bryant, W. R., and Dunlap, W. A., 1978. Consoli-dation characteristics and excess pore water pressures of Mississip-pi Delta sediments. Proc. Annu. Offshore Technol. Conf, Pap.3167:1037-1048.

Silva, A. J., Hetherman, J. R., and Calnan, D. I., 1981. Low gradientpermeability testing of fine-grained marine sediments. ASTM Spec.Tech. Publ., 746:121-136.

Silva, A. J., Jordon, S. A., and Levy, W. P., 1983. Geotechnical Stud-ies for Subseabed Disposal High Level Radioactive Wastes, Pro-gress Report No. 9, Contract No. 37-2235. Albuquerque (SandiaNational Laboratories).

Silva, A. J., Laine, E. P., Lipkin, J., Heath, G. R., and Akers, S. A.,1980. Geotechnical properties of sediments from North Pacific andNorthern Bermuda Rise. Proc. Mar. Technol. Soc, pp. 491-499.

Skempton, A. W., 1970. The consolidation of clays by gravitationalcompaction. Q. J. Geol. Soc. London, 125:373-411.

Terzaghi, K., 1956. Varieties of submarine slope failures. Proc. ConfSoil Mech. Found. Eng., 8th Texas, pp. 1-41.

Znidarcic, D., 1982. Laboratory determination of consolidation prop-erties of cohesive soil [Ph.D. Dissert.]. University of Colorado,Boulder, Colorado.

Znidarcic, D., and Schiffman, R. L., 1983. Constant rate of deforma-tion consolidation testing. Boulder (Department of Civil Engineer-ing, University of Colorado).

Date of Initial Receipt: 23 May 1984Date of Acceptance: 25 September 1984

750


10

15

20

25

-? 30

.cαΦ

•aE

35

co 4 0

45

50

55

60

65

70

• ••;• • • ' a .

al

a

a ^

a '

a a

a <f

aa

a a• <?•a

a ' *a

a a

• aa

a

• a "

• a

• ' • ' a

'•

aa a

aa

a

a• a *

a

β#

aI

aa.

•

•a>' a

a

a

••

• a "

a**

aa

a

a

a

a

a

aa

aa

• aa

a

a

a a

• aa

aa a

a #

a

a

a aa

• a

aa

•

•

•a

• ' • a

a a

a

• a

a

aa a

aa

a *a

a

a *

• aa

a

aa

a

aa

a

aa

a

r•

a

a

\

i

r+

"i

>

4->

5

nit

IAS

ub

ini

t IB

Su

bi

10 20 60 7030 40 50

Vane shear strength (kPa)

Figure 23. Laboratory vane shear strength-depth profile for Hole 576A sediments.

751


Table 8. Summary of unconsolidated-undrained (UU) triaxialtest results for Hole 576A sediments.

Core-Section

1-21-31-32-12-42-63-23-53-64-24-34-54-55-35-45-56-26-36-36-47-6

Sub-bottomdepth(m)

2.93.03.9

10.013.716.821.824.526.730.431.034.634.740.240.744.048.349.749.851.864.3

σo'(kPa)

10.911.314.737.751.763.482.392.5

100.8114.7116.9130.6130.9151.7153.6166182187.5188195243

Watercontent

(%)

153155154191160182121140114147158123130145105109101126.411499

117.7

Triaxialshear strength

(kPa)

14.713.714.312.811.415.617.618.929.616.718.326.642.126.639.332.833.024.230.029.038.0

su/σ0'

1.341.210.979.340.220.250.210.200.290.150.160.200.320.180.260.200.180.130.160.150.16

15

20

I 25

JC

α.Cl

ε 30

o

II 35

40

45

50

55

60

• Results of triaxial UU tests

_! 1_0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Figure 24. Normalized shear strength data comparing vane shear resultswith UU test results for Hole 576A sediments.

752


200

* 100

400

200

100

B

= 24

Sample 6-2 (2)

AA -̂" Sampl

)

e 6-2(1)

100 200

P' (kPa)

300 400

Figure 25. Example of isotropic consolidated undrained (CIU) triaxial test results from Hole 576Asediments located in Subunit IA (A, 24.70 m sub-bottom, c = 0, Φ = 34°) and Subunit IB (B,48.90 m sub-bottom, c' = 0, Φ = 26°). Other CIU test results are summarized in Table 9.

400

:* 200 -

800

Figure 26. Results of consolidated, anisotropic undrained (CAU) Ko triaxial test results from sediment locatedat 30.50 m sub-bottom ( C = 17.6 kPa, Φ = 23°). Other CAV-K0 test results are summarized in Table 8.

753


Table 9. Summary of CIU triaxial test results for Hole 576A sediments.

Test

no.

1

2

3

45

6

7

8

9

10

11

12

13

1415

Core-Section

1-4(1)

1-4(2)1-5(1)1-5(2)

2-6(1)2-6(2)

2-6(3)3-4(1)3-4(2)

3-4(3)

5-3(1)5-4(2)

5-4(3)6-2(1)

6-2(2)

Sub-bottom

depth

(m)

5.6

5.5

6.3

6.4

17.317.4

17.524.824.7

24.640.5

42.342.448.8

49.0

^0

(%)

192199

216

189

154153

155

113

115

127

134

103

103

124

125

H>f

(%)

138

131

164

164

122

137

105

76

89

121

130

78

91

102

109

(kPa)

20

20

22

22

57

57

57

85

85

85

145

155

155

180

180

σ3

(kPa)

392

441

392

353

540

491

601

588

490

392

621

690

414

598

549

"i(kPa)

294

294

294

294

393

393

393

294

294

294

442

306

228

294

294

(kPa)

98

14798

59

147

98208

94

196

98

179

384

186

304

255

B OCR

0.980.98

0.990.990.98

0.980.990.990.990.980.99

0.990.980.98

0.99 1

(kPa)

60.870.6

59.845.173.548.0

84.0132.398.053.962.5

145.083.0117.6

95.1

su/σ

o'

0.620.48

0.610.760.50

0.490.400.450.500.550.35

0.380.450.39

0.37

Note: w0 = initial water content (assuming 35 ppm dissolved salt in pore fluid); π>f water content at failure; σ0' = in situeffective stress; 03 = cell pressure; u\ back pressure; σc' = σ•$ - u\ = consolidation pressure; B = pore-pressure co-efficient; OCR = overconsolidation ratio; su = 1/2 (σ\ - σ 3 ) m a x = shear strength.

Table 10. Summary of CIU triaxial test results on overconsolidated samples from Hole 567A

sediments.

Sample

4-1(1)4-1(2)

4-1(3)

Sub-bottomdepth

(m)

29.2

29.529.6

wo(%)

145

148

144

Wf

(%)

101

101

96

σo'

(kPa)

100

105

105

σ3

(kPa)

442

559

589

"i(kPa)

393

393

393

σc'

(kPa)

49

98

196

B

0.970.960.98

OCR

842

(kPa)

94.1

108.8134.3

su/σ

o

1.92

1.11

0.69

Note: See Table 9 for explanation of symbols used in column headings.

Table 11. Summary of CAU triaxial test results for Hole 576A sediments.

Test

no.

1234

5678

Sample

4-2(1)4-2(2)

4-2(3)4-2(4)

5-4(1)

5-4(2)

5-4(3)5-4(4)

Sub-bottomdepth

(m)

29.829.4

29.529.6

42.642.842.9

43.0

n'(kPa)

143300498694

200

338

475

766

<73f

(kPa)

67

144249376

93170276

490

Multipleof over-

burden

Φ' = 23°

1.53.25.3

7.3

Φ' = 21°

1.52.5

48

^0

C" =

146

144144138

C" =

112111110111

Wf

(%)

17.6 kPa

118103

9382

29.8 kPa

98.589.9

81.6

"o(kPa)

149305

495682

202

331

475766

Qi(kPa)

65113173

223

82

120

164237

Pf(kPa)

121

232

384

521

150

244356

582

"f(kPa)

10.426.7

41.581.3

27.045.980.7

141.0

Af

0.080.120.12

0.18

0.160.190.25

0.30

Vf fO

0.440.37

0.350.33

0.410.360.35

0.31

Note: See Table 9 for explanation of symbols used in column headings, qç = deviation stress at failure; pf' = mean effective stressat failure; Hf = pore pressure at failure; Af = pore-pressure coefficient at failure.

754


Table 13. Summary of su/σo values for both the nor-mally consolidated condition and after correctingfor the state of overconsolidation.

Sub-bottomdepth

(m)

5.56.3

17.424.729.642.342.848.9

(W>NC

4.90.540.410.440.370.350.360.34

OCR

5.04.02 51.51.01.01.01.0

(s u/σ 0 ' )

1.11.10.650.530.460.350.360.34

5 U

(kPa)

2224374548545661

Note: For comparison with data in Figure 23, the shearstrength values (su) corrected for in situ conditions usingSHANSEP are also shown.

8 9 10

Figure 27. Relationship of the normalized shear strength with varyingoverconsolidation ratios developed using the results of CIU testson sediments at 28.70 m sub-bottom.

Table 12. Summary of su/σo values determined fromCIU triaxial tests and adjusted for the in situ KQcondition.

Core-Section

Sub-bottomdepth(m) OCR

Data from CIU test results

1-41-52-63-45-36-2

5.5 16.3 1

17.4 124.7 142.3 148.9 1

Data from CAU test results

4-25-4

29.6 142.8 1

CIU tests

su/σ0'

0.550.610.460.500.390.38

su/σ0'Ko condition

0.490.540.410.440.350.34

0.370.36

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Figure 28. Variations in the ratio of sü/σ^ with depth corrected for the

Ko condition. Profile developed using normally consolidated tri-

axial test results.

755


80

Figure 29. In situ shear strength-depth profile for Hole 576A sedi-ments calculated using the SHANSEP technique.

756


Percent quartz (Δ)Percent sand and silt (

50 55

2.5 3.0

Void ratio ( •)

Figure 30. Variations in void ratio (•), sand- and silt-sized fraction (o) and percent quartz (Δ) with depth in Hole 576A sedi-ments.

757


Orr

10

20

30

40

50

60

70 7 ,0 1 2 3 4

Compression index (Cc)

Figure 31. Compression index-depth profile determined using consoli-dation test results from Hole 576A sediments.

758

Documents

DRILLING PROJECT LEG 86, HOLE 576A1 The Marine ...Regional sea-floor morphology and near-surface seismic profile char-acteristics have been mapped and discussed by Damuth et al. (1983)