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DTIC ,IE COPYN-1 808
February 1990
By M. T. Millea
Sponsored By Office ofTechnical Note Naval Technology
LIQUEFACTION MITIGATIONTECHNOLOGY
A,.r-
NABSTRACT Seismically-induced soil liquefaction is a great threatto the Navy's huge shore facility investment located at the water-front. Improvement of potentially liquefiable soil beneath and sur-rounding structures is currently feasible. Using empirical designmethods, the construction of stone columns on relatively close cen-ters provides increased resistance to lateral loading from earth-quake waves and increases vertical drainage by reducing the radialflow path, which can increase the rate of dissipation of pore waterpressure. A background investigation of state-of-the-art soil im-provement methods was conducted. The Princeton Effective StressSoil Model was evaluated as a tool to aid the design of soil im-provement methods. Finite element models with and without stonecolumns were subjected to simulated earthquake excitation usingDYNAFLOW, a finite element computer model developed at Prin-ceton University to analyze dynamic soil-structure interaction prob-lems. Results indicate that the stone columns improved site condi-tions and decreased damage due to vibratory motion. The resultsalso indicated that each potentially liquefiable site is unique and willrequire individual analysis.
NAVAL CIVIL ENGINEERING LABORATORY PORT HUENEME CALIFORNIA 93043-5003
Approved for public release, distibution is unlimited.
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FEBRUARY 1990 Final - February 1988 to June 1989
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LIQUEFACTION MITIGATION TECHNOLOGY PE - 62233N
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6. AUTHOR(Sl
M. T. MILLEA
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Naval Civil Engineering Laboratory TN-1808Port Hueneme, CA 93043-5003
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Office of Naval Technology800 North Quincy StreetArlington, Virginia 22217-5000
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t3. ABSTRACT (Maximum 200 words)
4eismically-induced soil liquefaction is a great threat to the Navy's huge shore facility investment located atthe waterfront. Improvement of potentially liquefiable soil beneath and surrounding stnictures is currentlyfeasible. Using empirical design methods, the construction of stone columns on relatively close centersprovides increased resistance to lateral loading from earthquake waves and increases vertical drainage byreducing the radial flow path, which can increase the rate of dissipation of pore water pressure. A backgroundinvestigation of state-of-the-art soil improvement methods was conducted. The Princeton Effective Stress SoilModel was evaluated as a tool to aid the design of soil improvement methods. Finite element models with andwithout stone columns were subjected to simulated earthquake excitation using DYNAFLOW, a finite elementcomputer model developed at Princeton University to analyze dynamic soil-structure interaction problems.Results indicate that the stone columns improved site conditions and decreased damage due to vibratorymotion. The results also indicated that each potentially liquefiable site is unique and will require individualanalysis
I SUBJECT TERMSA 15. NUMBER OF PAGES
Liqucfactior mitigation, pore water pressure, soil, site improvement, stone columns, 38_ earthquake loading, effective stress, finite elements,PYNAF LOW, Cc2,r"-yv~ te v I& PRICE CODE
17. SECURITY CLASSIFICATION A SECb*Y CLASSIFICAI 19. SECURITY CLASSIFICATION 20L UMITATION OF ABSTRACTOF REPORT OF THIS PAGE OF ABSTRACT
Unclassified Unclassified Unclassified UL
NSN 7540-01-280-5500 Standard Form 296 (Rev 249)Proscribed by ANSI Sid 239-18290-102
CONTENTS
Page
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . .1
BACKGROUND...............................1
1960 Chilean Earthquake ..................... 11964 Alaska Earthquake. ..................... 11964 Niigata Earthquake ..................... 21968 Tokachi-Oki Earthquake ................... 21973 Nemuro-IHanto-Oki Earthquake .. ............... 21978 Mivagi-Ken-Oki Earthquake. ........ .......... 2
SEISMICALLY-INDUCED SOIL LIQUEFACTION .. ............... 3
STATE-OF-THE-ART TECHNOLOGY IN LIQUEFACTION MITIGATION ..... 3
New Construction. ........................ 5Existing Construction ...................... 7
PRINCETON UNIVERSITY EFFECTIVE STRESS SOIL MODEL. .......... 8
STUDY PROGRAM. ............................ 8
STUDY PROCEDURE ............................ 10
FINITE ELEMENT STUDIES........................11
Soil Masses...........................11Footing ............................. 12
SUMMARY AND CONCLUSIONS ........................ 13
REFERENCES .............................. 14
Acuession For
NTIS GRA&IDTIC TABUnannounced QJust i i cat io
Dist ribution/(4~~:~) Availabil1tV Codes
Ai and/orDist Special
INTRODUCTION
The Navy has huge facility investments in seismically active re-gions. Each year this investment is increased. The Navy, by the natureof its mission, must locate its facilities at the waterfront, encounter-ing a high water table, and often on marginal foundation conditions.
The present study will consider state-of-the-art methods for miti-gating seismically-induced soil liquefaction. Liquefaction plays a pre-dominant role in waterfront damage, often being the single cause ofwidespread losses. Methods applicable for improving sites with existingstructures will be evaluated for effectiveness using existing centrifugetest data. The usefulness of the finite element program DYNAFLOW todetermine the feasibility of techniques for improving potentially lique-fiable soil foundations beneath existing structures will also be consid-ered.
BACKGROUND
If a saturated granular material is subjected to cyclic loadingwith the reversal of shear stresses, the material will tend to compact.If drainage is impeded, the tendency to decrease in volume will cause anincrease in pore water pressure. If cyclic loading continues, the soilmay reach a condition of zero effective stress and will suffer a partialor complete loss of strength which is termed liquefaction. Seismically-induced soil liquefaction is a great threat to the Navy's ability tocarry out its mission in the event of a major earthquake.
Though the United States has not suffered a devastating earthquakein recent years, the seismic risk faced by the Navy is great, especiallyin the West and along the Pacific Rim. In Southern California, it isestimated that there is a 5 percent annual probability of a major eventoccurring that couid affect a number of Naval bases.
The significance of liquefaction is evident In the following summa-ries when we assess the damage caused in recent major earthquakes.
1960 Chilean Earthquake (Magnitude 6-8.3)
The most spectacular damage occurred in Puerto Montt, to quay walls,steel sheet piles, and sea walls. Liquefaction of the loose fine sandysoils was the primary cause of the failures.
1964 Alaska Earthquake (Magnitude 8.4)
There was severe damage at Anchorage, Cordova, Homer, Kodiak,Seldovia, Seward, Valdez, Klawock, and Whittier. Large-scale landslidesand liquefaction induced most of the extremely.heaydamgje and totaldestruction.
1964 Ntigata Earthquake (Magnitude 7.5)
There was severe damage in Niigata Port (West Harbor). The areasaffected were Additional Harbor, Yamanoshita Wharf, North Wharf, EastWharf, Central Wharf, South Wharf, Kurinoki River Landings, Bandai Is-land Wharf Shinano River Left Bank Bulkhead, and West Coast Bulkheads.Liquefaction caused most of the hea y damage.
1968 Tokachi-Okl Earthquake (Magnitude 7.8)
The ports affected were Hachinohe, Aomori, Hakodate, and Muroran.Damage was relatively light compared to that caused by the NiigataEarthquake. Most of the damage occurred to smaller scale structures.Liquefaction was not the primary cause of damage even though spoutingsand sediments were seen at several waterfront areas near the damagedstructures.
1973 Nemuro-Hanto-Oki Earthquake (Magnitude 7.4)
Severe damage occurred mainly in Hanasake and Kiritappu Ports.Nemuro Port, situated only 6 kilometers away from Hanasaki Port,sustained very slight damage. The damage was attributed to soilliquefaction.
1978 Mivagi-Ken-Oki Earthquake (Magnitude 7.4)
The areas affected were Shiogama, Sendai, and Ishinomake Ports, andIshinomaki and Yuriage Fishing Ports. The damage in Ishinomaki Portaccounted for approximately 90 percent of the total damage costs at portand harbor facilities caused by this earthquake. Gravity quay walls andpiers suffered various degrees of damage. The damage to sheet pile quaywalls was primarily due to liquefaction of fill materials. Liquefactionagainplayed a significant role in this earthquake. At sites whereliquefaction occurred, the damage to the port and harbor structures wassevere. Conversely, the damage to port and harbor structures was smallat sites where no liquefaction occurred.
Experience from recent earthquakes points out that liquefactiongreatly increased the amount of damage incurred by waterfront facili-ties. To date, Naval facilities have generally escaped damage fromearthquakes though the Navy did experience heavy damage at the KodiakNaval Station during the 1964 Alaskan earthquake: 1 foot of differen-tial settlement was noted beneath aircraft hangers; the sea wall re-ceived heavy damage; and the fill under asphalt aircraft ramps compact-ed. The damage was caused by soil failure due to the facility beingconstructed on 15 to 20 feet of fill where a seismically-induced porepressure increase would be expected to reduce soil stiffness and shearstrength. Although the United States has not had a large number ofearthquakes exposing Navy facilities to damage, the Japanese have hadseveral events demonstrating that seismic liquefaction is responsiblefor most waterfront damage.
2
Direct and indirect methods can mitigate liquefaction and assuresafety of structures and personnel. With the exception of several damsites, there has been essentially no experience with mitigation measuresin liquefiable soils beneath existing structures. No general method isavailable which is applicable for all conditions. Each site is uniqueand will require a uniquely engineered solution.
SEISMICALL.Y-INDUCED SOIL LIQUEFACTION
During an earthquake, cohesionless soils that, under ordinary cir-cumstances, provide adequate structural support may liquefy and settle.A loose sand subjected to seismically-induced vibratory motion tends todecrease in volume. If the sand is saturated and the drainage impeded,some of the interparticle stress is transferred to the water resultingin a rise in the pore water pressure. In general, the higher the inten-sity of the vibration, the greater the potential for an increase in porewater pressure. Shear resistance is lost as the pore water pressureapproaches the confining pressure on the soil. Differential settlementsand tilting may result, causing severe damage to structures. When thepotential for soil liquefaction exists, the engineer has no means toevaluate the associated risk that could be caused by an earthquake.
Though early studies of liquefaction pertain to instabilities re-sulting from a gradual rise in the water table, the term "liquefaction"has been extended to include failure to soil under cyclic loading condi-tions induced by earthquake vibrations or blasts. Loss of soil strengthmay come about by static or dynamic loading, but shear stresses leadingto liquefaction under cyclic loading may be much lower than shearstresses causing failure under static loading conditions. Continuousvibrations resulting in cyclic stresses cause a build-up of pore pres-sure which progressively reduces the effective strength. When the porepressure is equal to the total stress (i.e., the effective stress equalszero), the sand has lost its shear strength and is considered liquefied.Liquefaction may then be defined in terms of a loss of strength and ma-terial transformation of a saturated granular material Into a fluid.
The rate at which the pore pressure dissipates within a soil masshas a major influence on whether liquefaction will occur, particularlyunder cyclic loading. Pore pressure dissipation is a function of theradial drainage path, so the geometry of the soil mass is also impor-tant.
STATE-OF-THE-ART TECHNOLOGY IN LIQUEFACTION MITIGATION
There are three basic approaches to improve soil conditions to pre-vent liquefaction: (1) increase soil density, (2) increase effectiveconfining pressure, and (3) use particulate or chemical grouting to in-crease the stiffness and fill voids, preventing a reorientation of soilparticles to a denser state.
Table 1 lists mitigation measures and the site conditions whereeach could be used (Ref 1). 4
3
Table t. Improvement Methods for Liquefiable Soil Conditions
Method Case 1a Case 2b Case 3c
Bl1Sting X X
Vibratory probe X X
Vibrocompaction grout X XXd
Compaction piles X X X
Heavy tamping (dynamic compaction) X X
Displacement/compaction grout X X X
Surcharge/buttress X X
DrainsGravel X xdSand X xWick X X XWells (permanent dewatering) X X X
Particulate grouting X X X
Chemical grouting X X X
Pressure-injected lime X X X
Electrokinetic injection X X X
Jet grouting X X X
Mix-in-place piles and walls X X X
In-situ vitrification X X X
Vibroreplacement stone & sand columns x x xd
Root piles, soil nailing X X X
aBeneath structures.Not-underwater free field adjacent to structure.
cUnderwater free field adjacent to structure.The method has potential use for this case with special techniquesrequired which would increase the cost.
4
The selection of potential methods for site improvement and the applica-tions and results of the methods will depend on:
1. Location, area, depth, and volume of soil involved.2. Soil type(s), properties, and conditions.3. Site conditions.4. Anticipated earthquake loading.5. Structure type and condition.6. Economic and social effects of the structure.7. Availability of necessary materials (sand, gravel,
admixtures, etc.).8. Availability of equipment and skills.
The selection of potential methods also depends on the cost of themethod or technique and the length of time needed to stabilize a site.
New Construction
Though to date no experience with remedial actions exists on poten-tially liquefiable soils beneath existing structures, great strides havebeen made in the area of ground improvement on new construction sites.
The basic concepts of soil improvement have been around for centu-ries and are valid today. These are drainage, drying, densification,cementation, reinforcement, and heating. The most significant develop-ments in recent times are vibratory methods for densification ofcohesionless soils, new soil reinforcement concepts, and new injectionand grouting materials and procedures.
Extensive work has been done by Mitchell (Ref 2) in gathering datain the area of soil improvement to utilize marginal sites. Mitchellemphasizes the practical aspects of soil improvement, including consid-eration of soil types best suited for treatment, effective treatmentdepths, properties of treated soils, major applications, and relativecosts. The following paragraphs briefly discuss the major areas of siteimprovement (e.g., compaction, consolidation, grouting, soil stabiliza-tion, thermal stabilization, and reinforced soil).
Compaction. Thick deposits of loose cohesionless soils run therisk of developing excessive total and differential settlements and, ifsaturated, liquefaction under dynamic loading. In many cases improve-ment can be achieved by densification. In-situ densification of loose,cohesionless layers is generally accomplished using dynamic compactionmethods. This may be accompanied by displacement through the insertionof a probe and/or construction of a sand or gravel column. Methods in-clude blasting, heavy tamping, and vibrocompaction (referring to compac-tion piles and to the insertion of vibratory probes into the ground).
Densification of cohesionless soil layers requires the initial soilstructure be broken down and the particles rearranged in new packingarrangements. In saturated soils this may be accomplished by inducingliquefaction. Partially saturated soils are densified by collapse ofthe soil structure, allowing escape of gas from the voids.
5
Precompression. Weak and compressible soils, especially those thatundergo a large decrease in volume and increase in strength under staticload, are often strengthened by preloading prior to construction. Sur-charge loads or vertical drains may accelerate the process.
Earth fills are most commonly used as the preload. Other preloadsinclude water in tanks (or in lined ponds for larger areas), vacuumpreload pumping from beneath an impervious membrane, anchor and jacksystems, lowering of groundwater to increase consolidation pressure, andelectro-osmosis. The latter methods are more complex than earth fillsbut reduce stability problems and the need for large volumes of sur-charge fill.
Injection and Grouting. Grouting is most often used for groundwa-ter control, ground strengthening, and ground movement control. Due toits high cost, grouting is generally limited to zones of relativelysmall volume and to special problems that cannot be solved by othermethods.
The methods of injection are:
1. Permeation grouting in which the grout fills the soil pores.
2. Displacement grouting in which a stiff mixture fills the voidsand compresses the surrounding soil.
3. Encapsulation in which fragmented ground (naturally or hydrau-lically) is injected by grout that coats but does not permeate the indi-vidual. chunks of soil, forming a lens structure much like a card house.
Thermal Stabilization. Soil can be improved by heating or freez-ing. Moderate heating (100 0C) to fine-grained soils can cause dryingand strengthening if rewetting is prevented. Heating to higher tempera-tures (600 to 1000 oC) can cause permanent improvement by decreasingwater sensitivity, swelling, compressibility, and by increasingstrength.
Frozen, wet soil is much stronger and less pervious than unfrozenground. Temporary ground strengthening and support is accomplished byartificial ground freezing. One important application is freezing forsupport of open excavations. Some arctic areas require permanent groundfreezing around pile foundations in permafrost and under heated build-ings on permafrost.
Admixtures. The addition of admixtures to soil is the oldest meth-od of ground improvement and its use is widespread today. Chemical ad-mixtures, usually lime and cement, use ion exchange and cementation toimprove the properties of soils.
Chemical additives improve volume stability (swelling and shrink-ing), strength and stress-strain properties, permeability, and durabili-ty. Volume stability is accomplished by replacing high hydrationcations with low hydration cations, cementation, and waterproofing chem-icals. Strength and stiffness are increased by eliminating large pores,bonding particles and aggregates together, and by preventing swelling.Permeability is improved by modifying pore size and pore size distribu-tion.
6
Recent advances in soil improvements using admixtures include ex-tended applications of lime and cement in structural fills and with deepmixing methods, and the development of new materials.
Soil Reinforcement. The most intensely studied and advanced methodof soil improvement is soil reinforcement. Composites of earth withreinforcing inclusions and in-situ ground reinforcement have alreadybeen mentioned: sand and gravel compaction piles; piers, piles, and
walls constructed by deep mixing method; and ground strengthening byheating or freezing. Four other types of soil reinforcement commonlyused today are stone columns, root piles, soil nailing, and reinforced
earth.Stone columns are compacted columns, 0.5 to 1 meter in diameter, of
crushed rock or gravel installed in soft soils. They provide vertical
support for structures or embankments and increase shear resistance inhorizontal and inclined directions. Stone columns also act as drains.
Root or micropiles are reinforced (usually) concrete cast-in-place
piles, 0.1 to 0.25 meters in diameter. They are installed in groups of
inclined and vertical piles. Root piles support structures and stabi-lize inclined soil.
Supporting the ground by grouting reinforcing bars into the soil iscalled soil nailing. The purpose is similar to root piles and is usedprimarily for improvement of slope stability and support of excavations.
Reinforced earth uses metals and geotextiles as reinforcements. Itis a constructural composite material in which compacted fill and ten-sile reinforcement are built up in alternating layers. The reinforce-
ments are used to carry tension only.
Exi-sti ng Construction
Use of drains that relieve the build-up of pore pressure is themajor approach applicable to sites with existing structures. There arefour types of drains applicable to liquefaction mitigation: stone, sand,wick, and wells for permanent dewatering. Drains can be used in sand,silt, or clay soils. Primarily, stone or gravel drains are used to pre-vent liquefaction by dissipating pore water pressures nearly as quickly
as they are generated in earthquake-induced cyclic loading. Analyticaland experimental studies have shown gravel drains t-o be effective indissipating excess pore water pressure due to their high permeability(Ref 3, 4, and 5). Sand and wick drains may be iisod to supplement grav-el drains and to relieve existing pore water pressure in a confined lay-er of soil susceptible to l iqufactirmn. Grav I and sand drains are in-stalled vertically and used to control the not-underwater free fieldadjacent to a structure. Wheii Lhe fre field is underwater these drainsalso have potential, but special techniques would be required. Wick(trains can he installed at any angle and may ho used to relieve excesspore water pressure under an existing structure as well as in the freefield. Using wells f .r permanent dewatering is very expensive.
7
PRINCETON UNIVERSITY EFFECTIVE STRESS SOIL MODEL
In order to analyze the response of a large complex waterfrontstructure on soil in which seismically-induced pore pressures cause lossof soil stiffness and shear strength, and hence liquefaction of thesoil, a constitutive soil relationship capable of predicting soil behav-ior under generalized loading conditions was developed at Princeton Uni-versity (Ref 6). The authors demonstrated that implementing this effec-tive stress soil model into DYNAFLOW, developed by Professor Prevost,allows a rational analysis of complex dynamic soil-structure interactionproblems.
Professor J.H. Prevost of Princeton University (Ref 7) has devel-oped a finite element numerical procedure to analyze transient phenomena
in fluid saturated porous media. The saturated porous medium is modeledas a two-phase system consisting of a solid and a fluid phase. The sol-id skeleton may be linear, or nonlinear and hysteretic. Large deforma-tions may also be included. The fluid may be compressible or incom-pressible depending upon the intended application (blast, seismic,etc.). Time integration of the resulting semidiscrete finite elementequations is performed by using an implicit-explicit algorithm (Ref 8and 9). In order to remove the time-step size restriction associated
with the presence of the stiff fluid in the mixture, the fluid contribu-tion is always treated implicitly.
STUDY PROGRAM
Once a seismic stability problem has been verified, present stateof the art requires field tests be conducted to insure that an improve-ment method is applicable under specific site and soil conditions, andto verify that the method will perform as desired. This study will fo-cus on the use of stone drains to prevent the excessive build-up of porewater pressure during earthquake loading, thereby reducing damage to theNavy's ocean front structures as a result of liquefaction.
Under certain conditions, addition of stone columns to a foundationcan improve the ground when subjected to a strong motion earthquake.The mechanisms involve densifying the soil surrounding the stone column,increasing the shear strength of the foundation, and dissipating excess
pore pressure through radial drainage to the columns. Stone columnstypically replace 20 to 35 percent of the soil. being improved (Ref 10).
Densification of the surrounding soil takes place during construc-tion due to the heavy vibratory equipment used to install the columns.Stone columns constructed of coarse, open-graded stone are quite denseand not likely to liquefy. The shear strength of the stone column cansignificantly increase the lateral force resistance of the foundation.Stone columns on relatively close centers provide vertical drainagewhich greatly reduces the flow path and is effective in dissipating ex-cess pore pressures generated by earthquake loading. If the pore waterpressures generated in a soil mass by cyclic loading can be dissipatedto some extent as they are created, the danger of liquefaction may beaverted (Ref 4).
8
Construction techniques, site conditions, and the actual earthquakeloading determine the contributions from each of the stone columns im-provement mechanisms.
Seed and Booker (Ref 4) developed a design methodology to determinethe required geometry needed to install a system of stone columns capa-ble of dissipating pore pressures generated in sand layers due to strongmotion earthquake vibrations (Figure 1). In this method a design earth-quake is represented by N equivalent uniform stress cycles of a specificmagnitude and period. The resulting pore pressure ratio (pore pres-sure/initial stress) is determined based on the horizontal permeabilityof the sand, duration of the earthquake, unit weight of water, coeffi-cient of compressibility, and diameter of stone columns. The pore pres-sure ratio (pore water pressure divided by initial vertical stress) mustbe less than one to prevent liquefaction. The permeability of the drainis required to be at least 200 times greater than the soil beingdrained.
Extensive field evidence has shown that stone columns act effec-tively as drains for static load applications. The ability of stonecolumns to act as drains during earthquakes has not been demonstrated.The most appropriate method for validating the Seed and Booker designmethod would use field data from instrumented prototype situations. Thenonavailability of field data preempts such a study. As an alternative,the predictive capability of the Princeton Effective Stress Soil Model(PESSM) will be utilized. The PESSM has been proven capable of captur-ing the generation of excess pore water pressure in saturated sand de-posits during earthquakes (Ref 6).
This study is directed toward examining the predictive capabilitiesof the numerical procedure proposed in Reference 6. Using DYNAFLOW, thecapability of stone columns to improve liquefiable soil foundations toassure the safe performance of structures founded on them in the eventof earthquake excitation will be assessed. Site, soil, and accelerationconditions will be input to DYNAFLOW as well as the proposed course ofaction for the critical sites. DYNAFLOW will be used as a tool to de-termine if the mitigation techniques will reduce the risk of failure orassure that the consequences of a damaging earthquake will be tolerable.
Of particular interest is the value of the proposed numerical modelin adequately predicting the generation of excess pore water pressuresin saturated sand deposits beneath foundations during earthquakes, dis-sipation of the excess pore pressures through stone drains, and itsperformance in dynamic soil-structure interaction problems. The mostappropriate method for such a validation study would be to utilize fielddata from an instrumented prototype situation. However, such a study ispreempted by the scarcity of field data. An alternate method of valida-tion is provided by comparison of results from the numerical procedureswith state-of-the-art theory of gravel drainage systems to stabilizepotentially liquefiable sand deposits (Ref 4).
The PESSM was validated in Reference 6 by analyzing centrifuge soilmodel test data. Although imperfect in many respects, it is felt thatdynamic centrifuge soil model tests can provide a data base for cali-bration of numerical procedures. A number of dynamic centrifuge soilmodel tests have been reported and analyzed (Ref 11 and 12). Of partic-ular interest to this study is the dynamic centrifuge brass footing soil
9
model test reported in Reference 13. Reference 6 presents results of acomparison of the numerical method incorporated in DYNAFLOW and the dy-namic centrifuge tests.
STUDY PROCEDURE
The brass footing centrifuge test is the reference point by whichthe effectiveness of the stone columns will be measured. The soil wasplaced in a stacked-ring apparatus by pluviating the sand in layers intowater and then rodding to achieve the desired density. A brass cylinder(diameter = 113 mm) was placed on top of the saturated sand deposit(height = 151 mm, diameter = 406 mm) and consolidated on a centrifuge ata centrifugal acceleration of 80 g's. The deposit was then subjected tosinusiodal base acceleration. The corresponding prototype situation wasanalyzed using DYNAFLOW.
Figure 2 shows the finite element mesh used for analysis. The soilis discretized using 240 elements and the footing by using two rows of10 elements each. The soil parameters are given in Reference 6 and inthe analysis permeability = 2.5 x 10 x 80 m/sec to properly scale dif-fusion time. The footijg is godeled as a one-phase elajtic jolid withmass density = 8.5 x 10 /kg/m and Young's Modulus = 10 N/im.
The water table is located at the ground surface. Drainage of thepore fluid is not allowed to take place through the rigid bottom bounda-ry or the lateral side boundaries. Ground shaking is applied as a hori-zontal sinusiodal input acceleration at the bottom boundary nodes, witha maximum acceleration of 0.17 g (1.6677 m/sec ) and a frequency of 1 Hzfor 10 seconds (10 cycles).
The stacked-ring apparatus is used in the centrifuge test to simu-late free-field conditions. Each node in the computer model was as-signed four translational degrees of freedom: two for the soil skeletonand two for the fluid phase (pore water). In the free-field simulation,the nodal planes must remain horizontal and can only undergo parallelmotions. This is specified by assigning the same equation number toeach nodal degree of freedom on the same horizontal plane for the twoside boundaries.
Reference 6 used existing centrifuge test data which required per-forming laboratory triaxial tests to derive the material model for inputto DYNAFLOW. The specifications for the triaxial tests required thatthe specimens be constructed at the same relative density as the soilused in the centrifuge tests. Due to different preparation techniques,there were differences in void ratios between the respective soils.This affected the ultimate strength and moduli of the soil. Addition-ally, the authors of the centrifuge test report (Ref 13) acknowledgeddifficulties in precisely controlling the properties of the centrifugesoil deposit. These factors combined to generate a computer simulationthat caused much larger strains to be generated to reach an equilibriumstate of stress; however, it was only moderately weaker in ultimatestrength. The difficulties encountered in experimental procedure areinconsequential to the present study.
Two finite element meshes were designed and modeled in DYNAFLOW todetermine the effect of adding columns to reduce liquefaction potential.
10
The soil modeled is Leighton-Buzzard 120/200 sand, whose soil model wasderived and reported in the brass footing test (Ref 6). Horizontalsinusiodal base accelerations reaching 0.17 g were applied to the finiteelement meshes. This corresponds to the accelerations applied to thereferenced brass footing centrifuge test. Figure 3 shows the accelera-tion history input at the base nodes.
In order to determine the best modeling scheme to achieve realisticresults, several parametric studies involving a variety of modelingtechniques were tried. Parametric studies included the effects of vari-ation of stone column permeability, stone column depth and load applica-tion time steps. DYNAFLOW incorporates an algorithm that "slaves" nodaldegrees of freedom at different points in the finite element mesh, forc-ing those nodes to displace in the same manner. Modeling techniquesused to simulate the interface between the sand and stone column includ-ed nodal slaving across all of the stone column, nodal slaving acrosspart of the stone column, shared nodes, and double noding the stonecolumn to form a contact element. These techniques are described ingreater detail in the DYNAFLOW User's Manual (Ref 14).
The studies compare a homogeneous soil mass and homogeneous soilfoundation to ones including a stiffer, more permeable soil which willbe referred to as a stone column (Figures 2 and 4). The stone column isassigned a permeability 200 times that of the soil and a friction angle25 percent higher. The first study compared a homogeneous soil mass(MASS-H) to a soil mass containing a stone column (MASS-SC). The secondstudy compared the footing on a homogeneous soil foundation(FOUNDATION-H) to a foundation including four stone columns, two on eachside of the footing (FOUNDATION-CS) (Figure 2). The water table is atthe surface in all cases. DYNAFLOW has the capability to place the wa-ter table below the surface by defining the elements above the watertable as dry and below the water table as saturated. Consolidation tak-ing place due to seismic loading can cause a change in the level of thewater table. Within DYNAFLOW, this cannot be reflected in the finiteelement mesh by causing dry elements to become partially or fully satu-rated.
Horizontal and vertical effective stresses, shear stresses, andpore pressures were recorded for the elements designated in Figures 5aand 6a. Nodal displacements, velocities, and accelerations were record-ed for the nodes designated in Figures 5b and 6b.
FINITE ELEMENT STUDIES
Soil Masses
The control column (MASS-H) was designed with depth and elementsize comparable to the footing model. The column is made up of 1 by 1meter elements, five wide and twelve deep (Figure 4a). Gravity forceswere applied by consolidating MASS-H to 1.0 g vertical acceleration overa period of 10,000 seconds. This consolidation period was sufficient toensure no excessive pore pressure build-up, verified by monitoring porepressure and effective stresses within selected elements for 20,000 sec-onds. When consolidation was complete, sinusiodal horizontal base ac-celeration of 0.17 g was applied.
11
MASS-SC is the same size (5 by 12 meters) as MASS-H, but the centercolumn of elements is replaced with stiffer, more permeable stone columnelements (Figure 4b). MASS-SC was consolidated and shaken in the samemanner as MASS-H.
Results.
Effective Stress. Both soil masses experienced a decrease ineffective vertical stress when shaken. The outer elements of the twomasses behaved similarly (elements 7 through 10, Figure 6a). Elementsadjacent to the stone column in MASS-SC (elements 3 through 6, Figure6a) experienced a greater decrease in effective vertical stress than thecorresponding elements in MASS-H, though the amplitude of the higherfrequencies was smaller in MASS-SC than in MASS-H (Figure 7).
The elements within the stone column in MASS-SC (elements 1 and 2,Figure 6a) initially experienced a decrease in effective verticalstresF, but began to increase after the first second of shaking (Figure8). When shaking ended after 10 seconds, the final effective verticalstress within the stone column elements was greater than the initialvalue.
Pore Pressure. Pore pressure increase due to shaking was sim-ilar in the two masses for the first 1 to 2 seconds. After 2 seconds,MASS-SC dissipated excess pore water pressure while MASS-H continued tobuild up excess pore water pressure. The amplitude of the higher fre-quencies in MASS-H were larger than those of MASS-SC, in many elementstwo to three times larger (Figures 9 and 10).
Sumnary. It is of great value to see the limited zone affect-ed by the stone column. The addition of a stone column to a homogeneoussoil mass allowed a redistribution of stress from the nearby sand (onestone column diameter away) to the stone as the sand began to losestrength, but did not affect the stress in the sand a distance of twostone column diameters away. The stone column was also shown to be ef-fective in reducing the amount of pore pressure build-up due to seismicloading.
Footing
The second series of finite element meshes were designed to modelfoundations with surface footings. One mesh modeled a homogeneous soilfoundation (FOUNDATION-H) (Figure 2a) and the other a soil and stonecolumn foundation (FOUNDATION-SC) (Figure 2b). The foundations are 32.5by 12.1 meters and the footing 9.0 by 1.56 meters. The finite elementmodels were consolidated to 1.0 g vertical acceleration over a period of105000 seconds to simulate gravity forces. A static pressure of 1.30 by10 N/m was then applied to the top of the footing. The consolidationperiod was sufficient to ensure no excessive pore pressure build-up,verified by monitoring pore pressure and effective stresses within se-lected elements for 20,000 seconds. When gravity loading was complete,the mesh was subjected to a sinusiodal base acceleration of 0.17 g at 1Hz for 10 seconds (Figure 3).
12
Results.
Effective Stress. FOUNDATION-H (homogeneous foundation) expe-rienced large zones of liquefaction in the free field as well as somelocalized liquefaction beneath the structure. Great improvement wasachieved with the addition of stone columns, though FOUNDATION-SC expe-rienced some localized liquefaction (Figure 11). The majority of ele-ments monitored showed improvement when stone columns were added. InFOUNDATION-SC, both sand and stone elements experienced an increase ofup to 100 percent of the initial effective stress by the end of shaking(Figures 12, 13, and 14). Elements that liquefied in both models showedthe decrease in strength to be more gradual in FOUNDATION-SC than thosein FOUNDATION-H and with smaller amplitude of the higher frequencies.
Pore Pressure. Pore pressures were generated withinFOUNDATION-SC during the first second of shaking, though generally to alesser degree than in FOUNDATION-H. After the first second of shaking,excess pore pressures dissipated and, in general, the pore pressurewithin an element at the end of shaking was equal to or lower than theinitial pore water pressure (Figures 15b, 16b, and 17b). The influenceof the stone column on pore water pressure was found to affect elementstwo stone column diameters away.
Sumnary. Analysis of the effective stress time histories dur-ing shaking indicates a redistribution of the footing load to the stiffstone columns from the weakening sand. The addition of the stone col-umns to the sand foundation also proved beneficial in dissipating porewater pressure generated. The zone of influence of the stone columnswas larger in the soil/structure situation than in the soil masses.
SUNNARY AND CONCLUSIONS
The Navy has $25 billion worth of facility investments in areassusceptible to damage from seismically-induced soil liquefaction.Liquefaction plays a predominant role in waterfront damage, often beingthe single cause of widespread losses. Stone columns inserted into po-tentially liquefiable soil increase the overall shear strength of thefoundation and reduce the radial drainage path, which can be effectivein dissipating excess pore water pressures generated by earthquake load-ing.
The Princeton University Effective Stress Soil Model, incorporatedinto the finite element program DYNAFLOW, has the ability to predict thegeneration of pore water pressure in saturated sand deposits beneathsurface footings during earthquakes. Using DYNAFLOW, two finite elementstudies were conducted to determine the effectiveness of the inclusionof stone columns in saturated soil to improve site response when seis-mically loaded. In both studies, the addition of stone columns improvedsoil conditions and decreased damage due to the vibratory motion. Thestone columns appear to stiffen up the foundation allowing for addition-al resistance to lateral loading from earthquake waves. Additionally,when permeable stone columns are present, less seismically-induced porewater pressure is generated due to dissipation through the stone drains.
13
The numerical procedure shows the predominant mechanism for improvementis the stiffening of the ground. The ability to dissipate seismically-
induced pore water pressure is a secondary benefit. Experts in academiaand industry agree that stiffening of the soil is the major mode of im-
provement.In order to determine site response to an earthquake, it is impor-
tant to be able to accurately model soil conditions. The constructionof stone columns has a great affect on the conditions of the surroundingsoil, causing densification of the sand and smear between the sand andstone column due to infiltration of the small sand grains into the voidsof the stone column. Smear will change the permeability of the stonecolumn as well as its effective diameter and its radial drainage area.This becomes a complex modeling problem when the numerical model hasextensive analytical capabilities because it requires detailed input.Aside from the design and analysis of the stone column system, construc-
tion will be very expensive due to the large amount of stone necessaryto replace the soil (20 to 35 percent). Since stone columns are in-stalled vertically, they can only be put around, and not beneath, exist-
ing structures. Stone columns may or may not be of benefit if there arelocalized liquefaction zones beneath the structure. For these reasons
generic improvement guidelines are inappropriate. Individual analysisof candidate sites is necessary. This effort concludes our investigationof liquefaction mitigation technology (MS-5) of the Seismic Hazard Miti-
gation Task.
REFERENCES
1. Department of the Army. Technical Report REMR-GT-2: Improvement ofliquefiable foundation conditions beneath existing structures, by R.H.Ledbetter. Washington, DC, 1985.
2. J.K. Mitchell. "Soil improvement: State-of-the-Art," Tenth Inter-national Conference on Soil Mechan ics and Foundation Engineering, Ses-sion 12, Stockholm, Sweden, June 15-19, 1981.
3. Y. Sasaki, and R. Tanigighi. "Shaking table tests on gravel drainsto prevent liquefaction of sand deposits," Soils and Foundations, vol22, no. 3, 1982.
4. Earthquake Engineering Research Ceniter. EFRC 76-10: Stabilizationof potentially liquefiable sand deposits using gravel drain systems, byH. B. Seed and J. R. Booker. Univorsity of California, Berkeley, CA,Apr 1976.
5. Y. Yoshimi. "Protection of strnctures from soil liquefaction haz-ards," Geotechnical Engineering, vol 11, 1980.
6. Naval Civil Engineering Laboratory. Technical Report R-919: Evalu-ation and validation of the Princeton University Effective Stress Model,by J.H. Prevost, J.M. Ferritto, and R.J. Slyh. Port llueneme, CA, Dec
1986.
14/
7. J.H. Prevost. "Wave propagation on fluid-saturated porous media: Anefficient Finite element procedure," International Journal of Soil Dy-namics in Earthquake Engineering, Nov 1985.
8. T.J.R. Hughes and W.K. Liu. "Implicit-expicit finite elements intransient analysis: Implementation and numerical examples," Journal ofApplied Mechanics, American Society of Mechanical Engineers Transac-tions, vol 45, 1978, pp 395-398.
9. T.J.R. Hughes and W.K. Liu, "Implicit-expicit finite elements intransient analysis: Stability theory," Journal of Applied Mechanics,American Society of Mechanical Engineers Transactions, vol 45, 1978, pp371-374.
10. Department of the Army. Technical Report REMR-GT-7: Applicationsof the state of the art of stone columns - liquefaction, local bearingfailure, and example calculations, by R.D. Barksdale. (Prepared bySchool of Civil Engineering, Georgia Institute of Technology), Washing-ton, DC, 1987.
11. M.D. Bolton and R.S Steedman. "Centrifuge testing of microconcreteretaining walls subjected to base shaking," in Proceedings of the SoilDynamics and Earthquakes Engineering Conference, Southampton, U.K.,1982, pp 311-329.
12. P.C. Lambe. Dynamic centrifugal modelling of a horizontal sandstratum, Sc. D. Thesis, MIT, Cambridge, MA, 1981.
13. R.V. Whitman and P.C. Lambe. "Liquefaction: Consequences for astructure," in Proceedings of the Soil Dynamics and Earthquake Confer-ence, Southampton, U.K., 1982, pp 941-949.
14. Naval Civil Engineering Laboratory. UG-0014: DYNAFLOW User'sGuide, by J.H. Prevost, R.J. Slyh, and K.P. Hager. Port H{ueneme, CA,Nov 1988.
15
De - equivalent
De tributary areaD - Diameter
D Stone Column
Sand
RADIAL DRAINAGE TO STO COLUMNS
(a) Stone column drainage scheme.
STONE COLUMNS
(b) Plan view of stone columns for site improvement.Generally, 20 to 35 percent soil replacementis necessary.
Figure 1. Stone column geometry.
16
(a) Homogeneous soil foundation: FOUNDATION-H.
(b) Soil and stone column foundation: FOUNDATION-SC.
S-t r i ct ijr p
Stone column elements
i-eghon-Buzzard Sand elements
Figure 2. Foundation finite element meshes.
17
1.60---1.67
1.20{-
(1 0.801
4'
1.016
0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00
Time, seconds x 10 1
Figure 3. Sinusoidal base input acceleration.
18
,.... J.o
(a) Homogeneous soil (b) Soil mass withmass: MASS-H. stone column:
MASS-SC.
Stone column elements
Leighton-Buzzard Sand elements
Figure 4. Soil mass finite element meshes.
19
10.. .
1- . ._ . . . . .7.. . . . . .
_3 _ _ 6 12 _ _
(a) Elements monitored for effective stress, shearstress, and pore pressure.
A B C
E F
(b) Nodes monitored for displacement, velocity,and acceleration.
Figure 5. Foundation finite elements and nodes monitored.
20
A F
3 B G
5 4E
(a) Elements monitored for effective (b) Nodes monitored for displacement,stress, shear stress, and pore velocity, and acceleration.pressure.
Figure 6. Soil mass finite elements and nodes monitored.
21
1.00 -- 0.26
1.50
0
2.00 - 0.50
.O
0.75
. 0
3.50
3.86 1 ,t.00
0 ! 2 3 4 5 6 7 8 9 10
Time, seconds4
(a) Homogeneous soi~l mass: MASS-11.
0xa0.I 0.752~
I s
0.5 no0
I *•
2.00
uU
3.50
1.003.75 1 -- -
0 1 2 4 5 6 7 8 9 10
Time, seconds
(b) Soil mass wth stone column: MASS-SC.
Figure 7. Effective vertical stress in soil mass element 4.
22
2.19 0.34
S3.00j-.5 0.50
3*4 5400 8 9 |
4.00
4.720
0.75
5.00
6.00
6 .44 1.00a 1 2 3 4 1 6 78 9 10
Time, seconds
()Homogeneous soil mass: MASS-H.
0.75
6.90O
00
1 .C3
o .CO 1.25
10.00 -
rd 11.0D .
I .7513.)3 12'c3 --------
o I 2 3 4 5 6 7 8 9 10Time, second&
(b) Soil mass with stone column: MASS-SC.Note: This element is in the stone column.
Figure 8. Effective vertical stress in soil mass element 1.
23
7. U) - - - . -1.49
7 .00 -
1 .40
6.50 .
1.2
0 3
4.6.0001
0 3 4 6 7 091
.5
a.' 1.50*7.00
.0
51.10
5.00
47 1.0
240
12.13 1 1.53
12.00-1 .50
1.40 0o11.00-(
1 .30'n
10.00 f
1.2
* *
1.1
0. 1.0
7 940 1 2 3 4 5 698 910
Time, seconds
(a) Homogeneous soil mass: MASS-H.
11.40 - -- -- 1.44
11.00-*14
,10.501- 0,
x -1.30
10.00~
9.0 1.2
C6 A
1.20
8. .50-*
8 .00 1.0
8.55
_____ ____ ____4 _ _ _
(a) Homogeneous soil foundation: FOUNDATION-H.
_____________
(b ol n tneclm fudtonXONATO-C
StructureStone column elmet
Leghon-8zzar San eleentMonitred lemet -- no lquefctio
Figur Soi. Elantn withinth foondtin FOUNDATnexpriecin
# Mn;tord lemnt noliquefaction
26
1.18 -- 0.71
1.20
0.80
o 1.40
N 0.90
S1.602
I.US1.80
1.10
2.00 k1.20
2.07 I1.24
0 1 2 3 4 5 6 7 9 9 10Tim, second*
(a) Homogeneous soil foundation: FOUNDATION-Hf.
1.14 70.73
1.25 0.80
S1.50 IIU 1.00
I.?
2.0
2.25 .10 ~
2.07
0.84 0. 121.00
0.20
2.00
0.40
3.00 00
6j 4.00 .0.60
0> 5.00 .S
6Aq* '1
0.80
6.00
7.00 _____ 1.007 06 0 14 4 ? 8 9 10
Time, econd.
(a) Homogeneous soil foundation: FOUNDATION-H.
4.76 T0.60
6.00
0.80
7.00
N -
* 8.00 1.00
U 9.00
10.00 1
11.001.40
11.87 r 1.48
0 1 2 3 4 5 6 7 8 9 10Tim, -- "lds
(b) Soil and stone column foundation: FOUNDATION-SC.NOTE: This element is in the stone column.
Figure 13. Effective vertical stress in foundation element 8.
28
0 0
0.20
0.50
S0.40
I .00
0.80
2.00
1.00
TI". @*ondo
(A) Homogeneous soil foundation: FOUNDAI'ION-H.
0. 60 -T 0.031
1.00 0.50
V
1.00
3.00
U U.1 0.80
4.00
2.00
J: --- 2 .390 . 6 3 5 6 7 8 9 10
(b) Soil and stone column foundation: FOUNDATION-SC.
Fgure 14. Effective vertical stress in foundation element 10.
29
11 .91 - 1.49
11.001.4
N 1.30
aa0.0
*1 0.4 1.2
9.00
.08.00 l1.00
0 1 2 3 4 5 6 7 8 9 10Tine, seconds
(a) Homogeneous soil foundation: FOUNDATION-H.
10.11 1.16
10.001
9.50
00
9.0000
8.50
3.001
7.501
7 . 3 0. t. 8 40 1 2 3 4 5 6 7 8 9 10
Time, eod
(b) Soil and stone column foundation: FOUNDATION-SC.
Figure 15. Pore water pressure In foundation element 3.
30
I I IIi
11.8 1.00
M 10.00
.9.000
~1.208.00-
7.00-j
6 94 1o- - 0 o
Tilmsecondst
(a) Homogeneous soil foundation: FOUNDATTON-H.
9.01 1,299.00
8.501.20
* I
Co., 8.00
7.SO
97.00 10
6.50
0.90
1: 9,,, 0.860 1 2 3 4 S 6 7 9 10
"Tim.° qecond
(b) Soml and stone column foundation: FOUNDATION-SC.
Figure 16. Pore water pressure in foundation element 8.
31
4,90 1 .64
1.80
4.50.
1.60 0
4.00 1.
1.40
S3.5 D--
a. 1.2
3.00 1.20
0 1 2 3 4 5' 6 7 8 9 10
Time, meconds
(a) Homogeneous soil foundation: FOUNDATION-H.
4.00 I
-1.40
0
I.D
N -
2..2
2. 0.75
05 to
Tins. seconda
(b) Soil and stone column foundation: FOUNDATION-SC.
NOTE: This element is in the stone column.
Figure 17. Pore water pressure in foundation element 10.
32
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