GEOTECHNICAL INVESTIGATION AND ANALYSES

SLOPE FAILURE REPAIR UTILIZING BALED TIRE FILLINTERSTATE ffiGHWAY 30 WEST OF OAKLAND BLVD.

FORT WORTH, TARRANT COUNTY, TEXAS

Report to

Fort Worth DistrictTexas Department of Transportation

Fort Worth, Texas

By

TEAM CONSULTANTS, INC.Dallas, Texas

March, 2003

INTRODUCTION

Slope instabilities throughout Tauant County, Texas have been repaired using existingsoils or imported soils, with or without modifications in an effort to minimize costs andreconstruction time. Recent consideration for the use of baled tires as partial replacement for fillsoils used in slope repair projects appears to provide a significant improvement to long tenDstability while simultaneously providing for the beneficial reuse of waste tires. This is especiallysignificant where the in-situ soils are inherently unstable high plasticity clay and/or wheresurface or groundwater s~age may be a contributing factor to slope instability.

This study is a post-repair investigation of site conditions where baled tire fill has beenutilized for the repair of a failed slope along IH 30 west of Oakland Boulevard in Fort Worth,Tanant County. Texas. The project location is shown on the site vicinity map. Plate 1. in theIllustrations section of this report.

Project Description

A portion of the cut slope along the eastbound lane of ill 30 west of Oakland Boulevardfailed at some point during the first half of calendar year 2002. The failed slope area wassubsequently repaired by removing the slide spoils, stacking baled tires in the excavated slidearea, and covering the stacked bales of used tires with a portion of the excavated spoils. It is ourunderstanding that these cover soils were placed without compaction control and were ultiinatelysmooth graded to restore slope geometry and then over seeded to help reduce erosion. Since thateffort w~ completed in mid 2002, the slope has become revegetated and appears to be stable. . .

Purpose and Scope

The purposes of this study have been to evaluate subsurface conditions in the area of therepaired slope and to provide analyses 9f as-built slope stability factors of safety and any othergeotechnical considerations revealed by these studies which may be beneficial for incorporationin similar repairs in the future.

This study has included two continuous undisturbed and disturbed sample borings locatednear either end of the repaired area, laboratory analyses of selected soil samples for pertinentengineering properties, and computer-aided slope stability analyses to aid in evaluation of theeffects of using baled tires as partial replacement for fill soils in slope repair projects.

FIELD INVESllGAllON

. Subsurface conditions in the area of the repaired slope failure were evaluated in two

continuous undisturbed and disturbed sample borings completed on January 22, 2003, which arelocated approximately at the top of the slope on either end of the repaired area. Boring depthwas 30 feet in both borings. Approximate sample depths, descriptions of materials encountered

t

and soil classification (based on the Unified Soil Classifications System) are presented on thelogs of borings, Plates 2 and 3. Keys to descriptive terms and symbols used on the logs arepresented on Plates 4 and 5.

Undisturbed samples were obtained in each boring from the surface using thin-walledShelby tube samplers pushed in maximum one foot intervals. Disturbed samples were obtainedusing a split barrel sampler in conjunction with Standard Penetration test procedures(ASTM D1586). Standard field drilling and sampling procedures are presented in the Appendix.

LABORATORY STUDIES

Field soil classifications were verified by laboratory tests to detennine the liquid limit(TEX-I04E), plastic limit (TEX-I05E), and natural moisture content (TEX-103E)(ASTM D4318 and ASTM D-2216, respectively). The results .of these tests are located at thecorresponding sample depth on the boring logs. The engineering properties used in theembankment slope stability analyses were detennined from the results of consolidated-draineddirect shear and unconfined compressive strength tests perfonned on selected undisturbedsamples. Results are presented on Plates 6 through 9, and are summarized on Plate 10.

Laboratory test procedures followed the current testing standards of the Texas HighwayDepartment which closely follow current testing standards of the American Society for Testingand Materials (ASTM). Standard laboratory testing procedures are also described in theAppendix

GENERAL SITE AND SUBSURFACE CONDmONS

The area of the repaired slope failure appears to be in a cut area. The age of the cut isestimated to be more than 40 years but probably less than 50 years based on the approximatetime frame for the rn 30 construction. This area is characterized by gently rolling terrain.

Geolo9,Y

The project site is located in an area overlain by deposits of the Grayson Marl and MainStreet Limestone (undivided) and/or the Pawpaw Formation, Weno limestone and Denton clay(undivided) all of Cretaceous geologic age as shown on the Dallas Sheet of the Geologic Atlas of~ 1987. These formations vary in makeup but generally predominantly consist ofcalcareous clay and marl with thin limestone beds. Photographic evidence of the unrepairedfailed area obtained by TxDot personnel tends to indicate that exposed materials have verysimilar characteristics to those which are' used to describe the Weno limestone althoughdistinctions in these formations are not easily distinguished photographically.

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Subsurface Condi!i~

The results of our field and laboratory investigations indicate that materials encounteredat the locations of both borings vary from silty clay to limestone to shaIey clay and/or clayeyshale. One significant difference observed between the two borings was the presence ofslikensided features in boring B-1 (west end) between the depths of7 and 23 feet. The limestonestratum observed in both borings was characteristically weathered, fractured and dry. However,the materials encountered beneath the limestone in both borings was characteristically moist.Refer to the logs of boring for a more detailed description of subsurface conditions encountered.Both borings were reported to be dry at completion and no ground water was encountered duringdrilling.

ANALYSES OF RESULTS

General

Slope geometry used in these stability analyses was estimated to have a maximum heightof 20 feet and a constant slope no steeper than 3H:N. No topograp~c data was obtained as apart of this investigation and both the height and slope have been estimated for the purposes ofthis study.

Values used for unit weights and shear strengths of the soil and rock strata defined by thisstudy correspond to the values determined in the unconfined compressive strength and directshear tests perfonned on selected samples. The values used for unit weight and shear strength ofthe baled tires have been estimated from published data for equipment used to bale used tires andthe U.S. Environmental Protection Agency. Our assessment of the mode of failure is based upondiscussions with TxDot personnel and copies of photographs taken of the exposed failure zonefollowing removal of the displaced material in the failure zone. A copy of the black and whiteimages as received via e-mail is included in Appendix A to this report. Based on these sources,we believe this failure was probably a classic case of rotational failure. Evidence was observedwhich indicated underlying moisture sensitive clays, which appeared to be slickensided at orbelow a depth of approximately seven feet (see log of boring B-1), may have been strained to alimiting condition of residual strength and following a heavy rainfall event may have becomeoverloaded by the overlying weathered and fractural limestone stratum which likely becamehighly saturated. Other issues, such as a build up in pore pressures within the underlying claysoils, may also have contributed to this failure. Our feeling is that the condition that ultimatelyled to failure was progressive over a long period of time and could neither have been easilypredicated nor prevented. Our analyses ha.c; been based on the probability that this failure wasdeep seated and rotational. Our analyses based on this hypothesis follows.

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Slope Stability Analyses

Slope stability analyses for this study were completed using total stress analysismethodology. Soil strength parameters used in the analyses correspond to the values detenninedin the direct shear tests performed for this investigation, except that the cohesion portion of thestrength envelope was detennined by unconfined compressive strength testing and was reducedto limiting values or lower. Slope stability analyses were completed using a computer programoriginally developed for the Texas State Department of Highways and Public Transportation bythe Center for Transportation Research at the University of Texas. The program, UTEXAS2, hasbeen modified and expanded to also meet the U.S. Anny Corps of Engineers' requirements.

Our understanding of slope conditions prior to failure includes a cut profile ofapproximately 3H:IV and a maximum slope height on the order of 20 feel The soil stratigraphyobserved in the sample borings completed for this investigation tend to indicate shaley clays andclayey shales underlie weathered fractured limestone on both ends of the repaired area. Thedepth or thickness of the limestone is significantly thicker on the higher east end (see log ofborings B-2) than on the lower west end of the repaired area. Results of direct shear analyses onthe underlying shaley clays tend to indicate shear strength values (0=0) of from approximately100 psf to approximately 300 psf. Interestingly, the results of analyses on the clayey shales tendto indicate shear strength values (0=0) of from near zero to approximately 100 psf. It is easy tounderstand how a slope with almost no shear strength and an internal angle of shearing resistanceapproaching zero would fail, so using these limiting values in our calculations would not berepresentative of real world conditions. Although in some soils within a failure plane suchconditions may be approached, we have limited our analyses to average conditions and observedhow changes' i.n shear strength and angle of internal shearing resistance affect overalltwo- dimensional slope stability.

Our analyses has looked at peak values, residual values and variations of the strength andshear angles that would reduce stability to values of 1.0 or less. The limiting values are' thenreused with the slope profile being modified to include a repair using baled waste tires. Theanalyses for each case are described as follows.

The slope height and profile were analyzed for stability assuming circular shear failureand soil parameters up near the peak values observed in our laboratory studies. This scenario isrepresented by File TXD0010B. Results for this scenario using conventional circular shear arepresented in File OUT 0010B. The factor of safety is reasonable at 2.63.

Weathering and saturation eventually impact the slope negatively and soil parametersbegin to drop to residual values. The added overburden due to saturation and lowered strengthvalues are represented by File TXD0020B. Results for this scenario, again based onconventional circular shear, are presented in File OUT0020B. The factor of safety is 1.156.

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Finally, the angle of internal shearing resistance is reduced by the affects of saturation.When the air voids are filled and stresses are transferred to pore fluids the resulting strength andangle of internal shearing resistance drop rapidly and failure occurs. This scenario is representedby File TXD0030B. Results for this scenario, again using conventional circular shear, arepresented in File OUT0030B. The factor of safety is computed to be 0.997. A schematicdrawing showing the slope in profile is presented on Plate 11.

Although these studies have assumed a change in soil strength and angle of internalshearing resistance with time under conditions never measured or observed, it is consideredpossible that these conditions or similar conditions did exist at some time or in some portions ofthe slope. However, an exact representation is not intended nor is one possible. Thehypothetical strength and shear angle values used for these analyses are possible but representaverages. Other unknown variables not known to exist but considered possible which could havea significant influence on the rate of change in these assumed average strength and shear anglevalues include vibrations induced by nearby traffic and a buildup of hydrostatic pressures.During heavy rainfall events or during the subsequent runoff period when ground watermigration may be significant, these types of influences could exacerbate the rate of change in soilparameters and cause a rapid loss in slope stability.

The use of baled tires as replacement for soil backfill was implemented at this site as anexperiment in long term stability and as a potential beneficial re-use for waste tires. Theanalyses of slope stability which incorporates baled waste tires for replacement of soil baCkfill torestore the slope profile has some significant unknowns. These include mode of failure (circularor non-circular), shear strength of the composite stacked tire bales, and the angle of shearingresistance either internal to the stacks and/or bales or external between the stacked bales and thesupporting materials. Although the computer program is able to analyze either mode of failure,the mode selected for this analysis was non-circular failure simply because under circular failureanalyses the factor of safety is generally many times the computed value for non-circular failure.Therefore, the more conservative approach is to use the non-circular failure mode and makereasonable assumptions about shear strength and shear angle for the supporting materials. Usingsoil parameters and phreatic surface conditions which yielded a slope failure factor of safety, wereplaced a portion of the slope profile with stacked bales of used tires and reduced the supportingmaterial strength value to one half its value to simulate a worst case scenario. Both circular andnon-circular stabili analyses were conducted. However, only the more conservative (i.e. lowerfactor of safety) results observed in the non-circular analyses have been reported here. Thisscenario is represente y ile TXD 0 esu ts or s scenano usmg non-circular shearare presented in File OUT0050B. The factor of safety is acceptable at 1.721.

RECOMMENDA nONS

These studies have shown a considerable improvement in long term repaired slopestability is possible where baled tires are substituted for soil backfill in areas where highplasticity clays are predominant. The baled tires are porous and require little or no protection

5

from environmental elements and yet should be very durable and stable for a significant timeperiod. Costs for baled tires, although not thoroughly researched, should make their useeconomical and especially so in view of the added long tenD stability benefits gained. Otherfactors such as the beneficial re-use of an otherwise difficult waste to dispose of are considered abonus as well. Selection of baled tires for a partial replacement for fill soils used in slope repairprojects should be considered whenever economically practicable.

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