Atlantic Plant PAPERrev10!29!07

8/8/2019 Atlantic Plant PAPERrev10!29!07

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Paper No. 8.02a 1

MECHANICALLY REINFORCED EARTH FOR STEEP SURCHARGE SLOPES IN

PROXIMITY OF ADJACENT STRUCTURES TO IMPROVE COMPRESSIBLE SOILS

Hiren J. Shah, P.E. Hugh S. Lacy, P.E. Matthew B. Van Rensler, P.E.Mueser Rutledge Consulting Engineers Mueser Rutledge Consulting Engineers Duffield Associates, Inc.225 W. 34th Street 225 W. 34th Street 5400 Limestone RoadNew York, NY-USA 10122 New York, NY-USA 10122 Wilmington, DE-USA 19808e-mail: [email protected] e-mail: [email protected] e-mail: [email protected]

ABSTRACT

For a large scale expansion of an existing wastewater treatment plant in Virginia, constructing shallow foundations after surcharging

to pre-consolidate the compressible soils at the site was found to be the most attractive foundation option. A detailed geotechnicalinvestigation was performed to characterize the site stratigraphy and soil properties. Many of the tall and steep surchargeembankments were in close proximity of existing operating structures and utilities. Such tall and steep surcharge slopes were designedand constructed of mechanically stabilized earth (MSE). Due to the complex slope geometries, subsurface conditions and nearbycritical utilities and existing structures, each MSE slope design required multiple analyses to determine the required geo-syntheticstrength and embedment lengths, which were larger than what would be required for typical projects. A detailed investigation was alsoperformed to investigate sources adjacent to the plant site, for procurement of the large volume (over one million cubic yards) ofborrow material required for the surcharge embankments. Monitoring was performed during construction to ensure that thesurcharging program performed as designed and did not adversely impact the existing plant facilities.

INTRODUCTION

The Hampton Roads Sanitation District (HRSD) operates 13wastewater treatment plants in the south-eastern Virginia(Chesapeake Bay) area. The Atlantic Wastewater TreatmentPlant expansion project is the single largest capitalimprovement project undertaken by HRSD with a project costof $ 150M. The expansion will increase the capacity of theplant from 36 million gallons per day (mgd) to 54-mgd, withprovisions for a future expansion to 72 mgd. A number oflarge new structures are to be constructed as part of theexpansion, many of which are south of the existing plant. Thenew structures include:

Primary and Secondary Clarifiers Aeration Tanks

Preliminary Treatment Facility Acid Digesters and Control Building Chlorine Contact Tanks Dewatering Building Blower/Electrical Facility Odor Control Facility Plant Drain Pump Station Digested Solids Storage Tank Pump Station Hypochlorite Storage Building

An aerial photograph of the existing plant site along with theconstruction areas of the plant expansion is shown in Figure 1

The aerial photo is taken looking south and shows the existingplant to the north. The plant site is bounded by farm plots tothe south, north and west with wooded wetlands to the eastLake Tecumseh is located to the south east of the plant site.

Fig.1. Aerial photograph of overall site looking south


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SUBSURFACE CONDITIONS

A plan of the plant site is shown in Figure 2 on the followingpage. The existing plant, constructed in the 1970s, is shown inlight lines. The proposed plant structures are shown in heavylines. The geotechnical borings, cone penetrometer andpiezometer locations are also shown on the plan.

The site lies in the coastal plain province of southeasternVirginia. The geology of the area is characterized by coastaldeposits of sand and interbedded clays, silts and sands withthin lenses of marshy organic deposits. Site grades generallyrange between about El. +9 (NGVD Datum) in the north toabout El. +5 in the south. In the south-eastern portion of theexisting plant site, where a soil stockpile was located, existinggrades rose up to about El. +30.

The borings provided soil consistency and samples (splitspoon and undisturbed tubes) for visual classification andlaboratory testing. The cone penetrometer probessupplemented the boring information by providing acontinuous profile of stratigraphy and soil strength which wasespecially useful at this site due to the frequently inter-layered

sands, silts and clays in the upper 50 feet. A typical geologicprofile (at location A-A) is shown in Figure 3. This profile istypical of the plant site with variations in thicknesses of thestrata in the upper 50 feet, noted between different locations.Laboratory consolidation and triaxial shear strength tests wereperformed on undisturbed tube samples of the silty and clayeystrata to determine their consolidation and strength properties.The uppermost stratum is a miscellaneous fill. Below the fillthe following natural soil strata were found:

C1: normally consolidated, soft to medium clayey silt to siltyclay with fine to medium sand layers. The undrainedshear strength ranged from 200 to 1000 psf.

S1: loose to compact fine to medium sand with trace to somesilt and gravel.

S2: loose to medium compact silty fine to medium sand withshells.

C2: normally consolidated, soft silty clay to clayey silt withfine to medium sand layers. The undrained shear strengthranged from 200 to 500 psf.

M: This layer was found generally below 50 feet depth. It is astiff marl consisting of pre-consolidated clayey silts withsome fine sand and shells. Undrained shear strengthsranged between 1500 and 2500 psf. Over-consolidationratio (OCR) varied from 2.5 to 4.5.

The static groundwater table was seasonal and generallyoccurred within a few feet below the ground surface.

FOUNDATION EVALUATION FOR THE PROPOSEDNEW STRUCTURES

Due to the compressible normally consolidated nature of thesilts and clays in the upper 50 feet depth, the proposedstructures could not be constructed on shallow foundations as

it would result in several inches of settlement due to virginconsolidation of the fine-grained soils. Deep pile foundationswere evaluated and were found feasible, although these wouldneed to be installed to depths of 80 to 100 feet below groundsurface to obtain a compressive capacity of about 25 tonseach. The original plant structures were constructed in the1970s on shallow foundations after surcharging the site withsoil embankments in order to pre-consolidate the compressiblesoil strata. This alternative was evaluated for the proposedexpansion of the plant. The surcharging program workedeasily for the original plant structures since the site was emptyand undeveloped at that time. The proposed structures aresituated near the existing plant structures which needed to bemaintained in continuous operation. Therefore, in order for thesurcharging program to be feasible for the proposed structuresit had to prevent impact to the existing plant structures.

Cost analyses of the deep and shallow foundation alternativesfor the proposed structures were performed whichdemonstrated that the surcharging option with shallowfoundations would be significantly cheaper than the deep pilefoundation alternative. A detailed geotechnical investigationprogram consisting of undisturbed sample borings, cone

penetrations tests, test pits and laboratory testing was thereforeundertaken to evaluate the strength and consolidationproperties of the site soils to design the surcharging program.

SURCHARGE PROGRAM DESIGN

The low shear strengths of the shallow soil strata called forunreinforced surcharge embankment slopes to be no steeperthan 1V:2H to 1V:4H depending on the subsurface soistratigraphy and properties at the different locations. Many ofthe proposed structure locations were within close proximityof the existing plant structures and major utilities. Also, along

the east side of the existing plant, wooded wetlands werelocated near the plant and space between the wetlandboundary and the proposed structures was limited. In order tolayout the surcharge embankment to effectively pre-consolidate the compressible soils, the embankment slopesadjacent to the existing structures, utilities and wetlandneeded to be quite steep, with some slopes as steep as 1H:4VThese steep slopes could not be achieved by unreinforced soilembankments. Mechanically stabilized earth (MSE) mass wasnecessary to construct such steep surcharge soil embankmentsThe required height of the surcharge embankments was asmuch as 39 feet high. Global stability of such steep and talsurcharge embankment slopes became critical in design due tothe presence of the low shear strength soil strata. The planlayout of the surcharge areas along with the zones requiringthe MSE slopes are shown in Figure 4.

The toes of surcharge embankments adjacent to existingstructures were laid out by estimating the settlement at theexisting structures due to the influence of the surchargeembankments. The surcharge configuration that limited suchestimated settlement at the existing structures to no more thanhalf inch was selected as an acceptable configuration. Where


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Fig. 2. Plan of the Treatment Plant site with Geotechnical Investigation Program information


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Fig. 4 Plan of surcharge areas along with zones of MSE slopes


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there were no existing structures, utilities or wetlands inimmediate proximity of the proposed surcharge embankments,unreinforced slopes varying from 1V:2H to 1V:4H were ableto be laid out.

At certain locations adjacent to existing critical utilities, thehalf inch settlement criteria at the existing utility, due to theinfluence of the surcharge embankment, could not bepractically applied since the utilities were too close to theproposed surcharges. This occurred at a couple of locationssuch as at the northwest area of the Preliminary TreatmentFacility surcharge where the stub-out for a 48-inch diameterinfluent utility line was present and at the south of theChlorine Contact Tanks surcharge where a 54-inch diametersub-out for the chlorine tanks influent line was present. Atthese stub-outs, a temporary support scheme was designed andimplemented, in which the existing utility pipe had to becontinuously monitored during surcharge placement andremoval and the support scheme enabled the pipe to be liftedor lowered if the observed movement exceeded a quarter inch.One of these temporary pipe support schemes, near thePreliminary Treatment Facility (PTF) surcharge is shown inFigure 5. At this 48-inch diameter forked stub-out, the pipe

was supported by strap plates which were attached to threadedtie-rods with adjustable turnbuckles. The tie-rods wereattached by lug plates to steel beams which were supported ontimber cribbing beyond the excavation pit. The stub-out pipeportion was monitored at several locations and its elevationmaintained as the pipe settled due to the influence of theadjacent surcharge. This was done by adjusting theturnbuckles on the tie-rods whenever the movement thresholdof a quarter inch was exceeded. A similar scheme wasimplemented at the 54-inch pipe stub-out near the ChlorineContact Tanks.

DESIGN OF MECHANICALLY REINFORCED STEEPSURCHARGE SLOPES

The detailed design of the steep mechanically stabilized earth(MSE) slopes was performed based on the performancecriteria and design parameters provided in the contractdocuments. The surcharge embankments were to remain inplace for at least 12 to 18 months which required that the MSEslopes had to be designed for long term performancerequirements.

Selection of Type of Mechanical Reinforcement

Huesker Comtrac woven polyester geotextiles were chosenfor the primary reinforcement of the steep surcharge slopesdue to economic considerations as well as their availability ina wide range of high tensile strengths. Four different strengthsof Comtrac geotextiles were utilized due to the widely varyingslope height and angles as summarized in Table 1.

The long-term design strength of the woven geotextilereinforcement was based on manufacturer supplied reduction

Table 1. Geotextile Wide Width Tensile Strengths

factors (i.e., creep, installation damage, chemical andbiological durability) with cumulative reduction factorsranging from approximately 2.03 to 2.15. The resultingprimary reinforcement long-term design strengths ranged fromapproximately 1,753 to 4,892 pounds per foot.

ACF200 woven polypropylene geotextile manufactured byACF Environmental, Inc. was utilized as the secondarygeosynthetic reinforcement and slope face wrap.

Design Considerations

The mechanically reinforced steep surcharges slopes weredesigned to meet the majority of the recommended minimumfactors of safety as suggested by the U.S. Department ofTransportation Federal Highway Administration publicationMechanically Stabilized Earth Walls and Reinforced SoiSlopes, Design and Construction Guidelines, Publication NoFHWA NHI-00-043, dated March 2001 including:

Internal StabilityPullout Resistance: F.S. 1.5Internal Stability: F.S. 1.3

External Stability

Sliding: F.S. 1.3Bearing Capacity: F.S. 2.5Deep Seated Stability: F.S. 1.3Compound Stability: F.S. 1.3

The maximum primary vertical reinforcement spacing waslimited to 3 feet. In some cases, primary verticareinforcement spacing as close as 1.5 feet was required inorder for the steep surcharge slope to achieve the desiredglobal (i.e., rotational) stability factor of safety (1.3).

The minimum reinforcement embedment lengths as measured

from the front face of the slope were widely variable rangingfrom 10 to 105 feet. Due to the complex slope geometrysubsurface soil stratigraphy/properties and the proximity toadjacent existing structures and utilities, the reinforcemenembedment length to slope height ratio was as high as 2.8. As

Geotextile Wide Width Tensile Strength(ASTM D4595)

Comtrac 55.25 3,769 pounds/foot





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Fig. 5. Temporary support of utility pipe adjacent to Preliminary Treatment Facility surcharge


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a comparison, for stable foundation soils, this ratio is typicallyless than 1.

The mechanically reinforced steep surcharge slopes wereintended to have a life span of approximately 12 to 18 months.As a result, a non-vegetated wrapped face consisting ofACF200 woven polypropylene geotextile was utilized. Toreduce the potential for reinforced soils escaping out of theslope face, adjacent panels of the face wrap geotextile wereoverlapped at least 6 inches.

Fig. 6. Typical Face Wrap Detail

On slopes steeper that 1V:2H, 18-inch by 18-inch L-shapedwire forms were required to assist with construction andprovide additional surficial stability. A typical face wrapdetail is shown in Figure 6. The prefabricated forms consistedof 10-foot long welded wire fabric (4 x 4 W4 x W4).

Fig. 7. Wrapped Face of Acid Digester Steep Surcharge Slope

Under Construction

Adjacent wire forms were overlapped 2 inches on each endresulting in an effective wire form length of 9 foot 8 inchesThe face wrap geotextile was embedded at least 2 feet and 3feet beyond the front face of the upper and lower wire formrespectively. A photograph showing the construction progressof the face of a MSE slope is shown in Figure 7.

Analysis Methods and Procedures

The analysis of the mechanically reinforced steep surchargeslopes required consideration of the proposed surchargegeometry, subsurface conditions and other factors includinglimiting movements at nearby utilities and operational existingstructures. The nine (9) mechanically reinforced steepsurcharge slopes varied in overall exposed height fromapproximately 9 to 39 feet with the overall angle of the slopesvarying from 4V:1H to 1V:2H. Further, within a givensurcharge slope the angle varied by as much as 47 degrees.

The contract drawings provided a total of thirteen specificgeological cross sections for the various MSE slope locationsalong with the soil parameters, to aid in the design of the MSE

slopes. Due to the intended use of the reinforced steep slopesas a surcharge, a 100 psf surcharge was applied at the top ofeach slope.

At the Acid Digester location an 8-inch diameter high pressuregas line was located as close as 10-feet from the toe of thesteep reinforced surcharge slope. As a result, additionaprecautions were taken during the design of the referencedslope including the doubling of the required reinforcemenembedment lengths in order to limit movements at the gasline.

Based on the complex slope geometry, subsurface conditions

and nearby critical utilities and existing structures, each MSEslope design required multiple analyses to determine therequired geosynthetic strength and embedment lengths.

The analyses were performed utilizing the micro-computingsoftware Geosynthetic Reinforced Steep Slopes: ReSlopeVersion 4.0 and Reinforced Slope Stability Analysis: ReSSAVersion 2.0, both developed by Dov Leshchinsky, Ph.D.

ADAMA Engineering.

Three types of failure modes were analyzed includingrotational, translational, and three-part wedge limiequilibrium methods. The rotational analysis was performed

utilizing the Comprehensive Bishop method. Sliding(translational) along the toe of the slope and eachreinforcement layer and the three-part wedge was analyzedutilizing Spencers Method.

Results of Analyses

For each selected cross-section, approximately 20 entry andexit points for the circular failure planes were utilized in the


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rotational analysis. The typical results of the rotationalanalysis were displayed on a safety map. Such a safety mapgenerated for the Preliminary Treatment Facility MSE slope isshown in Figure 8. Based on numerous computer generatedruns, the type of rotational failure (i.e., internal, external orcompound) with the lowest factor of safety was assessed.Depending on the slope geometry and subsurface conditions,the majority of the critical rotational failure planes wereexternal (i.e., outside of the reinforced zone) or compound(i.e., failure circle intersecting a portion of the reinforcedzone). As a result, in some locations such as the PreliminaryTreatment Facility, reinforcement lengths on the order of 105feet were required in order to achieve a minimum global factorof safety of 1.3 making the rotational stability the criticalmethod of failure.

Fig.8. Preliminary Treatment Plant Rotational Global

Stability Factor of Safety Map

Due to the long reinforcement lengths required for therotational stability, achieving the minimum two-part sliding

factor of safety of 1.3 was achieved without difficulty.

SURCHARGE BORROW MATERIAL EVALUATION

A significant volume (over a million cubic yards) of borrowmaterial was required for the surcharging program in order toconstruct the various large size surcharge embankments.Obtaining acceptable borrow material from adjacent farm(Progress Farm) areas owned by HRSD was very attractivesince it would result in significant cost-savings, demonstratesustainability in use of resources and would also benefit theadjacent community due to reduced traffic, dust and noise.

Therefore a detailed geotechnical investigation was performedin the farm areas surrounding the site to determine sources ofacceptable borrow material for the surcharging program,where it could be returned upon completion of the surchargingprogram. A plan of the geotechnical investigation program isshown in Figure 9. The objective of the investigation was tofind sources within the Progress Farm areas where relativelyclean and thick sand deposits could be obtained at shallowdepths of excavation. The geotechnical investigation firstconsisted of cone penetrometer (CPT) probes and test pits to

locate such deposits. Additional deeper soil borings were thentaken at locations where thick and relatively shallow depositsof such clean sands were detected in the CPT probesGradations tests were performed on the samples collectedfrom the borings. Thereafter volumetric studies wereperformed which indicated that the volume of structural filand surcharge soils required for the entire project could beobtained from the Progress Farm areas.

MONITORING DURING CONSTRUCTION

During construction different tools were used for monitoringvertical and lateral movements and pore pressures. Formonitoring the settlements below the surcharge embankmentssettlement plates monitored by optical survey were theprimary instrumentation used and vibrating wire settlementcells served as secondary or check instrumentation. Severasettlement plates were installed around the base of eachsurcharge and were accompanied at spot locations bysettlement cells. A set of standpipe piezometers were alsoinstalled at each surcharge embankment to observe thegroundwater pressures as the surcharge fill was placed. One of

them was screened in the shallower clay stratum and the otherone in the deeper clay stratum. In addition, at a couple of theinitial MSE surcharge embankments that were placedvibrating wire piezometers were installed at the base of thesurcharge to monitor any pore pressure build-up below thesurcharge embankment since a drainage layer was notdesigned to be installed at the back of the reinforced zone ofthe MSE slopes. Two such vibrating wire piezometers wereinstalled at each cross-section, one placed at 30 feet behind theface of the surcharge and another one at 60 feet behind theface.

Settlement monitoring points were established at all existing

structures adjacent to the surcharge embankments, which weresurveyed regularly as the surcharge placement progressed andfor a period of time after the surcharge placement wascompleted. Settlement monitoring points were also establishedon the existing utilities after excavating to the top of theutilities. As discussed earlier in the paper, temporary supportswere installed at two utility locations which were monitored asthe adjacent surcharge embankment was constructed.

Inclinometers were installed typically at a distance of about 5feet from the toe of the MSE slopes at a spacing of 100 feecenter to center along the perimeter of the slopes, to monitorlateral movement distribution with depth below grade. Eachinclinometer was keyed into the stiff clayey silt stratum at 60feet depth which would experience negligible lateramovements due to the surcharge placement. The inclinometermonitoring frequency varied in accordance with the rate ofplacement of surcharge, with a reading collected for everycouple of feet of surcharge placement. The rate of lateramovement with time in the compressible strata was observedfrom the inclinometers.


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The surcharge placements started in the spring of 2006 andwere completed in the fall of the same year. Surchargeplacements were performed simultaneously in multiple areas.The surcharge placement began in the northernmost areas andprogressed south. It took about 3 months for placement of thelarger size surcharge embankments.

The primary consolidation settlement occurred immediatelyupon beginning of surcharge placement and continued withbuild-up of the surcharge. Completion of the primarysettlement occurred rapidly and was typically accomplishedwithin 2 weeks after the surcharge placement ended. Thereason for such rapid completion of the primary consolidationwas the frequent interlayering of natural sands within thecompressible clay strata. This interlayering of natural sandssignificantly reduced the drainage paths for the pore water inthe clays to dissipate as its consolidation occurred, evenwithout the use of vertical sand or wick drains. Based on theinformation obtained about the original surcharge programimplemented for construction of the existing plant in the1970s, such rapid consolidation settlement was also noted atthat time. During that original surcharging program, thesurcharge embankments were reportedly maintained for about

2 to 3 months before being removed. For the presentsurcharging program, the surcharge embankments weretypically maintained for about 6 to 8 months after completionof the primary consolidation settlement during which time asignificant portion of the secondary compression wasachieved.

The settlements underneath the surcharge embankments variedbased on the height and extent of the surcharge and thesubsurface stratigraphy. The maximum settlement under thetaller and larger surcharges of the Preliminary TreatmentFacility, Primary Clarifiers and the Aeration Tanks wasobserved to be about 16 inches. A graph of settlement vs. time

for the various settlement plates in the Aeration Tankssurcharge is shown in Figure 10. This graph is alsorepresentative of the other such surcharge areas.

Fig. 10. Graph of settlement vs. time - Aeration Tanks

Surcharge

The lateral movements were measured by inclinometerswhich were typically located about 5 feet from the toe of theMSE surcharge slopes. Lateral movement distribution withdepth was obtained from the inclinometers. The largest lateramovements were typically observed in the shallow claystratum which occurred at a depth of about 15 to 20 feet. Themost lateral movements were observed near the taller andsteeper surcharges. The maximum lateral movement from anyinclinometer was observed to be 2 inches. The lateralmovements in the inclinometers typically increased as thesurcharge placement progressed. After the surchargeplacement was completed, the rate of lateral movement withtime decreased at each inclinometer and became essentiallynegligible within 2 to 3 months of surcharge placement. Thisindicated that the steep surcharge slopes were globally stableand the lateral movements in the compressible soils werecontained. A graph from an inclinometer adjacent to the toe ofthe tallest surcharge (Preliminary Treatment Facility) is shownin Figure 11.

Fig. 11. Lateral movement vs. depth plot for an inclinometer

at the Preliminary Treatment Facility

The maximum lateral movement in this inclinometer was 2inches in the A axis (perpendicular to the surcharge) beforestabilizing. After the surcharge placement completed at thePreliminary Treatment Facility (approximately 11/20/06), theinclinometer stabilized within 3 months. The shallow C1 claystratum experienced the maximum lateral movement as wasanticipated during design. The lateral movement generallydecreased with depth; lateral movements were higher in theClay strata compared to the Sand strata.


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The piezometer observations (both the standpipe and thevibrating wire) indicated that the pore pressure build-up waslimited to a maximum of only a couple of feet. The rapid porepressure dissipation in comparison with the rate of surchargeplacement can be attributed to the frequent interlayering ofsands within the clay strata.

The performance of the existing structures adjacent to thesurcharge embankments were in accordance with the design.Settlements at the existing structures were observed to be lessthan inch. At the two utility locations that were temporarilysupported, the settlement monitoring observations requiredthat tightening of the turnbuckles(upon reaching the threshold of inch) on the support tie-rodsneeded to be performed about 2 to 3 times during thesurcharging program which was within the design estimatedvalues of less than 2 inches of total estimated settlement.

CONCLUSIONS

1. Employing surcharging for pre-consolidation ofcompressible soils at a developed site requires

consideration of additional factors as compared to using itfor an undeveloped site. These include impact to existingfacilities, performance of detailed geotechnicalinvestigations, additional design considerations,constructability issues and additional monitoringconsiderations.

2. A detailed on-site geotechnical investigation programhelped identify the soil stratigraphy and parameters indetail and enabled development of specific subsurfacedesign profiles for design of the various MSE slopes.

3. A detailed off-site geotechnical investigation program inthe adjacent farm sites revealed sources of large volume(over one million cubic yards) of borrow sand material

that could be extracted economically for project use. Inaddition to cost-savings, this also benefited thecommunity by resulting in reduced traffic, duct and noise.Returning the borrow soils to the farms after use alsodemonstrated sustainability of resources.

4. Close proximity of the existing structures and the futurefacilities dictated the need for providing tall (up to 39 feettall) and steep (up to 1H:4V) surcharge embankments.

5. Weak subsurface soil conditions and the steepness ofsurcharge slopes required MSE slopes at many of thesurcharge embankments.

6. Design of the MSE slopes required multiple analyses dueto complex slope geometries, steep faces, weaksubsurface soil properties and the need to minimizeimpact to adjacent existing facilities. Ratio ofreinforcement embedment length to MSE slope heightwas higher (up to 2.8) than what would be required forstable foundation soils (generally less than 1).

7. The inter-layered nature of the subsurface soil depositsplayed an important role in the successful application ofthe tall and steep MSE slopes and the surchargingprogram. The inter-layering of sand within the soft claydeposits helped provide additional soil strength to enable

stability of the surcharge slopes and also reduced the timerequired for surcharging by reducing the drainage paths.

8. The high strength-low strain properties of the HueskerComtrac woven polyester geo-textiles used for the MSEslopes successfully demonstrated the applicability of geo-textiles as an economic alternative to geo-grids for suchapplications.

9. Monitoring performed during the surcharging programdemonstrated the following:

a. Movement at the existing structures (less than inch) and utilities (less than 2 inches) inproximity to the surcharge areas were within theestimated values. Settlements of the surchargeembankments (up to 16 inches) were also withinthe estimated values.

b. Lateral movements (2 inches or less) at the toesof the MSE slopes indicated that global stabilitywas achieved during the surcharging program.

c. Pore pressures at the base of the surchargeembankments and in the subsurface clay strataremained low and not of concern, due to thetolerable rate of surcharge placement and due tothe inter-layered character of the subsurface

clays, silts and sands.

ACKNOWLEDGEMENTS

HDR Engineering Inc. and Hampton RoadsSanitation District for providing the opportunity towork on this challenging project.

Mueser Rutledge Consulting Engineers and the officepersonnel who worked on this project includingColleen Liddy, Ravi Gadhi and others.

Duffield Associates Inc. and office staff. ACF Environmental Inc.

Documents

Atlantic Plant PAPERrev10!29!07