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8/10/2019 AA Cu Ciment -Baraj http://slidepdf.com/reader/full/aa-cu-ciment-baraj 1/19 CEMENT DEEP SOIL MIXING REMEDIATION OF SUNSET NORTH BASIN DAM Rebecca F. Barron 1 , Christopher Kramer 2 , W. Andrew Herlache 2 , Jon Wright 1 , Howard Fung 3 , and Chu Liu 3   Abstract Cement Deep Soil Mixing (CDSM) is a state-of-the-art soil improvement method developed in Japan and gaining acceptance in the United States. The design team selected CDSM for the rehabilitation of the supporting embankment to San Francisco’s largest potable water reservoir, Sunset Reservoir. This project marks the first time the California Department of Water Resources, Division of Safety of Dams has approved CDSM to remediate a potentially weak foundation and thereby improve the seismic stability of an earth embankment dam. The design for the seismic remediation was completed in November 2004. Construction began in June 2005 and was completed June 2006. Sunset Reservoir is located in a densely populated residential neighborhood of San Francisco, with homes and businesses located immediately downstream of the dam. The reservoir has a storage capacity of approximately 177 million gallons or 543 acre-feet and supplies roughly 60 percent of the City’s water. The San Francisco Public Utilities Commission (SFPUC) owns and operates the reservoir. The seismic stability of the dam, a critical City facility, was the focus of the project. The SFPUC’s criterion is for the dam and reservoir to remain functional after a major earthquake on the San Andreas Fault, which is located 5 kilometers from the site. The 74-foot high earth embankment was built in 1938 on top of saturated native sands and silts. Geotechnical investigations revealed that these soils could be susceptible to significant strength loss during a major earthquake. This paper describes the design and construction process, including the initial geotechnical investigation, the selection of CDSM as the preferred remediation technique from various alternatives, the design of the remediation scheme, and construction issues and lessons learned. The paper also addresses the collaboration between the different agencies, consultants, and specialty contractors that led to the successful completion of the project. 1  California Department of Water Resources, Division of Safety of Dams 2  Fugro West, Inc. (formerly with Olivia Chen Consultants, Inc.). 3  San Francisco Public Utilities Commission Introduction This paper describes the evaluation, selection, design, and construction of Cement Deep Soil Mixing (CDSM) to retrofit the northwest embankment dam of Sunset North Basin in San Francisco, California. Originally developed in Japan, CDSM is a technique for in-situ ground improvement that is gaining acceptance in the United States. CDSM is a process to improve soil by injecting grout through augers that mix it with the soil, forming in-place soil- cement columns. Figure 1 presents two photographs of the CDSM equipment in operation on the embankment.

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CEMENT DEEP SOIL MIXING REMEDIATIONOF SUNSET NORTH BASIN DAM

Rebecca F. Barron1, Christopher Kramer 2, W. Andrew Herlache2,Jon Wright1, Howard Fung3, and Chu Liu3

 

 Abstract

Cement Deep Soil Mixing (CDSM) is a state-of-the-art soil improvement methoddeveloped in Japan and gaining acceptance in the United States. The design team selectedCDSM for the rehabilitation of the supporting embankment to San Francisco’s largest potablewater reservoir, Sunset Reservoir. This project marks the first time the California Departmentof Water Resources, Division of Safety of Dams has approved CDSM to remediate apotentially weak foundation and thereby improve the seismic stability of an earth embankmentdam. The design for the seismic remediation was completed in November 2004. Constructionbegan in June 2005 and was completed June 2006.

Sunset Reservoir is located in a densely populated residential neighborhood of SanFrancisco, with homes and businesses located immediately downstream of the dam. Thereservoir has a storage capacity of approximately 177 million gallons or 543 acre-feet andsupplies roughly 60 percent of the City’s water. The San Francisco Public Utilities Commission(SFPUC) owns and operates the reservoir.

The seismic stability of the dam, a critical City facility, was the focus of the project. TheSFPUC’s criterion is for the dam and reservoir to remain functional after a major earthquake onthe San Andreas Fault, which is located 5 kilometers from the site. The 74-foot high earthembankment was built in 1938 on top of saturated native sands and silts. Geotechnicalinvestigations revealed that these soils could be susceptible to significant strength loss duringa major earthquake.

This paper describes the design and construction process, including the initialgeotechnical investigation, the selection of CDSM as the preferred remediation technique fromvarious alternatives, the design of the remediation scheme, and construction issues andlessons learned. The paper also addresses the collaboration between the different agencies,consultants, and specialty contractors that led to the successful completion of the project.1 California Department of Water Resources, Division of Safety of Dams

2 Fugro West, Inc. (formerly with Olivia Chen Consultants, Inc.).3 San Francisco Public Utilities Commission

Introduction

This paper describes the evaluation, selection, design, and construction of CementDeep Soil Mixing (CDSM) to retrofit the northwest embankment dam of Sunset North Basin inSan Francisco, California. Originally developed in Japan, CDSM is a technique for in-situground improvement that is gaining acceptance in the United States. CDSM is a process toimprove soil by injecting grout through augers that mix it with the soil, forming in-place soil-cement columns. Figure 1 presents two photographs of the CDSM equipment in operation onthe embankment.

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Figure 1. CDSM Rig at Sunset Reservoir

Sunset Reservoir is a lined and covered offstream reservoir located in the southwesternregion

 the jurisdiction of theCalifor 

  that a maximum earthquakeon the

Roles of Various Parties

wnerhe San Francisco Public Utilities Commission is a department of the City and County

of San

  engineeringconsul

 

of San Francisco. It is the largest potable water reservoir in San Francisco, supplyingroughly 60 percent of the City’s water supply; it is owned and operated by the San FranciscoPublic Utilities Commission (SFPUC). The reservoir comprises the North and South Basins,which have a total storage capacity of approximately 177 million gallons or 543 acre-feet. Theembankment, located at the northwest corner of the north basin, is 74 feet high. The northbasin was built in 1938 using a combination of cut and fill. The south basin was built in 1960by excavating into relatively stable bedrock of the Franciscan Complex.

Sunset Reservoir’s storage capacity and dam height place it under nia Department of Water Resources, Division of Safety of Dams (DSOD). In 1998, as

part of the reservoir seismic retrofit, SFPUC initiated a field investigation and engineering studyto evaluate the seismic performance of the dam and reservoir.

In 2000, the SFPUC’s Consultants and DSOD concluded  San Andreas or Hayward Fault could result in strength loss of the foundation soils

below the northwest embankment of the North Basin.

OT  Francisco that provides water, wastewater, and municipal power services to San

Francisco. Under contractual agreement with 28 wholesale water agencies, the SFPUC also

supplies water to 1.6 million additional customers within three Bay Area counties. As owner and operator, the SFPUC retained the geotechnical and civiltants, managed the design phase, and provided contractual oversight during

construction. It was also responsible for monitoring the Quality Assurance and Control for theproject and working with the specialty contractor to minimize the disruption to the neighboringresidents and business, while maintaining the safety and the availability of water to itscustomers.

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Geotechnical and Civil Engineering Consultantswas retained by the SFPUC to provide

geotec

pecialty Contractorcontract was awarded to Gordon N. Ball/Yerba Buena Joint Venture

(GB/YB

egulatory Agencyf Safety of Dams is a division of the California Department of Water

Resou

and the Consultants, providing input for thegeotec

ogy and geotechnical engineering, as well asbreakt

Original Reservoir Construction

Sunset North Basin was built in 1938 utilizing cut and fill techniques, resulting in astorag

h was loose in someareas. According to as-built drawings and construction photos, the topsoil was stripped within

Olivia Chen Consultants, Inc. (Consultants)hnical and civil engineering services for the project. The Consultants performed the

geotechnical investigation, conducted engineering analysis, and designed the remediationscheme. The Consultants and the SFPUC jointly developed the project construction plans andspecifications. The Consultants also provided geotechnical observation and consultationduring construction.

SThe construction), the general contractor. The CDSM portion of the project was subcontracted to Raito,

Inc., a construction company specializing in subsurface in-situ soil improvements using CDSMand Dry Jet Mixing methods. Based in Japan, Raito, Inc. has been one of the leadingdevelopers of CDSM technology. GB/YB conducted all excavation safety monitoring,conducted all earthwork activities, and managed all interaction with the owner, consultants,and regulatory agencies. Raito, Inc. was responsible for conducting the CDSM groundimprovement and all verification testing.

RThe Division orces responsible for the supervision of dams larger than a certain size. It is DSOD’s

responsibility to assure that these dams and reservoirs are designed, constructed, operated,and maintained safe from failure due to natural static forces, as well as hazards such asfloods, earthquakes, landslides, destructive seepage, and serious operational malfunctions.To accomplish this, DSOD maintains a staff of engineers, geologists, and seismic specialistswho perform independent engineering analysis and monitor the construction of jurisdictionaldams. DSOD staff also performs periodic safety evaluations of all jurisdictional dams tomonitor their maintenance and performance. Currently, there are approximately 1250 damsunder the State of California’s jurisdiction.

DSOD worked with the SFPUChnical investigation and reviewing the field and laboratory results. DSOD performed an

independent engineering analysis of the existing embankment and foundation, and thedesigned remediation. DSOD reviewed and approved the remediation design and monitoredthe construction work with respect to dam safety.

Continuing changes in the fields of seismolhroughs in modeling technology, have resulted in a greater understanding of how dams

behave under both static and seismic conditions. DSOD is continually working to maintain aprogram that utilizes the latest state-of-the-art technology.

e capacity of 275 acre-feet and a 74-foot high earthfill embankment along its north andwest sides. The embankment has a crest length of 2,300 feet and a crest width of 11 feet.The upstream (interior) slope is 3:1 (horizontal to vertical); the downstream slope is 2.5:1, witha 10-foot wide bench at mid-height. The dam has 2.0 feet of freeboard.

The embankment was founded on a thick layer of dune sand, whic

 

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emban

Geotechnical Investigation

The geotechnical inves am included a review of thereviously performed work, field investigation, and laboratory testing. The Consultants

perform

Because of the Baldwin Hill Failure in 1963, Division 3 of the Water Code was amendedexisting offstream dams. A field investigation and embankment

stabilit

ever, portions of the foundation were identified as possibly beingsuscep

In 1998, as part of the SFPUC’s reservoir seismic retrofit, DSOD requested an updatednkment. A phased field investigation was begun in 1998 with

seven

kment’s footprint. Where the natural ground was below the bottom of the proposedreservoir, sandy material excavated from within the basin’s limits was used as backfill. Theembankment materials came from the north and south basin excavations and from theconstruction of Balboa Reservoir. The embankment materials consist of clayey gravel withsand (GC) and clayey sand with gravel (SC) with a rock fill shell on the downstream slope.Based on the specifications and construction photos, the finer grained material was placed bydump trucks, and then spread into 6-inch horizontal lifts before being compacted by dozerswith tandem sheepsfoot rollers. The rock fill was dumped in 3-foot layers and then compactedwith power rollers. Any voids between the separated rock fill were filled in with materialsexcavated from the reservoir.

tigation of Sunset North Basin Dp

ed the field investigation with input from the SFPUC and DSOD. Under the direction ofthe Consultants, an independent geotechnical laboratory performed geotechnical testing.

Previous Investigation

to include both new andy analysis were required to bring Sunset North Basin Dam into State jurisdiction. An

initial investigation and analysis were performed in 1969 with six rotary wash borings drilled todepths up to 90 feet. Standard Penetration Testing (SPT) was performed. Uncorrected blowcounts ranged from 11 to refusal, with 5 percent below 20. Soil samples were taken usingSPT and pitcher barrel samplers. Laboratory testing was performed on remolded and pitcherbarrel samples to determine material strength properties and to characterize the embankmentand foundation materials.

The analysis found the embankment to have adequate safety factors for both static andpseudo-static models. How

tible to strength loss during a large seismic event. Eight piezometers were installed infour of the borings to monitor the phreatic surface of the embankment and foundation.

Recent Field Investigations

stability analysis of the embaborings drilled up to 41 feet deep. SPT testing was performed. Uncorrected blow

counts ranged from 4 to refusal, with 41 percent below 20 for the foundation soils. Soilsamples were taken using the SPT and Modified California samplers. Laboratory tests wereperformed to obtain soil properties and material strength data. Subsequent phases were

performed in 2000, 2002, and 2002-2003 to obtain additional blow count data and materialproperties, and to better delineate the lithology of the foundation materials. A total of 20additional borings were drilled, ranging in depth from 21 and 95 feet. The materialsencountered ranged from the rock fill of the embankment, to poorly graded and silty sands tothe formational bedrock. The approximate locations of the borings from the investigations arepresented on the Boring Location Plan, Figure 2.

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Figure 2. Boring Location Plan

Geology, Subsurface Conditions, and Seismicity

egional Geologyvoir is located on the San Francisco Peninsula between the San Andreas

Fault,

ite Geologyat the Sunset North Basin embankment site consists of sheared sandstone and

shale o

RSunset Reser 5 km to the west, and Hayward Fault, 25 km to the east. The peninsula is underlain by

isolated Jura-Cretaceous outcrops of Franciscan Complex surrounded by Quaternary ageColma Formation (sand, silt, and gravel) and younger dune sand. The Franciscan Complex isa structural unit.

SBedrockf the Franciscan Complex (mélange), which is overlain by surficial deposits of silty sand

with clayey sand lenses, ranging in thickness from 5 to 15 feet. This silty sand is thought to bederived from the Pleistocene Colma Formation. Dune sand (poorly graded fine sand) overliesthe silty sand layer and ranges in thickness from 10 to 30 feet. The depth to bedrock rangesfrom 85 feet from the crest of the dam to 33 feet from the toe of the dam.

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Soil ConditionsNine lithologic units were delineated during subsurface investigation of the northwest

corner of North Basin. These units are described in the following paragraphs in order ofdecreasing elevation, and are shown in Figure 3 below.

Figure 3. Cross Section A

Silty Sand 1 (Fill): Approximately 5 feet of topsoil consisting of medium dense to dense siltysand with gravel.Gravel 1 (Rock Fill):  Five to 25 feet of gravel and rock fill below the downstream face of theembankment. The rock fill consists of medium dense to dense gravel, with varying amounts ofclay, sand, and rock fragments up to about 3 feet in diameter. The source of this material

appears to be select Franciscan Complex material excavated from the south basin area.Gravel 2 (Fill): Forty-five to 50 feet of medium dense clayey gravel with some sand within themain embankment. This unit also contains rock fragments up to 3 feet in diameter. Thesource of the material appears to be select Franciscan Complex material excavated from thesouth basin area.Poorly Graded Sand 1: A 10- to 15-foot layer of loose clean fine-grained native dune sand withlocalized medium dense areas was encountered below the downstream toe of theembankment.Poorly Graded Sand 2:   A 5- to 20-foot thick layer of dense to very dense clean fine-graineddune sand, generally located below the crest the embankment.Poorly Graded Sand 3 (FILL): Poorly graded dune sand placed and compacted during theoriginal construction to fill in low areas of the foundation.Silty Sand 2: A 3 to 7-foot thick layer of loose to medium dense saturated silty sand locatedbelow the poorly graded dune.Silty Sand 3: A 3 to 11 feet thick layer of medium dense to dense saturated silty sand, withinterbedded zones of clayey sand was encountered below Silty Sand 2 layer.Franciscan Complex: The Franciscan complex underlying the silty sand is composed ofmoderately to severely weathered, closely fractured to crushed, moderately strong to friable,and moderately hard to friable interbedded sandstone and shale.

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GroundwaterPrior to construction of the CDSM, there were 10 open standpipe piezometers

measuring ground water levels along the northwest corner of the embankment, with sets of twoto three piezometers nested together in some locations. The SFPUC performed regularmeasurements of groundwater levels in the piezometers. Historical piezometer readings haveshown that the regional groundwater levels within the foundation vary little over time.Historical readings also indicate the possibility of minor localized seepage (or perched water)within the embankment.

Seismicity and FaultingThe major active faults in the San Francisco Bay Area are the San Andreas, Hayward,

San Gregorio, Rodger’s Creek, and Calaveras. No active or potentially active faults are knownto cross the site. The distances from the site to major active faults in the area and theiranticipated maximum magnitude are presented in the following table.

Fault Approximate Distance

from Site (km)Maximum Moment

Magnitude

San Andreas 5 8.0

San Gregorio 10 7.3

Hayward 25 7.1

Rodger’s Creek 40 7.0

Point Reyes 37 6.8

Calaveras 42 6.8

These active faults are capable of generating strong ground shaking at the site during

an earthquake. A moment magnitude (Mw) 8 earthquake on the San Andreas Fault is thecontrolling seismic event. Using 84th  percentile deterministic attenuation relationships, theConsultants calculated a bedrock peak ground acceleration (PGA) of 0.97 gravities (g) at theproject site. DSOD accepted the seismic parameters after performing an independentevaluation with comparable results.

Seismic Design

Sunset Reservoir is a critical facility for the City of San Francisco. The embankmentperformance criteria were for the reservoir to be able to sustain minor damage, remainoperational, and suffer no catastrophic release of water during or after the earthquake.

The project team selected a maximum permanent seismic deformation criterion of 6inches, to be calculated using Newmark sliding block methodology. The Consultantsdeveloped site-specific horizontal response spectra, peak ground acceleration, and spectrallymatched input acceleration time histories for use with the Newmark model. Some cracking ofthe reservoir liner is anticipated at this level of deformation, but the reservoir should beoperable.

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Pre-Treatment Conditions

Loss of StrengthThe soils of concern for Sunset North Basin Dam were two silty sand layers, Silty Sand

2 and Silty Sand 3, which are located below the water table. The Consultants evaluated theselayers for potential significant strength loss using the Youd et al. (2001) method. Inputparameters included a moment-magnitude 8.0 earthquake on the San Andreas Fault and apeak ground acceleration of about 0.97g at the site. The results of the evaluation arepresented in Figure 4.

The Consultants concluded that Silty Sand 2 was susceptible to significant strengthloss, and that the majority of Silty Sand 3 was not, except in some localized areas. Thepredicted consequences of significant strength loss (prior to the retrofit) included settlementand slope instability. Using Ishihara’s 1993 method, the Consultants estimated thatearthquake-induced settlement could be up to about ½ foot. This settlement would not beuniform, and could reduce the reservoir freeboard in some locations. The settlement couldalso result in localized cracking of the liner and some distress to the roof structure.

Figure 4. Corrected N Values for Silty Sand

In addition to settlement, the temporary reduction in shear strength associated withsignificant strength loss could result in embankment slope instability during or after asignificant earthquake, if mitigation measures are not implemented.

For their independent evaluation, DSOD selected a more conservative approach. Theyselected a higher phreatic surface, extending from maximum reservoir elevation to the toe ofthe dam, to model a case of an ineffective reservoir liner. In the DSOD model, the lowerportions of Poorly Graded Sand 1 are also considered to be susceptible to significant strength

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loss. The following table provides a range of uncorrected field blow counts, averaged 30th percentile (N1)60 blow counts, and average percent fines content for the three layers.

Soil Layer NField

  Averaged (N1)60

(30th percentile) Percent FinesPoorly Graded Sand 1 4 to 55 (20 avg) 18 2

Silty Sand 2 4 to 44 (14 avg) 8 29Silty Sand 3 12 to 73 (37 avg) 24 44

Seismic StabilityThe Consultants evaluated the stability of the northwest embankment under static and

seismic loading conditions, including considering a reduction in shear strength of the SiltySand 2 during seismic loading. Idealized Subsurface Cross Section A, depicted in Figure 3,was the critical cross section because the critical layer is at its deepest and the embankment isat its tallest. They used computer software and the modified Bishop’s method of slices toevaluate a large number of circular and wedge-shaped failure surfaces.

Published data (Youd, 2003) indicate that significant strength loss of soils will require

some time to develop after the beginning of an earthquake. In addition, looser soils will losestrength sooner than denser soils. The Consultants divided Silty Sand 2 into two sublayers:looser and denser. Stage 1 represents the condition prior to and just after the beginning ofthe seismic event; at this time strength loss has not yet occurred in any layer. Stage 2represents the condition after the looser sublayers lose strength. Stage 3 represents thecondition after the denser sublayers lose strength. Typical failure surfaces for the casesanalyzed are shown in Figure 5.

Figure 5. Critical Pre-Treatment Failure Surfaces

The following table presents the strength parameters used to analyze the three stages

of seismic induced strength loss in the Silty Sand 2.

 AnalysisLooser

SublayersDenser

Sublayers

Stage 1 φ′  = 30 degrees φ′  = 30 degrees

Stage 2 Su,r  = 210 psf φ′  = 30 degreesStage 3 Su,r  = 210 psf Su,r  = 450 psf

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The parameter Su,r   represents the seismically-induced undrained residual shearstrength, which the Consultants estimated using the correlation presented in the paper bySeed and Harder (1990).

First, the Consultants evaluated stability for each stage, assuming reductions in shearstrength during Stages 2 and 3, but no seismic inertia loading. They then used accelerationtime histories in combination with the results of slope stability analyses to estimate themagnitude of permanent deformation using a sliding block model (suggested by Newmark,1965). The critical factors of safety and yield accelerations are presented in the followingtable. The most critical failure surfaces are shown on Figure 5.

Critical Failure SurfacesFactor of Safety

(no seismic inertia loading) Yield Acceleration (g)Stage 1, Downstream 2.6 0.46Stage 2, Downstream 1.9 0.26

Stage 3, Downstream 0.6(Not applicable when factor of safety is less than 1.0)

Stage 3, Upstream 4.7 0.63

For downstream failure surfaces (away from the reservoir), the consultant concludedthat severe deformations were possible as a result of the earthquake. For upstream failuresurfaces, the consultant estimated that permanent deformations would be less than about aninch. The Consultants further concluded that embankment foundation improvement would berequired to improve the seismic performance of the downstream slope of the embankment.

DSOD performed an independent analysis. Using the Consultants’ geotechnical data,they performed a post-seismic static analysis on circular and wedge-shaped failure surfaces.Residual strengths were assigned to the potentially weak layers based on the Seed andHarder method (1990). The post-seismic factor of safety was found to be less than one,confirming the risk of significant deformation after a strong seismic event.

Selection of Preferred Remediation Method

The Consultants initially considered the following remediation alternatives: CementDeep Soil Mixing (CDSM), jet grouting, compaction grouting, vibro-replacement, permeationgrouting, and excavation and recompaction. CDSM was selected after eliminating the otheroptions based on the following considerations:

•  Jet grouting, was eliminated because the Consultants concluded that theprocess could not be controlled to an acceptable degree and becausepreliminary estimates indicated it would be more costly than CDSM.

•  Compaction grouting is known to have limited effectiveness in soils with relatively

high fines content. The Consultants concluded that compaction grouting was notappropriate because the target layer, Silty Sand 2, had a median fines contentbetween 28 and 35 percent.

•  Vibro-replacement also is less effective for soils with high fines contents. Inaddition, the Consultants concluded that vibro-replacement equipment mighthave difficulties penetrating embankment soils to effectively treat the target layer.

•  Permeation grouting was eliminated because Silty Sand 2 was not permeableenough to allow uniform penetration of the grout.

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•  The SFPUC considered excavation and recompaction of the entire embankmentto be prohibitively expensive and lengthy compared to other methods and felt itcould not afford to have the reservoir taken out of service for an extended periodof time.

Considering the limitations of other methods, the SFPUC accepted the Consultants’recommendation to use CDSM for foundation remediation.

CDSM is a process whereby soil is improved by injecting grout through one or moreaugers that simultaneously mix the soil, forming in-place soil-cement columns as shown inFigure 7.

Figure 6. CDSM Construction Procedure

The CDSM would need to be performed in a regular grid of in-place columns toeffectively improve the target soils. Level benches would have to be cut into the embankmentin order to operate the CDSM rig. The grid would have to be designed to provide subsurfacedrainage paths to minimize buildup of groundwater behind the soil cement columns.

 Advantages of CDSM include:

•  High strengths can be achieved in the final soil cement product.•  CDSM has been used on similar projects to address seismic stability issues

associated with significant strength loss, such as the Jackson Lake Dam and theClemson Upper and Lower Diversion Dams. It has also been used on severalprojects within the Bay Area, such as at the Port of Oakland.

•  The zone of improvement can be controlled more effectively than jet grouting.•  Proven confirmation testing methods exist for assuring quality.

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•  The process would have a low impact on reservoir operations.

Disadvantages include:•  Temporary construction benches would have to be cut into the embankment

during installation.•  The process generates spoils that must be used onsite or hauled offsite.

Final Design

Consultants ApproachThe Consultants performed a series of stability evaluations to develop a CDSM scheme

that would meet the static and seismic performance objectives. They evaluated multiplelayouts with varying material properties prior to selecting a suitable treatment pattern.

The resulting treatment pattern is depicted in plan view in Figure 7, and in profile inFigure 8. The layout consists of multiple 47-foot square grids (blocks) of CDSM columns thatare placed in three rows (treatment zones) aligned parallel to the longitudinal axis of theembankment. The blocks consist of exterior and interior walls formed by adjacent CDSM

columns, as shown on Figure 7.

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Figure 7. Treatment Plan

The configuration required that the CDSM have an average 90-day unconfinedcompressive strength of at least 400 pounds per square inch (psi), and that 25 percent of SiltySand 2 be improved by CDSM. Available laboratory data indicated that a 90-day strength of400 psi will result if the 28-day strength is at least 300 psi. The resulting weighted-averageshear strength of the improved layer is approximately 7200 pounds per square foot (psf), orabout 75 percent higher than the strength of the underlying Silty Sand 3. The Consultantsrecommended the discrete block layout, with gaps between the individual blocks to provide aconduit for regional groundwater.

The recommended treatment zones extend at least 5 feet below the bottom of SiltySand 2, or to the top of bedrock when shallower. Site geometry allows three rows of CDSMblocks near the center of the embankment, decreasing to two rows on the east and west ends.Where only two rows were possible, the CDSM is keyed into the underlying bedrock to provideadditional resistance to sliding.

Figure 8. Post-Treatment Failure Surfaces (Section A)

In developing the design described above, the Consultants revised the slope stabilitymodel, assigning improved soil properties for zones to be treated using CDSM. They ignoredthe strength of unmixed soil within and between the grid blocks. Regarding the potential forCDSM cracking, the Consultants concluded that: the structural integrity of the blocks wouldlimit cracking associated with ground shaking; potential failure surfaces crossing through theblocks would experience insignificant deformation; and failure surfaces passing below theblocks, which bypass the CDSM, would have acceptable levels of deformation. The

Consultants repeated the staged analysis approach with the revised model. Two typical failuresurfaces are shown on Figure 8. In the downstream direction, the lowest static factor of safetyprior to earthquake loading (Stage 1) was 2.8, indicating the embankment is stable for thiscase. After Stage 3, the minimum factor of safety was still 2.8. For this case, the minimumyield acceleration is 0.41, resulting in less than 6 inches of calculated deformation after thecontrolling seismic event. Based on the analysis, the Consultants concluded that the CDSMtreatment scheme meets the seismic performance objectives.

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 DSOD Approach

DSOD’s analysis conservatively adopted a saturated foundation and a higher phreaticline within the embankment, taking into consideration the potential for the water table to risedue to the proposed remediation. The critical circular failure surface was determined by usingthe Modified Bishop method, while the critical block-slide failure surface was determined by theSpencer method. Post-seismic factors of safety for circular and wedge-shaped failure surfaceswere found to be greater than 2, with a corresponding minimum yield acceleration of 0.35.

The results of the stability analysis indicated acceptable factors of safety for the post-seismic condition and relatively high yield accelerations.

DSOD estimated the seismic deformation of the embankment with the proposedfoundation improvement using the Makdisi and Seed simplified procedure (1978). Based onthe calculated yield accelerations and the estimated maximum horizontal acceleration alongthe critical failure surface, DSOD estimated the seismic deformation to be less than 1 foot.This is within the current freeboard of 2.0 feet, satisfying the embankment performancecriterion of not allowing a catastrophic release of water during or after the earthquake.Therefore, DSOD agreed with the proposed design provided the CDSM was extended upwardinto the Poorly Graded Sand 2 layer.

Construct ion and Monitoring

Construction OverviewConstruction of the embankment stabilization began in May 2005 and was substantially

completed by December 2006. The work consisted of site demolition and grubbing,embankment excavation, CDSM construction, embankment reconstruction, restoration of siteimprovements, and re-landscaping. Embankment excavation, CDSM construction, andembankment reconstruction were completed for each treatment zone prior to beginning thenext. The construction proceeded from Treatment Zone 3 to Treatment Zone 1, from thebottom of the embankment to the top. The CDSM equipment required a 60-foot wide, levelsurface to operate, so the contractor had to excavate a flat bench for each of the treatmentzones. Safe excavation and recompaction of the soil was a major logistical challenge. Figure9 depicts the temporary construction of Bench 2. Material from the excavation was used tobackfill Bench 3.

The following constraints impacted the construction activities: space limitations for on-site temporary stockpiling of excavated embankment soil and construction staging, therequirement that the reservoir be kept in service during construction, and construction within adensely populated residential neighborhood. These issues were managed through closecoordination between GB/YB, Raito, Inc., SFPUC construction and engineering staff, DSODengineers, and the Consultants.

During construction, detailed monitoring revealed some issues that necessitated

modification of construction procedures and monitoring activities. Of particular interest are theissues associated with the embankment excavation and CDSM construction.

Temporary Embankment ExcavationTo address the stability issues associated with the temporary embankment excavation,

a detailed monitoring program and emergency response plan were developed. The monitoringprogram included daily monitoring of 10 survey hubs set along the crest of the embankment,weekly monitoring of three inclinometers installed adjacent to selected survey hubs, weekly

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monitoring of existing piezometers, and daily reconnaissance of the excavation to look for anyindications of possible instability. Planning for emergency response included defining the rolesand responsibilities of each party, disseminating emergency contact information for keyindividuals, discussing possible scenarios for heightened concern, establishing monitoringthresholds, developing communication protocols for addressing concerns, and preparing forpossible remedial measures that could be implemented if stability concerns arose.

During construction, rapid dissemination and evaluation of the monitoring data wasachieved using email and flexible communication protocols established by the SFPUC.Deformation monitoring generally revealed small movements (less than ¼ inch) and thetemporary cut slopes remained stable. On occasion, the survey data suggested moresignificant movements, but these were not consistent with reconnaissance observations andinclinometer data and were later determined to be errors in surveying and/or data entry.Localized seepage zones were observed on all three temporary benches; however,piezometer monitoring revealed no significant increase in the phreatic surface within theembankment. Drains were installed along each of the benches during reconstruction to ensurethe embankment remains dry.

Figure 9. Temporary Construct ion Bench 2

Cement Deep Soil MixingThe challenges during CDSM construction included limitations on space for construction

staging and rig operations, coordination during excavation and embankment reconstruction,assessing the effectiveness of the soil improvement, and evaluating the depth of penetrationinto weathered bedrock. The first two challenges required close coordination between GB/YBand Raito, Inc., and were met without significant cost or schedule impacts to the project. Thelatter two challenges were of particular concern to the SFPUC, the Consultants, and DSOD.

To assess the effectiveness the CDSM improvement, a detailed monitoring and testingprogram was established. The program included test sections utilizing several different mixdesigns with varied cement contents and water-cement ratios, monitoring of variousparameters during CDSM construction, full depth coring of the completed CDSM columns,unconfined compressive testing on selected CDSM core samples, test borings prior to CDSMconstruction or adjacent to completed CDSM columns, and test redrilling of cured CDSM.

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The monitoring program generally indicated that the CDSM at the site satisfied therequirements in the project plans and specifications. Specifically, the following points werenoted:

•  The completed CDSM satisfied the geometric requirements for plan location,configurations, depth, inclination, and overlap of adjacent elements.

•  Coring and testing indicated the minimum 28-day unconfined compressivestrength of the CDSM within the treatment zones was 179 psi (minimum 120 psirequired). The average strength for any complete core, in the treatment zone,was found to vary between 322 and 1177 psi (minimum 300 psi required).

•  Core recovery and uniformity of the CDSM was found to be satisfactory. Therecovery averaged 96 to 100 percent within the treatment zone (minimum 85percent required) and the percent of unmixed soil ranged from 0 to 15 percent(maximum 15 percent allowed).

While the CDSM achieved the goals established for the ground improvement,evaluating the penetration of the CDSM into the bedrock was of particular concern andrequired additional testing. The specifications required that some of the CDSM elementspenetrate at least 5 feet into bedrock; other elements were only required to reach the top of thebedrock. During the design phase, the project team anticipated that various CDSM drillingparameters, correlated to boring logs, could be used to assess the depth of penetration intobedrock. However, during construction, the project team could not establish a suitablecorrelation. In addition, comparison of boring logs and practical drilling refusal suggested thatthe CDSM rig was not able to consistently penetrate into bedrock. Therefore, additional testingand analyses were conducted.

The depth to bedrock below the site was known to vary significantly over a shortdistance. The original boring locations were not surveyed; therefore, the exact depth tobedrock at a given CDSM element location was uncertain. Additional borings were drilledadjacent to CDSM elements to determine the depth to bedrock and to allow correlation with theCDSM drilling parameters. The drilling parameters included the actual vertical penetration rate(speed), drive motor amperage, energy index (amperage divided by penetration rate), themeasured load on the cables that supported the augers and motors, and the visual observationof slack in the cables once the load was being supported by the subsurface materials. Thefollowing table summarizes the data where the new borings were drilled.

Depth of Penetration into Bedrock (feet)*

BoringNo.

CDSMElement

No.

Large Drop inPenetration

Rate

Spike inEnergyIndex

ZeroCableLoad

CableSlack

 Actual Depth ofElement into

Bedrock (feet)

CB-1 Z3-B6-T14 1.6 1.6 0.6 1.5 3.3CB-2 Z3-B3-T11 4.2 4.2 4.2 3.9 4.9CB-3 Z3-B2-T13 -0.5 -0.5 -2.5 -2.6 -0.5CB-4 Z3-B1-T13 3.5 3.5 3.5 3.5 3.5

* The depth below the bedrock surface, based on a significant change in the specific drilling parameter.

From the data, it was apparent that a suitable correlation between the depth to bedrockand drilling parameters could not be reliably established. In addition, the actual depth ofpenetration into bedrock at the boring locations varied significantly. The authors believe that

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these observations were likely the result of the variation in the depth of the bedrock along thewidth of one CDSM element (13 feet) and variations in the hardness and degree of weatheringof the bedrock.

Based on these findings, a test was performed to assess how strong the bedrock had tobe before encountering practical drilling refusal. The test involved re-drilling a cured CDSMelement while monitoring the drilling parameters. The test revealed that the CDSM rig wascapable of drilling into material with an unconfined compressive strength of at least 700 psi.

 Additional seismic deformation analyses confirmed that acceptable seismic performance wouldbe achieved if the CDSM columns were founded on rock with an unconfined compressivestrength of at least 700 psi.

On the basis of the borings and test results, the termination criteria was modified fromrequiring 5 feet of penetration into bedrock to achieving cable slack and a penetration rate ofless than or equal to 0.1 feet per minute.

CDSM CostsRaito Inc. estimated the CDSM work would be competed in about 4.5 months for a cost

of about $3.7 million. Actual construction of the CDSM was completed over a period of sixmonths, with 4.4 months for the CDSM construction and 1.6 months of down time. 21.4 million

pounds of cement were used to construct 4353 soil cement columns, for a total cost of about$4.5 million.

Conclusions and Lessons Learned

Cement Deep Soil Mixing was selected as the preferred alternative to mitigate apotentially weak foundation. This was a new technology for the SFPUC and DSOD; therefore,it was important to develop clear and effective specifications for its construction. It was alsoimportant for there to be enough flexibility in the procedures to address variations in theconditions encountered during construction. Thorough mixing, material strength, andtermination criteria were considered key components required to achieve a quality product.Verification testing was performed to ensure that the requirements for these criteria were met.

 Another essential element to the success of the project was the close coordination betweenthe SFPUC, the Consultants, the Contractors, and DSOD staff.

Key design considerations when applying newer technologies include:•  Designers must understand detailed construction aspects of the new technology

in order to anticipate the full range of potential problems related to specific siteconditions.

•  The unique details associated with newer technologies require special attentionto identify all of the potential failure modes.

•  A thorough geotechnical investigation, performed in phases, is critical tounderstanding the subsurface site geology and to obtaining reliable materialproperties and strength data.

•  Early communication and coordination between the consultant and the regulatoryagency is essential to promote understanding of the technology and regulatoryrequirements.

The project was a success from both the design and construction point of view. Thesite is located in a densely populated neighborhood in San Francisco, and the project was

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completed with minimal disruption to the residents. The design of the CDSM treatment toimprove the weak foundation was able to meet the seismic performance objectives that wereestablished for the project. The CDSM construction was completed in a manner that wasconsistent with the design, with strengths that were greater than required. The embankmentexcavation and reconstruction were successfully completed on a very constricted site, whileaccommodating the CDSM construction schedule. And finally, the changed conditions thatwere encountered during construction, and the challenges they presented, were evaluated andsuccessfully addressed by the SFPUC, the Consultants, and DSOD.

The project was also successful for other reasons as well. A new technology wassuccessfully used for a site that would have been difficult to remediate otherwise. The CDSMtechnology proved successful in addressing the difficult site constraints while achieving therequired improvement to the foundation soils. This project presents a model for taking newertechnologies from conceptual ideas to effective implementation through sound engineering andconstruction practices. The SFPUC, the Consultants, the Contractors, and DSOD, all workedtogether with an understanding of each other’s needs and concerns to achieve the commongoal of a good final product.

Lessons learned during construction include:•  Redundant/confirmatory instrumentation monitoring is necessary to help identify

the cause(s) of unanticipated conditions and to assess their implicationsregarding slope stability.

•  Emergency response planning is essential to identify the roles and responsibilityof the various parties involved, to disseminate emergency contact information, todefine communication protocols that allow for rapid evaluation and decisionmaking, and to anticipate and prepare for possible problems that develop duringconstruction.

•  Prompt communication between the contractor, SFPUC representatives,engineering team, and regulatory agencies are necessary to identify and respondto potential problems in a timely manner.

•  CDSM can be an effective tool to mitigate stability concerns associated withseismically induced significant strength loss for embankment dams.

•  Construction monitoring, coring, and strength testing can be used to verify thatthe required soil improvement has been achieved.

•  CDSM rig penetration into weathered rock can vary significantly and drillingparameters may not reliably indicate the depth of penetration into rock.

•  The width of the CDSM elements, the variation in the surface of the bedrock, andthe type and degree of weathering of the rock need to be considered whenestablishing CDSM depth termination criteria.

•  A CDSM depth termination criterion that is based on the performance of theCDSM rig may be necessary to achieve the desired results without imposing

excessive requirements on the CDSM subcontractor. However, the criterionmust be calibrated to the particular equipment being used and the subsurfaceconditions encountered.

•  The use of proper CDSM construction equipment and methods can result insignificant strength increases and relatively uniform ground improvement in looseto medium dense sands.

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 Acknowledgements

The authors wish to thank Dr. Robert Pyke for providing input and guidance during thedesign and Dr. David Yang of Raito, Inc. for information provided during design andconstruction.

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