evaluation of compaction effects on granular backfill using cpt

G E O T E C H N I C A L S P E C I A L P U B L I C A T I O N N O . 1 7 2

SOIL IMPROVEMENT

PROCEEDINGS OF SESSIONS OF GEO-DENVER 2007

February 18–21, 2007

Denver, Colorado

SPONSORED BY

The Geo-Institute of the American Society of Civil Engineers

EDITED BY

V. R. Schaefer, Ph.D., P.E. G. M. Filz, Ph.D., P.E.

P. M. Gallagher, Ph.D., P.E. A. L. Sehn, Ph.D., P.E.

K. J. Wissmann, Ph.D., P.E.

Published by the American Society of Civil Engineers

Quality Control and Constructability Session Organizer: Vern Schaefer

Evaluation of Compaction Effects on Granular Backfill Using CPTY. Tan, G. Lin, S. G. Paikowsky, and J. Fang

Ground Modification: How Much Improvement?S. M. Mackiewicz and W. M. Camp III

Laboratory Characterization of Jetting-Induced Disturbance ZonesMohammed A. Gabr, Roy H. Borden, Alex W. Smith,and Raymond L. Denton

Al Khaleej, Arabian Peninsula: Ground Improvement Based on Field ObservationsP. D. Scott and T. R. Skailes

A Proposed “Dredged Material Family of Compaction Curves” for Field Compaction Control Purposes

Allen W. Cadden, Dennis G. Grubb, David D. Miller, and Jason G. Mibroda

Aggregate Piers: Design and Case Histories Session Organizer: Kord Wissmann

Analysis of Uniformly Loaded Floor Slabs Supported by Rammed Aggregate Pier Elements

John P. Miller, Jason N. Richards, and Kord J. Wissmann

Load Test Comparisons for Rammed Aggregate Piers and Pier GroupsKord J. Wissmann, David J. White, and Evert Lawton

Use of Static and Seismic Deformation Criteria for Vibro Replacement Stone Columns: A Case History

R. A. Lopez and L. Shao

Stabilization of Failing Slopes Using Rammed Aggregate Pier Reinforcing ElementsJorge R. Parra, J. Matthew Caskey, Eric Marx, and Norman Dennis

Evaluation of Compaction Effects on Granular Backfill Using CPT

Y. Tan1, G. Lin2, S. G. Paikowsky3, and J. Fang4

1WPC, Inc., 2201 Rowland Avenue, Savannah, GA 31404; PH (912) 629-4000; FAX(912) 629-4001; email: [email protected], Inc., 2201 Rowland Avenue, Savannah, GA 31404; PH (912) 629-4000; FAX(912) 629-4001; email: [email protected] of Massachusetts Lowell, One University Avenue, Lowell, MA 01854;PH (978) 934-2277; email: [email protected], Inc., 2201 Rowland Avenue, Savannah, GA 31404; PH (912) 629-4000; FAX(912) 629-4001; email: [email protected]

Abstract: This paper examines the cone penetration testing (CPT) results prior to andafter two passes of deep dynamic compaction (DDC) to investigate compactioneffects on the loose saturated granular backfill between steel sheet pile walls and peatbogs. The site characterization results show that the backfill was significantlydensified after each pass of compaction. Two factors, (1) the existence of soft soillayers, and (2) severe liquefaction after DDC, greatly reduced compaction effects.The depth to which significant densification took place ranged from 6.5 to 9.5 m afterthe first pass of DDC, and 7.5 to 11 m after two passes. Finally, this paperrecommends parameters for the evaluation of densification depth based on thecomparison of the results from this study with those obtained from the literature.

INTRODUCTION

The U.S. Route 44 relocation project starts from the existing Route 44 at Carver,MA to the existing Route 3 near Plymouth, MA. The relocated roadway will be afour-lane divided highway with a typical median width of 18.3 m instead of thecurrent two-lane road. Parts of Route 44 relocation project at Carver, MA traveledthrough cranberry bog areas with deep peat deposits. At some stations, peat depositswere up to 10.5 m deep. Water table levels were near the existing ground surface.Figure 1 presents typical peat bog construction at this site. Because of its poorengineering properties and environmental concerns, all the peat in the proposedroadway sides were required to be excavated and then backfilled with granular soils,using sheet pile walls as retaining structures. The sheet pile wall was located about18.3 m off the proposed highway centerline. As the site was not dewatered during thisprocess, it was suspected that most backfill was in a loose saturated state, susceptible

GSP 172 Soil Improvement

to liquefaction during an earthquake. Two passes of deep dynamic compaction (DDC)were performed to transform the heterogeneous backfill soils into a more uniformlayer, which would consequently mitigate potential liquefaction hazards, increase soilstrength, and decrease compressibility and settlement.

Figure 1. Typical peat bog construction (modified from Tan 2005)

Using cone penetration testing (CPT) and seismic cone penetration testing (SCPT)prior to and after DDC, compaction effects on the backfill were evaluated (the CPTand SCPT results around stations 156+00 to 160+00 had been reported by Hajduk etal. in 2004). In order to verify whether all the loose, saturated backfill is densified, theinfluence depth for each pass of DDC is determined. In the mean time, observation onCPT data shows that two factors, (1) existence of soft soil layers, and (2) severeliquefaction after DDC, will reduce compaction effects. Finally, a parameter, n, isrecommended for the evaluation of densification depth. The authors believe that theparameter, n, can be used in other DDC design and construction.

SOIL CONDITIONS

The backfill depths at the project site range between 2.5 and 10.5 m. Based on thegrain size analysis results (refer to Figure 2), the backfill mainly consists of fine tocoarse sand, with trace amount of gravel and silt. It is considered liquefiable soilaccording to the procedures developed by Tsuchida (1970). Triaxial tests wereperformed on representative backfill samples, which were compacted in thelaboratory to similar relative density and moisture content as the in-situ backfillbefore and after DDC, respectively. The test results indicate its effective friction angle,φ′ , was around 32° at a unit weight of 18 kN/m3 before DDC, while 36° at a unit

MSE Wall

Backfill(inside DDC treated area)

Existing Grade

Route 44

On-site sand

Access RoadSheet Pile

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weight of 19.5 kN/m3 after DDC. The backfill was underlain by medium dense todense sand.

Figure 2. Grain-size distribution of the backfill (from Tan 2005)

DEEP DYNAMIC COMPACTION (DDC)

As a widely used ground improvement method, DDC was “rediscovered” by Mr.Louis Menard in French in 1965, who transformed the crude tamping method into arational compaction procedure. This method is carried out essentially by repeatedlydropping a very heavy tamper onto the soil surface from a relatively great height.DDC can be used for both unsaturated and saturated granular soils. The densificationprocess in unsaturated soils is the process of expelling air out from voids, therebydecreasing void volume and making soil denser. The mechanism of densification ofsaturated soils is based on dynamic consolidation (Ye et al., 1992). The energyproduced by DDC in soils destroys the soil structure and causes the water trappedbetween soil particles to be expelled out. As a result, cracks are formed, allowingrapid dissipation of pore pressure generated during DDC. Then, the soil consolidates,and soil particles are forced into a denser state.

The backfill at Rt. 44 was impacted by two passes of a 14.4-ton tamper with a basearea of 1.8 m2, dropped from a height of 18.3 m. Each tamping point was repeatedlyimpacted nine times. The time interval between the two passes was six days. Figure 3presents a typical DDC square grid-pattern with a spacing of 4.6 m and the locationsof CPT soundings used in this study. The second pass of DDC was conducted at thecenter spacing of the first pass. The distance between tamping locations and sheet pilewall was no less than 4.6 m to avoid unexpected large sheeting deflection duringDDC. Following the completion of each pass of DDC, all the craters caused by DDCwere filled.

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Grain size distribution curveMost liquefiable (Tsuchida, 1970)Potentially liquefiable (Tsuchida, 1970)


Figure 3. Typical DDC grid-pattern with CPT testing locations at U.S. Rt. 44

CONE PENETRATION TEST (CPT)

A series of cone penetration tests (CPT) were carried out inside the compactiontreated area and outside the compaction treated area to evaluate compaction effects.The initial evaluation test (EI) was conducted prior to the start of DDC. The secondtest (EV1) was conducted following the completion of the first pass. The subsequentthird test (EV2) was conducted following the completion of the second pass. In thispaper, only the CPT results inside the compaction treated area are presented anddiscussed. The CPT soundings in this study were located at the center spacing of thefirst and second pass of tamping points, and spaced 30.5 m apart along the central lineof the constructed roadway (refer to Figure 3).

COMPACTION EFFECT EXAMINATION

CPT Data

Figures 4(a), 4(b), and 4(c) present typical CPT results achieved before and afterthe DDC at stations 140+00, 142+00, and 144+00, respectively. The backfill mainlyconsisted of clean sands and gravelly sands. However, at station 142+00, layers ofsoft soils were identified at a depth of 9.5 to 11 m. The soft soils were the residualpeat which had not been replaced. This was also verified by the standard penetrationtest (SPT) performed by the Massachusetts Highway Department (MHD). Except fortopsoil near the ground surface, the cone tip resistance, qt, and side friction, fs, of thebackfill were significantly improved after each pass of DDC. For most of the backfill,the qt values were improved up to 7 times after the first pass, and up to 12 times aftertwo passes. The water table levels did not show significant changes because of thecut-holes in the steel sheeting, which were designed to allow the water table level on

Sta.140+00

(a) DDC Grid-Pattern

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Backfill

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Backfill inside DDC treated Area

Backfill outside DDC Treated Area

Sta.142+00

PEAT BOG

(b) CPT Sounding Location

30.5m 30.5m

Sta.144+00

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both sides of sheeting to be at the same level to keep sheeting structure in balance.The distribution of friction ratio, Rf, versus depth was more uniform after DDC. Atstation 140+00, following the completion of the first pass of DDC, the total verticalstress, σV0, along depth had some increase, while the second pass did not causeapparent changes. At station 142+00, there was small increase in the total verticalstress after the first pass. Similar to station 140+00, the second pass at station 142+00did not induce changes. At station 144+00, the total vertical stress along the depth hadsignificant reduction after the first pass, while it returned to its original state after thesecond pass. This reduction of total vertical stress after DDC could be attributed tothe severe liquefaction induced by the pore pressure, which had not dissipatedcompletely after DDC. If the maximum pore pressure generated during DDC cannotcreate cracks in the soil by expelling the water trapped between soil particles, porepressure cannot be drained quickly after DDC and remains at a high level. As a result,soil particles will lose contact with each other (liquefaction). The CPT results alsoindicate that the compaction did not induce significant densification effects on the in-situ medium to dense sand underneath the backfill.

Figure 4(a). Typical CPT site-characterization results at station 140+00

Figure 5 shows the comparison of tip resistance, qt, at stations 140+00, 142+00 and144+00 before and after DDC. In order to present clear comparisons, the measured qt

values at the depths of 3.5 to 6 m are discussed in greater detail. Before the DDC, themeasured qt values were close at all three stations. Following the completion of thefirst pass, the qt values at both stations 142+00 and 144+00 were still the same, butabout 3 to 4 MPa less than the values achieved at station 140+00. This could beattributed to the existence of residual peat at station 142+00 and the severeliquefaction at station 144+00. After the second pass, the qt values at station 144+00were around 3 to 6 MPa less than those at station 140+00, but 1 to 2 MPa larger thanthose of station 142+00. One possible explanation is that the residual peat at station142+00 acts as an impermeable boundary at the bottom of backfill. Compared to theother two stations (double-drainage), the pore pressure induced by DDC in the

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backfill at station 142+00 (single-drainage) could not dissipate quickly aftercompaction, which thus reduced densification effects. Another possible explanation isthat the residual peat could not provide strong confinement to its overlying backfillduring DDC. This suggests that prior to DDC, soft soils existing at relatively shallowdepths should be replaced. The comparison results also indicate that if backfillliquefied severely after DDC, an additional pass of compaction should be applied.

Figure 4(b). Typical CPT site-characterization results at station 142+00

Figure 4(c). Typical CPT site-characterization results at station 144+00

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Figure 5. Comparison of tip resistance, qt, at three stations

Influence Depth Evaluation

A criterion for ground improvement evaluation based on CPT tip resistance qt

values has been widely used. Dove et al. (2000) developed the ground improvementindex (Id), which is calculated in the following way:

1,

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aftertd q

qI (1)

where, qt,before is tip resistance before ground improvement, and qt,after is tip resistanceafter densification at the same location. The increase of qt values (Id>0.0) indicatesdensification effects. Figure 6 shows typical calculated Id at three investigated stations.Based on the investigation results at twelve stations, the depth of densification, D, atthe site was around 6.5 to 9.5 m after the first pass of DDC, and around 7.5 to 11 mafter two passes.

Figure 7 presents the relationship between square root of energy per drop and thedepth to which significant densification took place. Menard and Broise (1975)suggested that the depth of densification, D, could be calculated by hWD ⋅= , inwhich W is weight of tamper in tons and h is drop height in m. Many researcherslately pointed out that the equation proposed by Menard and Broise (1975)overestimates effective densification depth substantially and suggested that it shouldbe adjusted by multiplying a factor, n, to account for other factors that affect the depthof densification besides the mass of the tamper and the drop height, namely:

hWnD ⋅= (2)where, n is empirical coefficient less than 1.0. Leonards et al. (1980) suggested that nwas about 0.5. Mayne et al. (1984) pointed out that n was around 0.3 to 0.8. Gambin

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(1987) suggested that n was around 0.5 to 1.0. Ye et al. (1992) recommended that forcohesionless soils, n could be 0.5; for cohesive soils, it could be 0.35 to 0.5. For thegranular soils at this study, the determined n was around 0.35 to 0.55 with an averagevalue of 0.45 following completion of the first pass of DDC, while 0.45 to 0.65 withan average value of 0.55 after two passes. At the same tamping point, two passes ofcompaction resulted in greater densification depth than that induced by only one pass.Comparing the results from this study with those available from the literature, thedeveloped n values reasonably reflect the available experience.

Figure 6. Calculated ground improvement index, Id, at three stations

CONCLUSIONS

Based on evaluation of the CPT results prior to and after DDC, the followingconclusions were obtained:(1) DDC was very effective for densifying loose saturated granular soils at the site

studied. After two passes of DDC, all backfill had been improved by densification.(2) CPT is an effective tool to characterize densification effects of DDC. After

compaction, the tip resistance, qt, and side friction, fs, of backfill increasedsignificantly. The qt values could be increased up to 7 times after the first pass ofDDC and up to 12 times after two passes. The distribution of friction ratio, Rf,with depth was more uniform after each pass of DDC. The water table level didnot show significant changes because of the cut-holes in the sheet pile walls.

(3) To achieve desirable compaction effects on granular soils, the identified soft soillayers at relatively shallow depths should be completely replaced prior to DDC.Otherwise, the existence of soft soils may reduce densification effects.

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Figure 7. Relationship between square root of energy per drop and the depth towhich significant densification took place (after Leonards et al., 1980)

(4) If saturated granular soils are found to liquefy severely after the first pass of DDC,an additional pass of compaction should be applied.

(5) The depth to which significant densification took place ranged from 6.5 to 9.5 mafter the first pass of DDC, and 7.5 to 11 m after two passes.

(6) The empirical coefficient, n, equal to 0.45 is recommended for evaluatingdensification depth in granular soils impacted by one pass of DDC, and 0.55 forthose impacted by two passes.

ACKNOWLEDGEMENT

The authors would like to express appreciation to the following people andorganizations for providing related information: Dr. E. L. Hajduk of WPC, Inc.; Dr. N.Hourani and Mr. P. Connors of Massachusetts Highway Department (MHD); P. A.Landers, Inc., Massachusetts; TerraSystems, Virginia; Geotechnical Testing andResearch, Inc. (GTR), Massachusetts, and WPC, Inc.

REFERENCS

Chow, Y. K., Yong, D. M., Yong, K. Y., and Lee, S. L. (1992). “Dynamiccompaction analysis.” J. geotechnical engineering, ASCE, 118(8), pp. 1141-1157.

David, D. (1976). Deep compaction. Bulletin, D. David Engineers Ltd, Ramat Aviv,Isreal.

DeBeer, E., and Van Wambeke, A. (1973). “Consolidation dynamique par pilonnageintensif, aire d’Essai d’Embourg.” Annales, des Travaux publics de Belgique,No.5, Oct., pp.295-318.

0 2 4 6 8 10 12 14 16 18 20 22 24 26Square root of energy per drop, (wh)0.5, (ton.meter)0.5

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8

6

4

2

0

Co

mpa

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nin

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(de

nsifi

catio

n)d

epth

D,(

met

er)

20

18

16

14

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8

6

4

2

0

Co

mpa

ctio

nin

flue

nce

(den

sific

atio

n)de

pth

D,(

me

ter)

0 2 4 6 8 10 12 14 16 18 20 22 24 26Square root of energy per drop, (wh)0.5, (ton.meter)0.5

Malaysia (1st pass) (Chow et al.,1992)

Malaysia (2nd pass) (Chow et al.,1992)

Israel (David, 1976)

Belgium (DeBeer and W am beke, 1973)

France (G igan, 1977)

Sweden (Hansbo et al.,1973)

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Indianapolis, USA (Leonards et al.,1980)

Chicago, USA (Lukas, 1980)

Japan (Tanaka and Sasaki,1989)

Scotland (W est and Slocom be, 1973)

China (sandy soil) (Ye et al.,1992)

China (clayey soil) (Ye et al.,1992)

This study (1st pass)

This study (2nd pass)

D=0.55(wh) 0.5D=0.65(wh) 0.5

D=0.35(wh)0.5

D=(wh) 0.5, (M enard and Broise,1975)

D=0.45(wh)0.5

D=0.5*(wh) 0.5, (Leonards et.al.,1980)


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Gigan, J. P. (1977). “Compactage par pilonnage intensif de remblais de comblementd’un bras de seine.” Bulletin, de liaison des laboratories des ponts et chausses,Paris, France, vol. 90, July-Aug., pp. 81-102.

Hajduk, E. L., Connors, P. J., Miller, H. J., and Meng, J. W. (2004). “Verificationtesting of deep dynamic compaction between steel sheet piling using conepenetration testing.” International symposium on ground improvement, 9-10 Sept.2004, Paris, French.

Hansbo, S., Pramborg, B., and Nordin, P. O. (1973). “The Vanern terminal, anillustrative example of dynamic consolidation of hydraulically placed fill.” Lesedition sols-soils, No.25, pp.5-11.

Lee, D. H., Juang, C. H., and Ku, C. S. (2001). “Liquefaction performance of soils atthe site of a partially completed ground improvement project during the 1999 Chi-Chi earthquake in Taiwan.” Canadian Geotechnical Journal, Volume 38, number6, pp.1241-1253.

Leonards, G. A., Cutter, W. A., and Holtz, R. D. (1980). “Dynamic compaction ofgranular soils.” Journal of Geotechnical Engineering Division, Vol.106 No.GT1,pp. 35-44.

Lukas, R. G. (1980). “Densification of loose deposits by pounding.” ASCE, Journal ofthe geotechnical engineering division, GT4, pp.435-446.

Mayne, P. W., Jones, J. S., and Dumas, J. C. (1984). “Ground response to dynamiccompaction.” ASCE journal of geotechnical engineering, Vol. 110, No.6, pp. 757-774.

Menard, L. and Broise, Y. (1975). “Theoretical and practical aspects of dynamicconsolidation.” Geotechnique, Vol. 25, pp. 3-18.

Tan, Y. (2005). “Sheet pile wall design and performance in peat.” Doctoral Thesis,University of Massachusetts Lowell.

Tanaka, S., and Sasaki, T. (1989). “Sandy ground improvement for liquefaction atNoshiro Thermal power station.” Soils and found. Japanese society of soilmechanics and foundation engineering, Tokyo, Japan, Ser. 374, 37(3), pp. 86-90.

Tsuchida, H. (1970) “Predictions and countermeasure against the liquefaction in sanddeposit.” Abstract of the semina in the port and harbor. Research institute,Yokohama, pp. 3.1-3.33.

West, J. M., and Slocombe, B. E. (1973). “Dynamic consolidation as alterativefoundation.” Ground engineering, Vol.6, No.6, pp. 52-54.

Ye, S. L., Ye, G. B., and Han, J. (1992). Ground Treatment (in Chinese). TongjiUniversity, Shanghai, China.


evaluation of compaction effects on granular backfill using cpt

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