EVALUATIO OF U DRAI ED SHEAR STRE GTH O BUSA CLAY USI G FLATDILATOMETER TEST

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1 EVALUATIO OF U DRAI ED SHEAR STRE GTH O BUSA CLAY USI G FLATDILATOMETER TEST SI GH V. K. 1, CHU G S. G. 2, HO G Y.P. 3, KWEO H. J. 4 1 Researcher, National Research Laboratory of Soft Ground, Dong-A University, Busan, S. Korea 2 Professor, Department of Civil Engineering, Dong-A University, Busan, S. Korea 3 Postgraduate Student, Department of Civil Engineering, Dong-A University, Busan, Korea 4 Researcher, National Research Laboratory for Soft Ground, Dong-A University, Busan, Korea ABSTRACT: Busan clay, usually varying from to 7m thick, is widely deposited along the coastline in the Nakdong River deltaic area located west of Busan City in Korea. Despite many geotechnical investigations for various reclamation projects, the properties of the clay have not been yet elucidate due to their spatial variation and inadequate undisturbed sampling. The undrained shear strength of the clay is interpreted using the flat dilatometer (DMT) with various existing empirical equations. The results are compared with each other, and correlated with the corrected undrained shear strength obtained from the field vane shear test (S u(fvt) ). The results indicated that the K D based empirical equation proposed by Marchetti (198) slightly underestimates the S u(fvt) values on the clay.empirical equations based on S u -K D and S u -E D -I D relationshipsare developed for the clay. The undrained shear strengths estimated from thes u -E D -I D relation give the closest correlation. Keywords: clay, in situ test, undrained shear strength, DMT, FVT I TRODUCTIO The undrained shear strength is usually obtained from unconfined compression tests or unconsolidated undrained triaxial compression tests on undisturbed samples or from field vane shear test (FVT). Laboratory test results largely depend on the quality of undisturbed samples. Sample disturbance may result in the severe failure of the structure due to incorrect prediction of undrained shear strength such as failure of the Break-water at Busan New Port site that occurred during construction of the New Busan Port (Chung et al., 7). Thus, it is a common practice to undertake the field vane shear test (FVT) for the clay;however, it is necessary to appropriately determine the correction factor for the undrained shear strength of clay. Besides, the field vane shear test results can be affected by sand lenses, shells and seams. Based on the investigation on failure of the Break-waterat Busan New Port, Chung et al. (7) estimated the undrained shear strength of the Busan clay from back analysis.with the several field vane shear tests at five additional sites including the present site, they concluded that the correction factor proposed by Aas et al. (1986) is applicable to Busan clay. They found that the corrected strength ratio (S u,corr. /σ vo ) in the Aas et al. (1986)method varies between.22 and.3, however, the lower bound value.22 appears to be applicable for the design.hong et al. (7) evaluated clay at Busan New Port sites with flat dilatometer test, field vane shear test and CK U triaxial tests and found that S u(fvt) /σ v ranges from. to.22 whereas, S u(ckou) /σ v ranges from.3 to.3. The flat dilatometer test (DMT) is comparatively simple, rapid, repetitive and applicable to both clay and sand. The one of the main application is to estimate the undrained shear strength of the clay. The flat dilatometer test has been used in characterizing marine clays in the region including Korea by number of researchers (Chang, 1992, Kamei and Iwasaki, 199, Kim et al., 1997, Kim et al. 1, Lee and Seong, 1, Byeon et al., 4, Hong et al., 7, Lee et al. 8 etc.). However, the correlation varies location to location depending on the clay type. This infers the requirement of calibration of the flat dilatometer prior to use at the local sites. In the present study, the flat dilatometer test(dmt) and field vane shear test (FVT) were performed at Jangyu site. The undrained shear strength obtained from the filed vane shear testwas corrected according tosuggestionby Chung et al. (7). The DMT results were interpreted using various existing empirical equations and the predicted undrained shear strengths were compared and correlated with the corrected undrained shear strength obtained from the field vane shear test. Two new equations compatible to local site were presented, one using the S u(fvt) -K D relationship and 19

2 another using the S u(fvt) -E D -I D relationship. In the second equation, the material index (I D ) is incorporated so that the equation can give better estimation of undrained shear strength for different type of clays. DILATOMETER TEST I TERPRETATIO The flat dilatometer test (DMT) consists of pushing a flat blade located at the end of a series of rods. Once at the testing depth, a circular steel membrane located on one side of the blade (Fig. 1) is expanded 1 mm horizontally into the soil. The pressure is recorded at specific moments during the test. The blade is then advanced to the next test depth.the general layout of the flat dilatometer test is shown in figure 2. The principal and test procedure of DMT in detail can be found in Marchetti et al. (1). from equation 1 in soft, uncemented saturated clays compares fairly well with uncorrected field vane results, however, the equation is not recommended for OC cemented and/or fissured clays (Riaund and Miran, 1992). S u =.22 σ v (.K D ) 1.2 (1) Lacasse and Lunne (1988) showed that the measured value of S u varies depending on the type of test used. They suggested equation 2 to estimate S u for soft uncemented clays using field vane shear test. S u =.19 σ v (.K D ) 1.2 (2) Figure 2 General layout of the dilatometer test(marchetti, 198) Figure 1 The flat dilatometer Front and side view The interpretation of DMT is primarily based on two field readings, P (corrected contact stress) and P 1 (corrected 1mm expansion stress), from which three intermediate parameters, viz. material index [I D =(P 1 -P )/(P -u )], horizontal stress index [K D =(P -u )/σ v ], and dilatometer modulus [E D =37.4(P 1 -P )] are defined where u is the insitu pore water pressure prior to dilatometer insertion and σ v is the effective overburden stress(marchetti, 198). The other soil parameters are interpreted using these three intermediate parameters. Marchetti (198) suggested the first empirical equation (Eqn. 1) to estimate the undrained shear strength as a function of horizontal stress index K D. Several authors have shown that the undrained shear strength predicted Roque et al. (1988) considered the dilatometer insertion as a footing loaded horizontally to failure and proposed to use the classical bearing capacity formulas to estimate the undrained shear strength (Eqn. 3). S u = (P 1 - σ ho )/N C (3) Where, σ ho is total horizontal in-situ stress (σ ho = K.σ v +u ), N c = 7 for medium clay. To use in this equation, K =. was chosen for Busan clay based on the laboratory test on undisturbed samples. The measured undrained shear strength also depends on the clay type and thus varies from location to location. Various authors have developed different equations in order to make it compatible with local sites; for example, 11

3 figure 4. It is reported that the geotechnical properties of Busan clay vary appreciable with its depositional environment (Chung et al., 3, ). The detailed geotechnical properties and depositional environments of the Busan clay can be found in Chung et al. (3,, 7). based on field vane shear test and laboratory test at number of sites in Malaysia and Singapore, Chang (1992) suggested equation 4 to use with young marine clay. Su =.74 σ v KD1.2 (4) Similarly, based on the laboratory tests, Kamei and Iwasaki (199) suggested equation to use with Japanese clay. Iwasaki and Kamei (1994) also suggested equation 6 to estimate undrained shear strength for Japanese clay which is based on the dilatometer modulus (ED). Su =.3 σ v (.47KD)1.14 () Su =.18 ED (6) SOIL PROPERTIES AT THE TEST SITE The study site issituated atjangyu site,central-west ofthe Nakdong deltaic plain (Fig. 3). The clay is extended from ground level to about 32 m depth however, at the middle part (17. m to 24 m), clay is frequently interlayered with broken shell, clayey silt and sandy silt layers. Based onthe CPT based soil classification chart (Robertson, 199), 4 m to 17 m depth is classified as massive clay or silty claylayer, 17 to 24 m depth is markedby frequentlyinterlayering clay or silty clay, sandy silt and clayey siltlayers; and below 24 m depth is again clay or silty clay layer.the soil profile with various geotechnical properties of the clay from the test site are shown in qt (MPa) Soil profile Figure 3 Location map u and u2 (MPa) fs (MPa) Su(FV) corr. (kpa) 4 6 Wn (%) Clay/silty clay Sandy/clayey silt 1 Clay/silty clay 1 Shale/silt/sandy Clay 2 u u2 Clay/silty clay 3 Figure 4 Soil properties at the test site 111 Ip (%) γt (kn/m3)

4 Material index CLAY SAND Horizontal Stress Index Dilatometer Modulus SILT I D K D Figure DMT test results E D (MPa) TEST RESULTS A D A ALYSIS Figure shows the DMT test results in terms of three intermediate parameters. The soil classification based onmaterial index (I D ) indicates all clay until 27 m depth; however, clay between m to 17 m appears homogeneous. The dilatometer modulus (E D ) appearsmore sensitive to soil type compare to the horizontal stress index (K D ). The DMT results were interpreted using six empirical equations (Eqn.1 to 6) and compared with the corrected undrained shear strength obtained from the field vane shear test as shown in figures 6a and 6b. As shown in the figure 6a, the undrained shear strength estimated from Marchetti (198) equationappears very close to corrected undrained shear strength at upper massive clay layer whereas it underestimates at lower clay layer. The Roque et al. (1988) equation in other hand shows opposite trend, i.e., the estimated undrained shear strength appears close to the corrected undrained shear strength at lower clay layer and it overestimates at the upper massive clay layer.the Lacasse and Lunne (1988) method underestimates the undrained shear strength for the entire depths. Among the other two I D -based methods, Chang (1992) method largely underestimates and Kamei and Iwasaki (199) method overestimates the undrained shear strength (Fig. 6b). The E D -based method proposed by Iwasaki and Kamei (1994) estimates the undrained shear strength quite close to that of corrected undrained shear strength fromfield vane shear testhowever, the values fluctuate widely throughout the depths. Figure 7 shows the variation of σ v, K D and E D with undrained shear strength (S u(fvt) ).The relationship between σ v and S u(fvt) is linear and the regression line can be given by S u(fvt) =.28σ v. The relationship of S u(fvt) with K D and E D are quite poor, however, S u(fvt) - K D shows better correlation than that of S u(fvt) -E D. Figure 8 shows the linear relationship between S u(fvt) /σ v and K D 1.2 with -intercept. The regression line can be given by equation 7. S u =.12 K D 1.2 σ v (7) The relationship between S u(fvt) and E D until 17 m depth (marked by σ v = 3 kpa in Fig. 7) is linear whereas, below 17 m depth, the relation is very poor.this indicates the sensitivity of dilatometer modulus (E D ) with the soil type. Since the soil type is defined by the material index (I D ), it is plotted against thecorrected undrained shear strength (S u(fvt) )normalized by dilatometer modulus (E D ) as shown in the figure 9. The regression line for this relationship can be given by equation 8 with reasonable accuracy. S u = E D /(418 I D ) (8) 112

5 FVT (corr.) Marchetti (198) Lac. & Lum. (1988) Roque et al. (1988) FVT (corr.) Chang (1991) MC Iwa. & Kam. (1994) Kam. & Iwa. (199) S u (kpa) S u (kpa) (a) (b) Figures 6a & b Comparison between undrained shear strengths estimated from DMT with FVT σ' v (kpa) K D E D (MPa).4 y =.12x S u(fvt) (kpa) 3 4 S u(fvt) /σ' v S u =.28σ' v K D 1.2 Figures 7 Variation of σ v, K D and E D with S u Fig. 8. S u(fvt) /σ v - K D 1.2 relationship 113

6 E D /S u(fvt) y = 418x R² = I D Figure 9 E D /S u(fvt) - I D relationship S u(fvt) (kpa) Marchetti (198) Eqn. 7 Eqn Estimated S u (kpa) Figure 1 Comparison of estimated S u from DMT using equations 7, 8 and Marchetti (198) with S u(fvt) Figure 1 shows the comparison between estimated undrained shear strength from equation 7, 8 and Marchetti (198) with measured undrained shear strength from field vane shear test (S u(fvt) ). The undrained shear strength estimated from the equation 8 appears closerto the measured undrained shear strength (S u(fvt) )than other FVT (corr.) Marchetti (198) Eqn. 7 Eqn S u (kpa) Figure 11 Comparison of undrained shear strength profiles methods. The undrained shear strength profiles estimated from equations 7, 8 and Marchetti (198) are shown in figure 11 along with the corrected undrained shear strength (S u(fvt) ). Compare to Marchetti (198), the undrained shear strength estimated from the equation 7 shows slightly improved profile whereas, the equation 8 gives better estimation throughout the depths. Since the equation 8 is newly developed equation incorporating material index I D, it needs to be tried out with additional flat dilatometer tests (DMT) on different clays. CO CLUSIO The field vane shear test(fvt) and dilatometer tests (DMT) were performed on Busan clay at Jangyu site. The corrected undrained shear strength from FVT was compared with DMT results interpreted using various empirical equations. It is found that the K D -based empirical equation proposed by Marchetti (198) shows a close correlation with the corrected undrained shear strength obtained from the field vane shear test; however, it still underestimates the undrained shear strength. The E D -based method proposed by Iwasaki and Kamei (1994) method is also able to estimate the undrained shear strength close to that of corrected undrained shear strength obtained from the field vane test; however, the values fluctuate widely. The strength ratio,s u(fvt) /σ v,was found.28.the correlations of S u(fvt) with K D and E D were quite poorat the lower clay layer, in which the K D and E D values varied sensitively with soil type. Thus, two empirical equations were developed based on the S u(fvt) -K D and S u(fvt) -E D -I D relationships, whichcan be used in the local site. It appeared that Eq. (8) based on the S u(fvt) -E D -I D resulted in a better correlation throughout the depths. However, 114

7 further study is needed to prove whether the developed formula is applicable to different types of clays. ACK OWLEDGEME T This work was supported by the Korea Science and Engineering Foundation (KOSEF) NRL Program grant funded by the Korea government (MEST) (No. RA ), and by Dong-A University, Busan Korea. REFERE CE Aas, G., S. Lacasse, T. Lunne, and K. Hoeg. (1986). Use of In-situ Tests for Foundation Design on Clay. Use of In Situ Tests on Geotechnical Engineering 1: 1 3. Byeon, W. Y., Kim, Y. S. amd Lee, S. R., (4). Influencing factors for the estimation of undrained shear strength by Flat DMT. Journal of thekorean Geotechnical Society, V., No. 4, pp (in Korean). Chang, M. F. (1992). Interpretation of Overconsolidation Ratio from In-situ Tests in Recent Clay Deposits in Singapore and Ma1aysia. Canadian Geotechnical Journal, 28, Chung, S. G., Ryu, C. K., Jo, K. Y. and Huh, D. Y. (). Geological and Geotechnical Characteristics of Marine Clays at the New Busan Port. Marine Georesources and Geotechnology 23(3): Chung, S.G., Beck, S.H., Ryu, C.K. and Kim, S.W. (3). Keynote Lecture: Geotechnical Characterization of Pusan Clays, Korea-Japan Joint Workshop on Characterization of Thick Clay Deposits, Reclamation and Port Construction, ATC-7, April 8-1, Busan, pp Chung, S.G., Kim, G.J., Kim, M.S. and Ryu, C.K. (7). Undrained Shear Strength from Field Vane Test on Busan Clay. Marine Georesources and Geotechnology 2: Hong, S. J., Shin, D. H., Kim, D. H., Jung, S. J. and Lee, W. J., (7). Evaluation of undrained shear strength of Busan New Port clay by DMT. Journal of thekorean Geotechnical Society, Sp. Proceeding No.4, V. 23, No. 7, pp (in Korean). Iwasaki, K. and Kamei, T. (1994). Evaluation of In-situ Strength and Deformation Characteristics of Soils Using Flat Dilatometer. JSCE, Journal of Geotechnical Engineering, No. 499, III-28, pp (in Japanese).. Kamei, T. and Iwasaki, K. (199). Evaluation of Undrained Shear Strength of Cohesive Soils Using a Flat Dilatometer. Soils and Foundations, Vol. 3, No. 2, Kim, J. K., Kim, Y. U., Choi, I. G. and Park, Y. M., (1). Estimation of geotechnical characteristics at of the marine clay at Inchon International Airport marine clay using piezocone and dilatometer tests. Journal of thekorean Geotechnical Society, V. 17, No. 2, pp (in Korean). Kim, S. I., Jeong, S. S., Lee, S. R., Kim, D. S. and Kim, Y. S., (1997). Characterization of In-situ Properties of Korean Marine Clays Using CPTU and DMT. ISSMFE, Serial. 14, Hamburg, Germany, pp Lacasse, S., and Lunne, T. (1988). Calibration of Dilatometer Correlations. Proceedings of the 1st International Symposium on Penetration Testing, ISOPT-1, Orlando, Florida, Vol. 1, Lee, S.R. and Seong, J.H., (1). Estimation of Horizontal Coefficient for Korean Marine Clays by Flat DMT. Int. Con. on In-Situ Measurement of Soil Properties and Case Histories, Serial. Bali, Indonesia. Lee, S.R., Byeon, W.Y. and Kim, Y.S., (8). Reliable Estimation of Undrained Shear Strength of Korean Soft Clay using Flat DMT and ANN. The 3rd International Conference on Site Characterization, Taiwan, Marchetti, S. (198). In Situ Tests by Flat Dilatometer. ASCE Journal of Geotechnical Engineering, Vol. 16, No. GT3, Marchetti, S.; Monaco, P.; Totani, G. & Calabrese, M. (1). The Flat Dilatometer Test (DMT) in Soil investigations. A report by the ISSMGE Committee TC16. In Situ 1. Bali. Indonesia, 41 pp. Riaund, J. L. and Miran, J. (1992). The Flat Dilatometer Test.Report o. FHWA-SA-91-44, Federal Highway Administration, U.S. Department of Transportation, Washington, D.C. P. 12. Robertson, P.K., (199). Soil Classification using the Cone Penetration Test. Canadian Geotechnical Journal, Vol. 27, No. 1, pp Roque, R., Janbu, N., and Senneset, K. (1988). Basic Interpretation Procedures of Flat Dilatometer Tests. Proceedings of the 1st International Symposium on Penetration Testing, ISOPT-1, Orlando, Florida, Vol. l,