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Undrained shear strength of clean sands to trigger flow liquefaction: Reply1 M. Yoshimine, P.K. Robertson, and C.E. (Fear) Wride
The authors would like to thank Dr. Sladen for his valuable discussion on our paper. As pointed out in the discussion paper, there are many assumptions, approximations, and simplifications in the procedure of predicting undrained strength of sands to trigger flow liquefaction proposed in this study, and one should be mindful of these points when applying the methodology in practical use. In this reply paper, we would like to present our comments on the discussion points with additional information about the laboratory and field test data. First, the writer questioned the accuracy and reliability of the triaxial extension and simple shear tests because of the potential errors due to strain nonuniformity and suggested that the results of extension tests shown in Fig. 8 are implausible because the figure indicates sand at a relative density Dr as high as 80% may have an undrained strength ratio as low as 0.15. The lower boundary of the undrained strength shown in Fig. 8 is consistent with the triaxial extension tests on Kawagishi-cho sand. To see the test results in more detail, typical stress–strain curves and stress paths from the triaxial extension tests on Kawagishi-cho sand are plotted in Fig. DR1. The highest density of the data points in Fig. 8 is Dr = 79%, where actually the sand specimen at a density as high as Dr = 90% had some brittleness in triaxial extension with a minimum shear stress ratio around Su /pi′ = 0.2, as indicated in Fig. DR1. It should be noted that we selected the minimum shear stress as the undrained strength for flow failure from the viewpoint of stability criteria as discussed in the paper. Most of the undrained triaxial extension tests were terminated with necking of the specimens, therefore the ultimate steady states could not be observed. When necking occurred, the apparent mobilized friction angle had decreased and the stress path was bent downwards as shown in Fig. DR1 because the stress components were calculated using the average cross-sectional area of the specimen. Figure DR1 indicates that the stress path from the extension test follows the straight failure line up to the strain level of a few percent to 10% larger than the strain level at quasi-steady state (the minimum stress state), and necking occurred well after the quasi-steady state. Thus it is reasonable to say that measurement of the undrained strength at quasi-steady state
was not significantly influenced strain nonuniformity of Discussionsby 657 the specimen. The simple shear tests described in the paper were performed by means of a hollow cylindrical torsional shear apparatus. We believe that the simple shear test on hollow cylindrical specimen is very reliable because the dimension and shape of the horizontal cross section of the specimen are kept unchanged in the simple shear mode, and therefore there should not be any end restraint due to the friction on the rigid boundaries at the top and bottom of the sample (Yoshimine 1999). Although it is difficult to evaluate the quantitative strain nonuniformity in the specimens, there is no reason why triaxial extension and simple shear tests are less reliable than triaxial compression tests when the minimum undrained strength is concerned. Strain hardening during undrained shear after quasi-steady state is another issue related to the discussion on laboratory testing. When the undrained shear test on sands starts from relatively small confining stress levels, strain hardening towards the ultimate steady state inevitably occurred in monotonic loading conditions, as discussed by Yoshimine (1999). Figure DR2 is the result of undrained triaxial compression tests on Toyoura sand conducted by Verdugo (1992). In these tests, monotonic loading was applied to the sample until a very high stress level was attained due to the strain hardening, and then unloading was applied that resulted in catastrophic liquefaction with zero strength. This phenomenon was different from the zero stress state during the ordinal cyclic mobility; the sample was completely destroyed and reloading was impossible. Note that the strain reversal was very small (less than 1%) and the zero strength state was achieved at very large deformation (axial strain of about 25%). This test result implies that the fabric structure of sand particles rearranged during the hardening process in monotonic loading is very stiff only in one direction but is considerably brittle in other directions. If it is true, the ultimate state in pure monotonic loading in the laboratory could be very different from that of the failed mass in the field because of its turbulent nature and cyclic disturbances. This is one of the reasons why the authors considered that “it is unknown if hardening is possible in such conditions”. Although some researchers such as Meneses et al. (2000)
Received October 23, 2000. Accepted October 24, 2000. Published on the NRC Research Press Web site on May 31, 2001. M. Yoshimine.2 Department of Civil Engineering, Tokyo Metropolitan University, Minami-Osawa 1-1, Hachioji, Tokyo 192-0397, Japan. P.K. Robertson. Geotechnical Group, Department of Civil and Environmental Engineering, University of Alberta, Edmonton, AB T6G 2G7, Canada. C.E. (Fear) Wride. AMEC Earth & Environmental Ltd., 221 18th Street Southeast, Calgary, AB T2E 6J5, Canada. 1 2
Discussion by J.A. Sladen. This issue. Canadian Geotechnical Journal, 38: 652–653. Corresponding author (e-mail:
[email protected]).
Can. Geotech. J. 38: 654–657 (2001)
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DOI: 10.1139/cgj-38-3-654
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Discussions
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Fig. DR1. Undrained triaxial extension tests on Kawagishi-cho sand.
Fig. DR2. Undrained triaxial compression tests on Toyoura sand (Verdugo 1992).
examined the effects of cyclic disturbances on undrained characteristics of sand at large deformations, further research on this issue is necessary. Only Toyoura sand was used in the original paper to correlate the undrained strength ratio, Su /σ ′v, and the clean sand equivalent cone resistance, (qc1N)cs, from laboratory data because of the lack of element test data and CPT chamber testing on various sands. Recently, much additional data have been published as the result of the CANLEX Project (Robertson et al. 2000). The Su /σ ′v – (qc1N)cs relations from the CANLEX Project are generally consistent with that of this study. Note that relative density was used as a tracking parameter to develop the Su /σ ′v – (qc1N)cs relationship, and Su /σ ′v – Dr and Dr – (qc1N)cs relations are not necessarily the same for other sands. Hence the writer’s statement that “This essentially implies that the behaviour of all sands is uniquely related to relative density” is not applicable. The Su /σ ′v – Dr relationship could be strongly affected by the soil type, as suggested by Figs. 9 and 10. The Toyoura sand specimens for the element tests in this study were reconstituted by the dry-deposition or wettamping methods, whereas the Toyoura sand specimens for the CPT chamber tests (Tatsuoka et al. 1990) were reconstituted by the air-pluviation method. This difference in the mode of deposition may have little effect on the simple shear test results because the behaviour of sand in simple shear was intermediate between that in triaxial compression
and that in triaxial extension, i.e., the effects of anisotropy of the material may be minimal in simple shear conditions. In most of the CPT chamber tests, dry Toyoura sand specimens were used. Tatsuoka et al. (1990) ensured that the test results were not affected if the specimens were dry or watersaturated. The writer also questioned the reliability of the CPT data because “only for the Nerlerk case history were CPT data taken through the actual slide areas prior to failure.” The authors feel that the CPT data from the Jamuna Bridge case history are highly reliable at least to the same level as data from the Nerlerk case history. Figure DR3 shows the plane view of the west guide bund of Jamuna Bridge. The dredged area has a width of about 0.3 km and a length of about 3 km. The positions of the slide failures and the CPTs are also indicated in Fig. DR3. The CPTs were conducted on the slopes or on the bank typically within 50 m of the shoulder of the slope. The positions of the over 30 slides were randomly distributed along the bund, and therefore it seems that the whole area of the slopes had the same potential for flow failure. In fact, examination of the 22 CPTs revealed a surprising consistency in the data. To demonstrate this, the 22 CPTs were divided into the northern group (CPT1–CPT12) and the southern group (CPT13–CPT22). The minimum, maximum, mean, and mean plus and minus one standard deviation of the measured CPT tip resistance (qc) and the normalized sleeve friction ratio (F) of each group are compared © 2001 NRC Canada
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656 Fig. DR3. The plane view of the west guide bund of Jamuna Bridge (modified from Ishihara 1996). Arrows indicate positions of the slide failures.
in Fig. DR4. The same plots for all CPTs (CPT1–CPT22) are shown in Fig. 13 of the original paper. These figures show that the sand deposit in the failed zone (elevation –12 to +6 m) was very uniform in the horizontal and vertical directions. Thus it can be said that the CPT data are highly representative of the soil conditions prior to the failures of all the slopes. We agree that the case study from the Fraser River delta is somewhat less reliable if compared with the other two case studies because of the distance between the CPTs and the failure site, fewer CPTs, and the higher fines
Can. Geotech. J. Vol. 38, 2001
contents of the material. However, the authors’ opinion is that the case study from the Fraser River delta is still valuable because of the lack of an adequate database at present. The Fraser River sand was also studied in detail as part of the CANLEX Project (Robertson et al. 2000). Regarding the procedure of CPT data processing, the efficiency, reliability, accuracy, and limitations of the methodology are discussed in previous publications such as Robertson and Fear (1995), Robertson and Wride (1997, 1998a, 1998b, 2000), and Robertson et al. (1999, 2000). The “correction” factor for grain characteristics (Kc) for flow liquefaction should be studied further in the future, increasing our experience and database on flow failures of siltier deposits. At present the Kc value proposed for cyclic liquefaction could also be used for flow liquefaction as the first approximation with the background of the study by Ishihara et al. (1993), as explained in the original paper. Lastly, the writer suggests that in eq. [4] submerged density of the soil should be used in place of the bulk density for calculating shear stress in submarine slopes. This is correct when static stability of the slope is considered, whereas we assumed that the shear resistance of the moving mass on the residual slope was equal to the undrained minimum shear strength of the soil. In flow failures the pore water in the sliding mass moves together with the soil particles, and therefore the gravity force acting on the pore water also drives the sliding mass. In this case, the shear strength should be calculated on the basis of bulk density rather than submerged density.
References Ishihara, K. 1996. Evaluation of slope instability at the west guide bund in Jamuna Bridge project. Internal report, Jamuna Multipurpose Bridge Project, Bangladesh. Ishihara, K., Acacio, A.A., and Towhata, I. 1993. Liquefactioninduced ground damage in Dagpan in the July 16, 1990 Luzon earthquake. Soils and Foundations, 33(1): 133–154. Meneses, J., Ishihara, K., and Towhata, I. 2000. Flow failure of saturated sand under simultaneous monotonic and cyclic stresses.
Fig. DR4. Comparison of the CPT data from the northern and southern parts of the west guide bund of Jamuna Bridge.
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657 Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 126(2): 131–138. Robertson, P.K., and Fear, C.E. 1995. Liquefaction of sands and its evaluation. In IS-Tokyo ‘95, Proceedings of the 1st International Conference on Earthquake Geotechnical Engineering, Tokyo. Edited by K. Ishihara. A.A. Balkema, Rotterdam, The Netherlands, Vol. 3, pp. 1253–1289. Robertson, P.K., and Wride (Fear), C.E. 1997. Evaluation of cyclic liquefaction potential based on the CPT. In Proceedings of the Discussion Special Technical Session on Earthquake Geotechnical Engineering, 14th International Conference on Soil Mechanics and Foundation Engineering, Hamburg. Edited by P.S. Sêco e Pinto. A.A. Balkema. Rotterdam, The Netherlands. pp. 269–277. Robertson, P.K., and Wride (Fear), C.E. 1998a. Evaluating cyclic liquefaction potential using the cone penetration test. Canadian Geotechnical Journal, 35: 442–459. Robertson, P.K., and Wride (Fear), C.E. 1998b. Cyclic liquefaction and its evaluation based on the SPT and CPT. In Proceedings of the National Center for Earthquake Engineering Research (NCEER) Workshop on Evaluation of Liquefaction Resistance of Soils, Salt Lake City, Utah, January 1996. Edited by T.L. Youd and I.M. Idriss. National Center for Earthquake Engineering Research, Report NCEER-97-0022, pp. 41–87.
Can. Geotech. J. Vol. 38, 2001 Robertson, P.K., and Wride (Fear), C.E. 2000. Evaluating cyclic liquefaction potential using the cone penetration test: Reply. Canadian Geotechnical Journal, 37: 272–273. Robertson, P.K., Wride (Fear), C.E., Boulanger, R.W., Mejia, L.H., and Idriss, I.M. 1999. Liquefaction at Moss Landing during the Loma Prieta Earthquake: Discussion. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 125(1): 91–95. Robertson, P.K., Wride (Fear), C.E., List, B.R., Atukorala, U., Biggar, K.W., Byrne, P.M., Campanella, R.G., Cathro, D.C., Chan, D.H., Czajewski, K., Finn, W.D.L., Gu, W.H., Hammamji, Y., Hofmann, B.A., Howie, J.A., Hughes, J., Imrie, A.S., Konrad, J.-M., Küpper, A., Law, T., Lord, E.R.F., Monahan, P.A., Morgenstern, N.R., Phillips, R., Piché, R., Plewes, H.D., Scott, D., Sego, D.C., Sobkowicz, J., Stewart, R.A., Watts, B.D., Woeller, D.J., Youd, T.L., and Zavodni, Z. 2000. The CANLEX Project: summary and conclusions. Canadian Geotechnical Journal, 37: 563–591. Tatsuoka, F., Zhou, S., Sato, T., and Shibuya, S. 1990. Evaluation method of liquefaction potential and its application. In Report on seismic hazards on the ground in urban areas. Ministry of Education of Japan, Tokyo, Japan, pp. 75–109. (In Japanese.) Verdugo, R. 1992. Characterization of sandy soil behavior under large deformation. Ph.D. thesis, University of Tokyo, Tokyo, Japan. Yoshimine, M. 1999. Quasi-steady state: a real behavior?: Discussion. Canadian Geotechnical Journal, 36: 186–187.
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