UNDRAINED SHEAR STRENGTH OF LIQUEFIED SANDS FOR STABILITY ANALYSIS By Timothy D. Stark, 1 Associate Member, ASCE and Gholamreza Mesri, 2 Member, ASCE ABSTRACT: The postliquefaction shear strength of sands, called the undrained critical strength or s„(critical), is evaluated in terms of the critical strength ratio, s„(critical)/o-;, 0 . This allows postliquefaction stability analyses to incorporate the variation of s„(critical) with effective vertical stress instead of using a single value as proposed elsewhere. Comparison of back-calculated critical strength ratios and the cyclic stress ratios triggering liquefaction suggests that drainage occurs in most cases during the postliquefaction flow of liquefied sands. To evaluate the stability of an existing slope that is predicted to liquefy during a dynamic event, the original slope configuration and the constant volume s„(critical) must be used instead of the back-calculated partially drained j„(critical) values. The proposed procedure for estimating the constant volume i u (critical), using the results of field and/or laboratory tests, shows that the critical strength ratio is approximately one-half the yield strength ratio at the triggering of liquefaction for an earthquake magnitude of 7.5. It has been shown that the yield strength ratio at the triggering of liquefaction for a magnitude of 7.5 can be estimated by 0.011 times the equivalent clean sand blow count. Therefore, the critical strength ratio is 0.0055 times the equivalent clean sand blow count. INTRODUCTION The undrained behavior of a loose cohesionless soil is illustrated by the consolidated-undrained triaxial compression test results shown in Fig. 1. The monotonic loading test was performed on a uniform, clean, fine quartz sand, referred to as a banding sand (Castro 1969). The test specimen was initially consolidated to an equal all-around pressure cr^ of approximately 400 kPa (8,350 psf). The minimum and maximum void ratios of the sand are 0.50 and 0.84. The void ratio of the specimen after consolidation was 0.71, which corresponds to a relative density of 37%. The specimen exhibited an undrained yield strength s„(yield) of approximately 115 kPa (2,400 psf) at an axial strain of approximately 1%. After the yield strength was mo- bilized and the specimen liquefied, it deformed from an axial strain of about 1% to 19% in just 0.18 s. At an axial strain of approximately 10%, the deviator stress and pore- water pressure became essentially constant. The postliquefaction shear strength, called the undrained critical strength ^(critical) is approximately 40 kPa (835 psf). This critical strength was first defined by Casagrande (1936) while developing the critical void ratio concept. The undrained critical strength is the strength available after liquefaction has been triggered and is appli- cable to postliquefaction stability analyses. This paper reviews the current procedures for estimating the undrained shear strength of liquefied sands and then proposes a new approach for estimating ,?„(critical). ^sst. Prof, of Civ. Engrg., Univ. of Illinois, Newmark Civ. Engrg. Lab. MC- 250, 205 N. Mathews Ave., Urbana, IL 61801. 2 Prof. of Civ. Engrg., Univ. of Illinois, Urbana, IL. Note. Discussion open until April 1,1993. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on September 20, 1991. This paper is part of the Journal of Geotechnical Engineering, Vol. 118, No. 11, November, 1992. ©ASCE, ISSN 0733-9410/92/0011-1727/$1.00 + $.15 per page. Paper No. 2613. 1727 J. Geotech. Engrg. 1992.118:1727-1747. Downloaded from ascelibrary.org by The University of Western Australia M209 on 02/12/14. Copyright ASCE. For personal use only; all rights reserved.