Evaluation of Liquefaction Screening Criterion Based on Standard Penetration Test Values

Abstract

Liquefaction of saturated soil has often been one of the causes of earthquake damage to structures resulting in loss of life and property. The additional safety in the design of superstructure is not of any help in the event of liquefaction of foundation soil during an earthquake. It is therefore very necessary that possibility of liquefaction is examined before hand and necessary remedial measures are adopted against damages due to liquefaction of foundation soil. Complex behaviour of soil liquefaction phenomena makes prediction of liquefaction costly and time consuming task. Fortunately, liquefaction occurs only under certain conditions. To take advantage of this fact various criterions are suggested to screen liquefaction potential in different code of practices. These criterions proved very useful and handy for practicing civil engineers and eliminate specialized evaluation of liquefaction potential. Most commonly used criteria for evaluation of liquefaction resistance is based on the standard penetration test (SPT). Here an attempt is made to examine such provisions in Indian and US code of practices. IS 1893 part-1 and The Building Code of The City of New York 2004 suggests liquefaction evaluation is not required if SPT N value is more than 15 (near to ground) to 25 (depth of 15 m from ground). Detailed liquefaction analysis is carried out as per NCEER, 1997 (Youd et al., 2001), most common method proposed in most of the code. Analysis reveals factor of safety against liquefaction much lower than 1 (e.g. 0.32 only for N value of 16 at 2 m depth for maximum ground acceleration of 0.36 g). Which suggest these provision need to be evaluated. Further comparison is made with Eurocode8 and FEMA guidelines in which suggested SPT N value greater than 30 for no liquefaction for clean sand. As these provisions are only screening criterion therefore need to be conservative. If based on these quick and simple provision possibility of liquefaction is predicted, detailed analysis may be warranted.