Liquefaction Potential Analysis and Possible Remedial Measure for Existing Structure in Kathmandu Valley

Evaluation of liquefaction potential of soils using empirical relations based on field in-situ test data is a common practice in liquefaction study. In this study, Standard Penetration Test (SPT) data were used to examine the liquefaction potential of the Kathmandu Valley. In total 66 SPT data among the collected 102 from 33 locations were used for the analysis. The factor of safety against liquefaction (F L ) was calculated and found less than unity (F L <1) in 48 locations, which indicates the high probability of occurrence of liquefaction during the predicted scenario earthquake of magnitude 8 with peak ground acceleration 300gal. Similarly, mineralogical composition and physical index of field soil were compared to the commercially available sand and found more comparable with Toyoura sand. Two Centrifuge models in which one is saturated foundation soil model (Case1) and the other is desaturated foundation soil model by air injection (Case2) using the Toyoura sand were prepared in the laboratory. Both the model consisted of a metal plate at the top (representing the existing building structure in Kathmandu Valley) which is imparting the average contact pressure of 35kPa.  The prepared model was then set on centrifuge. The air was injected in case2 through the injector at centrifuge acceleration 50g. The estimated residual degree of saturation at desaturated area was 85% in case2. Both the models were tested in the centrifuge at 50g acceleration with imparting a simulated sinusoidal wave of frequency 40Hz and typical acceleration amplitude of 190gal. The test results showed that excess pore pressure was significantly reduced from 65kPa in case1 to 7.5kPa in case2 at the same location of the model. Similarly, vertical settlement is reduced approximately 50% in the case2 as compared with the case1. Test results of this study show the desaturation by air injection technique can be a better solution to control the foundation soil liquefaction and save the millions of structure standing over it in Kathmandu valley.