Soil Liquefaction Analysis in Risalpur Cantonment and DHA Peshawar Site, Nasitta Charsadda

Abstract

Soil liquefaction is an important geotechnical Phenomenon prevalent in saturated granular soils. These soils upon receiving dynamic loading (like earthquake) consolidate and the excess water liquidates the soil resulting in loss of strength and eventual settlement and collapse. There is not much awareness about this phenomenon and its mitigation in Pakistan. The research aims to highlight the Soil Liquefaction phenomenon and it will eventually assist the government authorities to develop guidelines for sustainable land development and put forth the remedial measures for soil liquefaction. Liquefaction potential of two sites, Risalpur Cantonment and DHA Peshawar site at Nisatta Charsadda, was assessed. These sites, within 20 km of each other, are located in zone 2B as per Building Code of Pakistan (BCP, 2007) and pose significant seismic hazard but they offer different liquefaction potentials. Liquefaction potential of these two sites was analyzed against earthquakes of magnitude 6.5 to 8.0 by the simplified Stress based approach using SPT which yields Factor of safety value of the site against liquefaction. Typical profile of subsoil in Risalpur constitutes of Silty Clay (CL - ML) with a thin middle layer of poorly graded sand (SP). Water table is deep and not encountered during boring but for the purpose of this study, we are considering a hypothetical situation where Risalpur’s water table suddenly rises during an event of flooding and heavy rains (2010 flooding). Results show that even when the water table is high, Risalpur soil is not Liquefiable except the middle SP layer which liquefies at earthquake magnitude 7.5 and above. However this layer will not cause any liquefaction related problems on the surface as its 16 ft deep bounded by non-liquefiable layers and only 3 ft thick. DHA site is located on bank of river Kabul. Subsoil profile consists of poorly graded sand (SP) overlain by silty sand (SM). Water table is 7 ft deep. Results show that DHA soil is highly liquefiable and liquefaction was observed till 30 ft depth at M=6.5 earthquake with the depth of liquefaction increasing with the earthquake magnitude. Significant settlement and lateral spreading is anticipated on the surface. Concluding the results, any construction project without considering liquefaction mitigation measures must be avoided at DHA Nisatta, Charsadda. Potential remedial measures are proposed in this study to counter the effects of liquefaction for future infrastructure Projects.