Liquefaction Analysis of a Bridge Site in Assam (India)

A rail-cum-road bridge with approach embankments is to be constructed on the Brahmputra river near Bogobil in Assam (India). The project site is located in Brahmputra’s alluvial plain and lies in an area of very high seismic hazard (zone V of Indian seismic zone map as per IS: 1893-1984) and traverse through the liquefiable ground. A two-level earthquake design criterion has been used: safety-evaluation ground motion of PGA of 0.6g, and a functional-evaluation ground motion of 0. lg. The liquefaction analysis has been carried using a “simplified procedure” originally developed by Seed and Idriss and progressively revised, extended and refined by others. The soil stratigraphy beneath the embankment consists of about 10m of recent alluvium of very loose to medium dense sand and silty soil overlying up to about 30m of medium to very dense deposits of sand and silty sand. Below this layer a dense to very dense sandy layer is encountered. The liquefaction assessment has been carried out for maximum embankment height of 2 lm which is to be constructed on the existing ground surface. The analysis indicated that the liquefaction is not likely to occur for functional evaluation motion. Soil strata under the embankments are liable to liquefy due to ground motion with PGA of 0.6g up to depths of about 14-18m with a total soil settlement of up to about 260 mm. Embankment stability has also been considered when the soil strata underlying the embankment undergo liquefaction. For this analysis, liquefiable soil layers have been assigned only the residual strength. For static case (i.e., no earthquake), the factor of safety ranges from 1.18 to 1.39. The bridge and abutments are founded on well foundations seated at about 55m below the river bed, conforming to minimum scour depth and grip length requirements. The average and normalized N-values at the bridge foundation levels are 86 and 39, respectively and, therefore, the bridge foundations are not susceptible to liquefaction