Development of liqucfaction potential map for Rangpur town

Abstract

Geomorphological unit and existing sub-soil characteristics indicate that the sub-soil of Rangpur Town is susceptible to liquefaction. To assess liquefaction potential of Rangpur Town, six boreholes were drilled and sixty-four borehole data were collected for 29 locations of the town. Sub-soil characteristics of Rangpur Town have been determined based on these seventy borehole data. It is seen that water table exists at the existing ground level (EGL) in most cases. SPT N-value varies from 1 to 10 for the depth I to 6 m from EGL. From 6 to 9 m depth, the SPT N-value varies' from 15 to 30. But there is a soft layer between 9 to 12 m in most cases and the SPT N-value varies from 8 to 10 for this layer. Below this layer, the SPT N-value is higher and in most cases the SPT N-value is over 50. Mean grain size varies from 0.02 to 0.35 mm for the depth from I to 6 m from EGL. For the depth 6 to 9 m, mean grain size varies from 0.13 to 0.47 mm. Mean grain size varies from 0.06 to 0.52 mm for the next 9 to 12 m depth. Mean grain size varies from 0.09 to 0.50 mm for the next 12 to 30 m depth. Fines content varies from 5 to 98% for the depth from I to 6 m from existing ground level. For the depth 6 to 9 m, fines content varies from 4 to 94%. For the next 9 to 12 m, fines content varies from 8 to 52%. For the next 12 to 30 m depth, the fines content varies from I to 30%. In general, the soil is silty sand from existing ground level to 30 m depth. The significant variation in the sub-soil characteristics indicates that sub-soil investigation is necessary for proper foundation design. Liquefaction potential analysis has been conducted for the above mentioned 29 locations using Japanese Code of Bridge Design and Chinese Criterion. Peak ground acceleration (am,,) for the liquefaction potential analysis was taken as 0.25g since Rangpur Town exists in Zone 3 of seismic zonation map of Bangladesh (Sharfuddin, 2001). According to Japanese Code of Bridge Design, liquefiable depth exists at 13 locations out of 29 locations. The maximum liquefiable depth is 5 m. However, the liquefiable depth varies significantly. Again, according to Chinese Criterion, there is liquefiable depth at 26 locations out of 29 locations. The maximum liquefiable depth is 10.5 m. In this case also the liquefiable depth varies significantly. It was seen that the liquefaction potential varied significantly in Japanese Code of Bridge Design and Chinese Criterion. Because in Japanese Code of Bridge Design both corrected Nvalue and soil characteristics are considered but in Chinese Criterion only corrected N-value is considered. Critical SPT N-value (i.e., SPT N-value corresponding to FL =1.0) has been determined based on both the Japanese Code of Bridge Design and Chinese Criterion. It was observed that corrected SPT N-value should be greater than 8 to 20 to avoid liquefaction. Liquefaction potential map was developed based on Iwasaki (1982) model. It is observed that, the liquefaction potential for 13% area is high, for 16% area is moderate and for 27% area is low. The liquefaction potential for the rest 44% area is very low. It indicates that ground improvement is indispensable for more than 50% area for Rangpur Town to avoid liquefaction. Keywords: Soil characteristics, liquefaction potential, liquefaction potential index.