Flow Slides of Underwater Sand Deposits in Jamuna River Bed

Abstract

When excavation was under progress by dredging through the sandbar deposit in Jamuna River in Bangladesh, a number of slips occurred underwater. Features of the slip s are first described herein together with the results of in-situ investigations on the ground conditions. In the fluvial deposit in the Jamuna riverbed the sand is known to contain several percent of mica mineral composed of plate-shaped grains. The inclusion of mica has been known to make the sand behave more strain-softening leading to increased vulnerability to flow type deformation. This was conceived to have been the seminal cause of the underwater slides. To confirm this aspect, the sand was recovered from the river site and triaxial tests were performed in the laboratory extensively. The outcome of the tests was compiled and arranged in a manner where the residual strength could be evaluated in a general framework of interpretation on sand behaviour. The results of the tests showed that the mica-containing sand from Jamuna River site exhibited contractive or strain-softening behaviour over a wide range of void ratio. The residual strength at the steady-state deformation obtained in the present test scheme was used to provide an explanation for the flow-type instability of the slopes in the light of what actually happened during the underwater excavation in Jamuna River.