Abstract:
ABSTRACT
Live capacity of Tarbela reservoir has threatened due to high sediment inflow. More than one-third of the volume of Tarbela reservoir has filled with sediment delta composed of low-plastic silt and silty sand deposits. With the yearly cycles of deposition, sediment delta is continuously reworking and moving closer to the dam. Excessive sedimentation coupled with the proximity of dam site to active fault system raised concerns about liquefaction of sediment delta as a result of seismic activities. This research study employs in-situ test data, laboratory test results, empirical procedure and numerical modeling to investigate liquefaction potential of sediments stored in Tarbela reservoir. Delta configuration and stratification was estimated using laboratory test results of samples collected during an offshore drilling and sampling project named “Sediment Management Study of Tarbela Reservoir (SMST)”, performed in 2013. Liquefaction susceptibility of material was evaluated using compositional criteria developed after various liquefaction case histories. Laboratory tests were performed on reconstituted samples to establish critical state and shear strength parameters. Two-dimensional model of problem was analysed in finite element based software, QUAKE/W, which estimates liquefaction potential using cyclic stress approach. Amplification of earthquake was observed near the surface of sediment delta. Silty sand deposits forming foreset and topset slopes of sediment delta were evaluated to initiate liquefaction under an earthquake with peak ground acceleration of 0.24g, extent of liquefied zones was limited to the depth of 18 meters from the surface of sediment delta. Occurrence of liquefied zones is consistent with the general observation that liquefaction initiates under low effective stresses.
