LIQUEFACTION PROBABILITY FROM MAGNITUDE 7+ EARTHQUAKE IN DHAKA MEGACITY, BANGLADESH

Liquefaction occurs when a granular material transforms from solid to liquefied state due to strong ground vibration during an earthquake. This leads to an increase in pore pressure so that the effective stress becomes zero and develops a liquefied state. Bangladesh is one of the most densely populated countries in the world which is surrounded by many active faults i.e., Dauki fault, where greater than 7.5 magnitude earthquakes have occurred more than once. Some researchers have suggested that the presence of a locked megathrust plate boundary in the eastern fold belt of Bangladesh could potentially develop a megaquake. Moreover, some large intraplate active faults are also known to exist in the central part of the country i.e., Madhupur fault, Dhamrai fault, etc. Geologically, the central part of Bangladesh is characterized by repeated sediment facies changes of sand, silt, and clay layers. The presence of trapped water in those layers is susceptible for generating overpressure to initiate surface manifestation of liquefaction even during a moderate earthquake.

Prior research mapped the liquefaction risk in Bangladesh's capital city using the liquefaction potential index. Based on 53 Standard Penetration Test (SPT) results, three zones were classified where the probability of surface manifestation of liquefaction were found in between 8% to 72% for an earthquake scenario having a magnitude of 7.0. The current study aims to find the liquefaction resistance from field measurements of shear wave velocity. The use of share wave velocity as field index of liquefaction resistance is well-founded because both the share wave velocity and Cyclic Stress Ratio are similarly, but not proportionally, impacted by void ratio, effective confining loads, stress history, and geologic age. In soils that are difficult to penetrate with SPT or to retrieve undisturbed samples, including gravelly soils, as well as at locations where borings or soundings may not be authorized, share wave velocity measurements are possible. The unplanned and vulnerable buildings are being built to accommodate the enormous amount of population at Dhaka, the capital city of Bangladesh. A moderate earthquake will lead to major structure collapse and the possible liquefaction would exacerbate the destruction and loss of lives at Dhaka city, Bangladesh.