Pile Foundation Seismic Design Approach for the Padma Bridge, Bangladesh

Padma River is 6 km wide at the location of the proposed bridge.The estimated riverbed scour is 62 m deep with a return period of 100 years. Due to the proximity of Bangladesh to the tectonic plate contact zones of the Himalayas, the site is prone to seismic events. The deep scour and seismicity made the foundation design very challenging. A detailed, state-of-the-art ground investigation was carried out down to a depth of about 160 m in the predominant micaceous sands encountered to establish the static and dynamic soil parameters. Undisturbed samples of sand were recovered using the GEL PUSH sampling technique. State-of-the-art insitu tests, the self-boring insitu friction tests (SBIFTs), cross-hole seismic shear wave and compression wavetomography, self-boring pressuremeter tests, and flat plate dilatometer tests were carried out. Cyclic triaxial loading tests, bender element tests, k₀-consolidated stress path compression and extension tests were carried out. A structural numerical model developed using MIDAS software comprising six 150-m-long spans was developed including the piles at each pier. Using the dynamic design soil parameters, the ground response was determined using SHAKE, with strong ground motions corresponding to a 475-year return period earthquake event. They were then input into the MIDAS dynamic soil-structure interaction model with the Penzien Method to design the piles and the superstructure, taking due note of the deep design scour depths. The analyses demonstrate that six 3-m diameter steel raking piles driven down to about 120 m are needed to support each bridge pier with the 150-m span truss girders placed on seismic isolation bearings.