Padma Multipurpose Bridge Project

Stratum Criteria DescriptionUnit 1a Clay + Silt ≥ 50% Clay or Silt with fine sandUnit 1b 20% ≤ Clay + Silt < 50% Very silty fine SANDUnit 2 Clay + Silt < 20% and Medium Sand < 10% Silty fine SANDUnit 3 Clay + Silt < 20% and Medium Sand ≥ 10% Slightly silty fine and medium SAND30. General Startigraphy of the Project area is:In the River along the bridge site is: Unit 1b from surface to -10m; Unit 2 from -10 mto -15m; Unit 1b from -15 m to -120 m. However, Unit 3 and Unit 1a also occurredlocally in the second strataOn the left bank, Unit 1a was found on the upper part up to depth of 10 to 15m. In thelower part of Unit 1a, Unit 1b was found.On the right bank very close to River, Unit 1b was found from the ground surfacethrough 120 m depth. In another borehole on the right bank located near servicearea, Unit 1a was found in the upper part up to a depth of 7 m followed by Unit 2. On the Charland, Unit 1a is found on the upper part up to a depth of 2 to 3m.31. Seismic zoning map of the Project area (Source: Bangladesh National Building Code, 1993)is presented in Figure 3-14. This map is prepared based on distribution of earthquake epicentres andmorphotectonic behaviour of different tectonic blocks of Bangladesh. Zone 1 represents the mostactive zone, Zone 2 represents intermediate seismic activity zone, and Zone 3 represents seismicallyrelatively quiet zone. Location of the River Padma in the Project area is located on the boundarybetween Zone 1 and Zone 2 with left bank is located in Zone 2 and right bank on Zone 1. On modifiedmercalli scale the Project area is located in an earth quake zone of Scale VI. There is, however, noevidence of major earthquakes in the Project area.32. Geomorphologically, the Project area is changing due to the erosion and accretioncharacteristics of the River Padma (See Figure 3-7). Many Charland (sand bars) are eroded andformed due to this process. Physical characteristics of the Padma and its distributaries are changingdue to oscillation of the Riverbanks, levee breaching and formation of mid-channel bars. River depthsbecame shallower at places. Many of the off takes were closed due to depositions of huge sedimentsat their mouths. Particularly the formation of large bar near Arial Khan River (distributaries of Padmalocated at 15 km upstream side on the right bank), the main flow of this reach is diverted along theright bank causing more erosion on the right bank. Hence, the width of the River at the Project site ischanging due to major bank erosion taking place along its right bank. Downstream of the Project site,between Mawa and Lauhajang, the Padma is very narrow. Here, the absence of any major bar closeto the bank allows relatively free flow of the water along the channel and does not cause any flowdiversion or concentration towards the bank. Between the downstream of Lauhajang and Munshiganjthe River is wide and has many in-channel bars. This part has the distinction of including most parts ofthe confluence of the Padma and Meghna Rivers. Some of the largest stable bars are located here.3-16