1. The strata at a flat site in an alluvial plain consist of 5m of gravel overlying 8m of soft alluvial clay (normally c

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1 The Strata At A Flat Site In An Alluvial Plain Consist Of 5m Of Gravel Overlying 8m Of Soft Alluvial Clay Normally C 1 (254.42 KiB) Viewed 15 times

1. The strata at a flat site in an alluvial plain consist of 5m of gravel overlying 8m of soft alluvial clay (normally consolidated). Beneath the clay is a sandstone rock mass of relatively high permeability. Assume the bulk unit weights to be: gravel above the water table 16kN/mº, gravel below the water table 18kN/m², clay 20kN/m2 and water 10kN/m? The ratio of horizontal effective stress to vertical effective stress is 0.3 (gravel) and 0.6 (clay). Calculate the values of water pressures and vertical and horizontal stresses, both total and effective stresses, at the following depths: 2m, 5m (in the gravel), 5m (in the clay) and 13m (in the clay), under the following conditions; and plot the water pressures and vertical stresses on graphs: (a) Assuming hydrostatic conditions with the water table at 2m below ground level. (4 marks) (b) Assuming the water table remains at 2m below ground level in the gravel but the piezometric head in the sandstone suddenly increases to ground level and remains there. Calculate the values of water pressures and vertical and horizontal stresses, both total and effective stresses, in the short term after the increase in piezometric head in the sandstone (6 marks) and in the long-term steady-state condition when water pressures in the clay have reached equilibrium. (6 marks) (C) If the coefficient of volume change of the clay is 0.0002 m²/kN estimate the magnitude of the increase in thickness of the clay layer in the long term due to the increase in piezometric head in the sandstone to ground level. (1 mark) (d) Describe with a drawing how effective vertical stress and the voids ratio change in the clay in response to the increase in piezometric head in the sandstone to ground level. (1 mark) (e) Explain what you would look for in the clay in your ground investigation to help predict the rate of time-dependent settlement after placing a fill embankment at the site, and the type of sampling to use. (2 marks)