Helwan University Fac. of Eng., Mataria
2nd Year Civil year 2012
Soil Mechanics Sheet (4), Water in soil
1- For the shown ground section , calculate and draw the “Total Stress”, “Effective Stress” and “Pore Water Pressure”
γ b = 1.7 t/m3 γsat = 1.9 t/m3
diagrams down to level (-7.00).
γsat = 1.8 t/m3
2 - A 12 m thick bed of sand overlaying a layer of clay 5 m thick. The ground water table is located 2 meter below ground surface. The sand above water table has a degree of saturation of 30%. The void ratio and the specific gravity of the sand are 1.1 and 2.72, respectively. The saturated density of the clay is 1.95 t/m3. The water table is lowered to a depth of 3.5 m below ground surface. If the 1.5 m of sand above the new water table position remain saturated, due to capillarity, determine the change in effective stress at the middle of the clay layer. 3 - A clay layer 10.5 m thick underlain by a bed of sand. The ground water table is 4.0 m below the ground surface. The specific gravity and void ratio of the clay are 2.72 and 0.85, respectively. If it is required to excavate a foundation trench in the clay, what is the depth to which the excavation can be safely carried out without the danger of the bottom becoming unstable under uplift pressure of the ground water. If the excavation is to be carried safely to a depth of 8.50 m, how far should the water table be lowered in the vicinity of the excavation. 4 - The soil at a site consists of silt whose water content is 40.5% and specific gravity 2.68. The ground water table is at ground surface. Find the depth of foundation D for a building weighing 5000t, such that the net increase of effective stress on the soil at foundation level does not exceed 0.8 kg/cm2. The foundation is a raft 20m long and 16m wide. 5 - The bed of a lake consists of clay whose water content is 50% and specific gravity 2.71. The water depth in the lake is 3m. Calculate the effective stress at a point 5 m below the level of the lake bed.