Calculate the total stress, pore water pressure, and effective stress versus dep

Question

Calculate the total stress, pore water pressure, and effective stress versus depth for the following soil profile. Note that when a piezometer (pressure meter) was inserted into the ground, the water level was measured a distance of 10 ft above the ground surface. The saturated unit weights of the sand and clay layer are equal to 132.4 pcf, and the moist unit weight of the clay above the ground water table is 122.4 pcf. Determine the depth D to which an excavation can be made that will cause the sand layer to boil. Ignore the shear strength of the clay layer in your calculations. Assume that once the soil is excavated the water will be kept at the same level in the excavated area Lo' 40 Sand

Explanation / Answer

Effective Stress(ES) at Sand layer interface = Total Stress(TS) - Pore Water Presure(PWP)

At z = 0m, TS = 0, PWP=0, ES = 0

At z = 10' , TS = 122.4* 10 = 1224 lbs/ft2, PWP = 0 , So ES = 1224 lbs/ft2

At z = 40', TS = 1224 + 132.4*30 = 5196 lbs/ft2, PWP = (40+10)* 62.4 = 3120 lbs/ft2, so, ES = 5196-3120 = 2076 lbs/ft2.

Now the depth D must be such that the effective stress becomes zero at the sand layer, so

2076 = 1224 + (D-10)*(132.4 - 62.4) => D = 22.171 ft

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