Question

2) Point A is located at the middepth of a 3.6 m-thick clay layer. The clay is overlain by a 7.5 m-thick sand layer. The groundwater table is located 3 m above the sand layer. The saturated unit weights of sand and clay are 20 kN/m and 19 kN/m, respectively. Assume the saturated unit weight of sand to be equal to its moist unit weight. a) What is the total, porewater and effective stresses at Point A? b) Due to a drought, the water table was lowered to top of the clay layer. Calculate the new total, porewater and effective stress values at Point A.

Answer 1

Hand Calculations are given in following images.

GE A zokn ml y sand 7.5m 3.6 m 19 km a) Total, porevater & effective stress at point A When water table at 3m above sand layer = Total stress, u= pore water pressure ö = Effective stress. V = Submerged unit weight = 79.51 KN3 ..) OA-A = (x45m) + x3)+P'x1.8. = (20x4.5)+ (20-9.44)x3)+(19-9.44) X1.8 A-4 = 137.112 kN/m2 - UA A = IW x H = 9.81 (3+1.8) = 47.08 kN/m2 91 T = 50- A U A = 137 112 - 47.08 I JA-A = 90.03 kN/m2 b) when water table dropdowns to top of clay layer JA-A = (8x7.5) = 20x7.5= 150 kN/m² = 180 + cx'x1.8) 150+(19-9.81) xi-8 - GA-A = 166.542 kN/m2

9 81 - UA-A - Dw x H = 9.81x1.8 = 17.65 kN/m2 TA-AE TA-A-UA-A SAA = 166.54 - 17.65 = 148.89 kN/m2 JA-A when water table 3 m above sand - 90.03 kun JA-A when water table on top of clay = 148.89 kn

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