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Question 3 An exploratory drill hole was made in a saturated stiff clay. It was observed that the sand layer underlying the clay was under artesian pressure Water in the drill hole rose to a height of H1 above the top of the sand layer. If an open excavation is to be made in the clay, how deep can the excavation proceed before the bottom heaves? we are given H-8 m, H1-4 m, and w: 32% Exploratory drill hole Saturated clay G, 2.70 Moisture content w H Sand

Answer 1

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Bottom of excavation is stable and hence the total stress acting upward and downward pressure is equal. Assume the safe depth of excavation as x therefore the thickness of clay over laying sand stratum is (8 - x) m calculate the total stress acting downward (b_T)_D = v_sat (8 - x) Calculate the upward pressure (pare water pressure) b_w = r_w h = 9.81 kN/m^3 times 4 b_w = 39.24 kN/m^2 Equate total pressure acting upward and downward r_sat (8 - x) = 39.24 (1) Calculate the saturated unit weight r_sat = (1 + w) Gs rw/1 + wGS = (1 + 0.32) 2.7 times 9.81/1 + (0.32) (2.74) = 34.96284/1.864 r_sat = 18.7568 kN/m^3 substitute in eq(1) 18.7568 (8 - x) = 39.24 8 - x = 2.092 x = 5.908 m The depth of excavation before the bottom heavers is 5.908 m