Question 2 (22 marks) As indicated on Figure Q2b, it is proposed to excavate a 1

Question

Question 2 (22 marks) As indicated on Figure Q2b, it is proposed to excavate a 10m deep cutting in a 12m thick deposit of sand (with c'-0 & 40°) overlying clay with an OCR 3. The results from the shearing stage of a consolidated undrained (CU) test performed on a sample of the clay are shown on Figure Q2a. Estimate the undrained shear strength and friction angle for this clay sample, assuming c-0. 250 200 150 Figure Q2a S 100 CU triaxial test 50 n pressure-150 kPa Pore pressure at failure -60kPa 10 15 Axial strain (%) If the side slope in the sand is 30° and the water table level is 2m above the surface of the clay, determine the short and long term factors of safety against a failure involving slippage along the upper surface of the clay deposit. Assume that the unit weight of both the sand and clay is 20 kN/m3' and use the results from the CU triaxial test to infer appropriate parameters for the clay. State any assumptions made. 10m 12m Sand 30° Water table Sand Clay Figure Q2b

Explanation / Answer

Undrained shear strength = 220/2 = 110 KPA approximately.

To get friction angle, plot effective stress Mohr circle.

sigma 3 = 150 + 60 = 210 KPA and sigma 1 = 220 + 150 + 60 = 430 KPA.

Plot Mohr circle passng through these 2 points..and then draw tangent line passing through origin. Angle made by tangent with horizontal is phy'.

There can be many slip planes along the upper surface of clay deposit. Please be specific or otherwise solution can be obtained through software only.

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