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5, A soil element inside a 20° natural soil slope carries total vertical stress (%) of 140 kPa, total horizontal stress (on) of 80 kPa and shear stress (Tvh) of 15 kPa Canticlockwise shear stress on vertical plane). The pore-water pressure (u) inside the element is 30 kPa. The critical state angle of friction of the soil is 30o and its undrained shear strength is 40 kPa. (a) Draw the Mohrs stress circles for this element in terms of total and effective stresses. Is the soil element at failure? Give reason(s) in support of your answer. [6 marks] It is anticipated that the pore-water pressure in the element may increase due to change in the groundwater flow condition. Compute the increase in pore-water pressure needed to cause the failure of the element, assuming that the total normal stresses and shear stresses in the element remain unchanged. [4 marks] Draw the Mohrs stress circle in terms of effective stresses at failure. Estimate the inclination of the failure plane with respect to horizontal and the magnitude of maximum shear stress in the element at failure. [10 marks] (b) (c)

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