1. The Office of Road and Rail has approved a plan from Network Rail to construct a new railway line motorway in the South East of England. The excavation of cuttings will be required at two sites on the proposed route; one site comprises soft clay with an overconsolidation ratio (OCR) of 1 and the other comprises very stiff intact clay with an OCR of 5. Excavation at both sites will be carried out rapidly and each cutting will have a design life of 100 years. (a) With the use of sketches explain the variation in total stress, pore water pressure, effective stress, shear strength, disturbing shear stress, and global factor of safety for the two sites during rapid excavation and subsequent design life. [12] (b) For the design of each cutting, which condition is going to be critical: short term or long term? Give reasons for your answer. [3] (c) The site with very stiff intact clay has a very long uniform slope inclined at (19+X)º. Steady seepage downhill occurs parallel to the slope with the phreatic surface at the ground surface. Assuming the slope can be approximated as infinite, estimate the depth of potential slip surface if the clay has a saturated density of (1800+Y) kg/m3 and the shear strength parameters given in Table Q1.1. Table Q1.1 Peak shear strength parameters Cp' = (4+Z) kPa pp = (23+X)º Residual shear strength parameters cy' = (2+Z) kPa ør = (12+X)º The variables X, Y and Z are functions of your URN, as defined on the cover page. [11] (d) Using the depth of potential slip surface calculated in (c), show that lowering the water table will improve the global factor of safety. Suggest at least three other measures which can be taken to improve the stability of the slope, using simple sketches to illustrate your answer. [7]

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