In thick-walled cylinder, the radial displacement of the surfaces makes use of the equation = Explain this equation and show how the equation is derived (2 marks) A thick-walled concrete drainpipe has an immer diameter of 760 mm and a wall thickness of 100 mm. It is designed to withstand an external soil pressure of 70 kPa and a maximum internal pressure of 200 kPa. The design utilises a safety factor of 4 against Rankine failure criterion Determine whether the pipe is safe (using the same safety factor) if the pipe is suddenly dry from drawing off water (i.e. no internal pressure). Assume that the material exhibits the same yield strength in tension and compression. (8 marks)