M23357-Soils and Materials 3

The construction of pumping station very close to a river bank requires a long excavation, running parallel with the river, 25m in width and to a depth of 7m (Figure 1) To enable the foundations of the intake works to be constructed in the dry a steel sheet-pile cofferdam is to be constructed between the river and the excavation. The properties of the River bed soil are presented in Table 1.

(a) Using Geosolve Wallap and the strength factor design method, calculate the penetration depth (d) needed to achieve a factor of safety of 1.25 against overturning. Include a prop if you think it is necessary, and choose its position.

(b) Validate this design penetration depth (d) with hand calculations, again using the strength factor design approach, taking into account seepage forces and water pressures.

(c) The design penetration depth (d) will also impact the flow of water into the excavation under the sheet pile. Sketch a flow net for your chosen design penetration depth and use it to determine the quantity of flow into the excavation.

(d) Critically evaluate the design approach followed from part (a) to (c) considering any design requirements or construction aspects that have been overlooked, or assumptions that may not be reasonable.