Ground water flow into excavations constructed by sheet pile walls should be minimized in order to save the cost of the provision of pumping systems or well points to lower the water table inside the excavation. In case a layer of impermeable material like clay is located slightly below the excavation, it may be desirable to drive the sheet piles further into this layer and the cost of further driving may be less than the cost of the provision of continuous pumping in the excavation. On the other hand, if there is no impermeable layer beneath the excavation, engineers may consider driving the sheet piles further so as to increase the flow path of groundwater into the excavation and this helps to reduce the amount of water flow into the excavation. Similarly, a cost benefit analysis has to be carried out to compare the extra cost of driving further the sheet piles with the reduced pumping costs.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.