8.5.5 Computation of the influence range of drawdo...
In Section 8.3，the influence range of drawdown is defined as the distances from which the drawdown is none.The influence range is thus obtained but is too large to be meaningful in engineering practice.Therefore，the reasonable influence range should be computed on the basis of s＝Sa（Sa is the allowable drawdown）.For example，if the objective of computing the influence range of drawdown is to estimate the influence on the supply of water or the amount of settlement，it is feasible to assume a value of Sa（e.g.Sa＝0.5 m）and compute the influence range.Note that drawdown does not necessarily cause settlement and the influence range of drawdown does not equal the influence range of settlement，either.
Figure 8.20（a）shows a diaphragm wall penetrating through the impermeable layer.Since pumping is confined to the excavation area，it will not have influence outside the excavation zone and the influence range of drawdown is minimal accordingly.Figure 8.20（b）shows the pumping is carried out outside of an excavation zone and the influence range of drawdown extends by a distance outside the excavation zone as a result.Figure 8.20（c）shows the pumping is executed under an impermeable layer.While water is pumped in a confined aquifer，the decrease of piezometric level in the confined aquifer may cause settlement，though the groundwater level above the impermeable layer does not necessarily come down.Figure 8.20（d）is a diaphragm wall located in a permeable layer and the well depth is less than the depth of the wall.As a result，pumping will not cause drawdown of groundwater level outside the excavation zone.Figure 8.20（e）is a case in which the well depth is larger than the depth of the wall and the influence range of drawdown may extend outside the excavation zone by a distance.
Figure 8.20 Influence of dewatering in excavations