8.5.4 Estimation of the number of wells
There have to be enough pumping wells at an excavation site to lower the groundwater level to a depth below the excavation surface.Generally speaking，the groundwater level has to be at least 0.5⁃1 m below the excavation surface.The design of deep wells is elucidated as follows：
（1）Compute the total quantity of water to be pumped in the excavation area
Assume the excavation site is an imaginary well，with a radius of Rw（see Figure 8.18）and compute the required total quantity（Qtot）of water to be pumped in the imaginary well using either equilibrium or nonequilibrium equations.The computation is discussed in Sections 8.3.1 and 8.3.2.
The radius of the imaginary well（Rw）can be computed by means of the equivalent area or perimeter，for example，
or
Figure 8.18 Radius of an imaginary well
（2）Compute the pumping capacity of each well
Determine the radius of each deep well and assume its water level to be hw，which should be a little lower than the water level，usually 0.5⁃1 m below the excavation surface，at the excavation center.Therefore，the capacity of each well（Qw）can be estimated following the method introduced in Section 8.4.3.
（3）Compute the number of deep wells
The number is
（4）Examine the assumed water level in each well
The total quantity of water and the number of wells determined，the assumed water level（hw）has to be examined.Under equilibrium and in a free aquifer，the water level in a well can be obtained following the methods introduced in Section 8.3.3.If the derived water level differs from the assumed hw，repeat step 2 to 4 till the computed and assumed values match each other.
（5）Examine the drawdown at the excavation center
Figure 8.19 An excavation site with pumping wells
Following the same method in step 4，compute the drawdown at the excavation center and comers and check if the computed values are smaller than the designed values.
As shown in Figure 8.19，there are four full penetration wells in an excavation site.To obtain the drawdown at the excavation center（point A）with the four wells pumping simultaneously，use Eq.8.1 or 8.4 to compute the drawdown of each well at point A and add up the values to find the total drawdown at point A if pumping time is not long and the drawdown curve is under nonequilibrium condition.