The difficulty of constructing large diameter wells which adequately penetrate the aquifer and which combine minimal resistance to inflow through the caisson with structural stability has led to some modifications. Two of these incorporate well screens of the type found in small diameter wells into large diameter wells. In both systems an unperforated large diameter caisson is sunk a certain distance below the water table. In the first system (Figures 53 and 54) one or more well casings with well screen are sunk vertically into the aquifer below the large diameter caisson by one of the methods described for small diameter wells. Since it is usually feasible to sink the casing to any desired depth, the possibility for placing the well screen in highly permeable material is good. The combination of increased depth and permeability can result in a well with increased yield. Since inflow is through the vertical pipe(s) the bottom of the well can be stabilized with a layer of concrete.
Fig. 53 Large diameter well with vertical casing and screen
Fig. 54 Sinking a casing and well screen into the bottom of a large diameter well to increase inflow
The second system (Figure 55), sometimes called a collector well, uses well screens which extend out horizontally and radially through the wall of the caisson. These screens are similar to large drive points and are forced out into the aquifer through precast ports in the caisson by means of jacks.
Fig: 55 Large-diameter well with radial well points