Changes

* Open-ended (hollow) bits without a footvalve (pure sludging) for layers of pure clay or gravel. In these conditions the presence of a footvalve may slow down progress, since clay has to be pounded into suspension and stones have to be ground to small pieces in order to enter the drilling tool through the footvalve.

If required, the upper part of the well can be reamed and cased with larger diameter pipe (3 or 5 inches), to accommodate larger pumps. A shallow (large diameter) [[rope pump]], for example, may require a wider well (although small-diameter rope pumps can fit in a 2" borehole), and submersible pumps commonly need at least 4".