Prior to beginning drilling an extensive pre-grout operation is generally undertaken to prevent environmental problems during construction and limit groundwater flows between the shaft and the formation. By drilling small diameter holes around the perimeter of the shaft and pressure grouting the formation it is possible to almost completely separate the surrounding water tables from the shaft construction.

Once the pre-grout operation is complete, surface casing is set to competent rock by excavating a large enough hole through the overburden to accept the casing and the drilling tools. This surface casing can be either steel casing grouted in place or cast-in-place concrete depending on the size of the shaft. A concrete pad is then generally poured to accommodate the drilling equipment and future equipment such as fans.

Lining of these shafts is generally accomplished using either a hydrostatic steel liner or a concrete liner. For shafts up to 2.4 m (8 feet) in diameter, hydrostatic steel liners are easily fabricated, hauled to the site and field welded in 40’ joints. Once the liner is in place a grout seal is pumped behind the liner to hold it in place.

3.05 m (10 foot) shafts can be either steel or concrete lined. The depth of the shaft generally determines the economic factors which govern the liner type. Shafts greater than 3.05 m (10 feet) in diameter are almost always concrete lined due to the enormous cost of a steel liner of that size.