REMOVAL OF WATER BELOW LINERS

When installing a liner into a borehole by eversion, the fluid below the liner is trapped by the descent of the sealing liner. In many situations the fluid (water or air) is forced into the formation. If the formation is very tight (i.e., has a very low conductivity) the liner descent is stopped by the fluid trapped in the hole. In that case, the water must be pumped from the hole below the liner. ( If air is trapped, it is usually vented via a tube or chain that is lowered into the hole down to the water table. If the hole is uncased, air below the liner can usually flow into the formation). In some situations, it is preferred to pump the water from the hole rather than force it into the formation.

The removal of water from beneath the liner can be done in several ways depending upon the size of the hole and the depth of the water table. For relatively large holes (>4" diam.) and a shallow water table (<20 ft below the surface), a "pump tube" is lowered into the hole to the bottom. As the liner is installed, a pump at the surface can draw the water from the hole. Once the liner is in place, the water level in the liner is pumped down until the pump tube can be slid from between the liner and the hole wall. The liner is then refilled to develop a good sealing interior pressure.

For deep holes (> 200ft.), the drag between the liner and the hole wall may be too great to slide the pump tube out of the hole. In that case, a "pump tube liner" is used to remove the water. The difference is that the pump tube is surrounded by a slender liner. A water fill of the slender liner allows the liner to be inverted from the hole by pulling upward on the tube. The inflated liner greatly reduces any friction on the tube.

For deep water tables, it is not possible to pump the water from the hole with a surface pump. In some cases, the presence of the pump tube in the hole allows water to flow between the liner and hole wall to exit the hole via any available fractures in the hole and not just those remaining below the descending liner. There are two pumping systems used for the deep water table. The first uses a pump tube liner with a slender pump at the bottom. The pump tube liner diameter is about 3" to allow a 2" pump to be inverted up through the interior of the pump tube liner after the hole is evacuated and the main liner has fully descended. The minimum hole diameter for a pump tube liner with pump is about 6 inches.

A second technique that has been used for the deep water table is to install a trimmie pipe pump down through a slender liner. This might be called a "trimmie pump tube liner". The trimmie contains a check valve at the bottom and a central tube that allows the trimmie to pump the water to the surface using an applied gas pressure in the trimmie. The advantage of the trimmie pump system is that it can be very slender (<1").

If the formation is very tight, a means must also be supplied to add water to the bottom of the hole as the liner is inverted out of the hole. Otherwise, the inverting liner will develop a full vacuum in the hole below the liner. For a deep hole, this produces an excessive pressure differential in the liner. One solution is to evert the pump tube liner down the hole between the liner and the hole wall to develop a flow path for water to be added below the ascending main liner. This is the patented "Duet" technique. A more simple version of that is to simply evert a slender liner alone (no interior tube). In that case, the dilated slender second liner prevents the liner seal of the hole and water can flow to the bottom of the hole from anywhere along the entire hole.

For a very tight formation, the water must be removed and added as the liner is installed and retrieved.

When installing a blank liner to temporarily seal a hole, it is not necessary to drive the liner to the bottom of the hole, if the bottom part of the hole is of very low conductivity. A timed blank installation is provided for that situation. In summary, the blank liner is emplaced until it has sealed the significant flow paths from the hole. This avoids the pumping procedure and also allows the blank liner to be more easily removed.

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