These consist of an array of bored wells and submersible pumps. The interaction between the cone of drawdown created by each well results in groundwater lowering over the desired area.
Deep well systems consist of an array of bored wells and submersible pumps. The wells act in concert - the interaction between the cone of drawdown created by each well results in groundwater lowering over a wide area. Because the technique does not operate on a suction principle, greater drawdowns can be achieved than with a single stage wellpoint system.
Each well consists of a bored hole (typically formed by a drilling rig) into which a well liner is inserted. The liner consists of plastic or steel pipe of which a section is slotted or perforated to form a wellscreen to allow water to enter; other sections consist of unperforated pipe (the well casing). Generally, deep well systems are installed in drift deposits and the annulus between the borehole and the wellscreen/casing is backfilled with filter media or formation stabilizer to form what is known as the filter pack.
The wells are generally sited just outside the area of proposed excavation (although for very large excavations wells may be required within the main excavation area as well as around the perimeter). A deep well system has individual pumps positioned near the base of each well; usually the pumps are electro-submersibles. The wellscreen and casing provides a vertical hole into which a submersible pump attached to its riser pipes can be installed (and also recovered, as and when required).
Once installed and pumped each well creates a cone of depression or drawdown around itself, which in a high permeability aquifer may extend for several hundred metres. The interaction between the cone of drawdown from each well produces the drawdown required for excavation over a wide area. Apart from pumping tests, deep wells are rarely used in isolation; the method relies on the interaction of drawdowns between multiple wells.
Generally, for most temporary works requirements:
The wellscreens, pumps and other materials are similar to those used for water supply wells. However, since the working life of a temporary works well will almost always be significantly less than the life of a water supply well, temporary works wells can be constructed by rather cheaper and simpler methods.
The initial cost of installing a deep well system is significant. A high standard of expertise in the design and control of installation procedures is required to ensure that the appropriate good practices are implemented throughout and to promote optimum and economic performance.
For more information on deep wells, please contact Stuart Well Services.