The purpose of a dewatering system is to control groundwater levels and to reduce pore water pressure in the ground.
OGI is able to analyse available geological data and the geometry of a construction site excavation to develop a conceptual model.
Using OGI’s experience and knowledge, combined with mathematical modelling techniques, OGI is able to establish the geological layers of significance and to design a suitable dewatering system to remove groundwater at the necessary flow rates to reduce pore water pressure down to the required levels so as to achieve the dewatering objectives of the project.
Following mathematical modelling, various practical design issues need to be considered to achieve an appropriate and efficient dewatering system as follows:
Well Installation Techniques
Dewatering wells and drainage can be installed by a variety of methods, the appropriate choice of which is influenced by parameters such as existing artesian pressure, required drawdown, geology, stability of ground conditions encountered, space available and location of sensitive structures and features such as potentially vulnerable buildings or rivers.
The well installation methods typically available include:
An excavator can be used to install sumps, large volume shallow dewatering wells or land drainage systems in shallow depth dewatering applications, typically 0.5 to 2m deep.
It is important that such installations have an appropriate filtration medium installed in order to avoid silty discharge water problems.
Jetting can be used in finer grained soils to install a large number of “well points” where the required drawdown in each well does not exceed 7m. Jetted well points are connected to a suction pumping system.
- Cable percussion (also known as shell and auger) drilling rig
Cable percussion can be used to drill through soils and small boulders for the installation of shallow to deep wells. A great advantage of this technique is that good quality site investigation data can be obtained.
- Rotary Drilling (air flush, water flush, mud flush)
Rotary Drilling can be used to drill through soils and through rock for the installation of shallow or deep wells. This technique is usually quicker than the cable percussion alternative but mobilisation costs are usually greater than cable percussion, the flush system can pressurise the ground and the quality of site investigation data is inferior unless expensive coring is undertaken (poor core results are produced in the weathered zone).
Appropriate selection, installation, commissioning and maintenance of the pumping system are critical to the success of a dewatering system.
The pumps need to have the appropriate water flow capacity and lift capability to remove water from the boreholes whilst having the ability to cope with low flow conditions that might occur as the ground dries out (at a later stage in a dewatering programme or during dry weather conditions).
Typical Pump types include:
- Borehole Pumps
For deep well applications drawdown in each well typically exceeds 7m, with flows typically up to 5 litres/second per well.
- Suction Pumps
For shallow wells (installed with well casing and appropriate filter medium) and traditional well points, drawdown in each well typically <7m.
- Eductor Pumps
For very low permeability, low flow, deep well applications.
- Sump pumps
For shallow surface drainage, often used where water clarity is not an issue. Sump pumps should not be used without adequate filtration as there is the potential for loss of fines and therefore ground loss. Sump pumps can be used to pump from filtered land drain systems.
Properly designed pipe runs allow the Construction Contractor to proceed with the construction works with the minimum of inconvenience, avoiding unnecessary time and special restrictions.
Tidy pipework maximises space within a congested excavation and increases safety.
Properly designed pipework allows individual pumps to be isolated, checked, maintained and replaced if necessary and for productive and unproductive wells to be identified.
Connection of discharge pipework to an inspection tank enables demonstration of water quality to regulatory authorities.
There are real benefits in designing a dewatering system to produce clean discharge water, not least of which is that relationships with the Environment Agency are improved, increasing the likelihood of obtaining consent to discharge to the natural environment.
OGI rigorously designs dewatering systems with abstraction filtration media to produce silt free discharge water, tailored specifically to the geology encountered.
A good filtration design should:
- Produce clear, environmentally acceptable silt free discharge water.
- Avoid the pumping of fines which can result in ground loss.
- Avoid the pumping of fines which can result in settlement.
- Avoid the clogging of wells which stifles well yield.
- Avoid the clogging of pipework reducing water yield.