DEWATERING TECHNIQUES

Dewatering involves controlling groundwater by pumping, to locally lower groundwater levels in the vicinity of the excavation.

The simplest form of dewatering is sump pumping, where groundwater is allowed to enter the excavation where it is then collected in a sump and pumped away by robust solids handling pumps. Sump pumping can be effective in many circumstances, but seepage into the excavation can create the risk of instability and other construction problems.

To prevent significant groundwater seepage into the excavation and to ensure stability of excavation side slopes and base it may be necessary to lower groundwater levels in advance of excavation. This is known as ‘pre-drainage’.

Pre-drainage methods include:

• Deep wells
• Wellpoints
• Eductors
• Vacuum wells
• Horizontal wells

Other specialist dewatering techniques are also sometimes used:

• Relief wells
• Artificial recharge
• Siphon drains

The selection of the dewatering technique or techniques at a particular site or country will depend on many factors. Groundwater Engineering’s team has decades of experience in dewatering projects around the world, and we provide a complete design and installation service to control your groundwater problems.

DEEP WELLS:-

A deep well system consists of an array of bored wells pumped by submersible pumps. Pumping from each well lowers the groundwater level and creates a cone of depression or drawdown around itself. Several wells acting in combination can lower groundwater level over a wide area beneath an excavation. Because the technique does not operate on a suction principle, large drawdowns can be achieved, limited only by the depth of the wells, and the hydrogeological conditions.

The wells are generally sited just outside the area of proposed excavation, and are pumped by electric submersible pumps near the base of each well. Water collection pipes, power supply generators, electrical controls and monitoring systems are located at the surface.

Groundwater Engineering provides complete deep well dewatering solutions:

• Design of dewatering systems
• Well drilling and installation
• Pumping tests
• Equipment sales and rental
• Monitoring systems
• On-site operation and maintenance

WELLPOINTS:-

wellpoint dewatering is widely used for excavations of shallow depths, especially for pipeline trench excavations. In appropriate ground conditions a wellpoint system can be installed speedily and made operational rapidly. A typical wellpoint system consists of a series of small diameters wells (known as wellpoints) connected via a header pipe, to the suction side of a suitable wellpoint pump. The pump creates a vacuum in the header pipe, drawing water up out of the ground. For long pipeline trenches, horizontal wellpoints may be installed by special trenching machines.

Wellpoints are typically installed in lines or rings around the excavation, and are pumped by diesel or electrically powered pumps, with associated header mains, water discharge pipes, power supply generators, electrical controls and monitoring systems.

Groundwater Engineering provides complete wellpoint dewatering solutions:

• Design of dewatering systems
• Wellpoint installation
• Equipment sales and rental
• Monitoring systems
• On-site operation and maintenance

EDUCTORS:-

The eductor system (also known as the ejector system) is specialist technique used to control pore water pressure in low permeability soils such as very silty sands, silts, or clays with permeable fabric. Eductors are typically used to help stabilise the side slopes and base of excavations in soils that would be difficult to dewater with wellpoints or deep wells.

Eductors have the advantage that they can allow vacuum-assisted drainage, to draw water out of poorly draining fine-grained soils. The system works by circulating high pressure water (from a tank and supply pumps at ground level) down the well to a small-diameter nozzle and venturi located in the eductor in each well. This generates a vacuum of up to 9.5 m of water at the level of the eductor. The vacuum draws groundwater into the well from where it is piped back to ground level via a return riser pipe and thence through the reservoir tank back to the supply pump for recirculation.

Groundwater Engineering provides complete eductor dewatering solutions:

• Design of dewatering systems
• Well drilling and installation
• Equipment sales and rental
• Monitoring systems
• On-site operation and maintenance

VACUUM WELLS:-

Vacuum wells are an adaptation of deep well systems. Each well in the system is pumped by a submersible pumps, but a vacuum is also applied to each well via a vacuum pump located at the surface. The application of a vacuum allows the wells to be more effective in reducing pore water pressures in poorly draining fine grained soils. In appropriate ground conditions vacuum wells can be a viable alternative to eductors.

Groundwater Engineering provides complete vacuum well dewatering solutions:

• Design of dewatering systems
• Well drilling and installation
• Pumping tests
• Equipment sales and rental
• Monitoring systems
• On-site operation and maintenance

HORIZONTAL WELLS:-

Horizontal wells for dewatering are of two principal types:-

• Horizontal drains installed by specialist trenching machines
• Horizontally directionally drilled (HDD) wells.
• Horizontal drains installed by specialist trenching machines

This technique uses a horizontal flexible perforated pipe, pumped by a wellpoint pump, to lower groundwater levels. The perforated pipe is installed by a special trenching machine. One end of the pipe is unperforated and is brought to the surface and connected to a wellpoint suction pump. The method can be very effective for dewatering long pipeline excavations.
Horizontally directionally drilled (HDD) wells.

HDD wells are used where groundwater must be abstracted from beneath inaccessible areas or from areas where the disruption associated with surface drilling is undesirable. Applications for HDD wells include:-

• Installation of permanent dewatering systems beneath existing built up (urban) environments.
• Pumping for remediation of contaminated groundwater without the risks of cross-contamination associated with vertical drilling
• Dewatering for tunnel construction
• Recharge wells to re-inject water as part of artificial recharge schemes.

Groundwater Engineering provides complete horizontal well dewatering solutions:-

• Design of dewatering systems
• Well drilling and installation
• Pumping tests
• Equipment sales and rental
• Monitoring systems
• On-site operation and maintenance