Wellpoint System

Wellpoint System

A wellpoint system consists essentially of a series of closely spaced small diameter water abstraction points connected, via a manifold, to the suction side of a suitable pump. The wellpoint technique is the pumping system most often used for modest depth excavations, especially for trenching excavations.

When creating a wellpoint system there are several factors that will need to be know before assembling a system.

Do you have a soil boring test from the excavation site? What soil type are you excavating, sand, gravel, clay, gray slit, unclean fill, etc.
Do you have a water source for jetting the wellpoints?
What is the rate or gallons per minute of water flow entering the excavation?
What is the distance of the water level below soil surface?
Where is the water coming from? Below, one side, two sides, three sides.
What is the length of the excavation?
What is the depth of the deepest part of the excavation?
What is the width of the excavation?
How far away will you pump the water from the excavation site?

In appropriate ground conditions, a wellpoint system can be installed speedily and made operational rapidly.

The level of expertise needed to install and operate a wellpoint system is not greatly sophisticated and can be readily acquired.

However, as with any ground engineering process, having experienced personnel to plan and supervise the works can be crucial in identifying and dealing with any change in expected ground conditions.

A wellpoint is a small diameter water abstraction point (the well screen), sometimes referred to as a ‘strainer,’

(So called because of the wire mesh or other strainers of the self-jetting wellpoint) through which the groundwater passes to enter the wellpoint.

They are installed into the ground at close centers to form a line alongside, or a ring around, an excavation.

The perforated wellpoint is typically about 0.7- 1.0 meter in length and of 40-50 mm nominal diameter.

Each is secured to the bottom end of an unperforated pipe (the riser pipe) of slightly smaller diameter; 38 mm diameter pipe is commonly used.

However, where the ‘wetted’ depth is limited due to the proximity of an impermeable surface,it is preferable that the length of the wellpoint should be shorter (usually 0.3-0.5 m) to restrict the risk of air intake at maximum draw-down.

Often it will be necessary to install wellpoints at closer centers to compensate for the lesser screened length of each short wellpoint.

Each wellpoint is connected to a header main (typically of 6″ diameter) that is placed under vacuum by a wellpoint pump.

The header main is normally made of high impact plastic, although steel pipe is sometimes used, especially when there is a risk of damage from construction activities.

The pipe is typically supplied in 6 meters lengths and is joined on site by simple couplings.