If present, a water level at a site can be monitored using a dipwell (standpipe). This is constructed from small diameter rigid open plastic tube, with a lower section of slotted pipe.
The dipwell is installed into a vertical borehole, created either by hand auger or mechanical corer. At the York project site, a lightweight 'vibro-hammer' corer unit was used, handled by two trained operators and powered by a diesel generator. The corer can be operated in areas with limited access, and is a relatively quick method for borehole construction. The system uses a percussion hammer to drive a 2m long 'window sampler' into the ground, into which the soil material passes. On reaching the required depth, the sampler is jacked out and the soil core it contains then described. In suitable conditions, a virtually continuous core is made available from which disturbed samples can be extracted for subsequent laboratory analysis.
Where retrieval of uncontaminated and continuous cores are important, the sampler can be replaced with a 'window-less sampler' that contains a plastic sleeve. The sampler is driven in and then jacked out as before, except that the core contained within the plastic sleeve can be removed intact for later study. Though this system has the disadvantage that a core is not immediately accessible for descriptive purposes, it is more representative of the undisturbed profile.
The borehole constructed is unlined, and therefore dipwell installation should be undertaken immediately to reduce the likelihood of borehole collapse. Depths of up to 10m can be achieved, especially where fine-grained fills and alluvium have sufficient cohesion to keep the holes open.
Once installed, the void around the dipwell is backfilled with excavated spoil, and replaced in the same order as it was excavated. At ground level a bentonite clay seal is created around the dipwell in order to limit surface water draining down the sides of the monitoring point. Finally, the clearly labelled dipwell is sealed with a removable plastic cap.
A water level is recorded by lowering the tip of a portable dip-meter down the dipwell; a light and audible signal indicates water contact. The level is measured off the graduated cable on the meter. The dipwell also provides a sampling point for the recovery of water samples that can then be analysed for water quality. Water samples are recovered using a bailer technique.
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Last updated: Thur Feb 28 2002