Armfield S12-MKII, Instruction Manual

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56
Exercise I - Water abstraction from a number of
neighbouring wells
Theory
From Exercise H,
(1)
The method of superposition allows the prediction of a complex situation by
considering it to be made up of a number of simple elements and superimposing their
resulting individual effects as described by the above equation. In the case of
neighbouring wells in the same aquifer, the linear relationships essential to the use of
this method are to be found in confined aquifer flow because the saturated depth of
the aquifer remains unchanged and the coefficient of transmissibility is constant.
The flow of ground water in an unconfined aquifer is always accompanied by a
change of the saturated thickness zone of flow. For ground water abstraction with
wells, the coefficient of transmissibility thus also depends on any previously existing
ground water movement, for example, that due to a neighbouring well. This means
that the method of superposition can be used only in its simple linear form if the
separate drawdowns are small compared with the saturated thickness of the aquifer.
Procedure
This experiment is carried out using an unconfined aquifer with water inflow at both
ends of the tank. The set up is the same as for Exercise H, but this time both
cylindrical wells are used. The method of superposition applies only to small
drawdown values, it is suggested that small Qo values should be used.
Draw the required flows from each well in turn, measuring the drawdown produced in
each case with the manometers. Now establish the combined well flow (both wells at
the same time) and measure the resulting water table drawdowns. It should be
possible, according to the principle of superposition, to synthesise this combined
water table pattern by adding the values obtained with each well flow independently.
For this experiment the drawdown close to the well should not exceed 25% of the
saturated thickness of the aquifer before drawdown.
It is also of interest to explore the drawdown due to much larger abstractions from
these wells. Although the superposition principle will not apply, it is possible to
determine the effect of a nearby abstraction on the cone of depression of another
well and to relate the size of these interactions to the relative flows discharged by the
wells.
Dewatering an excavation site
A deep excavation for the purposes of foundation construction or other below-ground
activity will frequently penetrate below the natural rest level for the water table in that
area. If the excavation is in permeable ground this will constitute an aquifer and the
excavation will fill with water to the local water table level due to ground water flow.
One method of keeping such an excavation dry is to sink a ring of wells around the
outside of the excavation site and to lower the water table locally by pumping the well
system.