GROUNDWATER LOWERING BY WELLPOINTS
Sometimes it is necessary to temporarily lower the groundwater level for construction of foundations. The process of lowering the groundwater is called dewatering and is accomplished by inserting wellpoints around the excavation for the foundations. A wellpoint system consists of an interconnected network of wells (pipes) installed around the perimeter of an excavation (Figure ). The wells are installed in rows and the spacing depends on the soil type and the hydraulic conductivity. The spacing in clean sands with water depth of about 5 m is about 1 m to 1.5 m. Reconsidering Figure , the drawdown, d, is
The radius of infl uence, R, of the depression cone is the radius at which the drawdown is zero. The fl ow rate or discharge between the limits r and R and h and H can be found from Equation as
Solving for h, we get
Therefore, at any point with coordinates (r, z), the drawdown is
The positive sign is used when water is pumped into the well and the negative sign is used when water is pumped from the well. The maximum drawdown, dmax, occurs at the well face, i.e., r 5 ro, and from Equation
The radius of infl uence of the depression cone is found from experience and can be estimated (Slichter, 1899) from
Equation does not have a theoretical basis and is not dimensionally correct. However, it has been satisfactorily applied in practice. An equation (Kozeny, 1933) that is dimensionally correct is
where t (sec) is duration for a discharge, qw(m3/s), n is porosity, and k is the average hydraulic conductivity (m/s). The predictions of R from Equations (6.30) and are normally signifi cantly different.
However, the discharge is not very sensitive to the accuracy of R because the changes in ln (Rr) are small for large changes in (Rr). The accuracy of R has signifi cant impact for drawdown near existing buildings. For closely spaced wellpoints, a two-dimensional fl ow analysis is required.