Tests for permeability characterization

Pumping tests

Pumping tests in wells for evaluating the coefficient of soil permeability are recommended for medium to high permeability soil masses, e.g., for sandy soils. Figure 1.44 summarizes the conditions of these tests and the expressions for the assessment of the permeability coefficient (Dupuit, 1863).

The tests consist of pumping water from a well at a constant flow rate and observing the effect of this pumping on the water level drop in piezometers or wells at specified distances. Figure 1.45 shows a layout for the observation devices suggested by Mayne et al. (2001). As can be seen, the observation piezometers (or wells) are arranged radially with respect to the pumping well. According to those authors: i) the nearest piezometers should be at a well distance of about 7.5 m; (ii) the furthest piezometers should be within the predicted limit of

Pumping tests in wells and equations for the estimate of the permeability coefficient

Figure 1.44 Pumping tests in wells and equations for the estimate of the permeability coefficient: a) unconfined aquifer; b) confined aquifer.

Pumping tests in wells (Mayne et al., 2001)

Figure 1.45 Pumping tests in wells (Mayne et al., 2001): a) layout of the observation piezometers (or wells); b) time intervals between readings for observation of the water level.

the lowering effect; and (iii) the two intermediate piezometers shall be positioned so that the water level lowering can be approximately defined (to this end, the distance between adjacent piezometers should increase progressively with the distance to the well).

The pumped water must be discharged sufficiently far from the test site to prevent recharge of the aquifer under study during the test.

The determination of the stabilized water level position in the piezometers requires observations of the water position in each of them over time. The time intervals between readings are shown in the table alongside Figure 1.45. The cited authors recommend that the pumping test should continue for at least 4 h after achieving stabilization. They also recommend pumping at three increasing flow rates. Upon completion of pumping, the progressive recovery of the initial ground water level should be observed.

The advantages of pumping tests in wells are very relevant: these tests allow characterization of the permeability of large volumes of the ground, thus incorporating certain permeability variations between different layers or sublayers of the aquifer and are susceptible to theoretical interpretation. However, from the previous description, it is clear that these are expensive tests, due, in particular, to the requirement of installation of the observation piezometers (wells), which involves time-consuming operations. Readers interested in learning further about these tests should refer to the study by Jimenez Salas et al. (1976).

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