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Measurement of surface permeability in situ: double ring test
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This test is used to determine the permeability of a water-permeable structure in situ. The apparatus consists mainly of two rings (Ø. 50 and 70 cm), a continuous water supply unit and an automatic system to maintain a constant water level.
The two rings are placed on the surface to be tested. They are “glued” to it for the duration of the test, to prevent water from penetrating between the rings and the surface. Conventional mortar or cement grout is used for glueing.
The test starts with the filling of the two rings. A constant water level is maintained both in the inner and the outer ring. The outflow time, that is, the time to be allowed for the water level to drop ± 1 mm in the inner ring (the time between the closing and opening of the water supply valve), is measured and recorded throughout the test.
NB: the measurements are made in the inner ring only, from which the water flows out vertically. The outer ring only serves as a “screen”, to minimize lateral outflow.
The minimum duration of a test is 30 min, to approximate the full saturation of the structure as closely as possible.
The infiltration rate or permeability (expressed in m/s) is given by the amount of water penetrating the soil per unit of surface area and time.