Test pits, used mainly in granular soils, allow determining the permeability of shallow soils. The test pit can be square or circular in shape.
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The operational phases consist of making an excavation, filling it with water and evaluating the flow rate required to maintain a constant level (constant head tests) or evaluating the drawdown of the water inside the excavation (variable head tests). The test pit can be square or circular in shape.
The following conditions are necessary for a correct execution of the test:
a) The soil must be previously saturated by introducing water in order to establish a steady flow regime;
b) the depth of the pit must be equal to about 1/7 of the height, from the bottom of the pit to the water table level;
c) the geometric dimensions of the pit must increase as the size of the soil grains increases. In particular, the side of the square (in the case of square-based pits) or the diameter of the circle (for circular pits) must be greater than 10-15 times the size of the significant particle size fraction;
d) it is preferable to perform the test in homogeneous, isotropic soils with a permeability coefficient k > 10-6 m/s.
Below are the formulas used to calculate the permeability coefficient:
Circular pit, constant head test
k=q/(π·d·h)
Variable head test
k=d·(h2 – h1)/[32·(t2 – t1)·hm]
Square pit, constant head test
k=q/[l2·(27·h/b+3)]
Variable head test
k=[(h2 – h1)/(t2 – t1)]·[1+(2+hm/b]/(27·hm/b+3)
Where:
q = absorbed flow rate at constant head;
h = ater height in the pit (h > d/4);
d = diameter of the pit;
hm = average water height in the pit (hm > d/4);
t2 – t1= time interval;
h2 – h1 = water level variation in the interval t2 – t1
b = side of the base of the pit.