DIRECT DETERMINATION OF k OF SOILS IN PLACE BY PUMPING TEST

The most reliable information concerning the permeability of a deposit of coarse grained material below the water table can usually be obtained by conducting pumping tests in the field.

Although such tests have their most extensive application in connection with dam foundations, they may also prove advisable on large bridge or building foundation jobs where the water table must be lowered.

The arrangement consists of a test well and a series of observation wells. The test well is sunk through the permeable stratum up to the impermeable layer.

A well sunk into a water bearing stratum, termed an aquifer, and tapping free flowing ground water having a free ground water table under atmospheric pressure, is termed a gravity or unconfined well. A well sunk into an aquifer where the ground water flow is confined between two impermeable soil layers, and is under pressure greater than atmospheric, is termed as artesian or confined well.

Observation wells are drilled at various distances from the test or pumping well along two straight lines, one oriented approximately in the direction of ground water flow and the other at right angles to it.

A minimum of two observation wells and their distances from the test well are needed. These wells are to be provided on one side of the test well in the direction of the ground water flow.

The test consists of pumping out water continuously at a uniform rate from the test well until the water levels in the test and observation wells remain stationary. When this condition is achieved the water pumped out of the well is equal to the inflow into the well from the surrounding strata.

The water levels in the observation wells and the rate of water pumped out of the well would provide the necessary additional data for the determination of k.

As the water from the test well is pumped out, a steady state will be attained when the water pumped out will be equal to the inflow into the well. At this stage the depth of water in the well will remain constant.

The draw down resulting due to pumping is called the cone of depression. The maximum draw down DQ is in the test well. It decreases with the increase in the distance from the test well.

The depression dies out gradually and forms theoretically, a circle around the test well called the circle of influence. The radius of this circle is called the radius of influence of the depression cone.