|The auger-hole method (1958)|
The method consists of pumping the water out of an auger-hole extending below the water table and then measuring the rate of the rise of the water in the hole. It is a widely used procedure to measure the saturated hydraulic conductivity in saturated soils. The measured result is dominated by the average value of the horizontal conductivity of the profile.
In its simplest form, it consists of the preparation of a cavity partially penetrating the aquifer, with minimal disturbance of the soil. After preparation of the cavity, the water in the hole is allowed to equilibrate with the groundwater; that is, the level in the hole becomes coincident with the water table level. The actual test starts by removing the entire amount of water from the hole and by measuring the rate of the rise of the water level within the cavity.
The auger-hole method is a rapid, simple and reliable method for measuring hydraulic conductivity of soil below a water table. It is mostly used in connection with the design of drainage systems in waterlogged land and in canal seepage investigations. The method, originated by Diserens (1934), was improved by Hooghoudt (1936) and later by Kirkham (1945, 1948), Van Bavel (1948), Ernst (1950), Johnson (1952) and Kirkham (1955).
The general principle is very simple: a hole is bored into the soil to a certain depth below the water table. When equilibrium is reached with the surrounding groundwater, a part of the water in the hole is removed. The water seeps into the hole again, and the rate at which the water rises in the hole is measured and then converted by a suitable formula to the hydraulic conductivity (k) for the soil. The auger-hole method gives the average permeability of the soil layers extending from the water table to a small distance (a few decimetres) below the bottom of the hole. If there is an impermeable layer at the bottom of the hole, the value of k is governed by the soil layers above this impermeable layer. The radius of the column of soil of which the permeability is measured is about 30–50 cm.
The use of this method is limited to areas with a high groundwater table (at least during part of the year) and to soils where a cavity of known shape can be maintained throughout the test. Hence in certain sandy soils it is necessary to use a perforated tube.
This treatment is mainly for practical purposes, so that the theory of the flow of water into an auger-hole has not been considered; only some background information is given, in order to make clear the reasons underlying the instructions and recommendations. The graphs and formulae given are largely based on Ernst’s publication (1950), having the least limitations, especially as regards the quantity of water that has to be removed from the hole. Moreover, with the help of these graphs the k-value can be computed quickly and easily.
In measuring hydraulic conductivity in the field, four phases can be distinguished, each having its own problems:
W.F.J. van Beers’ publication was published as ILRI publication 1 in 1958. The 6th edition from 1983 is available for download.
W.F.J. van Beers (1958) The auger hole method