Field-saturated hydraulic conductivity of unsaturated soils from falling-head well tests

Abstract

The field-saturated hydraulic conductivity ( K fs) finds use in many agricultural, environmental and geotechnical investigations and designs. It is commonly derived from falling-head tests in a well, but its derivation is sometimes based on assumptions that may not always hold. The aim of this study was to develop graphs for falling head percolation tests for 340 and 60 mm diameter wells—centred in two separate cylindrical soil tanks—for use in deriving K fs of a fine sand placed in the tanks. The graphs were obtained from results from the finite element program SEEP/W that was used to model percolation from the tank wells into sand, fine sand and silt of known K fs; the soils were all initially field-saturated, and the tests were carried out under (i) field-saturated (by spray irrigation) and (ii) draining conditions. For the modelled field-saturated tests on the three soils, the graphs of fall of head in the well, Δ H w, against the parameter K fs × Δ t, where Δ t is the time taken for the particular fall Δ H w to occur, were coincident. This indicated that for a given measured fall, Δ H w, for a particular head of water, H w, in a well, K fs could be calculated by dividing the parameter K fs × Δ t, by the observed time, Δ t, for the fall, Δ H w, to occur. For the modelled tests on the three draining soils, there was another inverse relationship between Δ H w and the parameter K fs × Δ t for small falls in well heads after start-up. There was good agreement between the K fs value of a fine sand measured in a cylindrical constant-head laboratory permeameter and that obtained using the graphs and the experimental results from both the field-saturated fine sand, and the same fine sand in draining mode at small falls of head in the well. This suggests that falling head tests on non-irrigated soils—initially field-saturated—could be used to determine K fs from early time falls in well heads. Since the time parameter greatly increases with increasing well size and decreasing pressure heads in the percolation well, it is quicker to carry out falling-head percolation tests in smaller test wells and at higher heads in the well. The procedure employed for obtaining K fs for the soils within the K fs range examined could be extended to the field.