Determination of coefficient of permeability from soil percolation test

Abstract

Hazelwood Ash retention Area (HARA) was established in 2004 to place dense phased ash from Hazelwood coal fired power station. The ash retention area underwent a tight scrutiny by Environment Protection Agency from early 2005 until May 2006 where approval was granted for placement of dense phased fly ash in HARA. During this phase, the permeability of 1.0 metre thick clay liner placed on the floor of proposed ash retention area was required to be proven not higher than the EPA standard permeability of 1x 10E-9 m/s for similar hazardous material placements in Victoria, Australia. The ash retention area was 240,000 m3 and laboratory permeability testing for samples obtained in a 50 metres grid of the clay liner was considered as expensive and time consuming. In addition, it was feared that the samples may be disturbed considerably during sampling for laboratory permeability testing. As a consequence resulting permeability for the liner would be higher than its natural permeability. Standard field tests conducted for determination of soil permeability such as constant head and falling head tests could not be conducted in a metre thick impermeable Clay liner due to the limitation of methodologies. A double ring permeability test method was then conducted on the floor of the ash retention area. The method resulted in serious errors due to leaking water from and among rings. This was mainly due to types and grading of the materials used in construction of the impermeable clay liner. Finally as an alternative method for testing permeability of the clay liner, a soil percolation test was proposed. Since permeability could not be directly measured from the soil percolation test, a method was developed to determine permeability from the existing soil percolation test data for the impermeable clay liner. Environment Protection Agency developed Soil Percolation Test was widely used for design of septic tanks for households in country areas throughout Australia. The method uses a 50 mm diameter standpipe piezometer with a screen at the bottom (Casagrande type) inserted in a 100 mm diameter borehole of 600 mm deep. Then the borehole is filled with water and a plunger in the standpipe measures drawdown verses time. The soil percolation rate is then calculated. There is a correlation between the rates of soil percolation to soil permeability in a range of different type of soils such as sands, clays, silts etc. However, soil permeability cannot be directly determined by the soil percolation test method. The paper describes the mathematical manipulations and assumptions used in developing a methodology of determining permeability using soil percolation test data set. In addition a limited number of laboratory tests were carried out from samples obtained from the locations where percolation tests were carried out to compare results of soil permeability from percolation tests and the laboratory permeability tests. The results of both tests were closely comparable and therefore the method can be successfully used to determine coefficient of permeability of impermeable earth liners using soil percolation test method.