The permeability of fine grained soils: an investigation into unconventional methods of measurement in the laboratory
The commonly used British Standard constant head triaxial permeability (BS) test, for permeability testing of fine grained soils, is known to have a relatively long test duration. Consequently, a reduction in the required time for a soil permeability test provides potential cost savings, to the construction industry (specifically, for use during CQA of landfill mineral liners). The overall aim of this research was to investigate and evaluate alternative short duration testing methods and equipment for the measurement of the permeability of fine grained soils. To meet this aim, two individual schemes were proposed within the research. The first scheme was to add to the understanding and assess the feasibility of an existing method of short duration permeability testing, known as the Accelerated Permeability (AP) test, and to suggest improvements, where possible. Four different fine grained materials, of a variety of properties were compacted at various moulding moisture contents to produced analogous samples for testing using three different permeability testing methodologies: BS, AP and RAP tests. Fabric analysis was carried out on specimens derived from post-test samples to assess the affect of testing methodology on soil fabric. Results produced indicate that AP testing in general underpredicts permeability values derived from the BS test due to large changes in soil fabric caused by AP test methodology. RAP testing, in general, provides an improvement to the AP test but still underpredicts permeability values. The potential savings in test duration are shown to be relatively minimal for both the AP and RAP tests. Recommendations for use of both the AP and RAP test in industry are made along with suggestions for further research. The second scheme was to develop a novel apparatus to allow a reduction in the duration of the falling head permeability test using elevated gravitational acceleration produced in a laboratory centrifuge. An apparatus was designed and produced to measure and record permeant flow through small soil specimens under flexible, no lateral strain conditions, using the falling head test method under conditions of accelerated gravity, compatible with a small desktop laboratory centrifuge available at Queens University Belfast. An inbuilt data acquisition system was designed and incorporated in the apparatus to monitor and record changes in head and flow throughout the test. A mathematical model was produced for analysis of permeability coefficient from the results derived. Preliminary testing was found to successfully predict the permeability of reconsolidated clay samples in a relatively short duration, showing good comparability with results of BS tests on analogous samples. Recommendations for further testing and future developments of this apparatus have been made.
Clays, permeability, geotechnical engineering
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