Time and scale effects in laboratory permeability testing of compacted clay soil
Date
1989
Authors
Javed, Farhat, author
Shackelford, Charles D., advisor
Jameson, Donald A., advisor
Doehring, Donald O., committee member
Abt, Steven R., committee member
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Abstract
Permeability (hydraulic conductivity) testing of clays in the laboratory typically requires a significant amount of time. It is hypothesized that the time required for clay permeability test can be reduced substantially through a statistical modelling technique known as "time series analysis". In order to test this hypothesis, permeability tests were performed on compacted samples of a silty clay soil in a standard Proctor mold (9.4 x 10-4 m3). The soil was separated into five different fractions representing five ranges in precompaction clod sizes. Constant-head permeability tests were performed on each of these five fractions. Tests were replicated five times for the time series analysis. The results of analysis indicate that time series modelling can significantly reduce statistical error associated with permeability data. It is demonstrated that the time required for clay permeability test can be reduced appreciably through time series modelling. Permeability tests also were performed on four soil fractions in a large-scale (0.914 m x 0.914 m x 0.457 m) double-ring, rigid-wall permeameter. The results of small-scale (Proctor mold) permeability tests indicate that the soil permeability does not vary much with a change in the precompaction clod size. Presence of large clods (> 25 mm), however, may result in side-wall leakage. The large-scale tests indicated that permeability is strongly related to the precompaction clod sizes. Permeability of the soil increased more than two orders-of-magnitude as the maximum precompaction clod size increased from 4.75 mm to 75 mm. Comparison of the results from the small-scale and the large-scale tests indicated that, for all soil fractions, the large-scale permeability was higher by more than an order-of-magnitude. As a result, there appears to be a scale-effect associated with laboratory permeability testing. This scale effect is more significant when soil contains considerable quantity of clods that are large relative to the size of permeameter. These results imply that the large-scale test is more capable of accounting for the hydraulic defects resulting from large clods. A more realistic evaluation of the field permeability of a compacted clay, therefore, may be possible in the laboratory if the permeameter is fairly large relative to the maximum precompaction size of clods present under field conditions.
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Subject
Clay soils -- Testing