Impact of Testing Procedure on Permeability of Compacted Soils

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 permeability test provides potential cost savings, to the construction industry (specifically, for use during Construction Quality Control (CQA) of landfill mineral liners). The purpose of this article is to investigate and evaluate alternative short duration testing methods for the measurement of the permeability of fine grained soils.

As part of the investigation the feasibility of an existing method of short duration permeability test, known as the Accelerated Permeability (AP) test was assessed and compared with permeability measured using British Standard method (BS) and Ramp Accelerated Permeability (RAP). Four different fine grained materials, of a variety of physical properties were compacted at various moisture contents to produced analogous samples for testing using three the three different methodologies. Fabric analysis was carried out on specimens derived from post-test samples using Mercury Intrusion Porosimetry (MIP) and Scanning Electron Microscope (SEM) to assess the effects of testing methodology on soil structure. Results showed that AP testing in general under predicts permeability values derived from the BS test due to large changes in structure of the soil caused by AP test methodology, which is also validated using MIP and SEM observations. RAP testing, in general provides an improvement to the AP test but still under-predicts permeability values. The potential savings in test duration are shown to be relatively minimal for both the AP and RAP tests.

Influence of testing on permeability of compacted fine soils

The British standard constant-head triaxial test for measuring the permeability of fine-grained soils takes a relatively long time. A quicker test could provide savings to the construction industry, particularly for checking the quality of landfill clay liners. An accelerated permeability test has been developed, but the method often underestimates the permeability values compared owing to structural changes in the soil sample. This paper reports on an investigation
into the accelerated test to discover if the changes can be limited by using a revised procedure. The accelerated test is assessed and compared with the standard test and a ramp-accelerated permeability test. Four different finegrained materials are compacted at various water contents to produce analogous samples for testing using the three different methods. Fabric analysis is carried out on specimens derived from post-test samples using mercury intrusion porosimetry and scanning electron microscopy to assess the effects of testing on soil structure. The results show that accelerated testing in general underestimates permeability compared with values derived from the standard test, owing to changes in soil structure caused by testing. The ramp-accelerated test is shown to provide an improvement in terms of these structural changes.

Measurement of permeability using a bench-top centrifuge

The commonly used British Standard constant head triaxial permeability test for testing of fine-grained soils is relatively time consuming. A reduction in the required time for soil permeability testing would provide potential cost savings to the construction industry, particularly in the construction quality assurance of landfill clay liners. The purpose of this paper is to evaluate an alternative approach of measuring permeability of fine-grained soils benefiting from accelerated time scaling for seepage flow when testing specimens in elevated gravity conditions provided by a centrifuge. As part of the investigation, an apparatus was designed and produced to measure water flow through soil samples under conditions of elevated gravitational acceleration using a small desktop laboratory centrifuge. A membrane was used to hydrostatically confine the test sample. A miniature data acquisition system was designed and incorporated in the apparatus to monitor and record changes in head and flow throughout the tests. Under enhanced gravity in the centrifuge, the flow through the sample was under ‘variable head' conditions as opposed to ‘constant head' conditions as in the classic constant head permeability tests conducted at 1 g . A mathematical model was developed for analysis of Darcy's coefficient of permeability under conditions of elevated gravitational acceleration and verified using the results obtained. The test data compare well with the results on analogous samples obtained using the classical British Standard constant head permeability tests.

Repeatability and reliability of new air and water permeability tests for assessing the durability of high-performance concretes

This paper reports on the accuracy of new test methods developed to measure the air and water permeability of high-performance concretes (HPCs). Five representative HPC and one normal concrete (NC) mixtures were tested to estimate both repeatability and reliability of the proposed methods. Repeatability acceptance was adjudged using values of signal-noise ratio (SNR) and discrimination ratio (DR), and reliability was investigated by comparing against standard laboratory-based test methods (i.e., the RILEM gas permeability test and BS EN water penetration test). With SNR and DR values satisfying recommended criteria, it was concluded that test repeatability error has no significant influence on results. In addition, the research confirmed strong positive relationships between the proposed test methods and existing standard permeability assessment techniques. Based on these findings, the proposed test methods show strong potential to become recognized as international methods for determining the permeability of HPCs.

Effectiveness of filters in reducing consolidation time in routine laboratory testing

The research reported here is based on the standard laboratory experiments routinely performed in order to measure various geotechnical parameters. These experiments require consolidation of fine-grained samples in triaxial or stress path apparatus. The time required for the consolidation is dependent on the permeability of the soil and the length of the drainage path. The consolidation time is often of the order of several weeks in large clay-dominated samples. Long testing periods can be problematic, as they can delay decisions on design and construction methods. Acceleration of the consolidation process would require a reduction in effective drainage length and this is usually achieved by placing filter drains around the sample. The purpose of the research reported in this paper is to assess if these filter drains work effectively and, if not, to determine what modifications to the filter drains are needed. The findings have shown that use of a double filter reduces the consolidation time several fold.

Hard coatings on elastomers for reduced permeability and increased wear resistance

In this study, three different elastomers, namely hydrogenated nitrile butadiene rubber, fluoroelastomer and silicone, have been subjected to two different hard metallised coatings by ion implantation process. The three different elastomers are commonly used in various seal applications, where reduced wear and gas permeability are essential in maintaining seal performance and functionality. Samples of these rubbers have been coated with chromium coating in one set of tests. In the second set of tests, samples of elastomers have been coated with tungsten carbide coating being deposited on all the three different elastomers. Wear, gas permeability and mechanical behaviour of the coated samples were compared with each other and with the control uncoated elastomers. All the coated samples showed good reduction in gas permeability. With the use of metallised coatings, there has been improved resistance to wear in all the coated samples. Adhesion strength and effect of coating on the elastomer have been investigated by mechanical testing. Mechanical tests revealed good adhesion of metal coatings on all the rubber samples, and there was no detrimental effect on the mechanical properties after coating. © 2012 Institute of Materials, Minerals and Mining.

Establishment of a preconditioning regime for air permeability and sorptivity of alkali-activated slag concrete

This study reports an experimental investigation designed to assess the influence of near-surface moisture contents on permeation properties of alkali-activated slag concrete (AASC). Five different drying periods (5, 10, 15, 20 and 25 days) and three AASC and normal concretes with compressive strength grades ranging from C30 to C60 were considered. Assessment of moisture distribution was
achieved using 100 mm diameter cores with drilled cavities. Results indicate that air permeability of AASC is very sensitive to the moisture content and its spatial distribution, especially at relative humidity above 65%. To control the influence of moisture on permeation testing, the recommendation of this paper is that AASC specimens should be dried in controlled conditions at 40 C for 10 days prior to testing. It was also concluded from this study that AASC tends to perform less well, in terms of air permeability and sorptivity, than normal concrete for a given strength grade. This conclusion reinforces the need to further examine AASC properties prior to its widespread practical use.

Design, construction and testing of an air-cycle refrigeration system for road transport

The environmental attractions of air-cycle refrigeration are considerable. Following a thermodynamic design analysis, an air-cycle demonstrator plant was constructed within the restricted physical envelope of an existing Thermo King SL200 trailer refrigeration unit. This unique plant operated satisfactorily, delivering sustainable cooling for refrigerated trailers using a completely natural and safe working fluid. The full load capacity of the air-cycle unit at -20 °C was 7,8 kW, 8% greater than the equivalent vapour-cycle unit, but the fuel consumption of the air-cycle plant was excessively high. However, at part load operation the disparity in fuel consumption dropped from approximately 200% to around 80%. The components used in the air-cycle demonstrator were not optimised and considerable potential exists for efficiency improvements, possibly to the point where the air-cycle system could rival the efficiency of the standard vapour-cycle system at part-load operation, which represents the biggest proportion of operating time for most units.