Permeability of Natural Clay Liners: Effect of Accelerated Permeability Testing on Soil Structure

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1 The 12 th International Conference of International Association for Computer Methods and Advances in Geomechanics (IACMAG) 1-6 October, 2008 Goa, India Permeability of Natural Clay Liners: Effect of Accelerated Permeability Testing on Soil Structure C. Anderson, V. Sivakumar Dept. of Civil Engineering, Queen s University, Belfast, Northern Ireland Keywords: Permeability, Hydraulic Conductivity, Soil Structure, British Standard Permeability Test, Accelerated Permeability, Scanning Electron Microscope ABSTRACT: A variety of clay specimens (Kaolin, Belfast Upper Boulder Clay and Belfast Sleech) were subjected to British Standards Triaxial Permeability Tests and Accelerated Permeability Tests in order to examine how the permeability predicted (under uniform effective stresses and conditions) varied depending on the testing method adopted. The specimens were then freeze-dried to preserve their post-test state, and examined using a Scanning Electron Microscope, in order to analyse their soil structure. In all cases the Accelerated Permeability Test predicted a value of permeability lower than that predicted by the British Standards Triaxial Permeability Test. Likewise, the soil structure present in the Accelerated Permeability Test specimens showed marked differences when compared to soil structure present in the British Standards Triaxial Permeability Test specimens; however the differences were not constant throughout the specimens. It was concluded that Accelerated Permeability Tests do result in soil structure degradation; however the full extent of this is not yet known. To ascertain this, it is recommended that a series of Accelerated Permeability and British Standards Triaxial Permeability Tests are performed on a variety of clays under varying effective stresses, along with a series of British Standards Uniaxial Compression Tests on these specimens post-test. 1 Introduction Permeability or hydraulic conductivity is an important engineering property of soil and is an essential value required for seepage, settlement and stability calculations (Mitchell et al., 1965). In other environmental problems, such as waste disposal, the importance of permeability is further increased. Leachate formed in the landfill from decomposing waste is a highly toxic pollutant, and if seepage of leachate occurs outside the landfill it can have extreme detrimental effects on the surrounding ground, groundwater and surface waters a considerable distance from the landfill. As a result, low permeable materials are used to line landfills, the objective being to act as a barrier, retaining the leachate within the landfill for collection and disposal under controlled conditions. There have been several advances in the design of barrier systems over the decades, including geosynthetics and geomembranes. However, one of the most common and economical solutions still involves the use of naturally occurring clays compacted into a liner (Mitchell et al., 1965; Tavenas et al., 1983b; Daniel 1984, Benson et al. 1994). The permeability of the compacted liner material is a major factor when determining the required thickness of the liner. For construction quality assurance of landfill liner integrity environmental regulatory authorities stipulate permeability testing to be carried out on samples from the compacted liner under a recognised standard test method, to ensure that placed materials achieve the desired low permeability. There are a wide range of procedures for measuring permeability using a variety of direct and indirect methods (Berryman and Blair 1986, 1987; Olson and Daniel 1981; Tavenas et al. 1983a). However in the UK, the specified test is the British Standard Constant Head Triaxial Permeability Test (BS1337-6: 1990), (the BS Test). This test is relatively time consuming, taking, in some cases, up to two months from the time of sampling to obtaining complete results. This in turn creates delays and additional costs in construction and in the majority of cases forces the site engineer into using less reliable comparison testing to justify continuation of earthworks operations while awaiting the results of laboratory permeability tests. Consequently, the environmental regulator in Northern Ireland, the Environmental Agency, commissioned a research contractor, Murray Rix Limited, to investigate the possible use of an accelerated permeability test (the AP Test) as a viable short duration alternative to the BS test. The AP test requires the same equipment as the BS Test however testing procedure differs in that it involves the combination of the saturation, consolidation and permeability stages, speeding up the availability of results. The prime objective of the research carried out by Murray Rix Limited was to compare, under controlled conditions, laboratory permeability tests using both the BS test method and the AP Test method on a range of materials prepared at water contents likely to be used in 1443

2 actual construction. The subsequent technical report derived from the research, (Murray 2003a) concludes that the AP Test produces permeability test data that is reproducible to a high degree and was suitable for acceptance testing of construction clay liners, however the AP Test method underpredicts (typically by an order of 2, but in some cases up to an order of 8), the results obtained from the BS tests. In the report, the recommendation that further research is required into the use of AP Test as a viable and reliable method of permeability testing is made. The experimental investigation reported in this paper endeavours to meet that recommendation and outlines a portion of the research that is currently underway at Queen s University investigating alternative methods of short duration laboratory permeability testing for fine grained soils. 2 Background During the BS Test, at saturation stage, confining pressures are raised in incremental steps half that of the effective pressure. Alternate steps allow full drainage of back pore water into the sample under a pressure slightly less than confining pressure. This process is repeated until measurement of pore pressure coefficient, B, indicates full saturation. This ensures that the applied effective stresses are not so high as to excessively prestress or over-consolidate the specimen. At the consolidation stage, confining pressure is increased by an amount corresponding to the required effective consolidation pressure above the back pore water pressure, and full drainage is allowed out of the sample. Both these stages generally require long durations for completion. In addition, both stages require regular human interaction, predominantly with the initial saturation stage, to monitor pore-water and pressure volume changes and subsequently to apply increases in confining pressure and back water pressures respectively. The final permeability stage requires adjustment of back pore water pressures at the top or bottom drainage lines to supply a suitable pressure deviation across the specimen, inducing a measurable flow that, when its rate becomes constant, is used to equate the coefficient of permeability. By comparison the AP Test involves the application of total confining pressures in one step. The top and bottom back pore water pressures are then applied immediately after, with a differential between top and bottom inducing the required flow and an average difference between back pore water pressures and the greater confining pressure pertaining a minimum effective consolidation pressure to limit sample swelling. It is the author s belief that a cause for the apparent under-prediction of permeability values produced from the AP Test is due to a disruption of the soil structure resulting from the initial application of total confining pressure during a time when the sample is in an unsaturated state. The high effective pressures resulting in unrecoverable yielding of the soil, leads to a decrease in void ratio and consequentially a reduction in the coefficient of permeability obtained. The verification of this hypothesis forms the basis of the experimental procedure outlined herein of which the two primary objectives are: 1. Execution of BS Testing and AP Testing on a variety of clays under controlled conditions, and comparison of permeability data produced. 2. Dissection of all tested specimens post-test to expose their inner structure, and analysis of soil structure using a Scanning Electron Microscope (SEM) to allow comparison between permeability testing method and soil structure for each soil material tested. 3 Sample preparation and testing 3.1 Material Selection A total of 6 samples were produced. One sample for BS Testing and one sample for AP Testing using 3 different soil types. The soil types selected for use were: 1. Kaolin 2. Belfast Upper Boulder (BUB) Clay 3. Belfast Sleech Kaolin is a commercially available material in a uniform homogeneous powdered state, and has been widely used for experimental soil mechanic research for many years. BUB Clay and Belfast Sleech are the predominant fine grained materials found in Belfast. They were chosen for investigation because of their high availability, and due to their high clay content and general low coefficient of permeability they would, in most cases, be deemed suitable materials for use as landfill liners after screening. 3.2 Sample Preparation Due to the high variability of the coefficient of permeability, it was deemed unsuitable to use undisturbed samples for the testing program; instead, as with the Kaolin Samples, recompacted samples were produced from the two naturally occurring materials. To ensure that the pairs of specimens were of common uniform structure and 1444

3 composition disturbed samples of the natural clay where dried and crushed to a fine powder. Screening of all the material using a sieve of aperture 425μm (BS 410: 1986, Wire Mesh Series) separated out any coarse material which was not included in specimen production. The British Standard Light Proctor compaction test was used to find the optimum moisture content of each material. Each pair of specimens were prepared at their specific optimum moisture content to ensure maximum dry density and a higher probability of continuity between samples. After the material was mixed to its optimum moisture content, the resultant soil mixes were immediately placed in airtight plastic bags and stored in a temperature controlled room for 24 hours to allow moisture equilibrium to be achieved throughout the soil mix prior to sample creation. Cylindrical specimens were produced to a height and diameter of 50mm. The prepared soil mix was placed in 50g layers into a 50mm diameter cylindrical mould and each layer was statically compacted under a force of 800N, until a height of over 50mm was reached, at which point the sample was trimmed carefully back to a height of 50mm using a wire cutter. To prevent the clay from sticking to the sides of the mould, and limit stressing and deformation of the sample, the sides of the mould were lubricated with a small film of silicon grease before compaction. (a) (b) Figure 1. Typical specimen for Permeability Testing (Kaolin Specimen) - (a) Side profile, (b) Plan profile. 3.3 Permeability Testing Procedure Both the BS Test and AP Test utilised the same equipment for the reported test, namely a Triaxial Test Cell and 3 GDS Automatic Pressure and Volume Control Units. Before testing commenced, the system was set up without a specimen to ensure no leaks from the cell, GDS s, valves or connections, as small errors in volume measurement of permeant flow through the sample will cause large inaccuracies in permeability data calculated. In addition, head loss across the system was measured and taken into account in the proceeding calculations of the coefficient of permeability. In addition the GDS used were calibrated for both pressure and volume measurements. All tests were carried out in a temperature controlled room, at 20º C, and the permeant used was de-aerated water. The arrangement of the cell and equipment is shown in figure 2. With regards to setting up each specimen in the triaxial cell for a test, the procedure for AP Test and BS Test is the same. Each GDS is flushed and filled with freshly de-aerated water. In addition, the pressure lines in the triaxial base and top cap are flushed thoroughly with de-aerated water to ensure there is no trapped air in the pressure lines, and both porous discs are saturated in de-aerated water under vacuum. The specimen is placed in an unused, leak-free, pre-soaked rubber membrane (excess water removed) using a membrane stretcher and excess membrane at the top and bottom of the sample is cut to provide just enough cover for the top cap and pedestal. The excess membrane is folded back onto the sample and the folded edge cleaned of any soil particles or water to ensure a clean contact between membrane and top cap/pedestal. Soaked filter paper is cut to fit the specimen diameter and placed on the saturated porous disk on the pedestal, on which the specimen is immediately set without entrapping air. The membrane is then carefully rolled down onto the pedestal, where a smear of silicon grease on the curved surface of the pedestal has been applied to improve the seal, and is then held in place using 2 rubber O-Rings. The procedure is duplicated for attaching and sealing the top cap to the specimen. The cell body is then assembled under freshly de-aerated water ensuring all air is displaced through the air bleed hole. Each GDS is then attached to a pressure line ensuring not to trap air in connections. 1445

4 Figure 2. Arrangement of cell and equipment for Permeability Testing. The BS Tests and AP Tests were conducted following the steps and procedures specified in (British Standard Institute, 1990) and the (Environmental Agency, 2003) Technical Report, respectively. However, to isolate procedural method as the variable in the investigation, the following pressures were used for both tests (permeability stage in BS Test): Cell pressure 660 kpa Bottom Pore Water Pressure 615 kpa Top Pore Water Pressure 605 kpa These values generate an average effective pressure of 50 kpa and a differential pressure of 10 kpa to generate a measurable flow upwards through the specimen. It should be noted that these are not the pressures specified in AP Test Technical Report (Environmental Agency, 2003b), where the pressure differential along the sample is 125 kpa and an average consolidation pressure of kpa is used. However, to comply with the guidelines indicated in (British Standard Institute, 1990) these pressures were altered. As stated previously, the British Standard Constant head permeability test comprises 3 stages. These are in order; saturation, consolidation and permeability. The saturation stage involves raising confining pressure in increments, with drainage lines closed, to monitor the change in pore water pressures, and calculation of Skempton s pore water pressure parameter, B. If the B value reached is less than 0.95 then the soil is assumed not to be saturated and a pore water pressure is applied to allow an inflow of water into the sample. Inflow of pore water was supplied through both ends of the sample and a bypass between top and bottom drainage/pressure lines allowed this pressure to be applied by one GDS only, ensuring no pressure deferential across the sample. Pore water inflow was pressurised 10kPa below the confining pressure to insure a small effective stress in BS test specimens at all times. Volume inflow is measured and when it begins to cease, drainage lines are again closed, a further increment of confining pressure is applied, and B is again calculated. This process was repeated in stages until the sample was saturated. As stipulated in (British Standard Institute, 1990), as average consolidation pressure for each test was to be 50kPa, increments of confining pressure were restricted to 25kPa, until a B value of 0.8 had been achieved, thereafter 50kPa was used until the soil was full saturated, and had reached a pore water pressure of 610kPa (confining pressure being 620kPa). Consolidation of the sample comprised increasing the cell pressure to 660kPa and allowing drainage from both ends (again using the bypass and monitoring using only one GDS), until pore water outflow was deemed complete. To complete the final permeability stage, drainage lines (including drainage bypass) are closed. Top and bottom pore water pressures are set to 605kPa and 615kPa respectively. Drainage lines from the top and bottom of the sample are then opened and flow into and out of the sample is observed until the rate becomes equivalent and constant. The AP Test is carried out by firstly applying the total confining and pore water pressures, with all lines closed. The cell is then pressurised by opening the cell pressure valve slowly. The inlet (bottom back pressure) valve, and then the outlet (top back pressure) valve are then opened. The rate of flow in and out of the specimen was recorded over time to obtain a continuous uninterrupted plot including any initial negative flow of water. The test was continued until such a point where the rate of inflow and outflow become similar and constant, plus a further 2500 min to ensure linearity of flow plots, and reliability in calculation of cumulative flow. 3.4 Post-permeability testing procedure To view any possible effect on soil structure disturbance, each sample was viewed using an electron microscope post-permeability testing. The permeability tests were stopped once suitable readings were obtained to provide an accurate calculation of cumulative flow. Pressures were reduced quickly but in stages down to atmospheric maintaining a small effective pressure in the specimen throughout this process. The triaxial cell was then 1446

5 disassembled and the sample carefully removed from the pedestal and membrane. The final wet mass was measured and recorded for density verification. The specimen was then cut into a cylindrical shape along the original axis of flow to a diameter of approximately 20mm in diameter using a wire cutter. To expose a cross section of the samples inner structure, shallow cuts were made around the circumference of the specimen and the sample was carefully pulled apart at these points. At all times the exposed surface was not touched or allowed to come in contact with the table or any other inanimate object, lest the surface be disturbed. The cut sample was then freeze dried and the exposed surface was coated in a thin film of gold to improve conductivity and image analysis inside the electron microscope. Due to the high variability of the coefficient of permeability, it was deemed unsuitable to use undisturbed samples for the testing program; instead, as with the Kaolin Samples, recompacted samples were produced from the two naturally the BS Test at saturation stage confining pressures are raised in incremental steps half that of the effective pressure, with alternate steps allowing full drainage of back pore water into the sample under a pressure slightly less than confining pressure until measurement of pore pressure coefficient, B, indicates full saturation. This ensures that the applied effective stresses are not so high as to excessively pre-stress or over-consolidate the specimen. At the consolidation stage, confining pressure is increased by an amount corresponding to the required effective consolidation pressure above the back pore water pressure, and full drainage is allowed out of the sample. Both these stages generally require long durations for completion. In addition both stages require regular human interaction, predominantly with the initial saturation stage, to monitor pore-water and pressure volume changes and subsequently to apply increases in confining pressure and back water pressures respectively. The final permeability stage requires adjustment of back pore water pressures at the top or bottom drainage lines to supply a suitable pressure deviation across the specimen, inducing a measurable flow that, when its rate becomes constant, is used to equate the coefficient of permeability. (a) (b) Figure 3. Typical specimen cut for EM analysis (Belfast Sleech specimen) - (a) Side profile, (b) Top profile 4 Results and discussion Figure 3 shows the inflow (from the bottom) and outflow (from the top) of a specimen of Belfast Upper Boulder Clay for both the BS Test and AP Test respectively. Both graphs are an accurate representation of the flow into the sample for the BS and AP Tests carried out on the other specimens of Kaolin and Belfast Sleech. Figure 3a shows cumulative flow for all three stages of the BS Test. For the saturation and consolidation portion of the test it should be noted that volume measurement was carried out using only the Bottom Pore Water GDS. However, flow was allowed through both the top and bottom of the specimen using a bypass. Incremental flows into the sample are clearly shown corresponding to the increases in back pressures after calculations of the degree of saturation, as is drainage from the specimen during the consolidation stage. The permeability stage is represented by the final linear portions of flow into and out of the specimen. The corresponding gradient for the linear portions for each BS Test yielded a value of cumulative flow used to calculate the coefficient of permeability of each specimen. 1447

6 (a) (b) Figure 4. Cumulative flow against time for permeability tests carried out on specimens of BUB clay, for both (a), the BS test and (b), the AP test. Figure 3b shows the cumulative flow for the total duration of the AP Test. It can be seen that initially there is a negative outflow from the top of the sample, representing an in intial inflow into the sample from both ends. Inflow from the bottom decreases until it becomes linear, and outflow changes from a negative gradient to a positive constant gradient represented by the linear portion of the plot. The point at which the rate of flow into the specimen corresponds to the rate of flow out is indicated on the graph. A further 2500 min of cumulative flow through the sample was then required to accurately determine a measurement of the gradient and the resulting calculated coefficient of permeability. Table 1. Summation of results from permeability testing data calculated from both BS and AP tests. Specimen Material Permeability Test Type Cumulative Flow, q [ml/min] Coeff. of Permeability, k [m/s] Bulk Density, ρ [Mg/m 3 ] Test Duration [min] Kaolin BS 1.995x x Kaolin AP 8.702x x BUB Clay BS 6.410x x BUB Clay AP 2.528x x Belfast Sleech BS 4.940x x Belfast Sleech AP 3.031x x Table 1 summarises results from the permeability tests carried out on each specimen as well as measurements of other controlled parameters of each specimen. It can be seen that on comparing the calculated coefficent of permeability for specimens of Kaolin, BUB Clay and Belfast Sleech, results from the AP test underpredicts those of the BS test by a factor of 2.26, 2.53 and 1.63 respectively. Results agree with observations reported in Murray (2003a), and Bulk Density of compacted samples show high coralation between similar specimens, therefore adding justification of valid comparison of permeability data between specimens. In all cases, results from AP tests were available in a lesser duration than the coresponding BS tests. It should be noted that in a non-research environment (i.e. comercial testing being carried out by a contractor), in all probability BS test duration would be significantly higher than stated in this study, as during saturation, when required, incremeatal increases in pressures were conducted outside the hours of a normal working day. In addition, sample sizes for permeability testing are normally 100mm in diameter and 100mm in length, which would take longer to saturate and consolidate and therefore the duration gap between AP and BS testing would increase. On cutting specimens post-permability tested specimens for electon microscope analysis, no differences in structural apperance were visable with the naked eye between specimens of same material. However, a trend was noticed during the trimming process whereby after freeze drying it was easier to cut into the BS tested specimens than AP tested specimens. This may indicate higher strength induced by the AP testing process, resulting from a change in soil structure. However, this is speculative suggestion and it is recomended that further studies should incoperate uniaxial compression testing of specimens to confim or disprove this theory. 1448

7 (a) Kaolin - BS Test (b) Kaolin - AP Test (c) BUB Clay BS Test (d) BUB Clay AP Test (e) Belfast Sleech BS Test (f) Belfast Sleech AP Test Figure 5. Images from Electron Microscope analysis showing representative pictures of clay structure for each specimen tested at a magnification of x40. Figure 4a and 4b show magnified cross sections of Kaolin specimens that have been subjected to BS and AP permeability testing respectively. Differences between specimen images are not overly apparent but from viewing a series of images, taken at various cross sections along the length of the specimens, it is evident that Kaolin BS specimen structure was more blocky in appearance, and resembled an idealised aggregated configuration in comparison with the sharp chaotic jagged surface visible on the AP Test. This may be due to a realignment of particles as a result of the higher effective pressures experienced initially and the subsequent rapid inflow of permeant under high back pressures. These factors leading to realignment of clay particles could lead to greater surface contact, inducing increased cohesion between particles, resulting in the jagged appearance when the specimen was carefully broken apart. This realignment of particles would lead to a change in micro structure of the soil fabric, increasing resistance to flow and reducing the permeability measured. The difference in soil structure resulting from BS and AP testing are more apparent in the images for BUB Clay and Belfast Sleech, shown in figures 4c and 4d, and figures 4e and 4f respectively. The aggregate structure of the soil matrix is more evident in these series of images. Also noticeable are the higher quantity of visible fissures between soil aggregates within the BS tested specimens, as compared to those within the AP tested specimens of BUB Clay and Belfast Sleech. A cause for the reduction in fissures within the AP tested samples is likely due to changes in the matrix pore structure occurring at elevated effective stresses, were aggregates are squeezed 1449

8 together so that flow paths becomes more tortuous, and permeability reduces as a result. 5 Conclusion In all cases the AP Test predicted a value of permeability lower than that predicted by the BS Test. In addition, the AP Test provides results in a shorter duration than similar tests carried out using the BS test method. This correlates with findings from previous research. Likewise, the soil structure present in the AP Test specimens show marked differences when compared to soil structure present in the BS Triaxial Permeability Test specimens. However, the differences were not constant throughout the specimens. It was concluded that Accelerated Permeability Tests do result in soil structure degradation. However, the full extent of this is not yet known. To ascertain this, it is recommended that a series of Accelerated Permeability and British Standards Triaxial Permeability Tests are performed on a variety of clays under varying effective stresses, along with a series of British Standards Uniaxial Compression Tests on these specimens post-test. 6 References Benson H. C., Zhai H., Wang, X Estimating Hydraulic Conductivity of Compacted Clay Liners, Journal of Geotechnical Engineering, 120, Berryman J. G., Blair S. C Use of digital image analysis to estimate fluid permeability of porous materials: application of two-point correlation functions, Journal of Applied Physics, 60, Berryman J. G., Blair S. C Kozeny-Carman relations and image processing methods for estimating Darcy's constant, Journal of Applied Physics, 62, BS Soils for civil engineering purposes. Consolidation and permeability tests in hydraulic cells and with pore pressure measurement (AMD 8261), British Standards Institution Daniel D. E.,1984. Predicting hydraulic conductivity of clay liners, Journal of Geotechnical Engineering, 110, Mitchell J. K., Hooper D. R., Campanella, J. K Permeability of compacted clay, American Society of Civil Engineers Proceedings, Journal of the Soil Mechanics and Foundations Division, 91, Murray E. J., 2003a. Validation of the accelerated permeability test as an alternative to the British Standard triaxial cell permeability test, Environment Agency. Murray E. J., 2003b Procedure for the determination of the permeability of clayey soils in a triaxial cell using the accelerated permeability test, Environment Agency Olson R. E. & Daniel D. E Measurement of the hydraulic conductivity of fine-grained soils (Philadelphia, PA, USA, ASTM, Philadelphia, Pa, USA) Tavenas F., Leblond P., Jean P., Leroueil S. 1983a. Permeability of natural soft clays. Part I: Methods of laboratory measurement, Canadian Geotechnical Journal, 20, Tavenas F., Leblond P., Jean P., Leroueil S. 1983b. Permeability of natural soft clays. Part II: Permeability characteristics, Canadian Geotechnical Journal, 20,