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Protecting Basal Liners with Geofabrics HPS Geotextiles
The function of the protective liner is to protect the liner from stresses, puncture and penetration from the overlying drainage media and waste: both short term dynamic loadings and long term static loadings. A protective material must protect the geomembrane through the permanent distribution of concentrated stresses on the geomembrane.
Geosynthetic protection layers have been in use since the late 1980s and now form the majority off liner protection methods. The driving forces behind the movement from mineral protection were lower costs due to void space savings and speed of installation, with a much lower risk of installation damage.
Once installed, a protective geotextile or goecomposite layer is placed before a stone leachate drainage blanket on the surface. It is important that the stone drainage blanket is free draining and has sufficient hydraulic conductivity to drain leachate over a large, relatively flat area. The secondary function of the drainage blanket is to provide a level of protection against the placement of the first layer of waste.
The selection of an appropriate geosynthetic protection layer is now made using the cylinder test. The cylinder test is recognised as an effective method for determining the effectiveness of a material in protecting a geomembrane against the long term effects of static point loads, it is designed to simulate as close as possible the conditions expected in the base of a landfill.
The introduction of the cylinder test transformed the way in which engineers specified geosynthetic protectors from a rule of thumb approach based on anecdotal experience from Germany and the United States to a method which acts as a design tool for a specific site.
Initially invented in Germany by the Quo Vadis group of laboratories, the German tests were carried out to suit the landfill design regulations defined by the German government research and standards institute BAM. The criteria was that the designs should be based on 60m deep landfills using a single type of angular stone of 16-32mm diameter. Whilst a significant amount of research was conducted, the final recommendation was that a minimum allowable geotextile for all conditions was 2000gsm. Whilst this test method was not fully implemented in Germany, the research and testing were continued by Geofabrics in the UK, and testing conditions were adapted to suit specific site conditions. The pass/fail criteria defined by the Quo Vadis group was retained.
The approach in the UK was to view each site as unique, viewing depths and stone selection independently. The key advantage of this approach is that the designer can balance cost and availability of an aggregate with a specific protection geosynthetic to find the optimum solution.
Initial experience in the UK had been positive in providing a method of comparing different geosynthetics for their suitability as protectors. However, problems arose in 1997 as the number of testing laboratories conducting the test grew and inconsistencies in the test apparatus, test method and reporting of results became evident.
To address this issue, a small team was formed to develop a detailed test methodology within the intension of bringing a degree of consistency to the various aspects of the test. This team included manufacturers, test houses, academia, and the environment agency. Initially published in March 1998, the Environment Agency test method is now the accepted method as a performance test to determine the optimum cover material for a geomembrane. Available for download on the gov.uk website the test method describes a full method for determining the effectiveness of a material in protecting a geomembrane against the long term mechanical effects of static point loads. The Environment Agency now require a cylinder test for every new cell.
The cylinder test is designed to simulate the cross section of the base of a landfill cell.
Cylinder Test Pass/Fail Criteria
The percentage strain of the three largest indentations is reported, along with the total average. The Environment Agency Pass/Fail criteria is that the average local strain of any single indentation must be ≤0.25%.