DESIGNING WITH THE CYLINDER TEST

Landfill closure techniques in recent years have matured and standardised. The use of a geomembrane-clay liner composite has become the most common form of engineered liner system in the UK. The standards for selection, production and installation of geomembrane liners is well defined. Recently the design criterion for geotextile protectors has improved. Engineers now have a researched and proven methodology for selecting a geotextile which will adequately protect the geomembrane from the overlying stone drainage blanket. The cylinder test or plate load test has now been adopted by the Environment Agency as the definitive proof statement for the suitability of a geotextile protector to a geomembrane in the base of a landfill. The author and colleagues carried out the necessary research and refined the test to enable the general acceptance of this useful tool. This paper includes a description of the test procedures sighting the critical elements and shares the experience of over 200 cylinder tests carried out on different geotextile/geomembrane/stone combinations for UK sites which have now been constructed. INTRODUCTION High Density Polyethylene geomembranes are extensively used as basal and capping layers for landfills and contaminated land containment sites. The material is readily available and produced at a consistent and high standard and the methods of production testing and installation are well established. Once laid a protective geotextile layer is placed before a stone leachate drainage blanket is placed on the surface. The stone drainage blanket has to be free draining and have sufficient hydraulic conductivity to drain leachate over a large relatively flat area. Its secondary function is to provide a robust defence against the placement of the first layer of waste. There is an incompatibility between a smooth membrane and a coarse drainage stone and it has been shown that this relatively stiff membrane is susceptible to stress cracking if strained over a long period of time. The function of the protective geotextile is therefore to present a relatively smooth surface and uniformly distributing the load applied to the upper surface of the geomembrane. The task of the cylinder test is to simulate as closely as possible the conditions expected in the base of a landfill. Figure 1: Simulation of the liner system at the base of a landfill THE CYLINDER TEST Over the last 5 years the method of specifying a geotextile protector has moved from a rule of thumb approach based on anecdotal experience from Germany and the USA to a method using a test which acts as a design tool for a particular site. The cylinder test was first invented in Germany and researched by the Quo Vadis group of laboratories. German tests naturally were carried out to suit the landfill design regulations defined by the German government research and standards institute BAM. The criteria was that designs should be based on 60m deep landfills using one type of hard angular stone 16-32mm diameter. Whilst a large amount of research was carried out the final recommendations was that the minimum allowable geotextile was a 2000g/m could be used and only in special circumstances. It is difficult to make quality geotextiles above 2000g/m and it is uneconomic to ask a contractor to lay two layers of geotextile. The conclusion was, at the time, that the test was too variable and the results were not commercially viable. It was at this time that the author and colleagues carried on the research and adapted the test for UK conditions. The pass/fail criteria defined by the Quo Vadis group was retained. The difference was that a more site specific design approach was acceptable in the UK where typically sites are 20-30m deep and acceptable stone types are between 5mm and 30mm in diameter. Consequently solutions were within the range of economically viable geotextiles. The key advantage of using the cylinder test is that a designer is able to balance the cost and availability of a particular stone with a particular geotextile to find the optimum solution. The Environment Agency Standard The standard set by the Agency entitled "A Methodology for Cylinder Testing of Protectors for Geomembranes" [Environment Agency, March 1998] has now been widely published and is now being used as a performance test to determine the optimum cover material for the geomembrane. This standard is available from the Regional Waste Manager, Warrington Office, Cheshire and describes in detail the apparatus, method, calculation and reporting required. There follows comments on key parts of the procedure. The idea of the cylinder test is to simulate the cross section of the base of the landfill. The most typical cross section is shown in Figure 2. Figure 2: Typical cross section of cylinder Figure 3: Assembled cylinder The construction inside the cylinder from the bottom up is: 1. Three load cells support a stiff circular metal plate these measure the direct load received by the geomembrane; 2. A thick rubber pad of a defined hardness this is to simulate a compacted mineral liner e.g.clay 3. A thin flat metal sheet of defined stiffness (usually 1.3mm thick grade 3 lead) this is a tell-tale sheet that deforms and maintains the deformed shape of the underside of the geomembrane 4. The proposed geomembrane (usually 2mm HDPE) 5. The proposed protection layer (usually a thick needle punched staple fibre non woven geofabric) 6. The proposed aggregate placed to a depth of 300mm this is the usual required depth in the landfill. 7. A separator geotextile and 50mm sand layer for load spreading 8. An upper steel plate with loading device The procedure The samples of stone, fabric and membrane are selected as typical and checked for obvious flaws. The cylinder built up to the cross-section as shown is often segmental in design so that the liner components can be placed and removed easily. Usually an additional layer of geotextile is placed around the wall of the cylinder to reduce the very high friction loads that occur. The placing of the stone is most critical to ensure reasonably consistent results. The aggregate is mixed to BS 812 and the stone is placed uncompacted in 3 layers to the final depth. Once the sand is in place it is tamped lightly to ensure a direct load thoughout the cylinder. The load is gradually applied over one hour up to the final load and then maintained at this load for the duration of the test. The gradual increase in the applied load is to allow some bedding down of the stone and to partially simulate the increase in load that would occur on site. The pressure calculation The load applied includes the dead load of the waste mass and any overburden load. Factors of safety are applied in addition which have been derived experimentally to simulate the long-term plastic deformation behaviour of HDPE sheet. It is known that the HDPE continues to deform under constant load and also at higher temperatures (plastic deformation). The factors required to take account of these are: 1.5 times the dead load for tests carried out at 40C and 1000hrs 2.25 times the dead load for tests at 20C and 1000hrs 2.5 times the dead load for tests at 20C and 100hrs Example: A landfill with 20m waste and waste density 1000kg/m3 and a 100hr test at 200C the test pressure would be 20 x 1000 x 9.81 x 10 x 2.5 = 490.5 kN/m (which is effectively the load received from a 49m deep landfill) Deformation measurement Figure 4: Method of measurement of lead sheet Once the cylinder has been carefully dismantled the metal sheet is removed to an engineers table mounted with horizontal and vertical vernier measuring devices. The aim is to find the maximum strain caused by the most damaging stone that was in contact with the protector. A minimum of three of the deepest indentations are selected. On each deformation two axes are drawn at right angles. Using the vernier device vertical measurements are taken at 3mm intervals across the indentation along each of the axes. Calculation of strain The undeformed length lu (shortest) is calculated from the first and last measurements across the axis. The deformed length ld (locus of curve) is calculated by using Pythagoras to calculate the slope lengths of each increment and finally adding the increments to give the total length. The strain is calculated as a percentage as shown in Fig 4: Figure 5: Deformed metal plate ready for measurement on the Engineers table Reporting A report is then prepared: Naming the laboratory; referring to the test method and any agreed variations; the conditions of the laboratory; the type and appearance of materials tested before and after test; record of the load and duration of the test, the results of the strain measurements. Results The percentage strain of the three greatest indentations is reported along with the average. The Pass/Fail criterion is at present 0.25% allowable strain for any one indentation. The significance of choosing a minimum of three indentation is to a) reduce the possibility of missing the worst indentation, b) help with deciding if a rogue result has caused a failure through bad sampling of the stone, for instance. Laboratory Records Apart from maintenance of records the most important record of testing for the laboratory is the calibration of equipment used. The most critical are the metal plate and the rubber pad. There is a specific annex to the report that defines the metal sheet to be used and a test that ensures that the plate deflects in a consistent manner under identical loads. The original material used by the Quo Vadis group was a thin organ pipe material that was not readily available in the UK. A test was devised to compare the indentation depth with a standardised method. This then allows laboratories the option of using other types of metal if they so choose. A spherical metal ball is clamped into a standard compression testing rig. A mini rubber pad is set up with the test metal on its sur