Neil Dixon, principal lecturer, Department of Civil and Structural Engineering, The Nottingham Trent University, D Russell V Jones, geotechnical engineer, Golder Associates (UK) Ltd and Andrew Nicholas, former student, The Nottingham Trent University, presently graduate engineer, Aspinwall & Company Ltd. This paper was first published in GE’s May 1997 edition.
Geomembrane panels are increasingly being used in slurry trench cut-off walls to help form low permeability barriers to control the Imigration of groundwater, leachate, chemical contaminants and gases. The need for an industry standard has resulted in the production of the ‘Draft Specification for Slurry Trench Cut-off Walls’ (1996) by The Institution of Civil Engineers, Construction Industry Research fk Information Association (CIRIA) and the Building Research Establishment (BRE).The new specification, together with the associated notes for guidance, are intended to provide a standard and consistent approach to the design, construction, testing and monitoring of cut-off walls and guidance on current best practice.
Typically, barriers to pollution migration are required to have a permeability less than 1x10-9 m/s, and since slurry walls often have difficulty reaching this requirement in the short term, the geomembrane plays a vital role in the barrier system. The permeability of the geomembrane in-service will only be as good as its weakest point, and hence work has been carried out to establish the permeability of the interlocking panel joints.
In order for the laboratory determined permeability of an interlock to have any meaning, the integrity of the interlock system must be maintained during installation and throughout the in-service life of the barrier. At present there is a dearth of information on the forces experienced by interlock systems during installation (ie as the connections are threaded together often over considerable lengths), or the forces induced in the joints during operation of the barrier. The connections between panels must have adequate strength to ensure that the interlock geometry is not compromised as this would lead to preferential flow paths and increased permeability. To ensure the method of interlocking geomembrane panels does not compromise the performance of the barrier, the draft specification requires that the strength of the interlock system is not less than the tensile strength of the geomembrane sheet.
This paper presents results of tensile tests carried out on three interlock designs typically used in the UK. Cross sections through the interlocks are shown in Figure 1, and nominal dimensions are provided. The locks were tested without the sealant systems which are usually constructed after the panels have been installed (ie to infill the voids in the interlocks and hence to provide the required permeability). Therefore, the tests are not intended to represent the in-service conditions, neither does the method of testing model the installation process. However, the index type tests undertaken, in addition to assessing whether the tensile strength of the interlock is greater than that of the geomembrane panel, provide a simple method of comparing the interlocks as structural systems.