Wonsy076 Geotechnical Engineering N: Landfill

b) Report on the use of geotextiles and geosynthetic clay liners in the construction of modern landfills.

In modern landfills, the waste is contained by a liner system. The primary purpose of the liner system is to isolate the landfill contents from the environment and, therefore, to protect the soil and groundwater from pollution originating in the landfill. The greatest threat to groundwater posed by modern landfills is leachate. Leachate consists of water and water-soluble compounds in the refuse that accumulate as water moves through the landfill.

Landfill liners are designed and constructed to create a barrier between the waste and the environment and to drain the leachate to collection and treatment facilities. This is done to prevent the uncontrolled release of leachate into the environment. The liner system of a landfill performs the vital task of retaining the leachate produced by the waste. The overall stability of a landfill may also be determined by the liner system.

Geotextiles: In landfill liners, geotextiles used for filtration, separation and protection which prevent the movement of small soil and refuse particles into the leachate collection layers and to protect geomembranes from punctures. These materials allow the movement of water but trap particles to reduce clogging in the leachate collection system. Usually geotextiles are placed at the tension surface to strengthen the soil. Geotextile also prevent damage to synthetic liners due to the rock aggregates hardness and angularity– comparative to the softness of some synthetic membrane liners.

Geosynthetic Clay Liner (GCL): Geosynthetics have revolutionalized many aspect of geotechnical design and construction. Geosynthetic clay liners are becoming more common in landfill liner designs. These liners consist of a thin clay layer (four to six millimeters) between two layers of a geotextile. These liners can be installed more quickly than traditional compacted clay liners, and the efficiency of these liners is impacted less by freeze-thaw cycles. GCL represent a potential alternative to a conventional compacted clay liner or as a means of augmenting the performance of a compacted clay liner as part of a composite liner system in the base of municipal solid waste landfills.

Geosynthetic materials also offer the following advantages:
• ease of use, and speed of deployment.
• a high level of manufacturing quality assurance – consistency and homogeneity.
• increase airspace in constructed cells.
• reduced requirement to quarry virgin gravels, clays and sands.
• reduced requirement to transport large volumes of gravels, clays and sands over urban, rural, or regional road systems.
• technically superior performance over many natural barrier and drainage materials when used alone.
• utilisation of recycled plastics that would otherwise occupy ‘dead’ space in a landfill cell.

A geosynthetic clay liner can be incorporated into different kinds of liner systems which are single single-liner systems, double-liner systems or composite liner systems. Composite-liner systems are more effective at limiting leachate migration into the subsoil than either a clay liner or a single geomembrane layer. A composite liner consists of a geosynthetic or geomembrane in combination with a clay line.

Figure 1: Double composite liner system

CASE HISTORY – GORDONVALE LANDFILL CAP, CAIRNS.
Cairns Water, a business unit of Cairns City Council, were given the responsibility of controlling and managing all aspects of capping the Gordonvale landfill - approximately 20km south of Cairns. After consulting with landfill design specialists (Golder Associates) and reviewing the performance of the options available, they selected a Bentofix® Geosynthetic Clay Liner over a 400mm thick compacted clay liner (CCL). Along with the many advantages of using a GCL over a CCL included the recent successful installation of a Bentofix® GCL at Babinda Landfill nearby. Bentofix® X2000 (a specialised capping grade of GCL) supplied by Geofabrics Australasia was used to cap the entire aerial extent of the landfill, with Geofabrics staff offering initial briefing andtechnical support to Cairns City Council employees who carried out the work in-house.

The GCL was laid directly onto a suitably prepared subgrade using an excavator and spreader bar supplied by Geofabrics. Rolls were run longitudinally from the base up the slope to the apex of the landfill. Adjacent roll had the standard 300mm overlap - no physical or mechanical joining is necessary. Transverse joints at the end of each roll also required a 300mm overlap, and were sealed with bentonite paste and protected from soil and foreign matter with geotextile cover strip (XROLL). Above the GCL barrier, a minimum of 400mm cover, a blended material comprised of approximately 50% shredded green waste and 50% sandy soil, was used as the confining cover soil layer.

The total project time required for complete installation of the 23,890m² of GCL was 6 weeks. Since the completion of capping works, Cairns Water staff have made comment on the significant reduction in leachate volume generated by rainfall infiltration into the waste body.