The Use of Geotextiles and Geosynthetic Clay Liners in Landfills

Report on the Use of Geotextiles and Geosynthetic Clay Liners in the Construction of Modern Landfills. Discuss a recent example.

Modern landfills are highly engineered containment systems that are designed to isolate surrounding soil and groundwater from the potentially harmful impacts of solid waste. Of particular concern is the segregation, collection and treatment of leachate, which consists of water and water-soluble compounds that accumulate as water percolates through solid waste.

In order to contain leachate, many modern landfills employ a composite liner system consisting of a geomembrane (or natural low permeability clay) overlying a compacted clay liner or geosynthetic clay liner (GCL). Geomembranes are relatively impervious polymer sheets (such as high-density polypropylene (HDPE) or polyvinyl chloride (PVC)) that slow the movement of leachate and permit its collection.

However, due to the angular nature of drainage gravel placed above the geomembrane and the large vertical stresses experienced due to dead weight and dynamic loads from earthmoving equipment, it is important to protect the geomembrane from puncture. This is achieved through the use of heavyweight geotextiles. Geotextiles are also used in modern landfills as a filter, preventing the movement of small soil and refuse particles into the leachate collection layer.

Underneath the geomembrane in many landfill systems is a secondary liner, consisting of either a geosynthetic clay liner (GCL) or compacted clay liner (CCL). Consisting of a thin layer of clay, typically sodium bentonite, sandwiched between two layers of a geotextile, GCL's are used as a hydraulic barrier to leachate movement. GCL's also have the advantage of being able to be used with a steeper slope than a CCL, allowing a greater waste storage volume.

A recent, local example of a modern landfill that employs a composite liner system is the Uleybury Landfill in Adelaide, South Australia (figure 1).

Figure 1: The Uleybury Landfill in South Australia is a modern landfill, situated in a disused quarry, that uses a composite liner system to protect the surrounding environment from leachate contamination [Source: personal photograph]

The Uleybury landfill uses a double composite liner system consisting of a HDPE geomembrane, protected by a geotextile layer, overlaying a geocomposite clay liner. Above the geomembrane is a drainage layer to ensure leachate is collected and treated (see figure 2).

Figure 2: The Uleybury Landfill utilises a double composite liner system to prevent the movement of leachate into the local groundwater [Source: Interpretative Sign photographed on site visit to Uleybury Landfill]

It is this liner system, combined with the baled waste operation, landfill gas management and future monitoring program that has lead to the Uleybury landfill being awarded the 2006 Case Earth Award.

Early Study Period 5 Post

So far this semester, Geotechnical Engineering N (can't explain the 'N') has investigated embedded retaining walls, earth dams and tailings dams, as well as landfill technologies.

The lecture on embedded retaining walls relied upon some concepts from Rock and Soil Mechanics (effective stresses, active/passive pressure, moment equilibrium) which was beneficial as it felt as though we simply 'picked up where we left off'. The examples covered in this lecture were particularly useful and I found the DVD of the basement excavation in Melbourne interesting. Following the largely theoretical nature of embedded retaining walls, the lectures moved into a very descriptive examination of earth dams, tailings dams and landfill technologies. Although I am a fan of equations, theories and numbers, I found these lectures very interesting as I want to be designing such structures in the future.

In regards to semester 1, I was very happy with how Rock and Soil Mechanics went and am very much enjoying and looking forward to the rest of Geotechnical Engineering N. In fact, I would be surprised if I missed out on more than 4 marks in the end of semester examination as I felt really confident with the previous material.

The content to be covered this semester will be particularly beneficial to me as I have been successful in obtaining vacation work in geotechnical engineering at Golders Associates. This represents a fantastic opportunity and every application discussed so far (sheet piles, contiguous pile walls, landfills, tailing dams) gets me thinking about what I may be designing in the future. This is a great source of motivation which has resulted in me spending many hours this past week in the computer pools designing an earth embankment.

In regards to assessment, I was pleased to discover that the practicals will be a greater focus this semester (50%) as it these that I learn most from, and put the most effort into throughout the semester (not just a cram at the end).

I apologise for the somewhat late entry for this early study period post (I inferred it meant anytime in the first half of semester until I talked to a few other students).

Potential Consequences of Failing to Control Seepage and Decant Water Level in a Tailings Dam

Tailings dams are used to contain the waste material of mining operations and represent the greatest environmental footprint of a mine. Although the size of tailings dams vary they can be as large as 4 square kilometres and, on flat terrain, are built in the form of a ring dyke. Tailings dams are used to dry out the slurried waste material of a mine and subsequently capture excess water so it can be reused. A ring dyke tailings dam spigots out the thickened slurry from the dam perimeter, allowing it to drain along a sloping beach towards a low point.

Although the preferred engineering option for such dams are similar to a water retaining earth dam, tailing dam embankments are progressively built using readily available materials in either an upstream, downstream, or centreline orientation as the mining operation progresses (figure 1).

Figure 1: Conventional embankment design techniques for a tailings dam [Source: www.tailings.info/conventional]

From a geotechnical engineering perspective, the downstream embankment design is preferred as it does not rely on the tailings material for its strength. Often a combination of high moisture content, low density and low stresses combine to produce a low strength tailings material. However, the downstream option is rarely used by mine managers as more and more material (and land) is required as the embankment is progressively built.

In all dams the designer aims to control the amount of seepage, seepage induced pore pressures and internal erosion/piping to ensure dam stability. This is often achieved through the application of drains (and filters) within the embankment to ensure the phreatic surface does not daylight on the downstream face of the embankment. If daylighting of the water table does occur then the effect is two fold; a reduction in shear strength and increased erosion/internal piping which can lead to a catastrophic failure of the embankment. Such a failure can have dire consequences to life and the environment, such as a large mudflow and the release of toxic/acidic tailings. One such failure occured in an Italian fluorite mine which sent 180000 cubic metres of material flowing towards the town of Stava, killing 268 people and causing 155 million Euros of damage.

During the design of a tailings dam embankment it is also important to cater for larger than expected inflows by designing spillways and bypass channels. Flood erosion is a problem common to all surface water impoundments, but overtopping is particularly important for tailings dams. This is because overflow across unconsolidated tailings can cause rapid erosion, leading to drainage from the tailings impoundment.

Overtopping occured in the Aurul S.A. tailings dam located in Baia Mare, Romania sending 100000 cubic metres of cyanide contaminated liquid into the Lapus stream (Figure 2). Described as the greatest European environmental disaster since Chernobyl the cyanide waters killed tonnes of fish and poisoned Hungarian drinking water supplies.

Figure 2: The failure of the Aurul Tailings dam to account for overtopping caused a great environmental disaster [Source: http://www.bristolbayalliance.com/mines_and_fish.htm]

The operation in Baia Mare revolved around the reprocessing of gold tailings from an old, unlined mine using sodium cyanide to recover the residual gold. The tailings stored in the Aurul impoundment had a cyanide concentration of 400 mg/L, but the designers constructed an embankment of inadequate height to account for melting snow and a heavy rain event. The rise in water level caused the dam crest to wash away, releasing the poisionous tailings. Except for the crest, the embankment remained structurally intact.