Sealing systems can be laid during construction to prevent contaminated water from moving in an undesired direction or to keep natural groundwater separate from contaminated road runoff and road construction seepage waters. In many places in which geomembrane barriers could be placed, spillage of petroleum and diesel from vehicles is a possibility. Sealing systems are used for sealing highways and embankments.
During the design of a sealing system the designer should take into account the sensitivity of the area, crossfall and alignment of the road. When the seal is placed on a slope, a very important part of the design procedure is the analysis of the slope stability as the shear strength between the layers of the sealing system may be much less than found between soil layers, thereby significantly reducing the factor of safety against slippage.
Liners are part of the sealing systems that consist of a base, a sealing layer and a protection layer. The base is that part of the construction on which the sealing layer should be placed and it can consist of natural soil or artificial aggregates placed on the natural soil. Materials selected for the granular base should not consist of sharp or large rock blocks that could damage the sealing layer. The base should be stable and compacted to at least 92% of optimum (Proctor) density. An important aspect is to ensure a planar base.
The sealing layer provides the low permeability of the sealing system. The required thickness depends on the sensitivity of the area that is to be protected and on the quality of the material used to make the sealing layer. The material of the sealing layer also depends on the purpose. Materials for sealing the pavement area will be different from those materials used for sealing the slopes of an embankment.
The protection layer is intended to protect the sealing layer from traffic (e. g. breakthrough caused by vehicle crashes), damage from the placing of coarse or sharp overlying material and negative climatic influences (e. g. freezing and drying). For this purpose natural materials such as soils, crushed rock and some artificial materials, such as concrete materials, are used. If necessary, the surface of the protection layer should also be designed against erosion due to high water flow velocities above it. Figure 13.34 shows an example where altered land use has increased run-off and flow speed to > 1 m/s over a trench so that the existing protection provided by 0/100mm crushed rock is no longer adequate. This could rapidly cut down to an underlying groundwater, damaging its quality. A high performance protection layer would be needed over a sealing layer in such a situation.
Retention tank
Infiltration tank
Fig. 13.32 Environment with extremely high sensitivity. A combination of various types of water treatment is included
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Fig. 13.33 Plan of environment with extremely high sensitivity. A combination of various types of water treatment is included |
For materials in the sealing layer, natural and geosynthetic barrier materials (GBR) can be used. The most common natural material used for sealing is clay, which is sometimes available on the construction site or in clay pits that are positioned in the vicinity. Materials available at the construction site can be enhanced with the addition of clean bentonite clays.
Geosynthetic barriers (GBR) can also be used. They come in various forms:
• polymeric geosynthetic barrier GBR-P;
• bituminous geosynthetic barrier GBR-B; and
• clay geosynthetic barrier GBR-C.
Four types of geosynthetic barriers application may be distinguished (prEN 15382, 2005): [31]
• high GBR on side slopes — where the GBR is installed above the drainage collection system as a high laying sealing system and covers the side slope of the road to prevent an overflow of the road surface runoff;
• deep GBR in central reserve — where the GBR is installed under the drainage collection system and covers the section in the central reserve, where sealing is required; and
• high GBR in central reserve: where the GBR is installed above the drainage collection system as a high level sealing system and covers the section in the central reserve where sealing is required.
Polymeric liners are supplied in rolls and must be joined on-site to form continuous sheets over large areas. This is a specialist task requiring the use of experienced personnel if one desires a reasonable confidence in achieving an effective water barrier.
Geosynthetic barriers are prone to damage by ultra-violet light and by vermin. The first can be overcome by ensuring that the material is covered in soil or other material rapidly after unrolling. Some are more resistant to vermin than others, but it is always sensible to consider ways of preventing damage from animals (perhaps by providing a light steel mesh cover a little above the placed geomembrane as a vermin barrier).
Clay sealing sheets are also available, especially when clay material is not present on the site. Their advantage compared to on-site materials is their precisely defined properties that allow easy design and construction. Typically, these comprise a thin (circa 1 cm) layer of rather dry bentonite formed between two geo-textile sheets. Supplied as a roll, these liners are unrolled on site and overlapped without seaming. Once buried and in contact with water the bentonite sorbs very strongly, causing significant expansion. This expansion develops an effective seal between the liner and the soils around it and between one roll of liner and another. If punctured, the bentonite expansion means that holes self-seal. Bentonite is an excellent sorbent of many species of heavy metals and some organics. Bentonite clay liners should be properly maintained and they should be prevented from drying out. If this happens, cracks up to some centimetres in width can appear and the sheet will no longer act as a the barrier. In that case bentonite layers can be more permeable than a sub-base.
prEN 15382 (2005) does not advise that geosynthetic barriers be connected to drainage systems when embedded in shoulders or slopes. Figure 13.35 shows a typical application of geosynthetic barriers. Details of technical solutions may be found in prEN 15382 and in RiStWag (2002).
Placing geosynthetic barriers on slopes with a thin cover of soil and lack of sufficient overburden to compensate for uplift pressures are elementary misapplications (Fig. 13.36).
