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Final Cover Systems in Landfill
The standard components within a final cover system
- An erosion control layer (top soil)
- A protection layer
- A drainage layer
- A barrier layer
- A gas venting layer
- A foundation layer
Design practices vary from site to site and as a result of national regulation and local conditions. State of the art design in landfill engineering has evolved from the use of natural soils to factory manufactured geosynthetics which allow for improvements in safety, reduced environmental impact and economic savings.
The level and type of protection required will depend on the topsoil used in the design. A geotextile must:
- Prevent dynamic puncture and abrasion during installation of restoration material placed by heavy plant. Restoration materials often consist of poorly defined uncontrolled materials discarded from other processes, such as quarrying or building, or other soil waste. These materials frequently include large angular rock fragments or other foreign objects, which could be damaging to the membrane. (The working strain of an LLDPE liner can be as much as 8% but this is required in service, and straining the membrane, either locally or globally during construction should be avoided).
- Cushion any sharp object in direct contact with the liner and prevent puncture both during installation long-term loading.
- Be flexible enough to allow differential settlement without loss of protection effectiveness.
- Retain tensile stress to reduce loads on side slopes while having good inter-face friction with the liner material.
- Be chemically resistant (if used under cap liner protection).
Index Test Correlation
Site-damage tests along with site simulation testing in the laboratory have shown that the stiffer and thicker a geotextile the greater its potential to prevent damage to a membrane liner.
Index tests are short-term repeatable tests used to compare different geotextiles with one another and to give a standard measure of quality during manufacture. Index tests were conceived to highlight certain characteristics in any one geotextile and the engineer must decide which of these tests are most relevant to the function the geotextile has to perform.
Site trials and laboratory testing have shown that the choice of geotextile cannot be based solely on one parameter but requires several different parameters to be sure that the highest level of protection is achieved.
Thickness: this relates to the ability of the geotextile to cushion both a dynamic load and a long-term load imposed by a sharp object. The geotextile distributes the load on the liner by allowing load spreading through its thickness. The importance of thickness should not be underestimated when designing for membrane protection when it is fundamental to use a fully three-dimensional product
The puncture resistance of a geomembrane can be improved by increasing the thickness of the membrane itself, however it has been shown that increasing the thickness of the protection geotextile has a greater improvement factor. The effect of increasing the thickness of the geotextile protector is significant, with huge improvements in protection levels achieved by nominal increases in thickness.
Dynamic perforation (cone drop): a dynamic test where a sharp cone is dropped on to a clamped geotextile producing a measurable hole in the geotextile. This simulates the damaging effect of a stone or sharp object being dumped onto the surface of a geotextile protector. The impact energy created in this test is 0.5kg.m (1-kg cone falling through 0.5m) represents approximately half the energy created by a 75mm rock fragment falling 2 meters during tipping and placing of restoration materials. The lower the cone drop hole diameter value for a geotextile the greater its ability to absorb dynamic loads.
Strength: it has been shown by various studies (Wilson et al 1996, Jones et al 1998) that an increase in strength has a significant impact on protection efficiency. However, this is only relevant if it is combined with other functions. A high strength thin material would not provide significant protection. A high load, high modulus material will significantly enhance the protection levels provided.
Fibre strength, inter-fibre friction and fabric construction are the most significant things that influence protection. There are two principal tests which measure strength and modulus, Tensile Strength and CBR.
- Static puncture test (CBR): it has been shown via research (Jones et al 1998), using the Environment Agency “cylinder” test method, that CBR relates directly to long term-membrane protection from strain.
- Tensile strength: this measures the in-plane, ultimate, tensile strength. Whilst this resistance contributes it does not replace the need if necessary, for an additional veneer design of a long steep capping slope. It is important to note that many non-woven geotextiles have differing tensile strengths (on each axis) and dependant on orientation loads will be transferred to the weakest direction.