Landfills nowadays each contain huge amounts of organic materials and hold a huge potential to pollute the local groundwater for generations in the containment systems upon which their design is based fail to function as intended.
The engineering of a landfill is no different to other engineered structures, in fact in many ways, especially due to its pollution potential it may be more important that it does not fail when compared to some other structures.
Landfill base liners are by nature buried once constructed and the opportunity to do repairs is extremely limited. Also, other structures may show visible signs of for example leakage, whereas a landfill may leak underground undetected for a long while until the damage is realised and by then there may be a substantial pollution plume already on its way underground to flow out into a river, or pollute a well or drinking water borehole.
The lining of a landfill is the foundation of a major civil engineering structure. If you think of a foundation of a tall building and how importantly engineers view the correct design of the piling for the foundations, you should then think of a landfill lining as equally if not more important.
Just as for the foundation of a multi-storey building great care is taken throughout the construction, the Engineer in charge of a landfill construction would be negligent if he did not require adequate checks to be made on all aspects throughout the design and installation of a landfill liner (or capping).
Carrying out all the necessary checking that the design is implemented and results in a properly built liner (or cap) in a methodical manner and without omissions and then to be able to show others subsequently that the quality of the materials used and the way they were placed will make a proper lining which is as the designer intended everywhere it is laid, is called Landfill Construction Quality Assurance (CQA).
CQA can only be applied once a competent design engineer has completed a design process which has resulted in a detailed specification for the materials to be used, and the thicknesses, depths and positions etc, of these materials when they are used.
This is what is called landfill geomembrane CQA, and it is normally carried out under the overall supervision of a client or purchaser’s professional representative (eg â€œEngineerâ€) who appoints an experienced CQA Engineer to carry out Construction Quality Control (CQC). The role of the CQC is the checker of the checker/tester which is usually the construction Contractor, assisted by an expert subcontracted testing laboratory.
The CQA Supervisor is best appointed to someone outside the construction Contractor’s organisation to ensure his/her independence.
Whilst geomembrane materials are relatively impermeable even when compared with low permeability clays, they will transmit a small amount of water even when perfectly installed.
The vapour transmission rates of the geomembrane materials used vary for different fluids, but for water they normally have a permeability in the region of 1×10^-15 m/sec. This sounds like a very low leakage rate, which of course it is, but for the large areas involved at most landfills the end result can be in the tens of cubic metres of leakage every day. This really does not matter in fact because during the design stage the lining designer will have ensured that this leakage will, by natural attenuation and dilution, cause minimal risk to the environment.
It is only if leakage rates increase substantially above this rate that problems will occur.
Unfortunately, if a landfill design is poorly carried out without a great deal of care being paid to construction quality (especially if only one thickness or one type of single barrier will be used), leakage can be hugely increased.
Just think how quickly a bath empties if you inadvertently knock the plug out while bathing!
In the realm of CQA, knocking the plug out without noticing when you did it would be called a lining defect.
It stands to reason therefore that leakage rates through a geomembrane are very significantly increased by the presence of even a few defects, and defects when present must be found and repaired before the job is finished.
In CQA plans in these defects are methodically identified and then as much as possible completely eliminated.
In CQA the defects that are usually identified and which the installer must prevent come from several sources, which typically take the form of:
Defective geomembrane sheeting
Defective seams resulting from inadequate seaming methods, or poorly trained installation staff
Damage to the geomembrane during construction due to inappropriate subgrade materials or from construction plant or careless site personnel
Damage to the geomembrane after burial due to inappropriate subgrade and/or cover materials, or due to excessive loading.
Does all his intensive CQA checking work?
The answer is yes, but it is never able to consistently always produce a perfect result â€“ what human activity ever is?
It does appear to be worth doing, as US Studies (Giroud JP and Bonaparte R, Leakage through liners constructed with geomembranes, Geotextiles and Geomembranes. Vol 8, pp 27-67. 1989) have shown that the frequency of defects in geomembrane installations can be significantly reduced by the use of rigorous construction quality assurance.
However, what this also means is that unless the surrounding ground around a landfill is known to be a clay which is very impermeable and which itself will retain the leakage, or the surrounding geology comes somewhere close to this ideal, a composite liner (geomembrane (2mm HDPE say), plus a clay liner below it is necessary.
A combination of the best CQA practise and a composite liner will then be considered capable of achieving the intended and very essential protection of the locality from the pollution capability within any modern landfill.
( Article inspired by the paper by D Hall and P Marshall, Golder Associates in The Planning and Engineering of Landfills, Midland Geotechnical Society, 1991, UK)
The above is provided for educational and entertainment purposes only, and the reader must not rely on the content of this article to plan or design a landfill or the CQA measures applied.