It is important that all landfill gas extraction wells (also known as gas extraction boreholes) are designed carefully in accordance with best practice and in this article we have tried to identify the most important aspect of design. We also recommend the United Kingdom reader to consult the UK Environment Agency Publication, “Guidance on the Management of Landfill Gas”.
All landfill gas collection and extraction systems must be designed individually for the site, taking into account the depth, shape and topography of the site, plus the data from the gas production flow rate modelling and a pumping trial.
It will be necessary to obtain an up to date site survey of the landfill, and to be in possession of the fullest possible information on the size, location, and likely dates of infilling of any remaining landfill cells or phases to be developed.
The landfill gas systems installed all around the world rely on suction to that the system does not cause excessive amounts of air to enter the landfill gas. Great care must be taken to ensure that gas migration control (particularly near the site perimeter) does not draw in air and cause fires in the waste by sucking excessive amounts of air into the waste mass.
So, if gas migration is a potential problem on the site, the first requirement of the landfill gas collection system will be to encircle the site perimeter 40 m inside the perimeter with wells spaced at 40 metre centres, and drilled to the base of the landfill, without penetrating the bottom liner. The intention is that these wells will intercept any landfill gas which would otherwise migrate out of the site. If there is no man-made or natural site development lining then a large amount of air will tend to enter this ring, making the gas extracted a low quality. So for old gassing landfill sites without a clay lining it is usually best to run separate pipework routes for the outer and inner landfill gas extraction systems.
In central areas of the landfill the wells may be set out at a wider spacing, and it is often preferred to space them at 50 metre centres. Each borehole is drilled to its full depth at approximately 325mm diameter and is then fitted with a high Density Polyethylene (known as HDPE or PE) pipe. The actual pipe diameter chosen will depend upon the depth, as usually the pipe used will be smaller at the base so that the lower pipe lengths can b lowered through the larger diameter upper pipes. All well pipes are horizontally perforated at regular intervals, to allow leachate and landfill gas to pass through, except at the top where the pipe will pass through the landfill restoration capping material.
Leachate well pipework is not usually wrapped with a filter geotextile, which if used, tends to blind up with fine material and the flow into the well then reduces or ceases.
Twin wall construction, the inner wall smooth and the outer wall convoluted and with a maximum outer diameter of 160mm has been used by some, however, concern exists that this type of pipe is so flexible that there is a tendency for the wells made in this material to bend after installation more than for the solid HDPE pipe equivalent. The problem after gradual well pipe bending, is that the pump may become stuck at the bend.
Although there are differences in views and practice adopted by LFG well drilling contractors, it is normal that no gravel or any other material is placed in the annulus between the well rising pipe. Gravel packs tend to prevent differential settlement between the liner and the well head which is a thing to be avoided. Furthermore it has been observed that where a gravel pack has been used with certain types of liner the entire well pipe structure has sheared, as a consequence of being too rigid.
The top of the landfill restoration cap (or liner) is usually set at approximately 2.5m below ground level to accommodate this thickness of covering soils materials, and the use of a well head with a sleeved top pipe is good, as when settlement takes place the wellhead slides over the liner and no stresses build up which otherwise usually brings problems at a later date. When finally installed with the lateral flange connections are placed on the floor of the pipe trench, and they are sealed around with bentonite slurry.