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How to Design a Permanent Geological Exposure in a Landfill

One of the most common locations for landfills are worked out quarries and quarries suitable for landfill are an increasingly valuable resource for this reason.

In a growing number of cases suitable sites include rare geological exposures of mineral bearing rock, or strata of regional importance which need to be kept exposed after landfilling for educational and also often for historical reasons.  In the United Kingdom these features are identified at planning permission stage and usually allocated Special Scientific Interest (SSI) status.

These SSI’s can result in conflict between conservation and waste disposal interests. The geological feature is usually below the intended restoration level and results in a low point being left in the restoration profile where the SSI is present.

Where quarries used for waste disposal contain Sites of Special Scientific Interest, it is necessary to maintain safe long term access to the geological exposure. The landfill operators will wish to minimise sterilisation of void space for the waste. These objectives can be met by the construction of a structure which limits land take and which maintains a safe barrier to the waste material.

However, it is possible to minimise the conflict and to provide for these geological SSI’s without undue difficulty, as we will describe.

The following list of considerations is broadly based on research described funded by the Nature Conservancy Council in the early 1990s, and has led to the identification of engineering measures designed to optimise landfill void in quarries whilst protecting, in the long term, geological Sites of Special Scientific Interest.

1.    To provide long term, safe, unhindered access to the geological exposure with minimal sterilisation of landfill void space for waste, it is necessary to provide an engineered structure which limits land-take and which maintains a safe and  secure perimeter barrier to the waste material. Long term slope stability must be checked by geotechnical analysis, but it is not the sole design consideration since the access to the exposure must remain drained, be free of leachate and free of significant concentrations of landfill gas.

2.   The presence of a geological exposure in a quarry used as a landfill may have a significant effect on the design and operation of the landfill particularly with respect to leachate management. In some cases in nations where leachate levels are not controlled by landfill site licenses it will be necessary to maintain leachate in the landfill at a much lower level than if a geological Site of Special Scientific Interest was not present.

3.   Natural drainage should be provided where possible to prevent the accumulation of surface water adjacent to the geological exposure. Where this is not possible or the base of the geological exposure is below the water table, pumping may be necessary to facilitate access to the exposure. In such cases it will be necessary to maintain the level of accumulated water below that at which it  will flow into the landfill to prevent the generation of unacceptable volumes of leachate. In addition, it will be necessary to minimise the volume of water which may become contaminated by leachate so rendering it unsuitable for discharge to the surface water system.

4.    It may be necessary to take measures to prevent the movement of leachate from the landfill site through or beneath the waste retaining structure towards the Site of Special Scientific Interest where it  may contaminate accumulating surface and groundwater. The measures may include excavation of the base of the landfill site to a lower level and maintaining leachate below the level of the  base of the geological exposure, reducing the leachate level by pumping and the construction of a low permeability leachate retaining structure keyed into the low permeability materials forming the base of the site.

5.     Landfill gas is flammable, is explosive if ignited in an enclosed space, and can also create an asphyxiating atmosphere. In Europe gas hazard sites (such as landfills) are controlled by the ATEX Directive and national regulations, such as the UK’s Dangerous Substances and Explosive Atmospheres Regulations. Landfill sites in most nations now have gas control systems, and with adequate control it is considered unlikely that landfill gas will accumulate in significant concentrations adjacent to a Site of Special Scientific Interest. However, this may not always be the case and especially if no landfill gas extraction is provided on the site, the area should be monitored for the presence of methane and carbon dioxide prior to access, and ATEX Rules applied as appropriate.

6.   Where the landfill perimeter slopes adjacent to the geological exposure are engineered and graded to a profile of less than 1:3 access by visitors on foot across mown ground should present no significant problems if all visitors wear suitable footwear. Where steeper landfill perimeter slopes are designed an engineered access route may be necessary in the form of a graded path across the landfill or the geological exposure or a purpose made staircase from original ground level to the floor of the exposure.

However, if the above criteria are met, there is no reason why a geological SSI and a landfill cannot co-exist without a significant conflict of interest.

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