The operator of a modern landfill site in responding to the requirements for minimising leachate generation is likely to fill the site rapidly within small constrained areas to reduce rainfall ingress. When the operational cell is complete to restoration levels it is likely to be capped to prevent further ingress of rain water. This means that even the oldest waste at the bottom is therefore likely to be in only the early stages of degradation.

Significant settlement can then be expected due to the following mechanisms:-

i. The load on waste in the lower levels imposed by waste above it, particularly for deep sites, will be several times greater than that imposed during the initial compaction process using mechanical compactors. This will result in continued, graded compaction through the waste. This mechanism for settlement is likely to be predominant during filling and immediately following capping.

ii. The degradation process will break down waste into a denser material.

iii. The production of gas will mean a net mass loss of possibly 18% assuming 150m3/hr of landfill gas at 1.15kg/m3 is abstracted from each tonne of waste.

iv. Removal of leachate from lower levels of waste can also cause further settlement as pore pressures are reduced.

Settlement is therefore inevitable and must be catered for in the design of the restoration capping and in the design of both gas abstraction system. It is the job of the landfill design Engineer to assess the site “condition” and determine the potential for further settlement so that he can be satisfied a suitable design is proposed.

The CQA Engineer will verify the degree of settlement on-site through the contractor’s survey etc and ensure that the landfill design Engineer’s requirements are implemented during construction of the restoration capping.

We will occasionally post capping design information on this blog. We hope that you will find these posts interesting.

To guarantee a landfill cap which will drain adequately and for effective gas collection, the landfill cap should incorporate a drainage layer above and preferably also, a gas collection layer below the cap’s low permeability lining system. 

The geosyntheic “Pozidrain” may be able to provide these functions with higher performance and lower cost than conventional crushed stone filters. The Pozidrain is also favoured by operators who want to squeeze the last once of waste into their landfill as the thickness of this material is much less than a stone layer, allowing more waste to be put in the landfill before the planning consented top of site levels are reached.

ABG Geosynthetics tell us that:

Pozidrain has been specially designed to be compatible with both HDPE and clay liners and to give the optimum performance over the whole life of the landfill capping. Pozidrain will enhance the performance of the GCL or HDPE liners by providing an additional barrier that prevents the majority of the water or gas reaching the liner. Pozidrain geocomposite drainage layer has a proven record in landfill capping.

If you are still wondering what Pozidrain material is, think of a very big area of a membrane material comprising many “egg boxes” joined together. Across the face remote from the low permeability landfill seal is “tack” welded a geotextile filter material. The egg box profile creates a continuous void which allows water penetrating the filter to drain away. A very simple but useful idea.

The Landfill CQA technician/supervisor must at all times ensure that the Pozidrain supplied meets the specified requirements, and is laid in accordance with the specification. This is likely to involve the checking, and acceptance of the contractor-provided supplier CQA records, plus some additional independent testing for verification of factory test data.

NORTHERN IRELAND INTERIM GUIDANCE ON LANDFILL CLOSURE: CAPPING AND RESTORATION:  The Environmental and Heritage Service haas released Interim Guidance on Landfill Closure: Capping and Restoration for EHS staff and operators. The procedures may be applied to both existing and new landfill sites. The guidance document sets out a brief overview of the main aspects to be considered when establishing the capping standards at landfill sites. More information here. 

There will be a strong workload for landfill capping contractors this summer due to the closure of many landfills during 2006, which has taken place as a result of the ending of the grace period for the transition from Waste Management Licences to PPC Permits under the IPPC legislation.

Many landfill owners which did not apply for PPC Permits, and instead closed their sites, were able to keep them open until the end of the year, and even those closing earlier missed the summer (good weather) earthworks window for 2006. Therefore, they have been lining up their projects for this summer.

Landfill owners are required to cap and restore their sites under all legislation (old Waste Regulations and within their PPC permits) throughout Europe.

Although most savvy operators are actually sealing their sites as early as possible, and before capping, in order not to lose valuable landfill gas, which is utilised almost universally now (at larger sites) for power generation and the electricity revenue that provides.

Nevertheless, by permanently capping the site as soon as possible they provide themselves with the best permanent installation for leachate extraction at the earliest opportunity. Leachate collection from the gas wells will be minimised as soon as possible, minimising leachate generated, and reducing leachate disposal costs. At the same time the installed permanent leachate pumps will ensure maximum landfill gas yield by avoiding some wells being effectively blocked to gas production, by the presence of standing leachate.

For those. like Rowe Environmental, working in landfill Construction Quality Assurance, the summer also looks like being a very busy time.

Ken Rowe - Rowe Environmental