Dense Asphaltic Concrete has been as a Landfill Lining membrane material for many years and although it is not commonly seen in the UK, it is used much more often in Europe, and particularly in Germany.

The Dense Asphaltic Concrete is formulated in a manner which ensures both very low permeability and possibly as low as 1 x 10^-11 m/s, and permeability to landfill gas is also lower than for clays.

This is an imporant point as the ability of clays to pass some methane gas is often overlooked for single clay liner designs. Let us not forget either that the primary motivation for the composite lining (clay/HDPE)systems which are the norm now throughout the British Isles, was originally the concerns about landfill gas movement through the clay membrane. It happens also to be far less likely that a pinhole in the HDPE will ever leak when in a composite arrangement that hole will be backed up by the clay geomembrane layer.

Asphaltic concrete is stable on steeper slopes than HDPE unless special measures are taken to support and/or reinforce the HDPE, and much less prone to the sort of slip plane development we often see between HDPE and clay and sand/HDPE on slopes and banking.

Another big assett when considering this alternative lining material is its robustness during the infilling of the first layer of waste. All CQA Engineers worth the name will have experienced HDPE lining damage which occurs when the top 300mm of leachate drianage stone is being emplaced, and then potentially can occur again when the compactor vehicle runs in with the first waste lift.

Resistivity checks completed once the leachate drainage stone over the membrane often identify small holes in the HDPE liner after the leachate stone has been spread and levelled. These tend to be caused by a moments lack of concentration which the driver may suffer. Unfortunaelty, one unfortunate jab downwords (as the backactor spreads the sand or gravel of the leachate drain over the HDPE geomembrane) can be all that it needs to created minor pin holes, and to see about a dozen occurring per hectare was not uncommon in CQA Reports, a few years ago.

A UK specialist contractor offering Dense Asphaltic Concrete in the UK is WALO UK. http://www.walo.co.uk/

Summer 2007 saw the environmental engineering consultants Enviros Consulting return to Wexford County Council’s Killurin Landfill Site, for further capping and restoration works.

This year sees the penultimate stage, of a 4 year involvement of Enviros with local authority Wexford County Council at this active domestic waste Landfill site.
Wexford originally commissioned Enviros back in 2004 to carry out extensive extension earthworks works to stabilise slopes and increase the existing landfill capacity. Over the following two years, further works were carried out to place the final cap on completed sections of the site, install surface water management systems, place restoration soils to a finished profile and grass seed the area .
Steve Last, Principal Engineer and Project Manager heads up the project, with Rowe Environmental providing CQA supervsion.

Outstanding Beauty
Killurin Landfill which has been operational since the early eighties lies some 8 miles outside of Wexford Town, and bordered by the River Slaney to its south, in what is a most outstandingly beautiful part of Southern Ireland.
The views across the surrounding areas, from within the site are extraordinarily beautiful.

Walking around the site again this year, you get a real feel for the necessity and value of the restoration works which were done last year, as the grass seeding is well established, it looks green and very natural, and fits in with the local existing environs.

The views across the River Slaney are undoubtedly superb.

2007 Capping and Restoration Programme
This year a further 10,000 sq,m of landfill area will be capped with clay restoration soils placed and hydro seeded.   Capping materials, being a locally won “marl” type clay, that meets specification limits on density and compaction, found within 1000 metres of the site, has been most fortunate indeed, given the scarcity of suitable materials.

A Geo-drainage layer is to be incorporated in the above-cap drainage system linked to a surface water collection system, along with the construction of surface water outfall points.

Erosion control matting will be veneered between the sub-soil and top-soil layer. Improvements to the gas management systems are also required.
An initial twelve week programme is on a two week break just now, but continues in the second week in August 2007 and all of works are hoped to be complete by mid- September. weather permitting…………………….. of course !!

The purpose of Landfill Construction Quality Assurance is to ensure that the landfill is constructed in full compliance with the specification. The primary responsibility for ensuring this, rests with the landfill construction (civil engineering) contractor. The CQA Engineer (provided by Rowe Environmental), on site, provides independent inspection and verification of the work, and receives all the contractor’s site construction quality assurance records.

We’ve been asked just where Landfill Construction Quality Assurance fits into the various components and stages of landfill development.

A typical landfill consists of the following subsystems such:

• Landfill liner
• Leachate collection and management system
• Landfill gas management system
• Landfill gas monitoring & leachate monitoring systems
• Final landfill cap
• Drainage system
• Road network

Their function is to secure the normal landfill operations and to control the anticipated emissions generated mainly by the decomposition of organic matter, such as leachate and landfill gas.

The appointed Landfill CQA Engineer is normally required to inspect all parts of the works constructed during the landfill development contract works which will control the potential emissions from the site. So, from the list above, the CQA Engineer is normally required to inspect, witness test, and verify the recording of all the above listed subsystems except for the road network. Even within the road network he/she would also inspect the work such that the road networks installed for access close to the development works will not be such as to risk damage the integrity of the liner, walls, or capping.

The widely used definition of geophysics is that provided by Sheriff in the Encyclopedia of Exploration Geophysics.

For the purposes of this site, we refer more specifically to the following definitions - these focus on Environmental and Engineering Geophysics:

1. Geophysics is: The subsurface site characterization of the geology, geological structure, groundwater, contamination, and human artifacts beneath the Earth’s surface, based on the lateral and vertical mapping of physical property variations that are remotely sensed using non-invasive technologies. Many of these technologies are traditionally used for exploration of economic materials such as groundwater, metals, and hydrocarbons.
2. Geophysics is: The non-invasive investigation of subsurface conditions in the Earth through measuring, analyzing and interpreting physical fields at the surface. Some studies are used to determine what is directly below the surface (the upper meter or so); other investigations extend to depths of 10’s of meters or more.

Both of these definitions have a common component, namely that geophysics represents a class of subsurface investigations that are non-invasive (i.e. that do not require excavation or direct access to the sub-surface). The exceptions are borehole geophysical methods that expand the use of holes already drilled to access the subsurface on a very localized basis.

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.