Leachate drainage layers are necessary in most waste landfill sites to minimise the accumulation of leachate within the site and they reduce the risk of contamination of surrounding ground and groundwater. A cheaper and environmentally preferable option is be the use of scrap vehicle tyres, but is their use permissible and what happens to them under pressure? A paper in the proceedings of Waste 2004 by A.P. Hudson, R.P. Beavan, and W. Powrie helps us to understand this.

Normally layers of whole or shredded tyres exhibit excellent drainage properties, but if tyres are used as the main drainage layer at the base of a landfill the concern exists that they may compress under the overburden stress from the weight of the waste above and cease to act as an effective drainage layer.

The results of a series of tests undertaken by the University of Southampton are reported by the above researchers as presented in their paper examining the compressibility and changes in hydrogeological properties of shredded and whole tyres subjected to a range of stresses typical of landfill conditions.

In the UK over 400,000 tonnes of used vehicle tyres are produced each year (Hird et al. 2002). The problem of disposing of used tyres has been made worse by the EU Landfill Directive which prohibited the disposal of whole used vehicle tyres to new landfills from 16 July 2003. The disposal of shredded tyres to landfill will be banned on 16 July 2006. There is, therefore, a need to establish alternative methods of re-use, materials/energy recovery and disposal of tyres.

The Landfill Directive permits used tyres to be utilised as engineering material in landfills. Use of whole or shredded tyres are often a cheaper and environmentally beneficial alternative to aggregates for the construction of landfill drainage layers or trenches. However drainage layers at the base of landfills will be subjected to high overburden stresses from waste subsequently placed above.

There is little published research indicating i) the extent to which tyre drainage layers will compress under such stresses, ii) the reduction in hydraulic conductivity due to compression and iii) the effect of tyre shred size on the compressibility and hydraulic conductivity of tyre layers. However, these atters have been addressed in their paper in a large scale compression cell in order to investigate the above.

The data demonstrated that tyre layers will compress under stress and this will result in a reduction of drainable porosity and hydraulic conductivity. The construction of any leachate drainage layer using whole or shredded tyres within a landfill would need to take into account the compressive behaviour of the material under load.

Countries that have specified a minimum hydraulic conductivity for landfill drainage layers generally give values of between 1 x 10^-3 and 1 x 10^-4 m/s.

However, this group found that shredded tyres would easily comply with requirements as low as 1 x 10^-3 m/s at stresses up to 600 kPa, but would only meet the most stringent requirements of some nations at stresses below 400 kPa.

The data presented in this paper demonstrate that the hydrogeological properties of whole and shredded tyres change according to the applied stress. In general the data indicates that shredded tyres are suitable for use as a drainage medium in landfill applications.

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/

Dense Asphaltic Concrete can be an extremely versatile product, suitable for many types of applications.

The ancient civilizations of Egypt, Babylon and Assyria all used bituminous lining materials and mortars for waterproofing and building, some more than 5,000 years ago, and many examples of their work remains intact even today.

More recently with the advances in hydraulic technology, asphalt has been shown to be an effective material for sealing Dams, Reservoirs, Canals, Water Catchments, Sea Defences, Coastal Groins, River banks and importantly for us - Landfill Sites.

WALO is a main UK supplier.

Current landfill site practice in response to the requirements of the Waste Regulations (UK) is to minimise leachate generation. To achieve this the operator fills the site, in a series of phases or cells which are raised as rapidly as input rates will allow to the top of the landfill, 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. Even the oldest waste at the bottom is therefore likely to be quite young and very little of degradation will have taken place to most of the waste.

So, under these circumstances significant settlement can then be expected, no matter how well the waste is compacted by the action of the site “compactor vehicles” (wheeled or sometimes tracked front-end shovels – often called “buldozers” by the public).

Settlement occurs 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, physical compression compaction throughout the waste. This mechanism for settlement is likely to be predominant during filling and immediately following capping.
ii.The degradation process breaking down waste into a denser material.
iii.Volume reduction due to volatilisation (carbon emission in landfill gas etc). The production of gas will mean in very approximate terms a net mass loss of possibly 18% in total waste mass assuming 150m3 of landfill gas is in time extracted from each tonne of waste at 1.15kg/m3.
iv. Removal of leachate from lower levels of waste may also cause further settlement which is probably due to the pore water pressures being reduced.

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

Determination of settlement rates and possible leachate levels is particularly pertinent to new sites and are important design parameters for the landfill gas Engineer to determine before he starts his system design.

If you need your landfill modelled for settlement, and settlement prediction provided, this is a service we provide regularly through our associated consultants. Just Contact us by email at news [at] landfillcqa.co.uk (Please replace [at] with @ ).

Although the following topic is not directly a landfill CQA issue we thought that it would be of real interest to many of our readers.

Compulsory site waste management plans (SWMPs) for construction projects over £300,000 in value, have been a legal requirement in England since April 2008, and are therefore needed for landfill works contracts. The intention is that the plans will forecast all waste produced on site and how much will be recovered or disposed of.

The SWMP should help businesses manage construction waste as part of a project rather than an afterthought. By planning for construction waste much earlier than has been the practice it should be possible to do much better in managing construction waste. Early experience that has been reported has suggested that in building projects it has been possible to achieve substantial savings which exceed the costs of producing and updating the site waste management plan.

A landfill site lining development or surface restoration project is undeniably a construction project, and almost all will exceed the threshold cost. Therefore, each project will need a site waste management plan, which starts with the designer and becomes the responsibility of the principal contractors on commencement of the work on site.

The regulations set out a range of offences relating to the failure to produce or implement a plan, punishable by a fine of up to £50,000 on summary conviction, or an unlimited fine on conviction on indictment.

On these landfill projects there will be little if any construction waste produced at all, apart from possibly some spoilt or unsuitable material, and the usual site facilities wastes. As ever, these otherwise laudable regulations make little sense in some applications, and in landfill in our view, we have such an example.

So how does a landfill contractor set about writing his site waste management plan after being awarded the contract and receiving the client’s design stage plan, for a landfill project?

What should the SWMP contain?

According to NetRegs, the level of detail that your SWMP should contain depends on the estimated build cost, excluding VAT. Their summary suggests the following:-

For projects estimated at between £300,000 and £500,000 (excluding VAT) the SWMP should contain details of the:

• types of waste removed from the site
• identity of the person who removed the waste
• site that the waste is taken to.

For projects estimated at over £500,000 (excluding VAT) the SWMP should contain details of the:

• types of waste removed from the site
• identity of the person who removed the waste and their waste carrier registration number
• a description of the waste
• site that the waste was taken to
• environmental permit or exemption held by the site where the material is taken.

At the end of the project, you must review the plan and record the reasons for any differences between the plan and what actually happened.

The contractor, or anyone else exporting waste from the site must still comply with the duty of care for waste.  However, because it will now be necessary to record all waste movements in one document, having a SWMP will help the site contractor’s management to ensure they comply with the duty of care.

For help in preparing your SWMP see the video on the Site Waste Management Plan hub page, or  Landfill Site Waste Management Plan page.