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.

In the design of municipal landfill leachate collection systems, some state regulatory agencies require carbonate content of leachate collection system aggregate not to exceed 15 percent by weight. This requirement comes from a legitimate concern about the possibility of aggregate degradation, or loss of mass due to contact with leachate.

Most involved in landfill design and development will have experienced as a result, the fact that in some areas it is difficult to find carbonate free stone within an reasonably economic distance from the site. Many potential aggregate sources have been eliminated for supplying drainage material, due to this stipulation in the specification, but is it really warranted?

While leachate in MSW landfills is capable of dropping to pHs of 6.5, and sometimes 6, it rarely falls below this other than for short periods. This does not seem to be so low that problems would necessarily be serious, and if any of the carbonate dissolved from the stone, the amount would presumably be low as the reaction would be self limiting due to the dissolved carbonate caused by the reaction being bound to raise the pH. High pH will not erode the carbonate so the problem is corrected.

There is not a huge amount of research work on this that we have been able to find. We would be very interested to receive comments if our readers have sources to research on this matter which are more authoritative than the paper I am about to refer to.

The best paper we have found which sets out to by experimentation over a reasonably extended time period (in this case just under 6 months) to investigate whether carbonate drainage stone, when submerged in leachate, will suffer damage, is the following paper:

Suitability of Carbonate Aggregate in Land fill Leachate Collection Systems; Christopher G. Rubak, PE John,O. Starke, PE William D. Upman, PG M. Merrill Stevens, PhD: Presented to the Nineteenth International Madison Waste Conference, September 25-26 1996, Dept of Engineering Professional Development, University of Wisconsin - Madison.

This paper summarizes a research project which evaluated the suitability of a carbonate aggregate with a municipal solid waste leachate. The tests were conducted over a 20 week period using site specific landfill leachate and collection aggregate. Laboratory bench reactors were constructed to simulate landfill conditions with leachate flowing through carbonate aggregate.

The reactors consisted of 12-inch diameter plexiglass cylinders each charged with 80 pounds of carbonate aggregate. Leachate was then circulated through the reactors. An anaerobic environment was maintained in the reactors by applying 0.5 Atmosphere of CO2.

Fresh leachate was added to the reactors on a regular basis to maintain a constant concentration level during the test. Leachate samples were analyzed to determine the change in dissolved solids throughout the test period. Aggregate material was measured before and after the test to determine net mass change. Chemical equilibrium speciation modelling was also performed and compared to the bench test results.

On the face of it this experiment showed that there was no need for concern about carbonate deterioration even down to the exceptional pH 3.0 (exceptional for an MSW landfill under good regulatory control, built to good current standards).

However, the strange thing about the experiment to the writer is that the leachate used was changed on only, I think, 3 occasions; other than on these occasions the leachate was simply recirculated.

I would have preferred to see results which would ensure that the natural circumstances of a landfill were replicated more closely, and that would have meant allowing fresh leachate to pass through the system all the time.

The views of our readers are encouraged. There is a commenting facility available on the Blog Site to enable you to very easily let us know your views on this.

Retaining Wall System Combining Sheet Piling and Geogrid Reinforcements Now Available from Northstar Vinyl Products, LLC and Tensar International

The new AquaTerra™ system combines sheet piling and geogrid reinforcements, offering considerable benefits to slope stabilization, marine and levee repair projects.

Atlanta, GA (PRWEB) December 2, 2007 - Northstar Vinyl Products, LLC (http://www.northstarvinyl.com/) and Tensar International recently unveiled the AquaTerra Retaining Wall System, a proprietary new concept that allows sheet piling and geogrid materials to be combined for the first time.

“AquaTerra is a solution for many complicated situations,” says Jeff Moreau, President of Northstar Vinyl Products, LLC. “AquaTerra is an excellent option when rock and hard soils present challenges, because in most cases, the sheet piles only need to be placed in a two foot deep trench. The system is also an excellent choice when piping and global stability issues are present because the sheet pile can be driven to a specified depth and then attached to the reinforcement system.”

Until now, there was not a connection for a ’sheet pile-to-grid’ system.  
As communities continue to expand, land once considered unusable because of steep grades is now being improved with the use of retaining wall systems. AquaTerra is already having a dramatic effect on how retaining walls are designed and installed. Retaining walls once reserved for steel sheet piling are now being designed with light gauge vinyl or composite sheet piling supplied by Northstar.

Upland retaining walls are currently comprised of two components: a fascia system and a soil reinforcement system. Fascia systems are typically masonry block or pre-cast concrete panel systems. Soil reinforcements are either a geogrid or geotextile material. Using a variety of methods, fascia systems are connected to the soil reinforcement. The purpose of the geogrid is to diffuse the soil load that would otherwise act on the fascia by causing the soil to “stack vertically” as opposed to stacking at its angle of repose. Once the retaining wall is built, there is little soil load against the fascia.

“For years, engineers have known that if they had a method to connect geogrid or geotextile to a sheet pile, the same benefits would apply, and shorter, lighter gauge sheet pile could be used in most retaining wall applications,” Moreau says. “Until now, there was not a connection for a ’sheet pile-to-grid’ system.”
AquaTerra also provides significant benefits to engineers designing tied back sheet pile walls. With AquaTerra, walls once reserved for steel, composite or heavy-gauge vinyl sheet piling can now be built with shorter sections of lightweight vinyl sheet piling. In addition, AquaTerra has no metal components, which is a huge plus in saltwater applications.

The versatility of the AquaTerra system lends itself particularly well to flood control and levee structures. The Modular Levee System is composed of two parallel sheet pile walls supported by layers of geogrid in the core. The geogrid mitigates the soil loading in the sheet piling and in many cases will allow the use of native soil in the core. The Modular Levee System by AquaTerra provides several key benefits:

• The footprint of the levee is dramatically reduced, solving many environmental and easement concerns.
• Lighter and shorter sheet pile sections reduce the cost of getting sheet pile material to remote sites;
• Lighter and shorter sheet pile sections don’t require the use of heavy machinery on an existing levee;
• Lighter and shorter sheet pile sections are less expensive that optional materials;
  The geogrid, in many cases, will allow the use of native fill in the core, saving the expense of trucking in non-native fill;
• Whatever the choice of fill, less of it is required; and,
• The AquaTerra Modular Levee System can be installed in less time than conventional designs.

BlogMaster: System looks useful: but is it available outside the US?

Leading drilling specialist Magpie has announced the deepest boreholes/wells for leachate extraction drilled to date.

SITA UK approached Magpie with a need for one of the most challenging landfill drilling projects undertaken to date.

Enderby Warren a closed landfill site in Leicestershire has existing leachate extraction chambers 74m deep. SITA wanted to retro drill some chambers adjacent to the existing chambers as a back-up to the wells. Magpie were contracted to drill two wells to an estimated target depth of 75m.

To Magpie’s knowledge this had not been achieved before in landfill in the UK. They completed the first well in just under two weeks, and on the second visit completed the second well in a week. This included drilling of the wells in 450/350mm diameters, with a double permanent steel installation. 406mm steel was installed to 50m and 273mm steel installed to final depth.

More on Magpie’s web site here.

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 !!