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.

A question for landfill construction quality assurance engineers which arises regularly is whether they should use Triaxial Compression Testing or Direct Shear Testing.

Well although triaxial compression testing is the most frequently used small scale direct shear testing does have certain advanatges and can be appropriate under certian circumstances.  

There is an interesting dicussion of this issue at the Engineering-Tips Forum. Where there is an excerpt from Holtz and Kovacs (section 10.5) that addresses this issue, as follows:

The advantages of the triaxial test over the direct shear test are:

• Progressive effects are less in the triaxial
• The measurement of specimen volume changes are more accurate in the triaxial
• The complete state of stress is assumed to be known at all stages during the triaxial test, whereas only the stresses at failure are known in the direct shear test

The triaxial machine is more adaptable to special requirements.

The advantages of the direct shear test are:

• Direct shear machine is simpler and faster to operate
• A thinner soil sample is used in the direct shear test, thus facilitating drainage of the pore water from a saturated specimen. 

Landfill Construction Quality Assurance places a dual requirement on engineers and technicians which is seldom the case for general earthworks projects. In landfill construction lining and capping soils data must demonstrate both structural strength capability, and provide a seal against the passage of water (leachate) and landfill gas, so reliable testing for the low permeability of these materials is also necessary.

We have been asked where the necessary skills can be obtained for Landfill Construction Quality Assurance work.

The core skill needed for Landfill CQA is an good appreciation of soils mechanics, as most of the CQA work relates to civil engineering earthworks, so a good grounding comes from the Soils Mechanics tought within any practically based Civil Engineering Course, at HNC, HND, or BSc/degree level.

From time to time, for those who need additional training there are also specialist Soils Mechanics and Geotechnical Engineering courses available.

In addition, experience of construction site work is also very important. It is during site work that a CQA Supervisor will obtain familiarity with the wide variety of proprietory geosynthetic materials available, and the various methods of testing employed to ensure contract compliance with the specified installation tests and inspections.

For geomembrane liners and cappings there are very few courses available, so most training in good practice for membrane testing is “on the job” and provided for CQA staff within the consulting organisations undertaking this work.

A good starting point for anyone seeking employment in Landfill CQA in the UK, would be to visit the Charterd Institute of Waste Management (CIWM) web site and look for the next courses in such subjects as “Advanced Landfill Management”.

Southampton University (UK) for example provides a good Soil Mechanics course here.