A mineral barrier system developed in the Netherlands overcomes many of the limitations of mineral liners.
Mineral barriers are commonly used alone or with other sealing components to protect the environment from hazardous leachate or gas, particularly on landfill sites.
The most common are compacted clay liners (CCL), bentonite enriched sands (BES) and geosynthetic clay liners (GCL).
Although these systems can achieve good results, they have their limitations. Lack of understanding of these can lead to failure, causing significant environmental impact that can only be solved with costly replacement.
Field trials at the Georgswerder Landfill Site in Hamburg showed mineral barriers can, under certain circumstances, lose most of their sealing properties as a result of desiccation, plant root penetration and shrinkage when used as capping.
Slope stability problems can occur when designing with bentonite-based materials and the low tensile strength of thick clays and BES can lead to differential settlement and stress cracking.
GCLs are moderately sensitive to being punctured during handling and installation, including the placement of cover material.
They need thick topsoil layers or geomembranes to protect them from desiccation, plant root penetration and shrinkage.
Cation exchange leads to a rapid loss of the swelling capacity of sodium bentonites, something that has not really been addressed in the UK.
Trisoplast is a patented mineral barrier developed in the Netherlands in the early 1990s. It consists of a mixture of granular fill (normally sand), bentonite and a special polymer manufactured by Trisoplast Mineral Liners. It is a durable, flexible, physically and chemically resistant and effective seal with permeability typically between 1x10 -12 m/s and 3x10 -11 m/s).
The material can be mixed onor off-site. It is laid in a single layer, ideally spread with a longreach excavator. Very low permeability is reached almost independently of the moisture content and the compaction rate, thus variations during installation are not critical.
This makes it easier to install on very steep slopes or where access for compaction equipment is difficult. Sufficient compaction is easily reached using a lightweight static roller or vibrating plate.
In the last four years about 80% of landfill basal liners in the Netherlands have used Trisoplast following extensive government sponsored research and approval.
Further research in Germany by an independent committee, AK Trisoplast, concluded that a 70mm thick layer of Trisoplast is equivalent to 0.5m of clay (according to the German standard procedures for the evaluation of landfill barriers). Consequently it has been approved as a replacement to 'traditional' clay barrier landfill caps.
Cation exchange and desiccation In cation exchange, high swelling sodium bentonite is gradually changed into calcium bentonite.
The replacement of the monovalent sodium ion Na +by the bivalent calcium ion Ca 2+ leads to a significant fall in swelling capability (from 25 to 30ml/2g for sodium bentonite to 8 to 15ml/2g in calcium bentonite tested to ASTM D5890).
This not only causes a tenfold increase in permeability but can, in combination with desiccation and cracking, cause a total loss of sealing function, as the remaining swelling capability of the bentonite is not sufficient to reseal the gaps. GCLs exposed to desiccation and cation exchange in the field have shown irreversible increases of permittivity by a factor of between 10 3and 10 4.The polymer in Trisoplast interacts with the bentonite and sand to form a web-like structure.
These bonds reduce the total cation exchange capacity of the clay gel in Trisoplast to 450meq+/kg compared with 8001200meq+/kg for pure bentonite, making it more stable and analogous to the relatively stable clay mineral Illite.
Excavations of Trisoplast barriers five to six years after installation have shown the material did not desiccate or crack, even with thin cover.
To better understand these capabilities, Trisoplast was subjected to drying and wetting cycles and compared with the behaviour of the natural clay barrier originally used and studied at the Georgswerder landfill site.
Laboratory experiments simulated conditions and results observed in the field. The immediate response of the soil water tension within the clay barrier to irrigation of the sample after the first desiccation period (tension > 600hPa) and the measured flow through the sample showed that while continuous desiccation cracks formed in the clay barrier, Trisoplast remained undisturbed, with no throughflow and soil water tension dropping very slowly.
Trisoplast did not show any cracks even after a second desiccation period with soil water tensions above 1,000hPa.
Durability of the polymer Testing carried out at the Dr R Wienberg laboratory in Hamburg, Germany has shown that the clay polymer gel is not measurably attacked or degraded when in microbiological or chemically active environments.
However, as little is known of the durability of the system outside the pH range of 4 to 10, a comparative study with other mineral liners is recommended.
An initial study of ATO-DLO in Wegeningen in The Netherlands indicates promising results as the polymer seems to have a positive and stabilising effect on the bentonite structure at lower pHvalues.
Long-term slope stability can be a problem when designing with clay-based mineral barriers. This occurs because of clay's generally very low friction angle.
This can be a particular problem with the very low shear strength of a pure bentonite layer.
The problem is addressed within GCLs by mobilising the tensile strength of fibres or threads in needle-punched or stitch-bonded GCLs respectively. The durability of these under the aggressive environmental conditions of a landfill is critically important for long-term slope stability.
Trisoplast makes use of the advantages of both non-cohesive and cohesive behaviour because its sandy structure gives high shear angles, with numerous contacts of the coarse grained particles, combined with high cohesion values resulting from the clay fraction. This means significantly steeper slopes can be designed, without the need for further reinforcement.
The heterogeneous nature of waste often leads to differential settlement, causing extra stress and deformation of the landfill cap, which can easily lead to cracking in clays and BES systems, and an increase in permeability.
Whereas GCLs can potentially cope with significant stretching depending on the geotextiles and bonding technique, it might become critical after cation exchange or desiccation has taken place.
To demonstrate the deformation capability of Trisoplast, a special testing device was developed to measure permeability after stepped deformation.
Tests were performed on a 25mm thick layer of saturated and unsaturated Trisoplast. Even at 10% biaxial deformation, permeability was not significantly affected.
Compliance with European directive
According to the EU Directive on the landfill of waste, a geological barrier is needed to provide sufficient attenuation capacity to prevent potential risk to soil and groundwater.
The landfill base and sides have to consist of a mineral layer giving at least equivalent protection to a standard geological barrier as clearly defined in the directive.
Where the geological barrier does not meet these conditions it can be completed artificially and reinforced to give equivalent protection. An artificially established geological barrier should be no less than 500mm thick.
A number of European countries have accepted that a 90mm thick Trisoplast layer used to reinforce 410mm of prepared subgrade, predominantly acting as a substantial barrier to diffusive contaminant transport, meets the above requirement.
This has been allowed because the two layers offer a better environmental protection than traditional mineral barriers.
While Trisoplast has not been used in the UK, there has been much independent research and testing and there is now considerable experience in the Netherlands and in Germany. As a result, it has been accepted in many European countries as a suitable replacement for traditional mineral barrier systems.
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