Landfilling of Waste

7. Liner Systems

7.1 Definition and strategy
7.2 Types of liner
7.2.1 Clay liners
7.2.2 Polymeric membrane liners
7.2.3 Bentonite liners
7.2.4 Composite liners
7.3 Choice of composite liner systems
7.4 Execution
7.4.1 Bottom and side liners
7.4.2 Liner subgrade
7.4.3 Liner protection
7.5 Quality assurance for liner systems

7.1 Definition and Strategy

Liner systems are used where leachate from a landfill must be collected in order to ensure that soil, groundwater and surface water are not polluted by the leachate.

Basically, a liner system must be impermeable and provide optimum environmental protection throughout the active phase of the landfill. With that in mind every liner system for a landfill should be designed as a composite liner system or another bottom design which provides the same or better environmental protection. A composite liner system consists of a primary and a secondary liner in direct contact.

Another bottom design with similar environmental protection that might be considered is the use of hydraulic barriers. These must be regarded as site specific cases and call for separate assessment; therefore, they are not further described in these Guidelines.

7.2 Types of Liner

The material for a liner must be capable of withstanding the physical impacts to which the liner is exposed during the construction process as well as physical and chemical impacts during the active phase of the landfill. Table 7.1 shows the types of liner that may be used in a liner system.

Summary of the types of liner that may be used in a liner system with specification of requirements, control, execution and quality assurance.

1) : Please refer to Annex B until a revised edition of DS/R 466 includes
bentonite liners.
L: Clay rate.
L_p: Plasticity index.
k: Permeability coefficient (saturated hydraulic conductivity).

7.2.1 Clay liners

Clay liners are low-permeable and cannot provide 100 per cent assurance against percolation. However, if a suitable material is chosen and the work is carried out meticulously, percolation can be reduced to a minimum. There are two types of clay liner: installed and in-situ liners. In-situ liners are in the form of natural deposits of clay. Installed liners are composed of two or more layers of homogeneous clay spread on the landfill area.

For requirements as to properties, control of materials, and the execution of clay liners, reference in general terms is made to the revised edition of DS/R 466.

The recommended layer thickness and quality assurance are specified in Table 7.1, which also contains a summary of the most important requirements and material controls.

7.2.2 Polymeric membrane liners

Plastic and rubber liners (polymeric membrane liners) must be chosen and installed so that they can resist the physical and chemical impacts to which they will be exposed during the construction work and throughout the active period of the landfill.

For a polymeric membrane liner to be selected for the relevant purpose, it must meet a number of material-specific requirements. These requirements will appear from the revised edition of DS/R 466; therefore, reference is made only to that document.

Polymeric membrane liners must have a minimum thickness of 1.0mm, and the number of joints to be made in the field should be as low as possible.

Material control

The long-term resistance of polymeric membrane liners to the chemical and biological impacts that apply to a landfill must be documented either by simulation laboratory tests or by producing material based on experience.

In addition, there must be documentation for the requirements for materials contained in the revised edition of DS/R 466.

Execution

Polymeric membrane liners should be installed and fitted so that they are protected against impacts that could lead to puncture and reduced resistance. The base for polymeric liners should be pre-treated so that it is level and free from stones and subsidence, cf. DS/R 466.

Welds and joints are the weakest points. The number of welds made in the field must therefore be as low as possible. Any welding must be carried out by plastics welders who have attended and passed one of the courses approved by the Directorate General for Employment Placement and Vocational Training within the area. The execution control must be stringent.

7.2.3 Bentonite liners

A bentonite liner may consist either of sodium or calcium bentonite in the form of granulates or powder laid out between two geotextiles, or of loose bentonite laid out with a spreader and ploughed down into an underlying layer of sand. Such a liner should contain about 10 per cent bentonite by weight.

In terms of installation it is more difficult to produce a uniform liner surface when using loose bentonite than in the case of finished liners (bentonite mats/carpets). Also, more tests are required of the permeability coefficient of the installed liner than in the case of bentonite mats/carpets.

For a bentonite liner to be used in a landfill, it must meet a number of material-specific requirements. These requirements will appear from Annex B and the revised edition of DS/R 466.

Material control

The long-term resistance (especially the chemical stability of the contents of sodium or calcium) to the chemical and biological impacts that apply to a landfill must be documented either by simulation laboratory tests or by producing material based on experience.

In addition, there must be documentation as specified in Annex B and in DS/R 466 (after revision).

Execution

The supplier of the bentonite liner should prepare a detailed installation manual. The installation manual should also include a description showing how the base and the protective layer (drainage layer, cf. Chapter 8) for the liner are to be prepared. When the revision of DS/R 466 comes out, the installation manual should adhere to these instructions.

The installation manual should include a description showing how the liner is connected tightly to pipes, wells, joints, etc.

7.2.4 Composite liners

A composite liner consists of two types of liner joined together in production. The composite liner must be made of materials that meet the requirements of DS/R 466 in terms of tightness, physical strength, as well as chemical and biological resistance.

When producing and installing a composite liner it must be ensured that the permeability coefficient does not become inferior to the lowest permeability coefficient for the liners included.

A handling and installation guide from the manufacturer must be available.

Material control

Controls must be made to ensure that the liner materials included will not affect each other in such a manner that the composite liner fails to meet the requirements listed in DS/R 466.

Execution

The supplier of the composite liner should have prepared a detailed installation manual. The installation manual should also include a description showing how the base and the protective layer for the liner are to be prepared.

The installation manual should include a description showing how the liner is connected tightly to pipes, wells, joints, etc.

7.3 Choice of Composite Liner Systems

It is assumed in the following evaluations of a composite liner system that the liners it consists of - the primary as well as the secondary liner - meet the requirements listed for the individual liners in sect. 7.2.

Clay liners

In-situ clay liners can function in a liner system only as secondary liner. The primary liner can be a polymeric membrane liner, a bentonite liner, or a composite liner.

An installed clay liner will not be capable of functioning as primary liner if the secondary liner is an in-situ clay liner.

If an installed clay liner is to be used in a composite liner system together with a polymeric membrane liner, a bentonite liner, or a composite liner, it must be used as secondary liner and one of the others as the primary one. The reason is the substantial mechanical treatment during the installation of the clay liner.

Polymeric membrane liners

Polymeric membrane liners may be used as primary liner in a composite liner system. Clay liners and bentonite liners should be used as secondary liner.

Bentonite liners

Bentonite liners in a composite liner system can act as primary as well as secondary liners. Bentonite liners will often be used as primary liner in connection with clay liners and as secondary liner in connection with polymeric liners and composite liners.

Composite Liners

As described in para. 7.2.4., composite liners consist of two types of liner joined together by the manufacturer. If each of the liners the product consists of meets the requirements described in 7.2 above, it can be used as a composite liner system.

If one of the liners in the product fails to meet the requirement, or if some of the properties have been altered by the joining, a composite liner can be used as primary or secondary liner.

Protective layer

As soon as a composite liner system has been installed, the liner must be covered with a protective layer consisting of drainage gravel that meets the criteria in the most recent edition of DS/R 466. This layer will then act as the drainage gravel layer of the landfill unit.

The spreading of the protective layer should be done with caution. No traffic may occur direct on the liner. It may thus be necessary to take special precautions - such as in the form of installation of a protective layer of considerable thickness during the construction works - where traffic is unavoidable. A separate description of this must be available before the protective layer is laid out.

7.4 Execution

7.4.1 Bottom and side liners

Bottom and side liners for a landfill should consist of identical composite liner systems. Therefore, when choosing composite liner systems, allowance should be made for the location and design of the landfill.

If the landfill is placed in a hole or an excavation, it is necessary to ensure that the slope angle is not excessive. The slope angle should be determined on the basis of Skempton’s formula, cf. DS/R 466. Steps must also be taken to ensure that the liners in a liner system have mutually satisfactory friction properties and are stable both in their dry and wet states, so that no tensile stress will occur in the liner.

Side liners

An effort should be made to ensure that the side liner has as few welds/ joints as possible.

7.4.2 Liner subgrade

Liner subgrade

The subgrade for the liner system shall be natural deposits and/or backfilling with properties ensuring that:

	no differential settlement arises which is greater, within a 4-metre straightedge, than the requirements for evenness of the finished liner surface;
	within the service life of the landfill no settlement or deformations due to loss of bearing capacity arise that cause the liner surfaces to no longer fulfil the requirement for unidirectional fall towards the leachate collection system; and that
	during execution of the work no wheel tracks produced are greater than the requirements for evenness of the finished surface.

7.4.3 Liner protection

Liner protection

The lower side of the liner system should be protected against puncture and tearing by stones in the natural deposits or the backfilling.

The top side of the liner system should be protected throughout by, as a minimum, an 0.3-metre layer of sand/gravel with a uniformity coefficient greater than 3, cf. DS/R 466, which also meets the design criteria in para. 8.5.1 hereof.

Climatic effects

Any drying-out of liner systems that include a clay liner should be prevented by moistening the liner surface and protective layer during dry weather periods - until the liner has been covered by the first layer of waste.

Freezing of bottom and side liner systems containing clay should be prevented by covering with protective gravel and/or waste.

Protective layer

Cf. sect. 7.3 above.

7.5 Quality Assurance for Liner Systems

Quality assurance

Quality assurance for liner systems is carried out as a check of the individual components of the overall project. At the start of the design phase a control plan is drawn up; the plan will specify who is responsible for the control as well as the extent of the control at the various stages of the project. The control should include the design itself as well as a control of materials and execution. The control must be planned, executed and interpreted by technicians with sufficient insight and background. Records should be kept of the checks performed. The records must contain data and evaluations and should be kept as part of the final design documentation.

In addition to control (material and execution control) of all the materials included in the liner system, the liner subgrade and the protective layer should be subjected to control.

In general, the first requirement for the quality of the liner system is the use of highly qualified staff for the design as well as the execution.