Landfilling of Waste

Annex B: Quality Requirements for Bentonite Liners

This Annex will apply only until the newest revised edition of DS/R 466 is to hand - with specific criteria for bentonite liners.

Before an order is placed the manufacturer of the liner must be able to substantiate that he is capable of delivering a product which meets, as a minimum, the requirements for material specified in Table B.1.

When producing the bentonite liner it must be ensured that the liner consists of a uniform layer of bentonite (same thickness of bentonite overall, no holes) laid out between two geotextiles.

Bentonite is a soft, rich clay consisting of the clay mineral montmorillonite, which is of volcanic origin. One of the most important properties of bentonite is its ability to adsorb water. The water molecules are bound electrically as dipoles in the diffuse double layer surrounding each clay mineral. The ability to bind water is directly related to the thickness of this double layer and therefore also to the ions that are bound to the surface of the clay mineral. Univalent (+) cations, especially sodium, are not tied quite as strongly to the clay mineral as divalent cations (++); therefore the double layer around the clay mineral will be thicker. The type of ions which are thus bound to the clay mineral (montmorillonite) is important to the ability of the bentonite to bind water. Obviously, the quantity of the special clay mineral montmorillonite in the bentonite is also relevant to the ability of the bentonite to bind water.

Requirements for bentonite liners

The ability of European bentonite (high proportion of calcium, Ca) to bind water is thus inferior to that of American (Wyoming) bentonite, which has a high natural content of sodium, Na. The ability of selected types of bentonite to bind water is listed below - expressed as the quantity of water (cm³) adsorbed per gramme of dry clay:

As the list shows, European bentonite can be improved by treatment with sodium ions.

Furthermore, the thickness of the double layer in the clay mineral will decrease as the ion strength in the soil/water around the bentonite increases. Therefore, leachate can reduce the thickness of the double layer both through its content of salts and substances that can exchange ions with the original substances (sodium, calcium) in the bentonite.

Most manufacturers of bentonite liners have a product which contains, nominally, 4.9 kg of bentonite per m² (not including additives) and a minimum of 4.4 kg/m² at 15% moisture content, not including additives. Besides, most manufacturers can deliver a product which contains precisely the quantity of bentonite needed. The above requirements for materials are thus minimum requirements and reflect the quantity of bentonite which will achieve a permeability coefficient of less than 5 x 10^-11 m/sec for the liner.

The bentonite must be arranged between two geotextiles, one above it and one beneath it. In some products the bentonite is fixed to the supporting geotextile by means of a glue; in other products it has been bonded together by needle-punching (a form of sewing) between the two geotextiles. If glue is employed to retain the bentonite, it must be ensured that the glue does not adversely affect the natural properties of the bentonite (the ability to bind water), and that the bentonite is not placed incorrectly or damaged in rolling-up, installation, or covering.

The geotextile which has primary responsibility for retaining the bentonite must have a certain thickness and must be capable of withstanding a certain tension, etc. It is therefore recommended that at least one of the geotextiles in the bentonite liner should have a density of not less than 135 g/cm².

Stronger geotextiles may be necessary where the subgrade is very uneven or in the case of steep slopes.

Most bentonite liners have permeability coefficients ranging from 1 x 10^-10 to 1 x 10^-12 m/sec. The permeability coefficient should be determined in accordance with DIN 18130 (a water pressure of 20cm and a load of 5 kN/m²) or ASTM D5084 (2 psi effective load).

The tensile strength should be at least 14 kN/m² determined in accordance with DIN 53857 T2 or ASTM D4632.

The elongation must be 15% in the direction of the mesh determined in accordance with DIN 53857 T2 or ASTM D4632.

The puncture resistance must be greater than 222 N determined in accordance with DIN 54307 or ASTM D4883.

It must be possible to carry out joining by simple overlapping of two pieces of liner (minimum 15cm). If loose bentonite is used in the joint, it must be laid out by machine so that a more uniform seal is ensured. The joining method must be stated prior to selection of liner supplier.

Documentation should be provided to the effect that the type of bentonite liner has been successfully installed as bottom liner in a landfill area in at least three cases. References must be included, stating the location, the name and address of the owner, the purpose, surface (m²), and the year.

The control of materials must be stringent.

Bentonite liners are supplied in rolls. Rolls that are damaged or show visible defects or are not declared correctly must be rejected by the developer’s supervision. The supervision must make sure that the rolls are stored in a dry place and covered by a waterproof tarpaulin.

Strips measuring 30cm x the width of the roll should be taken from a random selection of the rolls delivered in a consignment. Samples are drawn to determine the content of montmorillonite, the quantity of bentonite per cm², the permeability coefficient, and the tensile and puncture resistance. The said parameters should be established by a recognised geotechnical laboratory (accredited).

The control of execution must be stringent.

Bentonite liners are laid out by machine. An excavator is fitted with a tube and a chain. The tube is placed in the liner roll, and the chain is fixed to the tube. The machine lifts the roll so that the roll hovers 1-1.5 metres above the ground. The machine is driven to the place where the liner is to be laid. Here at least two men will grasp the edge of the roll and put down the loose end of the liner in the desired place. The machine is driven backwards slowly while the loose end is being held in place. There must be no driving on the liner itself.

In the case of bentonite liners where loose bentonite is used in the joints, the automated equipment for the laying of the loose bentonite must be calibrated separately on a flat subgrade and on slopes. 20 kg of granulated bentonite ( +/-0.5 kg) are spread evenly on 50m of overlapping joint. The equipment must be recalibrated once every week during the installation. In the case of manual laying of bentonite the overlapping joints should be divided into marked sections of 50m each. A 20-kg bag of bentonite must be spread evenly over this stretch. The mode of spreading and the walking speed must be the same throughout the stretch.

On slopes and gradients the liner is also laid by machine, but in such a manner that the machine is stationary at the top while a number of persons pull the liner from the roll. The various liner manufacturers have methods for reinforcement of joints on slopes. Alternatively, it is possible to have pieces of liner produced which are particularly long and which have such a strength that joints on slopes are avoided.

Special care is required in connection with joints around pipe penetrations and similar. Normally, these are made by cutting a hole (a cross with a sharp knife) in the liner where the pipe is to go through. Some loose bentonite may be placed close to the pipe. A piece of liner of suitable dimensions in relation to the pipe, and where the hole for the pipe is precisely cut, is placed on top of the bentonite.

As soon as it has been laid out the liner must be protected by at least 20cm of stabilised gravel. The surface of liner laid out in one day must not be so great that protective gravel cannot also be laid out.

Each manufacturer has prepared installation manuals that are peculiar to the individual products and where the laying and execution of certain pipe penetrations, adhesions to concrete, etc., are described. These installation manuals should always be observed.

Before the bentonite liner is delivered the manufacturer must submit details of quality control data for the rolls which the consignment will include. The desired information must, as a minimum, contain production dates, batch No., roll No., test results, and approval codes. The number of tests, the mean value, standard deviation, minimum average roll value, and maximum average roll value must also be included.

In addition, the contractor must prepare a drawing showing the location of liners and constructional details of the total area to be covered with bentonite liner. The drawing is to be used during the work to indicate the location of each roll No., so that it is known precisely where each individual roll is placed.

At least one control test of overlappings must be carried out on site.

Test box, cover layer material, bentonite.

Place a 15cm overlapping in the upper part of the test box.

Seal the edges with bentonite to prevent side leakage.

Place 30cm of cover layer material on top of the overlapping.

Fill the upper box with 45cm of water.

After the initial hydration period (six hours), remove any water that may have seeped into the lower box.

Over a period of seven days - at 24-hour intervals - measure the volume of water in the lower box and record it, indicating the time.

The permeability coefficient (k) for the joint is calculated as follows:

The permeability coefficient for the joint must be less than or equal to the permeability coefficient for the liner itself. If the test reveals major deviations - having regard to the uncertainty of the test - the overlapping method must be reconsidered.