Hydraulic and earth engineering – Earth treatment or control – Impermeabilization
Patent
1996-04-24
1999-01-19
Graysay, Tamara L.
Hydraulic and earth engineering
Earth treatment or control
Impermeabilization
405129, 405 38, E02D 312
Patent
active
058607722
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an improved clay liner to be used to form an impermeable barrier. In particular this invention relates to an improvement in the stability of the liner while being installed and also in the hydrated state while in use in steep slopes.
The present invention also relates to a method and an apparatus for the production of the clay liner of the invention.
BACKGROUND ART
Clay liners are widely used in the fields of Civil, Environmental and Geotechnical Engineering for waterproofing surfaces of soil, lagoons, foundation surfaces, and in particular constructions for the containment of pollutant or toxic waste. The known clay liners are generally formed by a layer of swellable clay between two fabric layers (geotextiles). The clay used in this type of liner is predominantly Sodium Bentonite, wnich, when it comes into contact with water or other liquids, will swell to several times its dry volume if not restrained by a confining stress. The swelling of the clay forms an impermeable barrier. The clay is usually confined between two fabrics and the clay and fabrics are held together by either bonding with water soluble adhesive or mechanical bonding.
In adhesive bonding (see U.S. Pat. No. 4,501,788) the two fabrics and the clay are held together by a glue which permeates through the clay holding the particles together and also holding the fabrics to either side of the clay layer. With mechanical bonding the clay is in dry form either powder or granular and is held loosely in position between fabrics by needlepunching the two fibres together (see EP-A-278419). Needlepunching is a form of bonding using barbed needles which drag the fine fibre threads from one fabric to the other through the layer of clay thus holding together the two fabrics with the loose clay between. The fibres dragged across are tangled with the fibres in the main body of the textile and not fastened in any way. The process is accomplished on a machine that uses two needle boards one on the top and one on the bottom. These boards contain hundreds of barbed needles and move up and down very quickly driving the rest of needles through the liner as it passes between.
Like all clays when it is hydrated, which it must be to maintain its impermeability, it becomes very slippery and therefore unstable on a steep slope. To overcome the instability, due to a shear strength failure through the clay layer, the fabrics on either side of the clay layer must be connected to one another in some way through the clay layer.
The fine fibres mentioned in the needlepunched material also increase the shear strength of the clay liner although this is not their primary function. Needlepunched material has a number of disadvantages. The strength of the fine fibres connecting the two fabrics is low unless the intensity of the needlepunching is increased considerably, and if this is done the increased needlepunching destroys the impermeability of the liner. A further problem with the needlepunching process is that a nonwoven fabric must be used on at least one side of the liner to obtain the fine fibres for the bonding. The nonwoven material allows a lateral flow of the liquid to be contained througn the plan of this material giving a problem with leakage at the edge, or overlap, of the liner. If a woven material is used, which is preferable in this type of liner, the weave would be damaged or destroyed by the barbed needles used in the process of needlepunching and weaken the liner.
In using this type of needlepunched liner on steep slopes there is also a problem with the movement of bentonite clay within the liner. The clay in its dry form, either powder or granules, is free to move and in the process of handling during loading, unloading and installation, tends to migrate causing light areas with little or no clay and potential leakage in the depleted area. This can also happen when the clay is in a state of hydration. When the clay is hydrated without cover. material to form the confining stress, the hydrated be
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Paoli Maurizio
Starita Piero
Graysay Tamara L.
Lagman Frederick L.
Laviosa Chimica Mineraria S.p.A.
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