This invention relates to a land disposal site for burying waste materials, and in particular to a capping for such a site.

It is known to cover waste material buried in a trench or depression in the ground by a capping which serves to prevent or limit the inflow of water into the waste material. Over a period of time settlement or subsidence of the waste material beneath the capping takes place, resulting in distortion of the capping. Such distortion can cause the formation of cracks in the capping. It will be apparent that the presence of cracks is a problem in that they may allow water to leak into and percolate through the waste material. If the waste material comprises hazardous waste, for example, low level radioactive material, leaching of dangerous substances into the surrounding soil can occur. Obviously this situation is unsatisfactory in that it can give rise to adverse environmental conditions.

It is an object of this invention to provide a capping for a waste material burial site in which the formation of cracks in the capping is prevented or at least inhibited.

According to the invention there is provided a land disposal site for the burial of waste material, the site comprising a depression formed in the ground for the reception and storage of the waste material, and a capping disposed so as to overlie the waste material deposited in the depression, wherein the capping comprises a water barrier layer having a peripheral portion comprising a substantially horizontal rim portion extending outwardly from the depression, the peripheral portion further comprising a sloping outer portion extending inwardly from said rim portion and inclined upwardly at an acute angle with respect to a horizontal plane.

Preferably, the barrier layer further comprises a sloping inner portion extending inwardly and upwardly from

the outer portion and inclined with respect to the horizontal plane at an acute angle which is less than that of the outer portion.

The outer portion of the barrier layer may be of substantially uniform thickness.

Alternatively, the outer portion of the barrier layer may have a thickness which converges from its junction with the rim portion towards a point at which it joins the inner portion.

Suitably, the angle of inclination of the outer portion with respect to a horizontal plane lies within a range of 10° to 20°.

Preferably, the angle of inclination of the outer portion is substantially 14°.

Preferably, the inner portion and the rim portion of the barrier layer are each of substantially uniform thickness.

The barrier layer preferably comprises a material formed from a mixture of sand and bentonite.

Preferably, the capping further comprises an overburden layer of material above the barrier layer and a layer of granular material below the barrier layer.

The waste material may comprise low level radioactive waste.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a cross sectional elevation of a waste disposal site provided with a capping according to one aspect of the invention,

Figure 2 is a cross sectional elevation of one half of the waste disposal site shown in Figure 1, showing the capping in greater detail, and

Figure 3 is a cross sectional elevation of one half of a capping according to a further aspect of the invention.

Referring to Figure 1, a waste disposal site comprises a depression in the form of an excavated trench 1 in the ground 2. Waste material 3 is deposited in the trench 1 and covered by a low permeability capping 4. The trench 1 may be suitably lined by a combination of mineral and geomembrane liners (not shown) and may be provided with drainage and gas venting systems (not shown) . The waste material 3 may be of a hazardous or'toxic nature, such as low level radioactive waste.

As best seen in Figure 2, the capping 4 has a peripheral portion comprising a rim 5 and a sloping outer region 6. The rim 5 is supported substantially horizontally on the ground surface 7 with the inner end of the rim laterally displaced outwardly from an edge 8 of the trench 1. Preferably, the outer region 6 has substantially parallel upper and lower surfaces 9, 10 respectively. An inner region 11 of the capping 4 extends inwardly from the outer wall 6 to the centre of the trench 1. The inner region 11 has substantially parallel upper and lower surfaces 12, 13 respectively.

The capping 4 may have a thickness in the region of 3 metres and comprises an overburden layer 14, an intermediate water barrier layer 15 and a granular fill layer 16. Typically, the overburden layer 14, typically soil, may have a thickness in the order of 1.8 metres and the granular fill layer 16, preferably formed from sand, may have a thickness in the order of 0.6 metres.

The barrier layer 15, which may have a uniform thickness of approximately 0.6 metres, comprises an outer horizontal rim portion 17 within the rim 5, a sloping outer portion 18 within the outer region 6 and an inner portion 19 within the inner region 11. Inclination of the outer portion 18 with respect to a horizontal plane is at an acute angle A which lies within the range of 10° to 20°, a suitable angle being substantially 14°. The barrier layer 15 is formed from a low permeability

material, preferably an engineered soil comprising a mixture of sand and bentonite. Typically, the engineered soil may be prepared by mixing seven parts of a fine sand with one part of a coarse sand and water. Bentonite is mixed separately with water in a high shear mixer and the resulting mixture is added to the fine/coarse sand mixture. The initial moisture content of the engineered soil may be within the range of 26 to 28%. Over a period of time the water content of the barrier layer 15 will change from the initial value until it comes into equilibrium with the environmental conditions. The equilibrium water content is dependent on the effects of climatic variation and/or changes in the sea or groundwater level.

A sloping inner portion 19 of the barrier layer 15 extends from the inner extent of the outer portion 18 to the centre of the trench 1. The inner portion 19 is inclined at an acute angle B with respect to the horizontal plane. Angle B is smaller than angle A and lies within the range of 1° to 5°, a suitable angle being substantially 3°.

Tests carried out to simulate the settlement or subsidence of the waste material 3 have shown that the barrier layer 15 will remain leakproof over a long period of time. The design of the barrier layer 15, particularly the provision of a sloping outer portion 18 around the edge of the capping, ensures that the layer will be subjected to compressive forces as the shape of the capping distorts due to settlement of the waste material 3. Thus, the formation of tensile cracks in the barrier layer is inhibited.

A further embodiment of the invention is shown in Figure 3. A capping 20 includes a sloping outer region 21 which has upper and lower surfaces 22, 23 respectively, which converge inwardly from a junction of the rim 5 with the outer region 21. Similarly, the thickness of an outer

portion 24 of the intermediate barrier layer 15 converges from the rim portion 17 towards the point at which it joins the inner portion 19 within the inner region 11. Thus, the barrier layer 15 has a thickened portion at a potentially weak spot at the base of the outer portion 24 which will inhibit the formation of cracks. The inner portion 19 and the rim portion 17 are of substantially uniform thickness. The average angle of slope A of the outer portion 24 and angle B of the inner portion 19 with respect to the horizontal plane correspond with those previously specified in relation to the embodiment shown in Figure 2. Above the barrier layer 15 is an overburden layer 14 and below the layer 15 is a granular fill layer 16.

The design of the capping according to the invention ensures that the formation of tensile cracks in the water barrier layer is prevented or inhibited, thereby ensuring that the capping will retain its integrity over a long period of use. External water, particularly rainwater, is therefore prevented, or at least inhibited, from entering the waste material so that potential environmental problems associated with the leaching of harmful substances into the surrounding soil will be avoided.