The invention relates to a method for protecting the soil against contamination caused by the contents of a storage tank present in or on the ground. The invention furthermore relates to a soil structure for protecting the soil against contamination caused by the contents of a storage tank present in or on the ground.

A structure of this type is known per se from European Patent Application No. 0 456 094, which not only discloses a structure of soil layers, but also a cable provided therein for delivering a signal if the storage tank exhibits a leak and the contents of the tank find their way into the soil. According to said European patent application an impervious layer or a foil which is impervious to the liquid in the storage tank is provided on a foundation consisting of asphalt, sand, concrete or the like. Practice has taught, however, that a comparatively rigid layer of asphalt or concrete may exhibit cracks, as a result of which the liquid from the storage tank may find its way into the soil layer to be protected. A foil provides insufficient protection, whilst sand or the like will become permeable after some time.

That is why it has been attempted to find a method for protecting the soil against such contamination and a soil structure which provides sufficient protection of the soil present under the storage tank.

The method according to the invention is characterized in that a sealing system forming a seal at its bottom side is placed in adjoining relationship with the surface of the storage tank present in or on the ground, which sealing system is built up of one or more layers, with the layer adjoining the bottom surface of the storage tank consisting of an oil-impregnated pulverized mineral having a hydrophobic surface. Under this layer, hereinafter referred to as layer 1, a barrier layer is provided, which barrier layer may consist of a foil or the 1 ike or of a polymer and smectite-containing water-moisturized layer 3, which will be described in more detail hereafter.

The use of layer 1 will lead to a satisfactory adhesion between the storage tank surface and the layer provided thereunder, and the hydrophobic nature of the material being used will provide a water- repellent effect. The term ^" storage tank ^" must be given a wide

interpretation, because it is also understood to include pipes and the like.

Furthermore it is to be preferred to provide a polymer and smectite-containing water-moisturized layer 3 which is impervious to water and to hydrocarbons under layer 1. It is especially preferred to provide a layer 2 containing particulate silicate between layer 1 and layer

3.

Thus it is in particular to be preferred to provide a sealing system which is built up of three layers, namely: a) a layer 1 of an oil-impregnated pulverized mineral having a hydrophobic surface, which adjoins the bottom surface of the storage tank; b) a layer 2 of a particulate silicate compound, which is provided under layer 1; and c) a polymer and smectite-containing water-moisturized layer 3 being impervious to water and to hydrocarbons, which is provided under layer 2.

Especially the smectite-containing layer referred to under c) is built up of a smectite-containing mixture of clay or a smectite-containing natural stone type, which contains more than 0.5 % by weight of one or more solid inert filler(s) having an average particle size larger than 0.05mm, 0.8-6.0 % by weight of a powdered solid activating agent and more than 0.2 % by weight of a water-soluble polymer, whereby the weight percentages have been calculated on the basis of the amount of smectite present in the mixture. A smectite-containing composition of this kind is known from W094/18284. Said PCT patent application does not disclose the method for protecting the soil against contamination caused by the contents of a storage tank present in the ground. Nor can a soil structure as disclosed in the present patent application be derived from said patent application.

As an alternative for the layer 3 described under c) a layer may be used which (1) contains a mineral clay selected from smectites having a three-layer structure and/or expansible sil icates, which may be partially activated with 1 % by weight (based on the weight of the mineral clay) of an activating agent selected from (i) an alkali metal or ammonia salts comprising an anion which forms water-insoluble precipitates with alkaline-earth etal ions and (ii) a smectite which is

overactivated with salts of this type, and (2) at least 0.6 % by weight (based on the weight of the mineral clay) of a water-soluble polymer which is capable of reacting with said clay and which has a molecular weight of 50,000 - 20,000,000, which mixture may form a gel which is capable of absorbing and releasing water after the addition of 1 - 5 % by weight (based on the weight of the mineral clay) of an activating agent selected from (i) an alkali metal or ammonium salts comprising an anion which forms water-insoluble precipitates with alkaline-earth metal ions and (ii) a smectite which is overactivated with salts of this type. A mixture of this kind is known per se from EP-A-0 335 653. From said European patent application a method is known for preparing a gel which is capable of absorbing and releasing water, but it is not known from said patent application to use a mixture of this kind as one of the layers in a soil structure for protecting the soil against leakage of hydrocarbons from a storage tank present in the ground.

A large part of the equipment used for transporting or storing hydrocarbons, such as various pipes and reservoirs, is placed on the ground or surrounded by the ground. An important requirement is that such equipment is effectively sealed in order to prevent contamination of the soil. In order to provide an effective protection several requirements must be met, such as the fact that the seal must protect the metal of the reservoir from the corrosive influence of aqueous mixtures, which may be achieved by preventing the reservoir from coming into contact with the water present in the ground. Furthermore the reservoir seal must have sufficient load-bearing capacity in order to be able to bear the heavy load, and be sufficiently flexible in order to be able to absorb any movements, both of the ground and of the reservoir. Furthermore the soil structure providing a sealing effect must provide sufficient isolation from hydrocarbons which are present in the reservoir and which may leak therefrom.

The above objectives may be accomplished by using a method according to the invention or by using a soil structure according to the invention.

The invention will be explained in more detail hereafter with reference to the appended drawing, in which:

Figure 1 shows a soil structure according to the invention.

The construction shown in Figure 1 will be explained in more detail hereafter.

In order to protect the subsoil 6 and the soil present thereunder against the contents of a reservoir consisting of bottom 8 and wall 7, a system of layers 1, 2 and 3 as described above is provided under bottom 8, whereby a textile cloth 5 may be provided between layers 2 and 3 and/or between layers 1 and 2. Furthermore a drainage 4 may be provided between layers 2 and 3. Said drainage may also be provided between layers 1 and 2. The further build-up of the layers, the compositions and thicknesses of the layers as they are preferably used will be discussed in more detail hereafter.

The new system for the soil structure according to the invention is in particular suitable for sealing the metal hydrocarbons- containing reservoirs against corrosion caused by the ground, more in particular against the corrosive action of the ground water. Furthermore the soil structure is sufficiently flexible to absorb any movements of the ground and it effectively prevents hydrocarbons that may leak out from finding their way into the soil layers which are protected against contamination.

More in particular the soil structure consists of several layers, preferably three layers, with the upper layer, which is in contact with the bottom or the wall of the reservoir, consisting of an oil-impregnated pulverized mineral having a hydrophobic surface. Said mineral preferably contains a corrosion inhibitor, in particular an oil- soluble amine compound. By impregnating the mineral with oil a satisfactory adhesion to the metal bottom of the reservoir and a proper adaptation to the shape of said bottom will be obtained. Oil is preferably added in an amount of 10-60 % by weight, based on the weight of the mineral. Furthermore the ground water is thus prevented from coming into contact with the metal of the bottom or the wall of the reservoir. Furthermore the water required for the smectite and polymer-containing layer is prevented from coming into contact with the bottom or the wall of the reservoir. The possible presence of a corrosion inhibitor reduces the risk of external corrosion of the metal holder considerably. Said layer 1 has a layer thickness of at least 0.3 cm, preferably of 4 - 6 cm. Layer 1 may also be mixed with a particulate silicate.

The middle layer 2 contains a particulate silicate, which is preferably mixed with a pulverized mineral having a hydrophobic surface. No oil has been added thereto, however. Said layer may also contain a solid silicate. When no pulverized mineral is added said layer may function as a drainage layer. Furthermore a drain pipe made of plastic material, such as polyethylene, may be provided. The second layer has a thickness of more than 1 cm, preferably 5 - 30 cm. Said layer 2 functions to take up the load applied by the container and to absorb any movements of the ground. The pulverized mineral having a hydrophobic surface that is added functions to absorb hydrocarbons that may leak out and it prevents ground water from permeating upwards.

The hydrophilic moisturized part of layer 3 forms a seal against the hydrocarbons that may leak out, so that any hydrocarbons leaking out are prevented from finding their way into soil layer 6. It furthermore forms a cohesive flexible layer, which can readily follow the deformations of the soil. The lower part of layer 3, which may still be dry, forms a solid layer which has a high bearing capacity and which forms a seal against ground water.

A textile cloth 5, which is preferably moisturized with water, may be provided in particular between layers 2 and 3. Said textile cloth (geotextile) ensures a uniform distribution of the load over the respective layers and an adequate separation of said layers. In principle this cloth may be provided between all layers.

The upper layer 1 of the soil structure according to the invention consists of an oil-impregnated pulverized mineral having a hydrophobic surface, which may contain a corrosion inhibitor. Hydrophobic surface minerals include talcum, Manusil (trademark) and pulverized oil- bearing materials. Furthermore mixtures of two or more such pulverized minerals may be used. The mineral oil which is added to the pulverized mineral is preferably a liquid or semi-solid hydrocarbon comprising more than 10 carbon atoms, or a mixture thereof. The term ^" oil ^" is also understood to include oil mixtures. Examples of semi-solid hydrocarbons are bitumen and wax. The impregnating oil must be used in such an amount that a fine coating of oil on the pulverized mineral particles is obtained, whereby the mineral can still be readily processed. Examples of corrosion inhibitors to be added which are compatible with the oil system and which are added to layer 1 are oil-soluble amines such as Nalco 192 and 169,

Nalca in, Cronox 100 and Dodigen 213. It is also possible to add the corrosion inhibitor only to the upper part of layer 1, this with a view to reducing costs.

Layer 2 consists of particulate silicate, preferably mixed with a pulverized mineral having a hydrophobic surface. The pulverized mineral having the hydrophobic surface may be one of the substances that are used in layer 1. Particulate silicates are sand, gravel and pulverized siliceous asphalt such as pulverized bricks or roof tiles. The particulate silicate itself may be a mixture of such materials. Layer 2 may also function as a drainage layer.

Layer 3 of the soil structure according to the invention is a clay and polymer-containing mixture as described in the aforesaid PCT patent application W094/18284 or in EP-A-0 335 653. Besides the active components, layer 3 may also contain an inert filler. Part of the bottom layer must be moisturized, so that the wet, flexible and hydrophilic layer thus formed provides a seal against hydrocarbons leaking out and rising ground water. The thickness of the moisturized layer is in particular more than 3 mm.

According to another embodiment of the soil structure according to the invention layer 2 consists of two sublayers 2.1 and 2.2, whereby sublayer 2.1 consists of a mixture of a particulate silicate compound and a pulverized mineral having a hydrophobic surface, and whereby sublayer 2.2 consists of a particulate silicate compound.

In a comparable manner also layer 3 may be composed of two sublayers 3.1 and 3.2, whereby the sublayer 3.1 adjacent to layer 2 has a low content of inert, solid filler and sublayer 3.2 has a higher content of inert, solid filler.