GB2168408A - Stabilisation of submerged soils - Google Patents

Abstract

Submerged soils upon which a base 1 of a heavy structure, or other element of a heavy structure, rests are stabilised by covering the area of submerged soil surrounding the base or other element of the structure with a liquid composition 2 which has a greater density than the surrounding water or other liquid in which the soil is submerged and which is capable of polymerising in situ to produce a polymeric material with a low modulus of elasticity. The composition may for example be a two-component epoxy or urethane resin. The preferred modulus lies between 5000 and 25000 kg cm<-2>. Flexible elements 3 may be provided in the carpet 2. <IMAGE>

Description

SPECIFICATION Stabilisation of submerged soils The present invention relates to a method of stabilising submerged soils for example in the seabed, carrying structures or parts of structures. It also relates to means for implementing such a method.

Submerged soils, such as the seabed, a lake bed or riverbed, commonly consist of sand or mud sediments which are more or less unconfined or unconsolidated. These sediments are permanently exposed to erosion from waves and from currents, and this erosion is difficult to control, in particular in deep waters.

Many civil engineering structures such as bridges, dams, anchorage points for pipe-lines and off-shore drilling platforms sometimes have to be built on submerged soils which are unstable and exposed to erosion. After a more or less prolonged period of time, the bases of these structures may be found to be resting only partly on the soil beneath them. As a result, the base and possibly the whole structure may become unstable and stresses appear which can eventually result in the failure of elements forming the structure.

To obviate the permanent danger of instability when the bases of heavy structures or elements forming parts of heavy structures are built on such soils, the areas surrounding the bases are at present protected for example with stones, sandbags or metal plates. Apart from the fact that such techniques can only be implemented with difficulty when the water depth exceeds a few metres, they can only slow down the erosion process without overcoming the risk of instability. The need therefore remains for a method which overcomes the above described problem which is preferably reliable, safe and easy to implement.

The aim of the present invention is to provide such a method and means for implementing it.

To this end, according to this invention a method of stabilising submerged soil which carries a base of a structure is characterised in that an area of the submerged soil surrounding the element is covered with a liquid composition, which has a density greater than that of the water or other liquid in which soil is submerged and which is capable of polymerising to produce a polymeric material having a low modulus of elasticity.

A so-called self-levelling liquid composition capable of polymerising is preferably used. The composition is sufficiently fluid under the conditions prevailing in the surrounding liquid to spread uniformly from the point where it is poured and form a carpet of polymerised material. The liquid composition preferably has such a composition as to produce a polymeric material with a modulus of elasticity of between approximately 5000 and 25000 kg/cm2, for example in the vicinity of 20000 kg/cm2. The modulus of elasticity may also have a value above or beneath this range depending on the type of submerged soil which is covered. The elasticity is related to the extent of cross-linking, and is controlled with the usual ingredients.Further, the various ingredients used in the composition are preferably selected so that the composition retains its ability to polymerise under water without foaming at temperatures which are generally between approximately 3"C and 5"C. It is important that the carpet of polymerised material should have a higher specific weight or density than the surrounding water or other liquid in order to adhere permanently to the soil surface.

Further, the ingredients of said composition are preferably selected so that the composition is capable of adhering to concrete and to metal, and of polymerising preferably without shrinking.

As examples of such liquid compositions, are two-component epoxy or urethane resins densified with fillers, and complemented with additives such as polymerisation retarders and dyes. What is imortant is to have a liquid composition which can be adapted in all circumstances to meet the requirements set out above. A person skilled in the art can easily achieve this by the use of products which are sold commercially.

Satisfactory results were obtained in sea water at depths of approximately 25 to 35 m by spreading around the base of a heavy structure a layer from 0.5 to 2.5 cm thick of a composition capable of polymerising into a polyurethane resin with a modulus of elasticity in the order of 20000 kg/cm2. Depending on the circumstances, the resin carpet is spread a distance of 5 to 25 m from the base of the element of the structure.

An example of a method in accordance with the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a perspective view of part of an off-shore structure and the surrounding seabed; Figure 2 is a cross-section through the part of the structure and the underlying and surrounding seabed when the composition has just been poured; Figure 3 is a cross-section similar to Figure 2, but at a subsequent time; and Figure 4 is a cross-section to a much larger scale through part of the structure, the layer of composition and the seabed.

An offshore structure has a base 1 resting directly in the seabed and surrounded by a layer or carpet of composition 2. Because of its low modulus of elasticity, the carpet of polymeric material has the property of closely adapting to variations in the surface of the soil resulting from localised soil erosion, in particular around the periphery of the carpet. The periphery of the carpet 2 follows progressively such variations as occur over a period of time and is subsequently covered partly or totally with fresh sediments, which contribute to stabilising the soil and preventing further erosion. Figure 3 illustrates this situation.

In this particular example of the invention, flexible elements 3 such as ribbons, strips, orfilaments of a polymeric material are incorporated in the liquid composition in such a manner as to extend above the surface of the carpet 2. These elements preferably have at least one end embedded in the carpet 2 while the other end or a centre part remains free to protrude above the surface of said carpet. These protruding elements have the main purpose of retaining particulate materials carried by currents and cause their deposition on the surface of the carpet, thus reinforcing the stabilizing effect of the latter. Figure 4 shows the details of the carpet and the elements.

The method of the invention can be practised with any appropriate means derived from present day technology. Such means preferably comprises in accordance with another aspect of the invention, apart from known storage means and known mixing means, means for transporting the liquid composition from the water surface down on the submerged soil and means for controlling visually the spreading of the liquid composition on the area of the submerged soil to be stablised.

Claims (7)

1. A method of stablising submerged soil carrying a base or other element of a structure, characterised in that an area of the submerged soil surrounding the element is covered with a liquid composition, which has a density greater than that of the water or other liquid in which soil is submerged and which is capable of polymerising to produce a polymeric material having a low modulus of elasticity.

2. A method according to Claim 1, in which the liquid composition before polymerisation is selflevelling.

3. A method according to Claim 1 or Claim 2, in which the liquid composition produces a carpet of the polymeric material having a modulus of elasticity of between 5000 and 25000 kg/cm2.

4. A method according to any one of Claims 1 to 3, in which flexible elements, such as ribbons, strips, or filaments, of polymeric material are incorporated into the liquid composition in such a mannerthatthe elements extend above the surface of the polymeric material.

5. A method according to Claim 4, in which the elements have at least one end embedded in the polymeric material formed from the composition while the other end or a centre part remains free above the surface of the polymeric material.

6. A method according to Claim 1, substantially as described with reference to the accompanying drawings.

7. Means for implementing the method of any one of Claims 1 to 6, comprising meansfortransporting the liquid composition from the surface of water or other liquid in which the soil is submerged down to the surface of the submerged soil, and means for controlling visually the spreading of the liquid composition over the area of soil to be stabilised around the element.