GB1580142A - Process for staking soil - Google Patents

Abstract

Steel reinforcing rods are driven in a grid-shaped formation into the soil layers to be nailed, the rods being placed at angles which correspond to the existing stress directions of the soil. An adhesive bond can be obtained if the reinforcing rods are ribbed to increase the skin friction and are grouted with cement over their entire length.

Description

(54) A PROCESS FOR STAKING SOIL (71) We, MESSRS. KARL BAUER SPEZIALTIEFBAU GmbH & Co. KG., a Germany company, of Wittelsbacher Strasse 5, 8898 Schrobenhausen, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method of stabilising soil, for improving the strength and stiffness of undisturbed soil, especially, but not exclusively, cohesive soil whose stability is adversely affected by artificial or natural encroachment.

Lack of stability of undisturbed soil is generally due to inadequate cohesion. To improve the cohesion, various processes have been proposed in the past, but the prior proposals which can ensure long-term improvement of the cohesion, are expensive and thus uneconomic. Known soil stabilisation methods include the injection of setting liquids, such as cement grout, use of ballast piles, and electrochemical and mechanical processes of other kinds. For making safe slopes and trench walls, injection tension anchorage processes of many types are known. The best known processes have the disadvantage that they are not equally suitable for all types of soil.

In accordance with the present invention there is provided a method of stabilising undisturbed soil, wherein elongate reinforcing members are placed in the soil by driving, drilling, vibrating, washing out or pressure and at angles selected in dependence upon the existing tension, compression and shear directions of the soil, the members being arranged in the form of a grid distributed over the soil to be stabilised.

The above method is widely applicable and economical, and is particularly suitable for low-lying building sites, embankments, and road structures, and for making safe slopes and walls of foundation trenches.

As the reinforcing members are inserted in the soil the adhesion or dowelling effect causes a composite body with increased shear stiffness and increased shear strength to be systematically produced.

The reinforcing members are preferably 3 to 15 m long. They may be solid rods or tubes and have profiles of any desired cross-section. In loose soils the reinforcing members may be coated with a setting constructional material to increase the friction between the members and the soil. The members may also be given a protective surface coating layer for protection against the effects of moisture and weather. This layer may consist of sprayed concrete. Prestresssing of the reinforcing members is also sometimes an advantage.

For further explanation of the invention, examples are referred to below.

Where the soil strength is mainly due to friction and dilatancy, for example dense sand and gravel, the stabilising effect given by the method of the invention, as with reinforced concrete, is mainly one of adhesive bonding.

The reinforcing members must, through adequate friction and length, take up an adhesive force corresponding to the tensile strength of the rod. Ribbed steel rods of 2 to 4 cm diameter are suitable. The rods are driven into the soil, in lengths of up to about 7 m by pressure and vibration, together with a hose, and cement is forced round them over their whole length to form a cement jacket. By this injection the friction is increased and corrosion protection is achieved.

The following formulae can be used for the purpose of dimensioning the reinforcing members and cement jacket: a. determination of the tension: Z S stda 1 Tm b. determination of the strength: Jl; Z'c d2 aF 4 where: 1 = length inserted dj = diameter of rods da = diameter of cement jacket Z = tension Tm = jacket friction aF = strength The combined effect of a number of rods is given by the formula Tn = SZi cos aj, whereby the resistance to slip is composed of the resistance of the rod Tn and that of the soil Tn. The soil resistance TB is cL + N tan . (a is the angle of the slip plane to the horizontal).

In the case of a soil whose strength is due mainly to undrained cohesion and very little friction (e.g. stiff plastic like clay), the reinforcement has mainly a dowelling effect. Lateral ploughing through by the reinforcing members has to be prevented by adequate cross-section width and sufficient length of excavation. The reinforcing member has to oppose adequate shear and bending strength to the lateral loading. A technically and economically optimum dowelling is achieved with members which yield plastically with the deformation of the surrounding soil. For this purpose thin-walled wide steel profiles, driven in by pressure and vibration, are suitable. The length is in principle unlimited, but in practice lengths of about 5 to 15 m are used. The width in cross-section is about 20 to 50 cm.

The complete sealing of the members in undisturbed soil also affords corrosion protection.

The formula can be used for dimensioning the members used for dowelling: a. determination of the resistance to ploughing Q S 4.14 dcn1 b. determination of the resistance to buckling

where: 1 = length inserted d = cross-section width Q = dowel lateral force M = bending moment = = cohesion MF = yield bending moment.

The slip resistance T, which is calculated from the soil component Tn = C' and the rod component TN, gives a rod component of TN = EQj sin aj, where a is the angle to the horizontal.

With mixed soils, the strength of which depends roughly equally on friction and cohesion, the combined effect of dowelling and adhesion should be aimed at. The combined dimensioning must then be approximated to an optimum by iterative combination of the processes. Tube cross-sections of about 5 to 20 cm diameter and up to about 20 m length are suitable, according to the dilatancy of the soil with or without cement injection.

Surfaces from which the reinforcing members protrude require separate strengthening according to the predominance of tensile adhesion or shear dowelling.

A predominantly loose or strongly over-consolidated coherent soil requires a skin a few cm thick on which load slabs to transmit the stabilising are supported. The skin serves mainly to seal the ground against weathering and reduction of cohesion. If ground water is present, flat drainage strings must be arranged behind the skin. The slabs must be dimensioned to prevent pressing into the soil and be at least slightly accommodating to form a force-locking bond. Sprayed concrete is envisaged as the material for the skin, but in principle other materials may be considered. The slabs can suitably be prefabricated of steel and concrete.

A mainly cohesive soil requires on the surface at which the reinforcing members protrude, a shear-stiff layer. According to the material, the thickness of the layer is chosen so that the dowelling conditions indicated are maintained. According to the superstructure, this layer is made of concrete or compressed earth. With the fill of an embankment, the soil stabilising takes up at the same time in this way the spreading thrust which the sub-soil might be unable to carry.

The reinforcing members must be arranged in a spatial grid in such a way that the global stability and stiffness are achieved to the required extent. The increased strength and stiffness values for the stabilised area of the construction site has to be estimated with the appropriate verifications. In the stabilising, the reinforcement members are dimensioned according to predominant adhesion or predominant dowelling. With mixed strength and load conditions, intermediate solutions are found analogously for verification and execution.

An earthwork with tension reinforcement must have adequate stability for all kinematically possible fracture mechanisms with slip joints through the unreinforced and reinforced region. With slip joints in the reinforced region the reinforcement tension components parallel to the joints are added to the thrust from soil strength. With adequate staking in respect of stability, the shear stiffness increases up to twice the value for the staked ground. Prestress would afford only an insignificant increase in stiffness and therefore need not be applied. With dimensions for soil reinforcing members 3 to 15 m long, a grid interval of 0.75 to 1.5 m has proved suitable.

Accordingly for verification of the stability of predominantly cohesive soils for slide joints through the staking region, the thrust forces have to be estimated in addition to the thrust resistance for soil strength. The shear stiffness can likewise be increased to about double, which results above all in a more uniform settlement basin.

To stabilise a 10 m deep foundation trench in silty sand and gravel of light stratification for temporarily making safe, the procedure is consequently as follows: - reinforcing members of ribbed steel diameter 28 mm grid interval 1.3 m, length 5 to 6 m; - sprayed concrete skin, 5 cm thick with reinforcement mat Q 131, in excavated sections of 1.3 to 2.6 m deep and 10 to 15 m long; - steel plates 15 cm diameter with nuts stressed to about 4 Mgf; - mouldings, mean diameter 10 cm of cement grout.

In subsoil staking for the embankment of a road or railway, the staking serves to reduce and even out subsidences on the embankment and to protect the surroundings from lateral movement of the subsoil. The staking consists accordingly of stakes, sheet piling HL 1, depth 6 to 8 m, grid interval 2 m for a compressed embankment filling of loose material up to 5 m high.

WHAT WE CLAIM IS: 1. A method of stabilising undisturbed soil, wherein elongate reinforcing members are placed in the soil by driving, drilling, vibrating, washing out or pressure and at angles selected in dependence upon the existing tension, compression and shear directions of the soil, the members being arranged in the form of a grid distributed over the soil to be stabilised.

2. A method according to claim 1, wherein the reinforcing members are made of steel.

3. A method according to claim 1 or 2, wherein the reinforcing members are 3 to 15 m long.

4. A method according to claim 1, 2 or 3, wherein the reinforcing members have other than cylindrical profiles.

5. A method according to claim 1, for stabilising vertical or steeply sloping earth works, wherein the soil surface is protected by a skin consisting of sprayed, thin concrete sheet and prefabricated slabs, the protective skin being mechanically locked to the reinforcing members.

6. A method of stabilising undisturbed soil substantially as herein discribed.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. protrude, a shear-stiff layer. According to the material, the thickness of the layer is chosen so that the dowelling conditions indicated are maintained. According to the superstructure, this layer is made of concrete or compressed earth. With the fill of an embankment, the soil stabilising takes up at the same time in this way the spreading thrust which the sub-soil might be unable to carry. The reinforcing members must be arranged in a spatial grid in such a way that the global stability and stiffness are achieved to the required extent. The increased strength and stiffness values for the stabilised area of the construction site has to be estimated with the appropriate verifications. In the stabilising, the reinforcement members are dimensioned according to predominant adhesion or predominant dowelling. With mixed strength and load conditions, intermediate solutions are found analogously for verification and execution. An earthwork with tension reinforcement must have adequate stability for all kinematically possible fracture mechanisms with slip joints through the unreinforced and reinforced region. With slip joints in the reinforced region the reinforcement tension components parallel to the joints are added to the thrust from soil strength. With adequate staking in respect of stability, the shear stiffness increases up to twice the value for the staked ground. Prestress would afford only an insignificant increase in stiffness and therefore need not be applied. With dimensions for soil reinforcing members 3 to 15 m long, a grid interval of 0.75 to 1.5 m has proved suitable. Accordingly for verification of the stability of predominantly cohesive soils for slide joints through the staking region, the thrust forces have to be estimated in addition to the thrust resistance for soil strength. The shear stiffness can likewise be increased to about double, which results above all in a more uniform settlement basin. To stabilise a 10 m deep foundation trench in silty sand and gravel of light stratification for temporarily making safe, the procedure is consequently as follows: - reinforcing members of ribbed steel diameter 28 mm grid interval 1.3 m, length 5 to 6 m; - sprayed concrete skin, 5 cm thick with reinforcement mat Q 131, in excavated sections of 1.3 to 2.6 m deep and 10 to 15 m long; - steel plates 15 cm diameter with nuts stressed to about 4 Mgf; - mouldings, mean diameter 10 cm of cement grout. In subsoil staking for the embankment of a road or railway, the staking serves to reduce and even out subsidences on the embankment and to protect the surroundings from lateral movement of the subsoil. The staking consists accordingly of stakes, sheet piling HL 1, depth 6 to 8 m, grid interval 2 m for a compressed embankment filling of loose material up to 5 m high. WHAT WE CLAIM IS:

1. A method of stabilising undisturbed soil, wherein elongate reinforcing members are placed in the soil by driving, drilling, vibrating, washing out or pressure and at angles selected in dependence upon the existing tension, compression and shear directions of the soil, the members being arranged in the form of a grid distributed over the soil to be stabilised.

2. A method according to claim 1, wherein the reinforcing members are made of steel.

3. A method according to claim 1 or 2, wherein the reinforcing members are 3 to 15 m long.

4. A method according to claim 1, 2 or 3, wherein the reinforcing members have other than cylindrical profiles.

5. A method according to claim 1, for stabilising vertical or steeply sloping earth works, wherein the soil surface is protected by a skin consisting of sprayed, thin concrete sheet and prefabricated slabs, the protective skin being mechanically locked to the reinforcing members.

6. A method of stabilising undisturbed soil substantially as herein discribed.