GB2354275A

GB2354275A - Building support construction and method

Info

Publication number: GB2354275A
Application number: GB9922057A
Authority: GB
Inventors: Robert David Essler
Original assignee: KELLER GROUND ENGINEERING
Current assignee: KELLER GROUND ENGINEERING
Priority date: 1999-09-17
Filing date: 1999-09-17
Publication date: 2001-03-21
Anticipated expiration: 2019-09-17
Also published as: GB9922057D0; GB2354275B

Abstract

A method of supporting foundations, embankments or the like on unsuitable strata, the method comprising the steps of: excavating though the unsuitable strata (1) to form a plurality of trenches (11) and filling the trenches with a supporting medium to form a plurality of underground walls (13). Also disclosed as a resulting product of the disclosed method is a support structure comprising a plurality of walls resting on a suitable strata and formed within an unsuitable strata.

Description

2354275 1.

SUPPORT MECHANISM The present invention relates to a support mechanism suitable for supporting foundations, embankments or the like. Specifically, the present invention is concerned with a support mechanism which can be used for supporting structures on weak or unsuitable strata.

When building structures, it is critical to provide good foundations for the structure. Ideally, the foundations need to be formed in firm underlying ground or strata. A firm strata will resist the load without failing or undergoing excessive compression. With suitable building ground becoming more and more scarce, there is often a need to form structures on weak or unsuitable strata such as soft clays, peat and even domestic refuse. Weak or unsuitable strata is not capable of withstanding the load from the foundations.

The present invention seeks to address the above problem, and, in a first aspect provides a method of supporting foundations, embankments or the like, on unsuitable strata, the method comprising the steps of: excavating through the unsuitable strata to form a trench; and filling the trench with a supporting medium to form an underground wall.

A plurality of walls may be formed by the same method. These walls may be interlocking walls, such that they form a mesh of walls when viewed from above ground. The underground wall or walls transfer the load from the foundation onto the firm or suitable strata which lies below the unsuitable strata.

Typically, the weak strata will be excavated down to the level of the suitable strata. Suitable strata can increase in strength with depth. Therefore, it may be 2 preferable for the excavated trenches to penetrate into the firm strata. Preferably, the trenches would penetrate at least 300mm into the firm strata.

Preferably, the supporting medium is introduced into the trenches in a liquid form. More preferably, it is pumped into the trenches. The steps of excavating the trenches and filling the trenches can be performed at the same time. This is advantageous as the strata is supported as the trenches are being excavated. The excavation could be carried out by a standard digger and the pumping of the supporting medium into the trench could be performed by a cement mixer and more preferably, a high shear colloidal mixer.

The supporting medium preferably comprises two components, one of the components is a supporting component which supports the side of the trench and the other component is a strength component which provides strength to the walls. For example, the strength component can be provided by cement and the supporting component can be Bentonite. These two components can be mixed together with mud or water and introduced into the trench in a single step. However, it will be appreciated that the trench could be dug under a Bentonite solution and the strength component could then be added into the trench after the trench has been excavated under the Bentonite.

Preferably, the supporting medium will be a medium which is often called "selfsupporting mud". Typically, this will be achieved by adding between 200 to 500 Kg of self-setting cement and between 20 to 60 Kg of Bentonite to I m 3 of mud or water. More preferably at least one of sulphate resilient cement, pulverised fuel ash (PFA) or Ground Granulated Blast Furnace Slag (GGBFS) will be added to the medium to improve the resilience of the supporting medium to attach by deliterious materials within the weak strata.

The arrangement of the plurality of walls will be determined taking into account the structure loading, the strength of the suitable strata and the strength or other, 3 characteristics of the other underground walls. A particularly preferable arrangement is if the walls are formed in a mesh-type arrangement. This mesh may be a rectangular grid where the angle between interlocking walls is 90'. However, this angle could be varied depending on the required use of the structure and the strata formation. For a symmetric arrangement, the plurality of walls might form a square grid, where the space in between parallel walls in orthogonal directions is kept constant. In the rectangular grid, the spacing between the walls in a first direction does not have to be the same as the spacing between parallel walls in a second direction which is orthogonal to the first direction.

Typically, the trenches may be excavated to a depth of between 4 and 10 m. However, deeper walls are possible. Preferably, the excavation depth will be 20 rn or less.

The separation between adjacent parallel walls will generally be between 2 and 6 m. The thickness of the walls will preferably vary from about 500 min to 1200 mm. More preferably, the thickness of the walls will be between 600 and 750 mm.

In a second aspect, the present invention provides a support structure for supporting a foundation, embankment or the like on suitable strata, the structure comprising a plurality of underground walls formed from a supporting medium, wherein said walls are formed in weak strata.

The present invention will now be described with reference to the following nonlimiting preferred embodiments in which:

Figure I shows a schematic cross-section of a foundation and supporting strata using a method according to an embodiment of the present invention; and Figure 2 shows a rectangular grid of supporting walls in accordance with an embodiment of the present invention.

4 In Figure 1, the ground directly underneath foundation 3 is weak strata 1. Weak strata I overlies suitable strata 5. The foundation 3 cannot be properly supported by the weak strata 1, in regions 7 and 9.

Firm strata 5 is capable of supporting the load from the foundation 3. However, firm strata 5 is a relatively long distance below the structure 3. It may not be desirable to fully excavate the site down to the firm strata 3.

To support the foundation 3, weak strata 1 is excavated to form a plurality of vertical trenches I I in the weak strata 1.

As these trenches I I are being excavated, a supporting medium formed by mixing I m 3 of mud with between 200 and 50OKg of cement and about 50kg of Bentonite is pumped into the trenches using a high shear colloidal mixer (not shown). The supporting medium is a self setting medium which hardens to form underground walls 13.

The trenches I I are excavated down to the firm strata 5. Thus, the load from foundation 3, is transferred through the weak strata to firm strata 3.

Figure 2 shows underground walls 13 in a square grid arrangement 14. The underground walls 13 are formed along X and Y axis such that the walls interlock at an angle of 90'. In this arrangement, the walls separate the weak strata into pockets 15.

The walls 13 of grid 14 are vertical such that the foundation (3 in Figure 1) sits on upper surface 17 of the grid 14.

Figure 2 only shows three walls formed along the x axis and three walls along the y. It will be appreciated that this structure can be expanded to accommodate the whole of foundation 3. Typically, the thickness of the walls will be about 600 and 750 mm. and parallel walls will be separated by a distance of about 2m to 6m. These parameters usually define how many walls are required in the grid to support a structure of a given size.

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Claims

CLAIMS:

1. A method of supporting foundations, embankments or the like, on unsuitable strata, the method comprising the steps ofexcavating through the unsuitable strata to form a trench; and filling the trench with a supporting medium to form an underground wall.

2. A method according to claim 1, wherein the supporting medium is introduced into the trench in a liquid form.

3. A method according to claim 2, wherein the supporting medium is pumped into the trench.

4. A method according to any preceding claim, wherein the steps of excavating the trench and filling the trench is performed at the same time.

5. A method according to any preceding claim, wherein the supporting medium comprises a component which supports the sides of the trench and a component which provides strength to the support medium.

6. A method according to claim 5, wherein the two components are separately introduced into the trench.

7. A method according to any preceding claim, wherein the supporting medium comprises cement, Bentonite and water.

8. A method according to any preceding claim, wherein the supporting medium comprises cement, Bentonite and mud.

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9. A method according to claim 7 or 8, wherein the supporting medium ftirther comprises at least one of the following: sulphate resistant cement, Pulverised Fuel Ash or Ground Granulated Blast Furnace Slag.

10. A method according to any preceding claim, wherein the underground wall is formed with a thickness from 500mm to 1200mm.

11. A method according to any preceding claim, wherein the trench is excavated to a depth of not more than 20m.

12. A method according to any preceding claim, wherein the unsuitable strata overlies suitable strata and said unsuitable strata is excavated down to the level of the suitable strata.

13. A method according to any preceding claim, wherein a plurality of underground walls are formed and said plurality of underground walls form an interlocking mesh of walls.

14 A method according to Claim 13, wherein the mesh is a rectangular grid of walls.

15. A method according to Claim 13, wherein the mesh of interlocking walls is a square grid of walls.

16. A method according to any of claims 13 to 15, wherein the separation between parallel adjacent walls is from 2m to 6m.

17. A support structure for supporting a foundation, embankment or the like on suitable strata, the structure comprising a plurality of underground walls formed from a supporting medium, wherein said walls are formed in weak strata.

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18. A support structure according to claim 17, wherein the underground walls comprise cement and Bentonite.

19. A method as substantially hereinbefore described with reference to any one of the accompanying figures.

20. A support structure as substantially hereinbefore described with reference to any of the accompanying figures.