GB2061355A - Soil stabilisation - Google Patents

Abstract

In a reinforced earth structure, a reinforcing/stabilising member 4 is connected to a facing unit 1 by a rod 5 having an enlarged portion of a diameter, which is a good fit in mating holes in member and unit during the building up of the structure. To relieve pressure on the facing, the rod is moved axially so that its enlarged portion is withdrawn from the mating holes and replaced by a portion of normal diameter which allows limited movement of the facing unit relative to the stabilising member. In an alternative arrangement, a rod has a portion of substantially elliptical section which can maintain the member and unit in a pre-determined alignment during building up to the structure. Rotation of the rod through 90 DEG will permit similar limited movement of the facing unit relative to the stabilising member. <IMAGE>

Description

SPECIFICATION Collapsible coupling earth structure This invention relates to reinforced earth structures and in particular to a method of forming such structures whereby stresses may be relieved.

A reinforced earth structure comprises a mass of material such as natural earth in which stabilising elements are embedded. These stabilising elements are usually strap-like members of steel or some other material, such as fabric webbing, having good tensile strength and are, in generai, attached to facing units which define at least a part of a structure. Such structures may be cuttings or embankments produced in connection with roadworks in which the facing units constitute retaining walls.

Stabilising elements interact with the earth mass such that destabilising forces on the mass place the stabilising elements under tension and the resultant compressive reaction acts to stabilise the mass.

Reinforced earth construction is advantageous in that soil can be contained by retaining walls of less massive construction than would be the case otherwise, especially with particulate soils such as sands and gravels where there is little or no cohesion between particles, frictional contact between stabilising elements at the soil acting to enhance the stability of the structure.

When forming reinforced earth structures it is usual to remove earth for some distance behind the location of a retaining wall and erect facing units progressively with their associated stabilising elements while, at the same time, introducing and consoliding an earth fill behind the facing units and around the stabilising elements until the desired structure is built up.

The compaction or consolidation of the earth fill in many cases gives rise to lateral pressures acting on the facing units and also to "locked-in" stresses between successive layers of the earth fill as this is built up.

It is desirable that the "locked-in" stresses be reduced to enable the shear strength of the earth to be fully mobilised in order to achieve a minimum pressure on the facing and thus an improved factor of safety. It is known that this may be done by permitting a limited forward movement of the facing at an appropriate stage of construction which also has the effect of relieving other undesirable lateral stresses. However, in the case of conventional reinforced earth constructions the connections between facings and stabilising elements would normally act to inhibit such movement.

It has been proposed that a coupling including a collapsible element be employed to connect a facing unit to a stabilising element so as to permit limited relative movement, with temporary supporting means (eg a prop) being utilised to prevent such movement during construction until such time as this is appropriate.

The use of temporary supporting means is frequently untenable for practical or economic reasons and the present invention seeks to overcome this difficulty.

According to the invention a reinforced earth structure comprises a facing unit extending substantially vertically, an earth fill behind the facing unit, a stabilising unit extending rearwardly from the facing unit into the earth fill, the stabilising unit being connected to the facing unit by a coupling including a member having an enlarged portion capable of retaining the stabilising element and the facing unit in substantially fixed relationship axially of the stabilising element and displaceable to permit limited relative movement between the stabilising element and the facing unit in a substantially horizontal direction.

The invention also provides a method of forming a reinforced earth structure comprising erecting a facing unit in a substantially vertical alignment, depositing an earth fill behind the facing unit and arranging a stabilising member to extend rearwardly from the facing unit into the earth fill, the stabilising element being connected to the facing unit by a coupling including a member having an enlarged portion capable of retaining the stabilising element and the facing unit in substantially fixed relationship along the axis of the stabilising element and displaceable to permit limited relative movement between the stabilising element and the facing unit in a substantially horizontal direction.

Embodiments of the invention will now be described with reference to the accompanying diagrammatic drawings of which: Fig. 1 is a sectional elevation through a reinforced earth structure including a facing unit connected to a stabilising element by one form of coupling, and Fig. 2 is a sectional plan view through a reinforced earth structure including a facing unit connected to a stabilising element by another form of coupling.

Referring to Fig. 1, a facing unit 1 of pre-cast concrete bounding an earth fill 2 carries a bracket 3 on its rear face (ie the face in contact with the earth fill). The bracket comprises a pair of angle members each having one arm extending substantially horizontally and spaced from that of the other member. A stabilising element 4 which extends substantially horizontally with the earth fill 2 comprises a steel strap having a hole formed towards one end thereof. This end of the stabilising element is positioned between the arms of the bracket 3, the hole being aligned with a pair of corresponding holes in the said arms, and a steel rod 5 is passed through all holes thereby linking the stabilising element to the facing.The rod 5 is of less diameter than the holes in the stabilising element and bracket and carries a sleeve 6 which is positioned so as to enter all the holes in which it is a close fit thus virtually inhibiting relative lateral movement between the stabilising element and the facing.

In the course of construction, facing units as 1 are erected in rows in a prepared foundation 7, which may be a concrete base extending along the length of the desired structure, earth fill is deposited behind the facing units and is compacted up to the level of bracket 3. Stabilising elements as 4 are laid on the earth fill and connected to the facing units as described above with the sleeves 6 of the respective rods 5 in place in the holes in the corresponding brackets and stabilising elements. Further earth fill is introduced and compacted, thus encompassing the stabilising elements and in frictional contact therewith.

After consolidation of the structure, each rod 5 is moved axially, either up or down, to disengage its respective sleeve 6 from the holes in brackets and stabilising elements. The smaller diameter of the rods themselves which now extend through the holes will permit relative lateral movement between respective stabilising elements and brackets (and thus of the associated facing units).

Lateral pressures caused by the earth fill deposition and compacting processes will cause movement of facing units to occur since the stabilising elements will be restrained by the friction of the earth. Within the limits imposed by the clearances between the rods and the holes through which they pass, the facing units can take up a new alignment which will have the effect of relieving "locked-in" and other undesirable stresses.

To reduce the likelihood of axial movement of rods 5 being hindered by earth packed around them, the rods are enclosed within vertically extending tubes 8a 8b which act as shields to keep earth from the immediate vicinity of the rods.

Where it is desired to protect the prepared foundation 7 from damage which might be caused by axial movement of rods 5 when driven downwards, as by hammer blows for instance, a pad 9 of shock absorbing material such as foamed plastics is positioned under each rod at the bottom of the lowermost tube 8b.

Further courses of facing units are erected on top of previous ones as required with earth fill being introduced and compacted and stabilising elements connected in progressive manner.

Generally it will be preferable to relieve the stresses in each course before the next one is completed but it is conceivable that stress relief might be effected with respect to facing units in two or more courses at the same time in which case it will be desirable that successive lengths of rod be coupled together in known manner, eg cooperating socket arrangements, to facilitate this.

In the arrangement shown in Fig. 2, facing units 1, earth fill 2, bracket 3, stabilising element 4 and sleeve 8 are disposed very much as previously described but the bracket and stabilising element are linked by a steel rod 10 having flats cut along its diametrically opposite sides for at least part of its length and with semi-circular section steel members 11 attached thereto, as by welding, to give an elongated, approximately elliptical, crosssection. Rotating the rod 11 through a quarter turn will vary the amount of lateral relative movement permissible between the bracket (and facing unit) and stabilising element along the axis of the latter and so can allow for re-alignment of the facing unit as before.

In both constructions the axial lengths of the enlarged portions of the rods at the spacing arms of the bracket would be selected to accommodate any vertical settlement of stabilising elements relative to facing units likely to be experienced in the course of construction and compacting of the structure.

Claims (8)

1. A reinforced earth structure comprising a facing unit extending substantially vertically, an earth fill behind the facing unit and a stabilising element extending rearwardly from the facing unit into the earth fill, the stabilising element being connected to the facing unit by a coupling including a member having an enlarged portion capable of maintaining the stabilising element in substantially fixed relationship axially of the stabilising element and displaceable to permit limited relative movement between the stabilising element and the facing unit in a substantially horizontal direction.

2. A reinforced earth structure according to Claim 1 in which the enlarged portion of the coupling member acts to maintain alignment between holes associated respectively with the facing unit and the stabilising element.

3. A reinforced earth structure according to Claim 1 or Claim 2 in which the coupling member is a rod capable of axial movement and having an axially extending portion of a larger diameter.

4. A reinforced earth structure comprising a facing unit, an earth fill behind the facing unit and a stabilising element extending from the facing unit into the earth in which a rod passes through holes associated respectively with the facing unit and the stabilising element, the rod having an enlarged axially extending portion of a diameter which fits closely in the holes.

5. A reinforced earth structure according to Claim 1 or Claim 2 in which the coupling member has a portion with an elongated cross-section.

6. A reinforced earth structure according to Claim 5 in which the coupling member has a portion of approximately elliptical cross-section.

7. A reinforced earth structure substantially as herein described with reference to Fig. 1 of the accompanying drawings.

8. A reinforced earth structure substantially as herein described with reference to Fig. 1 of the accompanying drawings.