GB2292763A

GB2292763A - Slope reinforcing structure

Info

Publication number: GB2292763A
Application number: GB9517667A
Authority: GB
Inventors: Samuel A Appleton
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-08-30
Filing date: 1995-08-30
Publication date: 1996-03-06
Anticipated expiration: 2015-08-30
Also published as: GB2292763B; GB9517667D0; GB9417413D0

Abstract

A slope reinforcing structure is formed from an intermeshing assembly of generally flat rigid steel mesh panel elements G1, G2 and G3 which are inserted through apertures in folded sheets of geogrid reinforcing material, T1, T2 and T3. These sheets of geogrid reinforcing material are embedded between successive soil layers 1, 2, 3 and the structure is constructed with the aid of a temporary slope former 4 which defines the required slope angle theta which is typically 50 DEG to 90 DEG . <IMAGE>

Description

SLOPE REINFORCING STRUCTURE AND METHOD The present invention relates to a slope reinforcing structure and to a method of constructing a reinforced soil slope.

Due to a vast increase in traffic volumes, many existing trunk roads and motorways, particularly in urban areas, have now exceeded their original design capacity. Consequently a need has arisen for the provision of additional traffic lanes by widening existing carriageways located on embankments and in cuttings. As originally constructed, such carriageways are normally bounded by earthwork side slopes having an inclination of 270 to 346 In order to widen the area of the carriageway to enable one or more additional traffic lanes to be provided within the confines of existing boundaries, it is necessary to increase the steepness of the side slopes.

Methods of constructing reinforced soil slopes (having an angle of 500 to 900 to the horizontal) are known, e.g. as described in EP-B-197,000 which discloses steel cage structures having a C-shaped cross-section for retaining the slope face, used in conjunction with horizontal layers of geotextile reinforcement and CH-A-680,078, which discloses similar cages having an L-shaped cross-section used in conjunction with horizontal layers of woven steel wire reinforcement. In both these known arrangements, the cages are lined with a geotextile material through which growth is possible. However, the steel cages used on the slope face in these constructions are bulky and are expensive to transport.

It is also known to retain the soil at the face of a reinforced slope within an enclosure formed by a wrap around extension of a flexible polymer plastic reinforcement grid known as a "geogrid". Examples of such geogrids include the Tensar (Registered Trade Mark) material made by Netlon Ltd.

(Registered Trade Mark) and Fortrac (Registered Trade Mark) made by Akzo (Registered Trade Mark). However, structures using flexible materials in this manner have the disadvantage that the wrap around geogrid tends to bulge at each layer resulting in a series of rounded steps with overhang and air pockets. Such features discourage plant growth. Plant growth is highly desirable both from an environmental point of view and in order to provide stability to the slope face by root growth. Furthermore, such flexible materials being normally composed of plastics are susceptible to ultra violet light degradation and to damage by fire and vandalism and are also environmentally unfriendly.

An object of the present invention is to provide an improved slope reinforcing structure and method.

In one aspect, the invention provides a slope reinforcing structure comprising face support by an upwardly extending embedded assembly of generally flat rigid mesh panel elements lying adjacent the inclined surface of the slope, lower mesh panel elements of the assembly being attached at their upper extremities to the lower extremities of adjacent upper mesh panel elements and said upper and/or lower extremities also being attached to embedded restraining means which extends from said assembly away from said surface of the slope and restricts or prevents outward bulging of the face assembly, in addition to the principal role of providing reinforcement for the soil body. Preferably the rigid mesh is a welded steel mesh.

Preferably, upwardly extending free ends of rods forming said lower welded steel mesh panel elements and/or downwardly extending free ends of rods forming said upper welded steel mesh panel elements extends through apertures in an embedded sheet of geogrid reinforcement material which is transverse to said rods, extends away from said assembly and engages the face of said assembly which faces said slope surface.

Conveniently, the upwardly extending free ends of the rods forming said lower welded steel mesh elements bear against the faces of said upper welded steel mesh panel elements which face said slope surface.

In a preferred embodiment, a mat or membrane of biodegradable material is located against the face of the assembly which faces away from the slope surface. This mat or membrane is preferably treated to facilitate plant growth.

In another aspect the invention provides a method of constructing a reinforced soil slope comprising the steps of: a) forming a lowermost row of generally flat rigid welded steel mesh panel elements at a predetermined inclination to the horizontal, and anchoring the bottom edge of said row; b) forming a compacted layer of soil which is bounded on the face by said lowermost row of steel mesh panel elements and is level with the upper horizontal rod of said row; c) laying a restraining means (geogrid reinforcement) and attaching it to the upper edge of said row;; d) forming a further row of generally flat rigid steel mesh panel elements at a predetermined inclination to the horizontal at the face of said compacted soil layer and attaching the lower edge of said further row to said restraining means (geogrid reinforcement) and/or to said upper edge of the lowermost row, and e) forming a further compacted layer of soil on top of the compacted layer of step b), said further compacted layer being bounded by said further row of step d), wherein the restraining means (geogrid reinforcement) is embedded in the soil layer of step b) and/or in the soil layer of step e) and restricts or prevents outward bulging of the resulting face assembly, in addition to reinforcing the soil block.

Preferred embodiments of the invention are described below by way of example only with reference to Figures 1 to 3 of the accompanying drawings, wherein: Figure 1 is a schematic side elevation of a slope reinforcing structure in accordance with the invention, Figure 2 shows a steel mesh panel element used in the structure of Figure 1, Figure 2A is a side elevation taken on Figure 2 also showing an alternative stepped steel mesh panel element, Figure 3 is a sketch perspective view showing the attachment between the steel mesh panel elements and geogrid reinforcing sheet in the structure of Figure l.

Figure 4 shows a steel mesh panel element according to a further embodiment. and Figure 5 shows a side view of the slope re-inforcing structure shown in Figure 1.

The structure shown in Figure 1 is suitable for forming a steep side wall of a motorway which will enable an additional traffic lane to be provided within the confines of existing boundaries. This normally calls for a widening of about 3.6 metres and the geometrv of a typical crosssection in the motorway cutting usually results in a retained wall height of between 2.6 and 2.3 metres. However, the invention is not limited to the formation of side walls for motorways, but may be utilised in any works of engineering construction requiring reinforced soil slopes.

Referring to Figure 1, the structure shown comprises layers 1, 2 and 3 of compacted soil the faces of which are supported by an assembly of 3 rows of rigid steel mesh panel elements Gl. G2 and G3 which form an assembly defining the slope angleb which is tvpically 50 to 90S. The assembly of steel mesh panels is restrained by sheets of geogrid reinforcing material Tl, T2 and T3 which are in the form of nets or are otherwise perforated to allow vertical rods v (Figure 2) of the intermeshing panels to penetrate a flap formed in the edge region of the geogrid as is more clearly illustrated in Figure 3. The geogrid material T includes apertures which typically represent at least 95,O of the surface area of the geogrid sheet T. As shown in Figure 1, the fold of each flap engages a lacing bar L1, L2 or L3 which lies on the outer face of the panel assembly. The face of the panel assembly is covered with soil (not shown).

The reinforced slope is constructed as follows: Firstly, all necessary excavation of existing earthworks is carried out, and then an adequate foundation is established and a level formation is prepared. Temporary extending slope formers 4 are set up to the correct toeline and supported at the required slope angle 0. Lengths of geogrid reinforcement T1 are laid on the formation with a flap folded under as shown in Figure 1. A steel lacing bar L1 is then inserted in the loop of the flap and the first row of mesh panels G1 are placed against the slope formers 4 with the free ends e (Figures 1 and 2) of their upright rods v (Figure 2) bent up and hooked into the lacing bar L1 and apertures (not shown in Figure 1) in the geogrid sheets.The geogrid reinforcing sheets may, for example, consist of high modulus polyester yarns (PET) interwoven into the form of netting, and covered with a protective layer of PVC. Alternatively, the geogrid reinforcing may for example be in the form of a mono-oriented grid formed from high density polyethylene (HDPE) with oval apertures by an extrusion process.

The tops of the mesh panels G7 are clipped on to lugs 5 (Figure 1) provided on the slope formers by disposable plastic clips (not shown). Open weave hessian (not shown for layer 1 but indicated at H in layer 2) is then laid against the inner face of the panels G7 and a shallow layer of loamy soil (eg of depth 200mm) is placed behind the hessian and loosely compacted by hand rammer. During this process, the geogrid sheet Tl is maintained taut. Further layers of soil are then built up and individually compacted by mechanical means until a layer 1 of the required height ("lift") of typically 500mm to 700mm is reached.

A flap of a further sheet of geogrid reinforcing material T2 is formed and the sheet is laid on the surface of layer 1 with the upwardly extending free ends of the rods v (Figure 2) extending through apertures in the flap as illustrated in Figure 3. A further lacing bar L2 is inserted in the fold of the flap and the next row of panels G2 is then intermeshed with the upper edge of the first row of panel Gl with the downwardly extending free ends of their rods v inserted through apertures in the flap.In this manner the lower edge of the row of panels G2 is restrained by the upwardly extending free ends of the rods v of the panels Gl. The tops of the panels G2 are temporarily secured to the lugs 5 of the slope former 4 by disposable plastic clips (not shown) and the geogrid reinforcing sheet T2 is tensioned and embedded in successive individually compacted shallow layers of soil until the complete layer of compacted soil 2 is built up. The slope formers 4 are telescopic having a telescopic length 9 extendable from the former 4 for supporting a further row of panels G3 which may be intermeshed with panels G2 and a further sheet of geogrid material T3 as shown. .The third layer of soil 3 is shown only partially completed and a length of split plastic tubing 6 is shown clipped over the free ends of the upwardly extending rods v (Figure 2). This feature which is included in all layers facilitates construcfion and enhances safety.

When a layer is built up to its full height, height, the length of plastic tubing 6 is removed to enable the placing of the next sheet of geogrid and row of mesh panels. The process can be repeated as often as necessary to build up a reinforced slope of the required height.

Finally, at the required height the slope formers 4 are removed and the exposed sloping face of the structure is mulched and seeded by hydra-seeding. In order to provide a medium to support the establishment of plant growth, the steel mesh panels G1, G2 and G3 are each backed by loosely woven natural jute sacking (hessian) H. As noted above, this layer of material is shown only for layer2 in Figure 1. Being made from natural fibre, jute fabric decomposes naturally to add organic nutrients to the soil, enabling vegetation to become established. The resulting root formations stabilise the slope face.

The steel mesh panels G1, G2 and G3 may either be protected against corrosion and designed to remain in position throughout the design life of the structure, or may be allowed to corrode at a rate which is sufficiently slow to enable front face reinforcement to be provided by plant and tree roots over the course of time.

The steel mesh panels are illustrated more clearly in Figure 2, and comprise an array of parallel horizontal steel rods h (spaced a distance D apart) which are welded at their intersections to an array of parallel upwardly extending steel rods v, the steel rods v extending by a distance C (eg 3.33 x D) from the uppermost horizontal rod and extending by a distance A of 0.33 x D, for example from the lowermost horizontal rod.

The panels may have a planar configuration as illustrated at G in Figure 2A or may have an offset upper portion as illustrated by G' in Figure 2A. A slope reinforcing structure formed from grids G' will have a stepped configuration. The panels are flat mesh panels and are not C- or L-shaped cages as are known in the prior art.

The intermeshing between the adjacent upper and lower panels is shown more clearly in Figure 3, which depicts a sheet of geogrid reinforcing material T folded around a lacing bar L. The relatively long free ends of the upwardly extending rods vl of the lower panels penetrate the geogrid sheet T via apertures a and bear against the horizontal rods h2 of the adjacent upper steel mesh panel. The relatively short free ends of the downwardly extending steel rods v2 of the upper steel mesh panel lie adjacent the rods vl and similarly extend through apertures a in geogrid sheet T.

The intermeshing between adjacent upper and lower panels is also shown in Figure 5. A sheet of geogrid reinforcing material T is also shown folded around a lacing bar.

It should be noted that it is preferred, but not essential for the adjacent upper and lower steel mesh panels to be connected to each other. In an alternative embodiment, not shown, the panels G2 of Figure 1 could be attached to geogrid sheet T2 at a position to the right of that shown, to form a terraced structure. Furthermore, it is not necessary for the assembly of panels Ci, G2 and G3 to be restrained by embedded sheets of geogrid material; for example cables attached to suitable anchoring points or other restraining means could be used.

In another embodiment, shown in Figure 4, the steel mesh panels are constructed more simply in order to reduce manufacturing costs. Here, the upwardly extending steel rods v and the horizontal steel rods h are arranged in a simple lattice with the upright steel rods v extending from the uppermost horizontal rod h by a distance of C=2hD, and from the lowermost horizontal rod h by a distance of A=D.

Finally, it is noted that in the case of low existing embankments, (i.e. having a vertical height of up to 4.5 metres) the reinforced soil block forming the widened embankment should be founded at or below the toe level of the existing embankment. Conversely in the case of high existing embankments (i.e. having a vertical height of greater than 4.5 metres) the height of the reinforced soil block forming the widened embankment should be kept to a minimum, consistent with the geometry of widening (normally of the order of 4.0 metres). In this case, the weight of the reinforced soil block should be transferred through the remainder of the embankment fill material to a suitable stratum below toe level by means of end bearing concrete mini piles or stone columns.

Claims

1. A slope reinforcing structure comprising an upwardly extending embedded assembly of generally flat rigid steel mesh panel elements lying adjacent to the inclined surface of the slope, lower mesh panel elements of the assembly being attached at their upper extremities to the lower extremities of adjacent upper mesh panel elements and said upper and/or lower extremities also being attached to embedded restraining means which extends from said assembly away from said surface of the slope and restricts or prevents outward bulging of the assembly, in addition to providing reinforcement for the soil body.

2. A structure according to claim 1 wherein the flat rigid mesh panel is made from steel mesh.

3. A slope reinforcing structure according to claim 1 or claim 2 wherein upwardly extending free ends of rods forming said lower steel mesh panel elements and/or downwardly extending free ends of rods forming said upper steel mesh panel elements extend through apertures in an embedded sheet of geogrid material which is transverse to said rods, extends away from said assembly and engages the face of said assembly which faces said slope surface.

4. A slope reinforcing structure according to claim 3 wherein the upwardly extending free ends of the rods forming said lower steel mesh panel elements bear against the faces of said steel upper mesh panel elements which face said slope surface.

5. A slope reinforcing structure according to claim 3 or claim 4 wherein said sheet of geogrid material is wrapped around a rod member which lies against said face of said assembly.

6. A slope reinforcing structure according to any preceding claim wherein a mat or membrane of biodegradable material is located against the face of said assembly which faces away from said slope surface.

7. A slope reinforcing structure according to claim 6, wherein said mat or membrane of biodegradable material is treated to facilitate plant growth.

8. A slope reinforcing system according to any preceding claim wherein downwardly extending rods of the lowermost mesh panel elements have free ends which are bent upwards and are anchored to the ground by anchoring means which engages the bent portions of the upwardly bent free ends.

9. A slope reinforcing structure according to any preceding claim wherein said mesh panel elements are constructed of welded steel mesh.

10. A slope reinforcing structure as claimed in any preceding claim wherein said mesh panel elements have an upper portion which is offset with respect to a lower portion and said assembly has a stepped configuration.

11. A slope reinforcing structure substantially as described hereinabove with reference to Figures 1 to 3 of the accompanying drawings.

12. A method of constructing a reinforced soil slope comprising the steps of: a) forming a lowermost row of generally flat rigid mesh panel elements at a predetermined inclination to the horizontal, and anchoring the bottom edge of said row; b) forming a compacted layer of soil which is bounded by said lowermost row of mesh panel elements and is level with the upper horizontal rod of said row; c) laying a restraining means (geogrid reinforcement) and attaching it to the upper edge of said row;; d) forming a further row of generally flat rigid mesh panel elements at a predetermined inclination to the horizontal at the surface of said compacted soil layer and attaching the lower edge of said further row to said restraining means (geogrid reinforcement) and/or to said upper edge of the lowermost row, and e) forming a further compacted layer of soil on top of the compacted layer of step b), said further compacted layer being bounded by said further row of step d) wherein the restraining means (geogrid reinforcement) is embedded in the soil layer of step b) and/or in the soil layer of step e) and restricts or prevents outward bulging of the resulting assembly, in addition to reinforcing the soil block.

13. A method according to claim 12 wherein said restraining means is a sheet of geotextile material which is sandwiched between the soil layers of steps b) and e).

14. A method as claimed in claim 12 or claim 13 wherein a slope reinforcing structure as claimed in any of claims 2 to 9 is formed.

15. A method of constructing a reinforced soil slope substantially as described hereinabove with reference to Figures 1 to 3 of the accompanying drawings.