GB2255581A

GB2255581A - Retaining structure

Info

Publication number: GB2255581A
Application number: GB9109459A
Authority: GB
Inventors: Leslie Norman Allen
Original assignee: ARMITAGE BRICK Ltd
Current assignee: ARMITAGE BRICK Ltd
Priority date: 1991-05-01
Filing date: 1991-05-01
Publication date: 1992-11-11
Anticipated expiration: 2011-05-01
Also published as: GB9109459D0; GB2255581B

Abstract

A retaining structure comprising an outer face (1) and, extending along behind the outer face and inwardly therefrom, a plurality of cells, each cell comprising a pocket (7) attached to the outer face and compactable material (8) enclosed within the pocket and retained therein by upper and lower surfaces of the pocket, the outer face or an outer surface of the pocket adjacent to the outer face, an inner surface (9) of the pocket remote from the outer face, and end retaining surfaces. <IMAGE>

Description

RETAINING STRUCTURE AND METHOD OF FORMING A RETAINING STRUCTURE The present invention relates to a retaining structure and to a method of forming a retaining structure.

Retaining walls are constructed to separate two distinctly different ground levels or for such other functions as the creation of storage areas, and river and dam retention and bank stabilization. The wall generally extends vertically or at an angle close to the vertical.

Natural banks of earth of other flowable material settle at an angle of repose depending on the material and generally somewhere between 100 and 500 to the horizontal.

The slope thus has significant horizontal width and it is also likely to become unstable, particularly under wet conditions, and therefore dangerous. Retaining walls prevent the waste of horizontal space and mitigate the danger to some extent. However, it is known that rigid retaining walls are prone to catastrophic failure, which may result in a sudden release of the materials from which the wall is built and the material which they are retaining. The consequences can be extremely dangerous.

One relatively recent method of forming retaining walls is to use reinforced earth techniques, wherein straps connected to elements forming an outer face extend rearwardly from the face into a compacted earth mass.

Retention is due to the weight of the mass and the frictional engagement between the straps and the material of the mass. In order to obtain sufficient anchorage it is necessary for the straps to extend a considerable distance into the earth mass, so requiring a large amount of excavation work behind the outer face. Reinforced earth structures may not be as prone to catastrophic failure as rigid wall structures, but nevertheless they are capable of such failure, with potentially dangerous result.

The present invention seeks to provide a different type of retaining structure.

According to one aspect of the invention a retaining structure comprises an outer face and, extending along behind the outer face and inwardly therefrom, a plurality of cells, each cell comprising a pocket attached to the outer face and compactable material enclosed within the pocket and retained therein by upper and lower surfaces of the pocket, the outer face or an outer surface of the pocket adjacent to the outer face, an inner surface of the pocket remote from the outer face, and end retaining surfaces.

According to a further aspect of the invention a method of forming a retaining structure comprises building an outer face and, as the building progresses, attaching a first length of pocket material to the outer face to extend along behind the outer face and inwardly therefrom, placing over the pocket material a layer of compactable material and compacting this in position, folding an inner part of the pocket material upwardly to form an inner surface remote from the outer face so as to retain the layer of material in position and then towards the outer face to overlie at least part of the layer of material so as to form a first cell, attaching a second length of pocket material to the outer face to extend along behind the outer face and inwardly therefrom and overlying the layer of compacted material, compacting a second layer of compactable material over the second length of pocket material and folding the second length similarly to the first length so as to form a second cell, and repeating the process to form a vertical series of cells extending the full length of, and behind, the outer face.

The invention is thus based on the principal of forming a series of horizontal, purpose-designed closed cells of selected compactable material immediately behind and positively connected to the outer face by the pocket material that forms the cell wall. The dimensions of the closed cells in any particular environment may readily be determined using standard structural design analysis methods and techniques.

The invention also extends to any and all of the novel concepts that are disclosed herein.

The invention will be better understood with reference to the following more detailed description, and the accompanying drawings in which: Figure 1 is a schematic illustration of a first retaining structure according to the invention; and Figure 2 and 3 show respective retaining walls of different heights.

Referring to Fig 1 this shows a retaining wall 1 between a lower ground level 2 and an upper ground level 3.

A foundation or strip footing 4 is built along the length of the required wall, the ground on the high level side of the wall having first been excavated to an appropriate distance from the wall. A number of anchor bars 5 are secured in the footing at appropriate horizontal intervals.

A first course 6 of facing material is then laid on the foundation and integrated as necessary with the anchor bars.

A first length 7 of pocket material is laid over the first course and secured in any suitable manner to the anchor bars. The pocket material extends along behind the outer face material and inwardly therefrom for a required distance. A second course 10 of facing material is then laid. A layer of compactable material 8 is then placed over the pocket material and is compacted into position.

An inner part 9 of the pocket material is then folded upwardly to form an inner surface remote from the outer face so as to retain the layer of compacted material in position, and is then folded towards the outer face to overlie part of the compacted layer of material so as to form a first cell. Compaction of the material is completed after this folding operation.

A second length 11 of pocket material is then placed over the second course 10 and again secured to the anchor bars by any appropriate method. The second length is laid over the layer of compacted material, and a further course 12 of outer face structure is erected. A further layer 13 of compactable material is laid over the second length of pocket material, and this is then folded upwardly and then towards the outer face to retain the filled material.

Compacting is completed so as to form a second cell.

Additional cells and courses of outer face material are constructed as will be apparent from the figure, to build the required structure.

That structure is a retaining wall having an outer face that is reinforced by the anchor bars 5, and a plurality of closed cells of fill material extending along behind the outer face and inwardly therefrom. Each cell comprises a layer of fabric with the two folds as described, so closing part of the upper surface of the fill material, the remainder of the upper surface of that material being closed by the pocket fabric of the cell immediately above.

Tests of the structure have shown it to be extremely stable. Any failure that does take place occurs very gradually and it is found that the outer face can become extremely misshapen while the structure still fully maintains its capacity for retaining the soil behind the structure.

The materials that may be used in the structure may vary very widely, and will obviously be chosen with the particular situation in mind. The outer face may, for example, be of brickwork, blockwork, stone or other masonry; may be of concrete, which can be mass, reinforced or prestressed and cast either in situ or precast, with a solid, open or crib face; may be timber e.g, sheet materials counter-battened by sections extending vertically, horizontally, or at an angle; may be metal sheets such as plain or profiled metal sheeting manufactured from aluminium, coated or stainless steel or other metals; or may be plastics, composites, or any other suitable material shaped and/or patterned to achieve the desired appearance. In some cases the outer face may simply be pocket material folded upwardly and around the front face of the compacted material.A soft face is thus achieved which may be filled with pockets of soil for planting and or grassing. A soft face could also make the structure capable of use in a sandbag-type situation.

The outer face may either extend vertically or it may be battered to lean slightly backwardly from the vertical.

This would generally be preferred for tall walls.

There are many suitable materials from which the pockets could be formed. Generally speaking the material is required to be flexible, substantially inert, durable and of sufficient strength to meet the requirements of the situation. Suitable materials include geotextiles of plastics or natural materials, geogrids of polypropylene of other plastics materials, woven metals, welded mesh, or suitably strong fabrics such as, for example, the geofabric sold under the trade name Terram. The pocket material will desirably have voids in order that moisture may permeate through the material to drainage locations at the bottom of the stack of cells. In some applications, however, this may be neither necessary, nor required (for example in dam or river bank constructions) and impermeable materials such as high density polyethylene sheeting could then be used.

The compactable material filling each cell will preferably be material selected for good draining and compaction characteristics, for example a Department of Transport Type 1 or Type 2 road sub-base material. This, however, is not essential and various types of earth, sand, gravel or other compactable material could be used.

Figure 1 shows the top of each cell being formed partially by the pocket material that forms the base of the cell lying immediately above. The amount of overlap necessary will depend on the particular environment and materials and can readily be determined empirically. It may be desirable to have a positive connection between the overlapped pockets, and there are many different ways in which this could be done. For example, when relatively open materials such as geogrids or welded mesh are used interlocking may be effected by metal bars threaded through voids in the two pockets and interlinking with the material of the pockets. Where fabrics are used for the pockets then these may be stitched or otherwise secured together in appropriate locations.It is, of course, possible for each cell to be closed at both its top and bottom surfaces by a length of pocket material unique to that cell, and in this instance the upper layer of pocket material as shown in Fig 1 will extend back to the anchor bars and be suitably secured thereto.

The method of securing the pocket material to the anchor bars may vary and will be such as to suit the pocket material being used. If a geogrid or relatively open pocket material is used then it may simply be slipped over the anchor bars to be held thereby. Other types of material may be secured by fasteners extending between the pocket material and the anchor bars, the fasteners being any suitable form of loop or tie.

The dimensions of the individual cells may vary widely. There is effectively no limit to the length of the cells, which may each be as long as the outer face structure. It may be desirable, particularly for long structures, to use a multiplicity of shorter cells which abut each other end to end. At each end of the structure the end of the cell must be closed in some manner. This can be by a further outer face structure, or by sections of the pocket fabric folded and secured as necessary. Closure of the end of intermediate cells is effected by the material of the immediately adjacent cells, although independent closure by pocket fabric is also possible.The height of each cell may vary, although it is presently considered that the practical maximum height for any one cell is approximately 0.5m and the practical minimum height is approximately 1OOmm. The maximum width of any cell, ie the distance behind the wall that any cell extends need not be more that a maximum of 2/3 the total wall height, and is desirably as short as design considerations will allow in order to reduce the cost of the structure.

In designing a retaining structure according to the invention it is recommended that the following steps be taken. First, the material of the outer face should be selected to have the characteristics and appearance required for the particular environment. The positive horizontal disturbing forces from the retained materials and surcharge loading should then be calculated, and the width and depth of the closed cells determined. From this there can be calculated the maximum tensile forces that will be experienced by the pocket material, and a suitable material can then be selected. Cell depth can be adjusted if necessary should material selection prove difficult.

The outer face and connections to the cells should then be checked for ability to span between pocket locations and strength of connections at each level. The positive connections themselves, together with the overlap between adjacent pockets should then be checked. Finally, the overall stability of the structure needs to be checked, considering parameters such as factor of safety, sliding, overturning, bearing pressures and slip circle failure.

No difficulty has been experienced in building structures of significant height, and Figs 2 and 3 show by way of specific examples retaining structures using a reinforced and grouted cavity brick wall as the exposed face and a geotextile or geogrid forming the pocket material for the closed cells. Specific variation of and modifications to, the invention and the materials used therein. are numerous and will be apparent to those skilled in the art.

Claims

1. A retaining structure comprising an outer face and, extending along behind the outer face and inwardly therefrom, a plurality of cells, each cell comprising a pocket attached to the outer face and compactable material enclosed within the pocket and retained therein by upper and lower surfaces of the pocket, the outer face or an outer surface of the pocket adjacent to the outer face, an inner surface of the pocket remote from the outer face, and end retaining surfaces.

2. A method of forming a retaining structure comprising building an outer face and, as the building progresses, attaching a first length of pocket material to the outer face to extend along behind the outer face and inwardly therefrom, placing over the pocket material a layer of compactable material and compacting this in position, folding an inner part of the pocket material upwardly to form an inner surface remote from the outer face so as to retain the layer of material in position and then towards the outer face to overlie at least part of the layer of material so as to form a first cell, attaching a second length of pocket material to the outer face to extend along behind the outer face and inwardly therefrom and overlying the layer of compacted material, compacting a second layer of compactable material over the second length of pocket material and folding the second length similarly to the first length so as to form a second cell, and repeating the process to form a vertical series of cells extending the full length of, and behind, the outer face.