GB2309992A

GB2309992A - Providing a layer of soil on a non-horizontal face

Info

Publication number: GB2309992A
Application number: GB9602647A
Authority: GB
Inventors: Peter R L Brinckman; Hendrick A Jas; Nigel Edwin Wrigley; Limited Netlon
Original assignee: Netlon Ltd
Current assignee: Netlon Ltd
Priority date: 1996-02-09
Filing date: 1996-02-09
Publication date: 1997-08-13
Also published as: GB9602647D0

Abstract

In order to provide a soil veneer 17 on a steep face of a geoengineering construction 14, a geogrid or geotextile 18 is secured adjacent the top of the face, and soil retaining elements 19, formed of geogrid or geotextile, are secured to the main geotextile or geogrid 18. A soil veneer 17 is formed on the face in such a way that the soil retaining elements 19 project out from the original face and to the outside of the new face, thereby supporting the weight of the veneer 17 by means of the main geogrid or geotextile 18.

Description

PROVIDING A LAYER OF SOIL ON A NON-HORIZONTAL FACE Background to the Invention In general terms, the present invention relates to providing a layer of soil on a non-horizontal face, of an angle up to an including 900 to the horizontal.

The background to the invention and the invention itself are described with reference to the accompanying drawings, in which: Figures 1 to 6 are schematic vertical sections through a number of different reinforced soil slopes, providing steep faces; Figure 7 is a schematic vertical section through a geoengineering construction providing a sloping face provided with a layer of soil; Figure 8 is a schematic vertical section through a construction in accordance with the invention; Figure 9 is a detail of the construction of Figure 8, as indicated by IX in Figure 8; Figure 10 shows the application of the invention to the type of construction shown in Figure 2; Figure 11 is a schematic vertical section showing another arrangement in accordance with the invention, in the course of installation; and Figure 12 is a schematic vertical section showing yet another arrangement in accordance with the invention.

Figure 1 It is now common practice in civil engineering to construct steep, reinforced soil slopes. A typical example is illustrated in Figure 1. The slope is constructed on existing ground 1, or on a prepared foundation, the surface of which may or may not be horizontal. The angle e of the face 2 of the slope to the horizontal is greater than can be constructed with an adequate factor of safety against failure using the soil fill 3 of the slope alone. The soil 3 may be any suitable material including granular fills, clay, industrial or domestic waste, etc. The slope is reinforced with layers 4 of a reinforcing material which may be of any suitable material including geogrids, geotextiles and metal strips. Sufficient reinforcement is included to provide stability and an adequate factor of safety to the slope. The length of each layer of reinforcement, which may be different for each layer, is sufficient to anchor the reinforcement into the body of the fill behind all possible failure planes.

The detail design and construction of the face region 5 of the slope may take many forms, including, eg: Figure 2 Wrap-around: external formwork. Each layer 4A of the main reinforcement 4 is returned up the face of the slope to retain the face and returned back into the slope adjacent to the next uppermost layer 4B of the main reinforcement. It is generally necessary to use some form of temporary formwork against which the face of the slope can be formed. If the required vertical spacing of the layers of the main reinforcement 4 is large (eg > 0.5m) then it may be necessary to incorporate short layers of secondary reinforcement 6.

At point 7 where a layer of secondary reinforcement 6 meets a layer 4A of main reinforcement 4 as it returns up the slope, the two materials may or may not be joined together.

If it is desired that the face of the slope should be vegetated, then the soil 3A in the region of the face must support plant growth and may be different from the soil 3 in the main body of the structure. If the soil 3A in the region of the face is sufficiently fine to wash out through the face, then the face may be lined with a layer 8 of a suitable geotextile or other material, eg turf.

Figure 3 Wrap-around: internal formwork. As Figure 2, except that suitable blocks 9 of material, eg filled sand-bags or blocks of turf, are used to form the face of the structure. The liner layer 8 is not required, but secondary reinforcement 6 may be required (not shown).

If the face is to be vegetated, there is no need for soil 3A which will support plant growth behind the blocks 9. It may be advantageous to use stakes 11 (as shown in Figure 5) in constructing a slope according to Figure 3.

Figure 4 Metal formwork. Main reinforcement 4 terminates at the face of the structure, which is formed by using cages of metal mesh, eg as in EP 0 197 OOOA (Textomur). The liner layer 8 is generally required and secondary reinforcement 6 (not shown) may be required.

Figure 5 No wrap-around. As Figure 3, except no wrap-around. If blocks 9 are filled sand-bags or turf blocks, it will generally be advantageous to use stakes 11 made of wood, metal or plastic to hold them and the layers of reinforcement 4 in position during construction.

In a structure of Figure 5, each block 9 may be small, as shown, or may be large, eg as tall as or greater than the vertical spacing between layers of main reinforcement 4. The blocks 9 may take the form of gabions or concrete blocks. If concrete blocks, then these may be mortared together or dry built with no connection together or connected, eg by shear-keys or pins. Also the concrete blocks 9 may be formed, eg in open box-form, to provide openings or open ledges on the face in which vegetation can grow.

Figure 6 If the angle e of Figure 1 is such that the soil 3 alone would be stable at the face, but with an inadequate factor of safety or a potential for deep-seated failures, it is possible to construct a satisfactory reinforced slope consisting only of soil 3 and layers of reinforcement 4. There may be a layer of soil 3A that supports plant growth and/or a layer 12 of an erosion control material fixed to the face of the slope, eg by tying to the reinforcement 4 or pinning with stakes 13 or U-shaped pins. Normally it is unnecessary for the soil 3A in the face region to be different from the main fill 3 if the erosion control material is filled with a good growing medium, even though the thickness of this material will be limited to circa 20 mm or 200 mm depending on whether the erosion control material is a mat or a cellular structure (see a paper by Montanelli and Rimoldi, in Go11994).

The above description covers the design and construction of soil slopes with face angles up to and including 900 to the horizontal, but in all circumstances it is necessary for the total structure to be sufficiently deep for the length 1 of the reinforcement to be sufficient. There are, however, cases in which this is not possible, where the structure is to be built against an existing or pre-formed face, eg against an existing rock, concrete or brick-built face or against the lining system of the sides of a waste management pit or the sides of the cover system of a full waste pit or dump.

In the former case, ie construction against an existing rigid face, if the objective is simply to encourage vegetation of a rock-slope, then a net, grid or mesh of plastics or metal can be draped over the face and fixed at regular intervals to the face. This will encourage wind-blown soil and seed to settle in gaps and ledges of the face, and between the net, grid or mesh and the face to give, eventually, natural vegetation. Alternatively, a satisfactory structure with short reinforcement lengths can be built, provided each layer of main reinforcement 4 is anchored to the existing rigid face, eg with rock bolts.

In the latter case, ie construction against the sloping sides of the lining or cover system of a waste pit or dump, the use of fixings to the face is not possible because the integrity of the lining or cover system must be maintained, but the limiting angle of friction within the lining or cover system, or between it and the soil to be placed on it, is generally such that only very shallow soil slopes, eg less than 15 to the horizontal, can be placed on the lining or cover system without reinforcement.

Figure 7 If reinforcement of a soil layer (soil veneer) over a lining or cover system is required, it is generally installed as shown in Figure 7.

The sides of a geoengineering construction such as a waste pit or dump 14 are formed into a "benched" profile with trenches 15 running across the slope at the required vertical intervals. The lining or cover system 16 is installed on the face of the slope, including in the trenches. A soil layer or veneer 17 is placed over the lining or cover system 16, reinforced by one or more layers of reinforcement 18 acting as a tension member.

Down-slope tensile forces induced in the reinforcement 18 by the soil veneer 17 trying to slide down over the lining or cover system are resisted by the reinforcement 18 being anchored in the trench 15 by the weight of soil above the reinforcement 18.

If "benches" of a sufficient horizontal depth can be constructed, the reinforcement 18 may be anchored on the surface of the bench without a trench 15.

For the soil veneer 17 on the uppermost section of the slope, the anchorage trench 15 for the reinforcement is formed behind the crest of the slope. For slopes of relatively low height, this may be the only anchorage trench 15 needed for the reinforcement of the whole soil veneer 17.

If the crest of the slope is relatively narrow with a reverse slope beyond it that is also to be covered with a soil veneer, the reinforcement 18 may be continuous over the crest and down the slopes without a trench 15.

The reinforcement 18 is generally a geogrid, to give good interlock with tie soil of the soil veneer 17, but it may include or altfrnatively be an erosion control material. Also, thee may be a layer of erosion control material on or in the surface of the soil veneer 17.

With the system ills-,rated in Figure 7, both the thickness t of the ssil veneer 17 and the angle of the slope to the horizontal e are limited by the friction properties of the soil used in the soil veneer 17.

The invention is concerned with extending the range of soil veneers that can be installed on slopes in terms of veneer thickness and/or the angle of the slope up to approaching 900 and 900 itself. The slope in general can be steep, say over about 4cO The invention is particularly suitable for use where for aesthetic and/or environmental and/or regulatory reasons, a soil veneer is required to support good vegetation which is thicker and/or steeper than can be achieved with the system illustrated by Figure 7. The invention may be used to provide a reinforced soil veneer on the sloping sides of the lining system or cover system of a waste dump or pit or on any other form of face, in which latter case it may be used to minimise the number of fixings required between the reinforcement and the face.

The Invention The present invention provides a method as set forth in Claim 1 or 13 and a geoengineering construction as set forth in Claim 14. The remaining Claims set forth preferred and/or optional features of the invention.

The tension member will be under tension. It can be secured only adjacent the top of said face, or it can be secured at intervals down the face, for instance at 3 metre intervals.

Figure 8 Figure 8 shows by way of example a preferred embodiment of the invention.

Figure 8 is the same as Figure 7 except that there are soil retaining elements in the form of additional pieces 19 of reinforcement material which extend through the thickness of the soil veneer 17. These additional pieces 19 of reinforcement are fastened to or integral with the main reinforcement 18. In general, each additional piece 19 is flexible, and if not supported on the soil veneer 17 below, the piece 19 would bend down under the weight of the soil veneer 17 above the piece 19.

The main reinforcement 18 may be a single layer of a single grade of reinforcement. However, the tensile stresses in the main reinforcement increase from a low level at the base of a section of slope to a high level at the top of that section of slope near to the anchorage trench 15. It may therefore be advantageous to provide a reinforcement system with increasing strength up the slope, or up a vertical section of the slope if the main reinforcement is secured to the slope at large vertical intervals. This may be achieved for the whole main reinforcement, top to bottom, or for a or each corresponding vertical section (ie each section generally vertically above that section below it) of the main reinforcement, eg by joining. together short lengths of reinforcing material with differing strengths with higher strength material at the top of a section of slope than at the bottom. At joints a spare length of one of the materials may be left beyond the joint to be bent outwards to form one of the additional pieces 19 of reinforcement. Alternatively, if a single grade of reinforcement is to be used, it is possible to choose a grade such that, for example, three layers are needed to resist the tensile forces in the top one third of the slope section, two layers are needed in the centre one third and only one layer is needed in the bottom one third. With this, three layers would be anchored in the trench 15, the top layer would then terminate one third of the way down the slope section, the second layer would terminate two thirds of the way down the slope section and the bottom layer would terminate at the base of the slope section. As each section terminates, its end may be bent outwards to form one of the additional pieces 19 of reinforcement.

The additional pieces 19 of reinforcement may be any suitable material and need not be the same type or grade of reinforcement as the main reinforcement 18. In Figure 8, they are shown extending horizontally through the soil veneer 17, but they may extend through the soil veneer 17 at any suitable angle, eg at 900 to the face of the slope. Adjacent to the main reinforcement 18, an additional piece 19 of reinforcement may extend parallel to the main reinforcement 18 for a short distance before extending outwards through the thickness of the soil veneer 17.

The additional pieces 19 of reinforcement may be attached to the main reinforcement 18 by any suitable means, eg sewing, glueing, tying or hog-ringing.

Figure 9 A preferred embodiment for main reinforcement 18, additional pieces 19 of reinforcement and the joint between them is illustrated by way of example in Figure 9.

In Figure 9 both the main reinforcement 18 and the additional pieces 19 of reinforcement are uniaxially oriented geogrids in accordance with GB 2 073 090A.

They are joined together with a bodkin 20 by the method described in GB 2 078 833A. The main reinforcement 18 and the additional pieces 19 of reinforcement are not necessarily the same product grade. However, as an alternative, all or some of the-additional pieces 19 (say every second or third piece 19) can be integral with a section of main reinforcement 18, the main reinforcement being formed in sections with each section secured to the one above with a suitable tensile fixing, each section having its lower (or upper) portion bent out to provide the additional piece 19.

When designing and constructing a soil veneer system as shown in Figure 8, it can be desirable to incorporate a drainage system between the soil veneer 17 and the slope face. This may be incorporated into the design of a waste pit or dump lining or cover system 16.

Alternatively, it may be achieved by incorporating a manufactured layer of drainage material or a layer of free-draining granular material between the soil veneer 17 and the slope face, eg as shown by 21 in Figure 9.

Here the drainage layer is shown below the main reinforcement 18. However, particularly if granular material is used, the drainage layer may be between the main reinforcement 18 and the soil veneer 17, in which case it may be designed as part of the soil veneer 17.

If the soil veneer 17 is to be constructed on a slope to which fastening is possible, eg an existing rock face, then the anchorage trenches 15 may not be needed.

Instead the main reinforcement 18 may be anchored by fastening to the face, eg the main reinforcement 18 may be a continuous length of reinforcement from the top to the bottom of the face, fastened to the face at the top only or fastened to the face at various levels.

Alternatively, the face may be covered by a number of short lengths of main reinforcement 18 with each length fastened near its upper edge to the face.

It can be seen that the face of the soil veneer 17 together with the additional pieces 19 of reinforcement illustrated in Figure 8 resembles the reinforced face of the slope illustrated in Figure 1. It can therefore be readily understood that any of the face details shown in Figures 2 to 6 and described above can be used in the construction of a reinforced soil veneer in accordance with the invention.

A preferred embodiment is a reinforced soil veneer in accordance with the invention having a face detail as illustrated in Figure 5, particularly with the blocks 9 being blocks of turf. In this particular embodiment, the blocks 9 may extend through the full depth of the soil veneer 17, ie form the whole of the soil veneer 17, or they may extend only part-way through the soil veneer 17 with a layer of soil 3 making up the remainder of the thickness of the soil veneer 17.

Figure 10 Figure 10 is an illustration showing how one of the face details shown in Figures 2 to 6 can be used. In this case, the specific face detail is that of Figure 2.

Main additional pieces 19 are secured to the reinforcement 18 and are sufficiently long to be brought up the outer face of the soil veneer 17 and taken back into the soil veneer 17. There are further, shorter additional pieces 19A which just extend to the outer face of the soil veneer 17 and are not attached to the main reinforcement 18.

Figure 10 also illustrates a compress able block 22, eg of polystyrene foam, as described in more detail below.

General Although the preferred material for the main reinforcement 18 is a uniaxially oriented geogrid, any suitable material may be used, eg woven geogrids or metal meshes, grids or strips. In particular, woven geotextiles may be used. Woven geotextiles are not normally usable for the construction of reinforced soil veneers by the method illustrated in Figure 7 because of their poor interlock with soil. However they may be used in the method of the invention.

The additional pieces 19 of reinforcement may be fastened to the main reinforcement at any suitable time. This may be as early as when the main reinforcement is manufactured, in which case they may be manufactured integrally with the main reinforcement. Or it may be as late as during the construction of the reinforced soil veneer as the construction reaches the level of each additional piece 19 of reinforcement. A preferred embodiment may be to pre-assemble the pieces of main reinforcement 18 and the additional pieces of reinforcement 19 together at the construction site prior to placing the main reinforcement 18 on the slope.

If the main reinforcement 18 is an extensible material, eg a polymeric geogrid or geotextile, then allowance may need to be made for the strains that will take place down-slope in the main reinforcement 18 during construction. Means for accomplishing this include: Anchoring the top of a length of main reinforcement 18 in trench 15 or to the slope then pre-straining the bottom of the length downwards prior to constructing the reinforced soil veneer.

or Anchoring the top of a length of main reinforcement only lightly, then constructing the lower portion of the reinforced soil veneer, then pre-straining the top of the length upwards before anchoring it in its final position and constructing the remainder of the reinforced soil veneer.

or Founding the base of the reinforced soil veneer on a compressible block, eg of polystyrene foam, for instance the block 22 illustrated in Figure 10, that will compress as the construction of the reinforced soil veneer progresses, allowing the reinforced soil veneer to slide down-slope, thus straining the main reinforcement 18 to the level necessary.

or Founding the base of the reinforced soil veneer on a stack of sheets of, eg wood (for instance 20 mm thick boards), then removing these sheets one by one as the construction progresses, thus allowing the reinforced soil veneer to slide down-slope and straining the main reinforcement to the level necessary.

Any suitable thickness of soil veneer 17 can be used.

In general however it is preferred that the thickness is greater than about 200 mm. The additional pieces 19 should project sufficiently far to retain the soil veneer 17, normally to the outer face of the soil veneer 17 or at least to adjacent the outer face, and, as indicated above in Figure 10, they can be taken up the outer face. Very roughly, the additional pieces 19 should terminate within about 20 mm of the outer face of the soil veneer 17 or within a distance of the outer face which is about 10% of the thickness of the soil veneer 17.

Figure 11 Figure 11 illustrates by way of example how the soil veneer 17 can be applied to a very steep face. In this case, by way of example, the reinforcement 18 is secured adjacent the top of the face by suitable means (eg a firm rock bolt) indicated at 23, and its bottom is strained downwards before beginning the application of the soil veneer 17. In this case, by way of example, the additional pieces 19 are shown attached so that they project upwards. The relevant additional piece 19 is bent down so as to be horizontal, a course of concrete blocks 24 is placed in position, for instance on the outer portions of the pieces 19, soil 17 is filled in behind the blocks 24, the next additional piece 19 is bent down, and the procedure is repeated, thereby forming a faced soil veneer. The blocks 24 can be concrete blocks or breese blocks, and need not be secured in position in any way. The additional pieces 19 can extend to the outer faces of the blocks 24.

Figure 12 Figure 12 illustrates using the detail of Figure 3 (in part) and the detail of Figure 5 (in part) in the soil veneer 17. Also, there are separate lengths of the main reinforcement 18 down the face, each secured by suitable means 23, the main reinforcement 18 thereby being in separate vertical sections.

Example I A slope at an angle 6 of 450 with a vertical height (h) of 4 m is covered by a 0. 9 m thick (t) veneer with a face detail as shown in Figure 5 consisting of 0. 5 m wide turf blocks and soil. The slope is covered with a waste dump cover system and the minimum angle of friction () between the veneer and the cover system is expected to be 130 The average density (6) of the veneer will be 15 kN/m3.

Weight of veneer (W) = h x 8 x t (per m width) Sin o = 76. 4 kN/m Gravitational force down slope (P) = W x Cos 6 = 54 kN/m Friction force up-slope (F) = W x Sin 5 x Tan = 12. 5 kN/m Out-of-balance force to be taken by main reinforcement (T) = P - F = 41. 5 kN/m This load can be carried by a tension member or main reinforcement formed by two layers of a proprietary geogrid "Tensar" (trade mark) SR110 anchored at the top of the slope. The upper layer would terminate half-way down the slope. 1. 3 m long pieces of additional reinforcement, using a lighter grid of proprietary geogrid "Tensar" (trade mark) 5R55, would be fastened to the main reinforcement as shown in Figure 9 at 0. 5 m vertical centres to profide face stability. SRi 10 and SR55 are uniaxially-oriented, high density polyethylene geogrids, manufactured by Netlon Limited, with respective short-term strengths of 110 and 55 kN/metre width.

Example II Instead of the "Tensar" (trade mark) SR110 used in Example I for the tension member, the tension member or main reinforcement is formed by one layer of a proprietary woven geotextile "Stablilenka" 200, manufactured by Akzo, with a short term strength of 200 kN/metre. 1. 3 m long pieces of additional reinforcement using either the same geotextile or a lower strength geotextile, is sewn to the main reinforcement at 0. 5 m vertical centres. The operation of sewing these pieces of additional reinforcement to the main reinforcement could conveniently be carried out at the factory where the geotextile is manufactured.

* * * * * The present invention has been described above purely by way of example, and modifications can be made within the spirit of the invention. The invention also consists in any individual features described or implicit herein or shown or implicit in the the drawings or any combination of any such features or any such generalisation of any such features or combination.

Claims

1. A method of providing a layer of soil on a non-horizontal face, comprising: securing a tension member so that it extends down said face; providing soil retaining elements which are secured to the tension member, for retaining a layer of soil on said face; and forming a layer of soil on said face in such a way that the soil retaining elements project out from said face and at least to adjacent the outer face of the layer of soil so formed, whereby said retaining elements subject said tension member to tensile force and the layer of soil is supported by said tension member.

2. The method of Claim 1, wherein the tension member is a geogrid.

3. The method of Claim 1, wherein the tension member is a geotextile.

4. The method of any of the preceding Claims, wherein the soil-retaining elements are formed of geogrid.

5. The method of any of the preceding Claims, wherein all or some of the soil retaining elements are integral with relative sections of the tension member, the tension member being formed in sections with each section having a lower or upper portion bent out to provide a said soil retaining element.

6. The method of any of the preceding Claims, wherein at least a section of the tension member is secured only adjacent its top and comprises a plurality of portions, namely at least a higher tensile strength portion adjacent the top of said tension member or section thereof and a lower tensile strength portion adjacent the bottom of said tension member or section thereof.

7. The method of any of the preceding Claims, wherein at least a section of the tension. member is pre-strained prior to completing the formation of said layer of soil on the tension member or section thereof.

8. The method of any of Claims 1 to 6, wherein at least a section of the tension member is secured only adjacent its top and the bottom of the tension member or section thereof is pre-strained downwards prior to forming said layer of soil on the tension member or section thereof.

9. The method of any of Claims 1 to 6, wherein at least a section of the tension member is secured adjacent its bottom and is then pre-strained upwards and is secured adjacent its top, prior to completing the formation of said layer of soil on the tension member or section thereof.

10. The method of any of Claims 1 to 6, wherein at least a section of the tension member is secured only adjacent its top and the base of said layer of soil on the tension member or section thereof moves downwards as the formation of said layer of soil on the tension member or section thereof progresses.

11. The method of any of Claims 1 to 6, wherein at least a section of the tension member is secured only adjacent its top, and the base of said layer of soil on the said tension member or section thereof is founded on compressible material which compresses as the formation of said layer of soil on the tension member or section thereof progresses, allowing said layer of soil to move downwards and straining the tension member.

12. The method of any of Claims 1 to 6, wherein at least a section of the tension member is secured only adjacent its top, and the base of said layer of soil on the tension member or section thereof is founded on a stack of moveable members, the moveable members being removed one by one as the formation of said layer of soil progresses, thus allowing said layer of soil to move downwards and straining the tension member.

13. A method of providing a layer of soil on a non-horizontal face, substantially as herein described with reference to, and as shown in, any of Figures 8 to 11 of the accompanying drawings.

14. A geoengineering construction having a non-horizontal face on which a layer of soil has been provided by the method of any of the preceding Claims.