Patent Publication Number: US-5156496-A

Title: Earth structures

Description:
This application is a continuation of application Ser. No. 274,841, filed Nov. 22, 1988, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The invention is concerned with improvements in or relating to earth structures of the kind frictionally stabilised by a plurality of elongate stabilising elements extending rearwardly from a facing of the structure into an earth mass. 
     BACKGROUND OF THE INVENTION 
     In such structures the earth is stabilised throughout the mass by frictional engagement with the stabilising elements which are usually in the form of strips. This interaction enables the earth mass to behave as an elastic material with greatly improved resistance to failure. The facing of such a stabilised earth structures can be relatively light and has hitherto largely been built up from either V-shaped channel members of 3 mm steel or relatively thin concrete panels which are connected to the forward ends of the stabilising strips. However, even such relatively light panels represent a major element of the cost of the structure and there is a need for stabilised earth structures having a less costly facing. 
     It has been proposed to retain the earth at the facing of a structure stabilised by layers of embedded mesh by continuing the mesh cover the facing. Thus each element of the mesh which extends rearwardly to stabilise the earth has a corresponding continuation forming part of the facing, which may lead to a rather more substantial facing than required by the earth pressures. Furthermore the stabilising elements of the mesh embedded in the soil tend to separate the mass into layers, giving rise to less uniform stabilisation of the earth than when using strip stabilising elements. 
     According to one aspect of the invention there is provided an earth structure having a plurality of elongate stabilising elements extending rearwardly from a facing of the structure into an earth mass, the facing comprising at least two laterally spaced support members each connected to a pair of said stabilising elements at lower and upper points of the member, and a mesh cover spanning the lateral space between the support members, the mesh cover having a front portion supported by the support members against forward movement under earth pressure, and at least one substantially horizontal rearwardly projecting portion for stiffening the cover against such forward movement. 
     The invention also provides a facing panel for such a structure, comprising at least two laterally spaced support members each having means for securing said lower and upper point of the member to said stabilising elements, and a mesh cover spanning the lateral space between the support members, the mesh cover having a front portion supported in use by the support members against forward movement under earth pressure, and at least one substantially horizontal rearwardly projecting portion for stiffening the cover against such forward movement. 
     The invention also provides a facing for an earth structure, the facing comprising an array of facing panels according to the invention assembled to provide a continuous facing. 
     With such an arrangement, the support members are located and supported by the elongate stabilising elements embedded in the earth, and the separately provided grid cover may be of relatively lightweight construction designed to accommodate the earth pressures at the facing. Thus the structure may be relatively inexpensive and is particularly useful when the structure is to be temporary or of low height e.g. 3 to 6 meters. 
     The support members are preferably generally C-shaped, each member having a front part at the facing of the structure and a pair of lower and upper rearward extensions interconnected by the front part. The rearward extensions of the support members will normally each be relatively short and connected to a respective stabilising element and for this purpose each extension may be formed with a vertical hole for receiving a connecting bolt. Thus it is particularly advantageous if each support member is formed as a strap or belt in which a suitable hole may be provided. The support members will thus be located at intervals across the structure corresponding to the lateral spacing of the stabilising elements; such intervals normally range from 0.5 to 2 m although for very high structures, the intervals may be less than 0.5 m. The length (height) of the support members will normally vary from 0.3 to 1 m. The support members may be formed of mild steel or half mild steel or equivalent and may optionally be galvanised. 
     The facing of the structure may be vertical or it may be at an angle to the vertical e.g. a slope of 4:1, 2:1 or 1:1 (vertical:horizontal). The stabilising elements will generally extend substantially horizontally in the earth mass, and therefore the rearward extensions of the support members will also be substantially horizontal, even if the facing is not vertical. 
     Preferably the mesh cover is also generally C-shaped and thus has relatively short lower and upper rearwardly projecting portions interconnected by and integral with a substantially flat front portion. The provision of two rear projections contributes to further stiffening of the cover against forward movement. In general, it is preferred that the cover comprises a mesh facing bent rearwards along its upper and lower edges to provide tow mesh stiffening projections. Thus the shape of the mesh cover will tend to correspond to that of the support members which support the cover. The length of mesh cover will normally be sufficient to span two, three or more support members and will generally be at least 2 m and no greater than 10 m to facilitate transportation. 
     The mesh cover will normally be formed of mesh elements or bars arranged at right angles to each other and may for example be steel wire mesh of the type conventionally used to reinforce concrete. The laterally extending mesh bars, which in practice will normally be horizontal, may be spaced at intervals ranging from 5 to 30 cm and preferably from 10 cm to 20 cm. The diameter of thebars will normally range from 5 mm to 15 mm and preferably from 6 mm to 10 mm. The mesh bars extending upwardly of the facing may be spaced at intervals generally a bit less than the lateral bars, for example at intervals ranging from 5 cm to 30 cm and preferably from 5 cm to 15 cm, e.g. 10 cm. Their diameter will usually be slightly less than the lateral bars, ranging from 4 mm to 12 mm and preferably from 5 mm to 8 mm. The steel used may be mild steel or half mild steel or equivalent and may optionally be galvanised. 
     It is desirable, although not essential, for the mesh cover to be arranged such that its lateral bars are on its outside face and its upwardly and rearwardly extending bars are on its inner face. Thus in the preferred C-shaped from of the cover the lateral bar or bars of the upper stiffening projection are located above the rearwardly projecting bars, and the lateral bar or bars of the lower stiffening projection are located below the rearwardly projecting bars. This arrangement can be particularly advantageous in securing the mesh cover to the support members, since the mesh cover may be arranged with the lateral bars of its front portion located behind or inwardly of a respective support member and the lateral bar or bars of its stiffening projection located outwardly of the rear extension of the support member. By threading the support member into the cover in this way, the rear extensions of the support members will each tend to lie in the lateral space between adjacent rearwardly projecting bars of the mesh cover i.e. in the same plane. 
     A porous sheet, such as a fine mesh, geotextile or, more preferably, a geogrid, may be placed behind the mesh cover of prevent fine soil particles from escaping past the facing. Such a sheet will also permit hydroseeding (or equivalent) of the facing. If crushed rock is used as the backfill then such a sheet will probably not be necessary. 
     The structure facing will usually consist of a plurality of rows arranged one above the other, each row comprising support members at lateral spacings across the structure and one or more mesh covers spanning between the support members. The support members of adjacent rows are preferably connected to each other, and in a preferred embodiment the rear extension of such support members are separated by a vertical gap in which is located the forward end of a stabilising element connected, e.g. by a vertical bolt, to both rear extension. If the lateral bars of the mesh cover stiffening projections of adjacent rows are outwardly arranged as discussed above, then these bars will ensure the presence of such a vertical gap between the support member rear extensions. 
     The facing will normally be sufficiently flexible to accommodate settlement of the stabilised earth both during and after construction. The preferred C-shape of the support members allows them to flex, and in a particularly preferred embodiment the front part of each support member is formed with an angular bend, preferably at the median line between the top and bottom of the member, which, when there is relative vertical movement between the top and bottom of the member, causes forward bowing of the cover to take place relatively uniformly. This arrangement can thus ensure that bending occurs in one horizontal line across the structure, avoiding uneven bulging or flexing of the support members which could make the facing unsightly. 
     The elongate stabilising elements may be any such elements used in earth stabilisation. In a preferred embodiment, the stabilising elements are steel strips as described in United Kingdom Patent No. 1563317. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Certain preferred embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: 
     FIG. 1 is a perspective view of part of a structure according to the invention; 
     FIG. 2 is a section through the structure; 
     FIG. 3 is a view of the connection between adjacent rows of the structure; 
     FIG. 4 is a sectional view of a second embodiment of structure according to the invention; and 
     FIGS. 5 to 8 are sectional views showing stages in the construction of an embodiment similar to that of FIG. 4. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, the structure 1 has a facing 2 formed of a plurality of laterally spaced support members 3 each connected at top and bottom to a pair of elongate galvanised steel stabilising elements 4 extending rearwardly into an earth mass 5. A mesh cover 6 spans the spacing between the support members. 
     SUMMARY OF THE INVENTION 
     Each support member 3 consists of s strap having a front part 7 and integral lower and upper rear extensions 8 and 9. The straps may typically be formed of 40 mm by 5 steel strips bent to the illustrated C-shape. Each strap has an angular bend 10 midway of its front part so that the strap bows slightly forwardly. The angular bend provides a hinge to accommodate relative vertical movements of the top and bottom of the strap which may be caused by settlement of the stabilised earth backfill. If desired, however, the bend 10 can be omitted. 
     The mesh cover 6 fits behind the support straps 3 and in this embodiment is of sufficient length to span across four such straps. The cover has a front portion 15 formed of vertical bars 14 and lateral bars 18, the vertical bars being bent to form integral lower and upper rear projections 16 and 17. The cover is connected at each end thereof to similar covers, the connections 11 between the covers being spaced away from the adjacent support straps. The lateral spacing between the support straps on each side of this connection 11 may be reduced to compensate for the loss of stiffness caused by the connection. A geotextile filter cloth 12 fits behind the mesh cover 6 to prevent fine soil particles escaping from the facing. 
     FIG. 2 shows three rows 13a, 13b and 13c of support straps 3 one on top of the other. 
     FIG. 3 shows a connection 14 between the straps 3 of adjacent rows 13a and 13b. The laterally extending bars 18 of the cover are arranged outwardly of the bars 14 , the cover being bent such that one of the lateral bars 18a is located at the outside of the bend. Referring to the arrangement of the lower row 13a, the cover 6 is threaded on to its associated support strap 3 such that the lateral bar 18a is located inwardly of the strap, while the lateral bar 18b of the rear projection 17 is located outwardly of the strap. Thus the upper rear extension 9 of the strap lies in the same horizontal plane as the upper rear projection 17 of the mesh cover. The lateral bar 18b ensures a vertical gap between the straps of the adjacent rows 13a and 13b, and a stabilising element 4 is located in this gap and bolted to both straps. The stabilising element is in the form of a steel strip having an integral thickened portion 19 through which a bolt 20 passes. 
     The connection between the mesh cover 6 and the support strap 3 in the upper row 13b is a mirror image of the row 13a connection, except that the lateral bar 18c of the lower rear projection 16 of the upper row is displaced slightly to the rear so as to nest with the bar 18b of the upper rear projection 17 of the lower row. An additional horizontal bar 21 may be located between the upper and lower rows to maintaining the gap near the front of the facing and assist construction. 
     FIG. 4 shows another structure in which the facing has a 1:1 slope rather than being vertical. 
     The construction sequence will now be described with reference to FIGS. 5 to 8. Once the foundation has been prepare pegs or bars 30 are driven into the ground to provide alignment and support for a first course 31 of support straps 3 which are then placed in position together with the mesh covers 6. A first row 32 of stabilising elements is bolted to the lower rear extensions 8 of the support straps, as shown in FIG. 5. 
     Referring to FIG. 6, backfill is placed and compacted on the row 32 of stabilising elements up to the level of the second row 33 of elements. These elements are placed at a 5% slope and loosely bolted (finger tight) to the upper rear extensions 9 of the first course 31 of support straps. The elements are used to adjust the alignment of the support straps as necessary, and are then secured by placing on their rear portions a sufficient amount of backfill to maintain the alignment as previously adjusted (see FIG. 6). The second row 33 of stabilising elements may then be disconnected from the support straps to allow a second course 34 of support straps to be installed, whereafter the row 33 of stabilising elements is reconnected by bolting through the rear extensions of both the first and second courses of straps. 
     The facing is then backfilled to the extent shown in FIG. 7 i.e. right up to the first course 31. The earth is hand tamped int his front region to ensure compaction. 
     Backfilling is continued up to the level of the next row of stabilising elements as shown in FIG. 8 and the procedure for installing these is the same as for the previous row. This construction sequence is repeated for subsequent course of straps together with their mesh covers until the structure is completed. 
     While this invention has been illustrated and describe in accordance with a preferred embodiment, it is recognized that variations and changes may be made and equivalents employed herein without departing from the invention as set forth in the claims.