EARTH STABILISATION

Abstract: A stabilised earth structure has a substantially vertical front face (6) and is stabilised by layers of substantially horizontal strips (1) extending rearwardly from the front face. Each strip (1) is bolted at its rear end to an anchor member (3), which may consist of a plate arranged at right angles to the line of the strip, or of a plate arranged parallel to the strip.

Description: "Earth stabilisation" This invention concerns improvements in or relating to reinforcements for use in stabilised earth structures. The technique of stabilising earth structures by incorporation of spaced flexible reinforcements in the earth mass has become well-established. The basic principles of this procedure were set out in British Patent No. 1039361 of Henri Vidal and a large number of structures of this kind have been build all over the world. The reinforcements stabilise the mass virtually completely by frictional forces, both between the reinforcements and the adjacent fill particles and between those particles and the remainder of the fill. The reinforcements are so spaced that such frictional forces are transmitted throughout the fill and tension qenerated in the reinforcements opposes significant horizontal movement of the fill particles. The tensile strength of the reinforcements must be sufficient to withstand the horizontal forces generated by the weight of the fill and any loads placed thereon, such as a road and road traffic. In order to retain the elastic properties of the stabilised earth structure, it is necessary that any modified form of the reinforcement should be flexible, in order to retain frictional contact with the fill and accomodate earth movements. It has been found that an earth wass stabilised in this way can be built with vertical sides up to substantial heights and the earth behaves as a material having predetermined elastic properties capable of accomnodating significant vertical settlin g - movements wit'-ou*- failure. An unstabilised block of earth has a tendency to fail in the well known way first described by Coulomb along a plane from the foot of the block at an angle of α = Tr/ + T (where (j) is the angle of friction) normally about 63° to the horizontal. The mass of earth above this plane is often termed the "Coulomb wedge" or "active wedge". In older techniques, where a vertical wall was required, this _ra. provided by a relatively massive wall structure at the vertical face resisting overturning primarily by its weight. Using the techniques of British Patent No. 1069361, the vertical sides of the earth block merely need protection from erosion and are commonly provided with relatively thin cladding elements attached to the exposed ends of the reinforcements. Another type of stabilisation system which has been used employs anchor members embedded in the earth mass which are connected to the facing by tie rods. In general, the tie rods have negligible frictional interaction with the earth and stabilisation is effected by constraining the earth mass between the anchors and the facing. In this case, the Coulomb wedge is substantially unchanged and the anchors must be placed at the rear of this zone. The reinforcements used in the technique of British Patent 1069361 are, most efficiently, strips but differently shaped reinforcements are possible provided they are capable of mobilising frictional forces adequate to stabilise the mass. The strips or other reinforcements are generally incorporated in the fill in layers, the structure normally being built up by placing a layer of spaced- strips on a flat compacted layer of earth, compacting a further layer of fill on top of the strips and placing a further layer of strips, this procedure being continued until the structure has reached the requi r ed height. It is found that the presence of the reinforcements according to the Vidal technique changes the properties of the earth mass to the extent that the boundary of the active zone is substantially nearer to the vertical face of the mass than in the case of unreinforced earth. Recent experiments have shown that, surprisingly, the position of the boundary of the active wedge, which is, in fact, the line of maximum tension in the reinforcements, runs almost parallel to the vertical face, except for the region near the foot of the structure. Thus it has been found that the boundary of the active wedge lies, for the greater part of the structure, at a distance about 0.28H ( ± 0.02H) from the face (where H is the height of the structure) . In such a structure, the reinforcements have always had a length of at least 0.7H which means that a length of reinforcement of at least 0.4H extended beyond the active wedge into the resisting zone, i.e. the zone not liable to failure. In low or medium heiqht walls, the length of the reinforce¬ ments is normally greater relative to height, e.g. 0.7 to 1.2H, so that in such cases even more of the reinforcement lies in the resisting zone and simply serves to mobilise sufficient friction in the earth mass to resist movement of the stabilised active wedge. The surface area of reinforcement in contact with the fill is calculated to ensure that the reinforcements cannot be pulled out. Substantial safety factors are always applied, however, and it has not been previously appreciated how little of the length of the reinforcements lay in the active zone. The reinforcements have always been designed to present a substantially uniform frictional surface over their length. Typical!v these have been stripe of stainless or galvanised s-_.ee!, sometimes provider " with transverse bars to incr^as.? frictional contr.ct. When it is appreciated that only 0.3H of the length of the reinforcement is required to stabilise the active wedge and the remainder, amounting to 0.4H or more, functions simply to retain the reinforcement in the zone behind the active wedge, the retaining zone, it becomes possible to consider alternative ways of retaining the rearward parts of the reinforcements in the retaining zone which might result in savings of materials and hence costs. It will be appreciated that the length of the reinforcements contributes significantly to the cost of the structure both in terms of the material of the reinforcements and also the depth of fill which has to be moved and compacted to construct the wall. It is believed that in any εtablised earth structure, the flexible reinforcements should extend to a distance of least 0.45H, preferably at least 0.5H, in order maintain the desired characteristics of the mass except near the toe of the structure, where this could be reduced to 0.35H or, more preferably, 0.4H. Beyond a distance of 0.8H however, it is now believed that frictional contact with the fill is unnecessary even in low walls. As indicated above, previous designs have used reinforcements of length 0.7H or greater and having uniform characteristics along their length. It has nov? been found possible to use shorter reinforce¬ ments, for example having a length of 0.65H or less, more preferably about 0.5H, provided the reinforcements are designed to ensure their retention in the retaining zone. It is thus possible to provide in the active wedge, only sufficient frictional contact between the reinforcements and the earth to stabilise the active wedge (with applied safety factors) while designing the rearward section of the reinforcements to resist pulling out c f the retaining zone. Tb.s can he aσheived b a d opting a τιi*ed technology coirbin.ng the Vidal tec ~ ni -ue using reinforcements having uniform properties over their whole length with no elements providing massive interaction with earth on the one hand and anchor - tie rod technique on the other hand. It is thus possible to design reinforcements with a frictional zone over substantially their whole length and having a terminal anchor attached thereto such reinforcements being generally shorter than corresponding flexible reinforcements used in the Vidal technique for any particular job. According to the present invention, therefore, there is provided a flexible reinforcement for earth stabilisation having a front end to be placed at or near the front surface of a stabilised earth mass and a rear end to be situated at the rear of said mass, said reinforcement having a frictional zone capable of significant frictional interaction with earth over its length and having an anchor member attached to the rearward end thereof. In this way, the reinforcements can be shorter from front to rear than conventional reinforcements, thereby saving on the material used for the reinforcements (commonly steel) and/or on the volume of fill handled in building the wall. While the frictional zone of the reinforcement can be any of the types suitable for use in the Vidal technique, it is preferably a strip. The material of the reinforcement may also vary. High tension metals are preferred, e.g. steel; however, suitable precautions must be taken against corrosion, e.g. galvanisation. If desired, the strip can carry ribs which increase its frictional interaction, as in British Patent 1563317. The anchor member may be a mass of material exercising passive interaction with the earth, e.g. a concrete or ιret-1 plate- arranged -..i. its ■ ia..e at right angle-- tc the line of reirforcement. Alternatively, the anchor member may be an anchor plate, normally arranged with its plane parallel to the line of reinforcement e.g. substantially horizontal, and resisting movement by frictional interaction with the earth. This may be of metal or concrete and will normally be rigid (in contrast with the flexible friction zone of the reinforcements) . The anchor member may be attached directly to the end of the frictional zone or may be connected thereto by a tie member exerting substantially no frictional interaction with the earth. Direct attachment is preferred. According to a further feature of the invention there is provided a stabilised earth structure having a substantially vertical front face and comprising layers of substantially horizontal reinforce¬ ments having a frictional zone extending from the said front surface rearwards, and an anchor member attached to the rearward end thereof, layers of compacted earth being between said layers of reinforce¬ ments, the frictional force mobilised between the reinforcements and the earth in the frictional zone being sufficient to resist horizontal movement of the earth, while creating a tension in the reinforce- ments, which is less than their tensile strength, the said anchor member providing sufficient pull- out resistance to maintain the stability of the structure. In general the line of maximum tension in the reinforcements in such a structure will be in substantially the same position as in a conventional Vidal structure i.e. at about 0.3H. It will be appreciated that in such a structure, the reinforcements may be shorter than conventional uniform reinforcements used in the Vidal technique which have the same frictional capacity per unit length as th_._ frictional zone of the reinforcements used accordi"- to the invention, rcvided the same number of re nfcrrements is used in each case. The reinforc Tiertf may also be s' rter than the non-frictional tie rods in the above described anchor system, since in the latter case the Coulomb wedge (at least at the top of the structure) extends significantly deeper into the earth mass and thus requires stabilisation of a greater volume of earth, thereby increasing the amount of fill to be handled during construction. As indicated above, the present technique can result in a considerable saving in the amount of fill handled and, in many cases, the volume of earth excavated to accommodate the wall. The invention is particularly applicable to low or medium height walls for example from 4 to 15 metres in height or for the upper 15 metre sections of high walls. Over that range, the ratio of the lenqth of the frictional zone of the reinforcements L to the height of the wall H is conventionally from 1.3 to 0.7. It is found that, in a structure according to the invention, L/H can be significantly reduced, e.g. by up to 50%. On the other hand, it is necessary to consider the function of the stabilised earth wall as a gravity wall which must resist overturning forces. In general L/H should not be reduced below 0.45, preferably not below 0.50. While, in theory, the number of reinforcements can be varied infinitely, in practice any stabilised earth construction system will use standard panels having uniform appearance and possessing a limited number of points for attachment of reinforcements. This leads to a tendency for lower walls actually to have a greater ratio L/H than higher walls. In such cases L/H, in systems according to the invention, may well be higher than 0.5; in general, however, L/H will not be above 0.8. In some cases L/H may not be the same throughout the structure but will be less for reinforcements near the toe, provided the stability of t structure is maintained. Ir general, in order t~> ensure adequate fr- ' ct onal interaction in the active zone, it is desirable that in this zone, at least 2%, and preferably at least 5%, of the area of the bed of earth on which each layer of reinforcements is laid is covered by the material of the reinforcements and that there are at least 4 such layers in the structure. The invention is now described by way of illustration only with reference to the accompanying drawings in which; Figure 1 shows in plan view one embodiment of a reinforcement according to the invention. Figure 2 shows a further embodiment of a reinforcement according to the invention. Figure 3 shows in diagrammatic section a stabilised earth structure according to the invention. In the reinforcement shown in Figure 1, the front section 1, consisting of 60 x 5 mm plain galvanised steel strip, is jointed by a bolt 2 to an anchor member 3. The front end of the reinforcement is secured by a further bolt 4 to the tab 5 embedded in a concrete facing unit 6. If the reinforcements form part of a 10.5 metre wall, L/H can be 0.5 and the plain section 1 should therefore, in such a structure, be about 5.25 metres in length. In terms of pull-out resistance, the resistance provided hy the anchor and strip is equivalent to two ribbed high adherence strips having the same total length i.e. 5.25 metres. The total length of 2 such strips would thus be 2 x 5.25 metres. The saving in steel which may be acheived is thus of the order of one third. Alternatively, it can be seen that the reinforcement accordinq to the invention is equivalent to a single- ribbed high adherence strip of significantly greater length, e.g. about 8 metres, which would require a far greater depth of fill and consequent increases in costs. In reinforcement shown in Figure 2 the strip and anchor are as shown in Figure 1 except that the anchor member 3 is replaced by a flat anchor plate 3'. In the wall shown, in Figure 3, the line 7 joins points of maximum friction in the reinforcements 8 according to the invention. The anchor members 9 attached to the reinforcements 8 define the rear of the wall. The line 10 shows the position of the ends of conventional reinforcements of the same material as the front sections of the reinforcements according to the invention (used in the same numbers and spacing) . The line of maximum tension 7 is approximately the same in both cases.

Claims: CLAIMS 1. A flexible reinforcement for earth stabilisation having a front end to be placed at or near the front surface of a stabilised earth mass and a rear end to be situated at the rear of said mass, said reinforcement having a frictional zone capable of significant frictional interaction with earth over its length and having an anchor member attached to the rearward end thereof. 2. A flexible reinforcement as claimed in claim 1, wherein the anchor member is a plate arranged with its plane at right anαles to the line of reinforce¬ ment. 3. A flexible reinforcement as claimed in claim .1, wherein the anchor member is a plate arranged with its plane parallel to the line of reinforcement. 4. A flexible reinforcement as claimed in claim 1,2 or 3, wherein the anchor member is directly connected to the end of the frictional zone. 5. A stabilised earth structure having a substantially vertical front face and comprising layers of substantially horizontal reinforce¬ ments having a frictional zone extending from the said front surface rearwards, and an anchor member attached to the rearward end thereof, layers of compacted earth being between said layers of reinforce¬ ments, the frictional force mobilised between the reinforcements and the earth in the frictional zone being sufficient to resist horizontal movement of the earth, while creating a tension in the reinforce¬ ments, which is less than their tensile strength, the said anchor member providing sufficient pull- out resistance t > maintain the st^bi_ity of the structure. 6. A stabilised earth structure as claimed in claim 5, wherein L/H is less than 0.8, where L is the length of the reinforcements and K is the height of the structure. 7. A stabilised earth structure as claimed in cliam 5 or 6, wherein the structure is between 4 and 15 metres in height. 8. A stabilised earth structure as claimed in any of claims 5 to 7, wherein in the frictional zone of the reinforcements at least 2% of the area of the bed of earth on which each layer of reinforce¬ ment is laid is covered by the material of the reinforcements, and wherein the structure has at least four such layers. 9. A stabilised earth structure as claimed in any of claims 5 to 8, having flexible reinforcements as claimed in any of claims 1 to 4.

Inventors: VIDAL HENRI, VIDAL, HENRI
Applicants: HOLMES MICHAEL JOHN, VIDAL HENRI, HOLMES, MICHAEL, JOHN, VIDAL, HENRI
Application year: 1989
Main classification: E02D29
IPC codes: E02D5, E02D17, E02D29