METAL STRIP FOR USE IN STABILISED EARTH STRUCTURES

Abstract: A rolled metal strip 1 for use in stabilised earth structures has at periodic intervals along its length thickened reinforce­ ment regions 7 formed during the rolling operation. The strip is cut into required lengths such that each strip length has an end reinforced region 6 through which an aperture 5 is then formed to receive a bolt 3 passing through a bracket 4 of a facing 2. The strip may include transverse ribs 8 at intervals on both faces of the strip to assist engagement with the sur­ rounding soil.

Description: This invention relates to a metal strip for use in stabilised earth structures, for example of the type disclosed in GB-A-1069361. A preferred type of strip for use in such structures is a rolled steel strip, and particularly a strip provided with transverse ribs as disclosed in GB-A-1563317 (on which the precharacterising part of claim 1 is based). In use, the ends of the strips are attached to facing units such as those disclosed in GB-A-1324686. These facing units are interlocked to provide a generally vertical surface. The connection between a strip and a facing unit is usually formed by a fastening element such as a bolt which passes through an aperture in the strip. In order to ensure that the aperture does not unduly reduce the tensile strength of the strip, it has been proposed in GB-A-1563317 to reinforce the end of a strip by means of welding one or more plates to the end of the strip and to form the aperture in the reinforced region. This presents a number of problems in practice, however. The general use of high tensile strength carbon steel means that welding can be difficult and the high heat required can produce crystallization having a corrosive effect. This can result in hairline cracks not visible to the naked eye. Furthermore, it is common to galvanize the steel strips but this can be difficult with welded reinforcement plates. Prior to galvanizing, the strip passes through an acid bath and it is difficult to remove all of the acid from between the welded plate and the strip. The presence of residual acid is undesirable and may damage the galvanization, leading to corrosion problems. It has also been proposed in GB-A-1563317 to hot forge the end of the strip to a required configuration. However, this is a relatively expensive process and the forging may change the property of high strength steels in particular, leading to e.g. brittleness. To deal with these problems, the invention provides a rolled metal strip for use in stabilised earth structures, the strip comprising thickened portions formed during the rolling operation at periodic intervals along the length of the strip, and a reinforcement region located at or near one end of the strip having therein an aperture suitable to receive fastening means to locate the strip in a stabilised earth structure, characterised in that at least some of said thickened portions are elongate portions between 40 mm and 100 mm in length, the strip being cut to a desired length with one of such elongate portions being located at or near one end of the strip to serve as the reinforcement region having said aperture formed therethrough. Thus, the strip will have elongate thickened portions provided along its entire length, spaced apart by no more than the distance corresponding to one full revolution of the forming rolls. Typically, the maximum spacing would be about 1400 mm for a roll of this nominal circumference but preferably two or more thickened portions are provided for each revolution, thus giving spacings of e.g. 700 mm or 350 mm. Another possibility is to provide a roll of 1000 mm circumference with spacings of 500 mm between pads. Greater spacing results in lesser increase of weight per metre of the strip, and thus less use of material, but increases wastage when the strip is cut to length for use. The result of providing a reinforcement region at or near the end of the strip length with an aperture formed therethrough is that there need be no separate forming steps after rolling. It will be appreciated that to avoid cutting wastage in the event that reinforcement regions are required at both ends of the strip, each thickened portion might have a longitudinal extent at least twice that required for use at one end of a strip length. In this way, a single cut through the middle of a thickened portion would ensure that each severed strip length will terminate in a suitable reinforcement region. However, since the number of thickened portions intermediate the ends of a strip length will not be used, it may be preferable to reduce the amount of material and have thickened portions of a size suitable for use at one end only. Thus, the cut would not be through the centre of the thickened portion but would instead be made at or near one longitudinal end of the thickened portion e.g. through the thickened portion, or through its junction with the unthickened strip, or through the unthickened strip at a short longitudinal spacing from the thickened portion. The length of the thickened portions of between 40 mm and 100 mm can be compared with preferred strip cross-sectional dimensions of about 60 x 5 mm, 50 x 5 mm or 40 x 4 mm. Thus, the length of the thickened portions is many times the thickness of the strip, for example of the order of 8 to 25 times the thickness of the strip. This can be contrasted with the dimensions of preferred transverse ribs as disclosed in GB-A-1563317 whose dimension in the longitudinal direction of the strip is of the same order as the thickness of the strip. It is, in fact, preferable that the strip with thickened portions incorporate the transverse ribs also. For the 50 x 5 mm strip mentioned above, there may be ribs of, say, 3mm high and 5mm in the longitudinal direction of the strip. These may be provided at 50 mm intervals on both faces of the strip, with the ribs on one side offset by 25mm with respect to those on the other. The ribs need not be provided continuously, and for ease of production will preferably not be formed in the thickened portions. Apart from that, the ribs could be provided in groups of say four - two on each face of the strip - spaced apart by a larger distance of 100mm or more. The thickness of the thickened portions will depend on the stress to be experienced in use, but for typical applications the thickness could be about 8mm for the 5mm thick strip. In general, the increase in thickness will be of the same order as that of the thickness of the strip. e.g. a 1.5 to 3.5 mm increase for a 4 or 5mm thick strip. Where ribs are used, the thickened portions will generally give a similar increase in strip thickness to that provided by the ribs. The thickened portion preferably increases in thickness from the adjacent unthickened strip gradually at each end, preferably rising to maximum thickness over about 1/4 to 1/13 of its overall length, to assist in rolling. In some instances, it may be preferable to increase the thickness of the strip in two sections. For example, the use of two generally straight sections of different gradient might minimise the effect of a sudden discontinuity during rolling. The thickened portions should also preferably be symmetrical about the laterally extending central plane of the strip. Otherwise, in the transition from one thickness to another the strip centre plane may shift for a short time in the rolling operation, leading to unwanted vibrations. However, absolute symmetry is not essential, so that although the increase in thickness might take place on both faces of the strip with the longitudinal profiles of the two faces being substantially the same as each other, these profiles might be longitudinally offset. Preferably, the thickened portions will extend over the entire width of the strip, as any transverse ribs would also do. However, rolling of a thickened portion results in a reduction of width. For the 50 x 5mm strip with 8mm thick thickened portions, the maximum reduction in strip width may be about 4 to 5 mm. This is acceptable. The size of aperture which can be provided in the thickened portion serving as the reinforcement region will depend on many factors, but the diameter is likely to be substantially greater than the thickness of either the basic strip or the reinforcement region. Thus although the diameter of the aperture will vary it will generally be at least 10 mm. For the 5mm strip with an 8mm thick reinforcement region, an aperture of nominal 12.7mm diameter may be used. The strip may be of any suitable metal, bearing in mind the condition of forming and of use. ASTM A36 mild steel may be used, or higher strength steel such as ASTM A572, grades 40, 50, 60 or 65. Typical basic cross sections may be 40 x 5mm, 60 x 5mm, 50 x 6mm and 50 x 8mm for the ASTM A36 mild steel. With the higher strength steel, cross sections of 50 x 4 mm or 50 x 5mm are possible and it is in the context of these that the reinforcement of the ends may be of particular importance. The strip can be rolled using conventional apparatus but with the final rollers being suitably profiled to give the thickened portions. The existence of the thickened portions of increased thickness may have a tendency, in some cases, to cause bunching during the rolling process but since the duration of forming the thickened portions is relatively very short in most cases, there will generally be no insurmountable problems. It may be desirable to monitor and if necessary vary the roller drive velocity at appropriate points for example by terminating or varying the current supply in the case of electrically driven rollers. The cutting and handling of the strips subsequently may be by any convenient means. The lengths of strip cut could be e.g. from 3.5m to 11.5m or more. It will be seen that the process for forming strip lengths with reinforcement regions at either end, has a number of advantages over the known processes using e.g. welding. Furthermore, the strips themselves may have improved properties leading to more reliability in the stabilised earth structures themselves. The invention also provides a stabilised earth structure including facing units to which are attached rolled metal strips as discussed herein, each strip being attached by fastening means received by the aperture formed through the strip. An embodiment of some of the broad aspects discussed above will now be described by way of example only and with reference to the accompanying drawings, in which:- Fig. 1 is a side view of a length of strip joined to a facing element; Fig.2 is a plan view of the strip; Fig. 3 is an enlarged side view of the strip; Fig.4 is a diagrammatic view of the rolling process; and; Fig. 5 is an enlarged view of the last part of the rolling process. Referring now to Figures 1 and 2 a strip 1 of high strength steel A572 (ASTM) is joined to a facing 2 of a stabilised earth structure by means of a bolt 3 passing through a bracket 4 secured to the facing 2 and through an aperture 5 formed in a thickened reinforcement region 6 at the end of the strip 1. The reinforcement region 6 is provided by the major part of one of a number of elongate thickened portions 7 spaced regularly along the strip at about 700mm intervals. The strip has a nominal thickness of 5mm and a nominal width of 50mm. Each thickened portion 7 has a maximum thickness of 8mm for a central portion 50mm long, and tapers at either end to the 5mm thickness over a length of 10mm. In an alternative embodiment the length of taper could be as little as 5 mm, while the central portion could be 55 mm long. Greater lengths of taper e.g. 25 mm are also possible. In the illustrated embodiment the tapering occurs in a single generally straight section, but it could also occur as two sections of differing gradient. The thickened portion 7 has a reduced width, the reduction being about 5 mm maximum in the central portion. Reinforcement region 6 is formed by one tapered part and the central portion of a thickened portion 7. The strip is also provided with transverse ribs 8 of known type, each extending a few millimetres high and a few millimetres in the direction of the length of the strip. The ribs 8 between the adjacent thickened portions 7 are arranged in groups spaced apart by about 175mm. The ribs in the groups alternate on the two faces of the strip, the spacing between two adjacent ribs being about 25mm. There are a number of groups of four ribs and, in this arrangement, one group of three ribs between two adjacent thickened portions. Each thickened portion 7, and thus reinforcement region 6, is symmetrical having identical and aligned profiles on both faces of the strip. However, in an alternative arrangement the profiles on the two faces could be longitudinally offset relative to each other. For example there might be an offset of about 3 mm in a case where the length of taper is 5 mm. With reference to Figures 4 and 5, the strip is formed in a generally conventional manner. A hot billet 9 from a continuous casting process passes through sixteen sets of rollers 10, to produce a strip of reduced thickness. It is then passed through final profiling rollers 11 which produce the final strip 1. The circumferences of the rollers are provided with grooves 12 to produce the ribs 8 at the required intervals. However, two larger profiled regions 13 are also provided to produce the thickened portions 7. The circumference of the rollers is about 1346 mm but the strip is extruded by about 4% during rolling, so that this corresponds to about 1400mm in the finished strip. Thus, the thickened portions are provided at the required intervals of 700mm since two are formed for every rotation of the rollers. The rolled strip is then cut into suitable lengths with a thickened portion at one end at least constituting a thickened reinforcement region 6 through which an aperture 5 is punched. The thickened portions 7 intermediate the ends are not punched. The strips can be galvanized by known means to improve corrosion resistance.

Claims: A rolled metal strip (1) for use in stabilised earth structures, the strip comprising thickened portions (7) formed during the rolling operation at periodic intervals along the length of the strip, and a reinforcement region (6) located at or near one end of the strip having therein an aperture (5) suitable to receive fastening means (3) to locate the strip (1) in a stabilised earth structure, characterised in that at least some of said thickened portions (7) are elongate portions between 40 mm and 100 mm in length, the strip (1) being cut to a desired length with one of such elongate portions (7) being located at or near one end of the strip to serve as the reinforcement region (6) having said aperture (5) formed therethrough. A strip as claimed in claim 1, wherein the cut is made at or near one longitudinal end of the elongate portion serving as the reinforcement region (6). A strip as claimed in claim 1 or 2, wherein the elongate portions (7) increase in thickness gradually at each end thereof. A strip as claimed in claim 3, wherein the increase in thickness takes place in two generally straight sections of different gradient. A strip as claimed in any preceding claim, wherein the elongate portions (7) are symmetrical about the laterally extending central plane of the strip. A strip as claimed in any of claims 1 to 4, wherein the increase in thickness of the elongate portions (7) takes place on both faces of the strip, the longitudinal profiles of the two faces being substantially the same as each other but longitudinally offset. A strip as claimed in any preceding claim, including transverse ribs (8) provided at intervals on both faces of the strip between said elongate portions (7). A stabilised earth structure including facing units (2) to which are attached rolled metal strips (1) as claimed in any preceding claim, each strip being attached by fastening means (3) received by said aperture (5) formed through the strip.

Inventors: VIDAL, HENRI, WEINREB, DANIEL
Applicants: SOCIETE CIVILE DES BREVETS DE HENRI VIDAL
Application year: 1986
Main classification: E02D29
IPC codes: E02D29, E02D17, B21B1, E02D5