SUPPORT ARRANGEMENT BACKGROUND OF THE INVENTION
 This invention relates to a support arrangement suitable for use in an underground tunnel.  In mining, tunnels are formed through rock to provide safe and effective pathways for equipment and personnel. The integrity of a tunnel is therefore of paramount importance. To address this requirement various arrangements can be used to provide tunnel support.
 For example a rock surface which defines a tunnel can be coated with a cementitious mix which is reinforced with mesh or the like. Support shields can be employed, as appropriate. Compound assemblies of timber, beams and packs can also be used.
 A support arrangement should be able to cater for tunnel movement, for example if closure of a hanging wall takes place. One approach is to make a support arrangement so strong that physically it is capable of resisting tunnel closure to a substantial extent. An alternative approach is to provide a support arrangement which is capable of exhibiting at least a limited degree of yield. The present invention is concerned with this type of support arrangement.
SUMMARY OF THE INVENTION
 The invention provides a support arrangement for a tunnel which includes a plurality of arches positioned at spaced intervals from one another along a length of the tunnel, wherein
each arch has two legs and a curved overhead section supported by the legs, bracing structure which interconnects adjacent arches and, in respect of each arch, yielding ground-engaging supports which are respectively engaged with the legs of the arch.
 The tunnel may have a nominal roof curvature and, preferably, each arch section is configured, within acceptable limits, to have a corresponding curvature.
 Each arch section may be adjustable in curvature. This may be achieved by forming each arch section with a number of curved modules which are relatively movable thereby to increase or reduce the curvature of the arch section.
 Alternatively, each arch section is formed from modules of selected curvatures or shapes so that the modules can be assembled to follow the roof curvature.
 Each arch section may include a centrally positioned key element.
 Each arch may be fixed to a body of rock in which the tunnel is formed by means of ground anchors. Use may be made of rock bolts which, particularly for use in poor quality rock, may have a yielding characteristic.  The support arrangement may include mesh which overlies the arches.
 A settable material may be positioned between the arches and an opposing surface of the tunnel, i.e. the rock face.
 The settable material may be cementitious and may be provided inside containers such as flexible bags.
 The bracing structure may comprise a plurality of beams which extend in a longitudinal direction and which interconnect adjacent arches.  As indicated each arch has two downwardly extending legs which are respectively brought into ground-engagement with yielding supports. Each yielding support may be of any appropriate kind and may for example include hydraulically adjustable components which, under load, can yield in a satisfactory manner e.g. telescopically. In another approach each support includes a timber element which can yield at a controlled rate when subjected to axial loading. These examples are non-limiting.
BRIEF DESCRIPTION OF THE DRAWINGS
 The invention is further described by way of examples with reference to the accompanying drawings in which :
Figure 1 is a view in a longitudinal direction of a tunnel which includes a support arrangement according to the invention,
Figure 2 illustrates how the support arrangement can be adjusted,
Figure 3 is a perspective view of a compound support arrangement in accordance with the invention, displaced from a tunnel in which it is erected, so that various constructional details are more readily visible, and
Figure 4, which is similar to Figure 1 , illustrates a different form of a support arrangement of the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
 Figure 1 illustrates somewhat schematically a body of rock 10 through which is formed a tunnel 12. The tunnel has a curved or arcuate hanging wall 14 and a footwall 16. Sides of the tunnel are formed by substantially vertical walls 18 and 20 respectively. The tunnel is supported in accordance with the principles of the invention through the use of support structure 24 of the general kind shown in Figure 3. It is to be understood however that Figure 3 illustrates one example of the support structure and that in use the support structure can vary from installation to installation.
 The support structure 24 includes a plurality of arches 26A, 26B ... 26N which are spaced apart from one another by a distance 28 and which extend in a longitudinal direction 30 of the tunnel 12.
 The various arches 26 are substantially identical. Each arch, see Figures 1 and 2, has an overhead curved section 32 which is made from curved modules 34 and 36 which are interconnected by means of a curved beam 38. The modules 34 and 36 can be fabricated with different degrees of curvature and appropriate modules can be selected according to a particular requirement. Despite this a substantial amount of adjustment can be achieved, as is shown in Figure 2, by extending the modules in telescopic fashion.
 Figure 2 illustrates in dotted outline the modules 34 and 36 wherein the spacing provided by the interconnecting beam 38 is relatively small. As the modules are moved apart a different shape of curvature results and a greater portion of the beam 38 is exposed. Given the rough conditions which prevail in underground excavations "smooth" arcs are not necessarily defined by the tunnel and, at least for this reason, it is not essential for the arches,
when extended or relaxed, to follow precisely curved arcs. Often gaps will exist between an upper surface of an arch section and an opposed surface of the hanging wall. In many instances these can be "filled" by means of wooden beams or other load-bearing devices which can transfer load from the tunnel to an arch section.  The spacing 28 between adjacent arches is set by the use of supporting structure 40, see Figure 3, which, in this example, consists of a plurality of tubular members 42 which are slotted into engagement with the respective arches. Each tubular member can be positioned between an adjacent pair of arches and can be engaged with these arches e.g. in a spigot and socket fashion. Alternatively each tubular member can include a flange at each respective end and these flanges can be bolted or otherwise fixed to corresponding surfaces of the pertinent arches. In another possible form of construction the arches have apertures or are formed with eyelets into which the elongate members 42 can extend. Subsequently these members are fixed using appropriate fasteners such as clamps to the arches. In this way one tubular member can for example interconnect three or four arches to one another.  Each arch has downwardly extending legs 50 and 52 which are close to the sidewalls 18 and 20 respectively. Each leg is directly supported by a corresponding elongate support 54 which abuts the footwall 16 and which, at an upper end, bears against a lower end of the corresponding leg. Each support 54 has a yielding capability and can yield in a controlled manner when subjected to an axial loading in excess of a predetermined value. This enables a support to yield under load as opposed to supporting an increasing level of load until a value is reached at which the support fails catastrophically. Each support may be of a kind known in the art and may for example comprise a hydraulically activated device which can yield telescopically; a timber element which can yield in a controlled manner, or a combination of
steel and timber components which, under load, yield in a controlled manner. Preferably each support is extensible so that it can be actuated to urge the arch section which it supports upwardly into firm engagement with the roof.
 In a different approach use is made of preloaders. This type of device comprises a metallic housing which is inflatable by means of water pressure to expand and thereby place a component to which it is engaged under compressive or axially directed loading. A preloader is then used to bring an arch section into sound load-bearing engagement with a hanging wall.
 Figure 4 illustrates support structure 80 which is substantially similar to the support structure shown in Figures 1 to 3. Where applicable, similar referenced numerals are used to indicate similar components.
 The structure 80 is located in a tunnel 12 which has a hanging wall 14 and a foot wall 16.
 The support structure 80 is formed from a plurality of arches 80A, 80B ... 80N (see Figure 4A). These arches are spaced apart from one another in a longitudinal direction of the tunnel.
 The various arches are substantially identical to one another. Each arch has opposed upright legs 82 and 84 respectively which are supported on the ground by respective supports 54. These supports are used in the manner which has been described, ie. to provide a yielding characteristic for the arch structure and also, initially, to position the arch structure correctly relative to the rock face formed by the tunnel.
 At a central upper location each arch 80 has a respective key element 88 which is slotted between and bolted to opposing flanges 90 and 92 extending from the uprights. A longitudinally extending supporting structure, which is similar to the support structure 40 shown in Figure 3, is provided between adjacent arches. Figure 4B illustrates how a tubular member 42 is engaged with an arch 80.
 The arches are preferably anchored to an adjacent body of rock using one or more ground anchors 98 as shown in Figure 4C or 100 as shown in Figure 4D.
 If the rock quality at an arch is poor then a yielding anchor 98 of the kind shown in Figure 4C is used. This is a so-called "double bulb resin anchor" wherein strands of a cable 102 are deformed to define distinct anchor points 104, 106, etc. The cable is covered with a debonding agent such a grease (not shown in Figure 4C) and once inserted into a borehole 1 10 in the body of rock is stressed through an arrangement 1 12 comprising a bearing plate 1 14 and a barrel and wedge anchor 1 16 as is known in the art. This arrangement allows a degree of yielding to take place.  On the other hand, if the rock, adjacent an arch, is of good quality then an anchor 100 of the type shown in Figure 4D is used. This anchor has a cable tendon 120, an expansion shell anchor 122 at one end, and an arrangement 1 12 similar to that shown in Figure 4C at an opposing end.
 Once the arches are erected they are elevated to a suitable position using the supports 54. Thereafter wire mesh 130 is placed over the arches and the supporting structure 40. Voids 132 between the mesh 130 and an opposing surface of the body of rock around the arches are filled with flexible containers, eg. grout bags 134. These bags are placed, initially
empty, in position and are then expanded and filled by means of a pumping process with an aerated cementitious mixture 136. After the cementitious mixture has set the supports 54 are activated to preload the supports to a desired operating level, eg. 20 tons. The anchors 98 and 100 are used, as appropriate, taking into account the nature of the rock body adjacent the arches. Thereafter the anchors are stressed to a desired operating level.
 The invention, in each form, has a number of benefits. The arches and support structure can be erected fairly rapidly to provide a substantial degree of roof support and this can be enhanced, as appropriate, by including load-bearing sheet material, which in one form of the invention comprises the mesh 130 shown in Figure 4 over the arch sections. Depending on the type of rock in which the tunnel is formed the cementitious filler 136 is employed. The arch modules 34, 36 can be adjusted on site, with a reasonable degree of tolerance, to tunnels which have different arcuate shapes. As noted the gap between the arches and the tunnel surface can be filled with the cementitious filler. The use of the key elements 88 is important because the length of each key element can be adjusted to fill a gap between the respective opposing flanges 90, 92. The supports 54 which are coupled to the legs of the arches can be actuated to bring the arch sections into load-bearing contact with the tunnel roof and, as the loading exerted on the arch sections by the tunnel increases, e.g. due to closure, the supports 54 can yield to a substantial extent and thereby carry the load without catastrophic failure.