STABILIZATION OF EXCAVATIONS

Abstract: A method for shoring walls of open earth excavations comprises excavating the earth to a predetermined distance, anchoring earth anchors in boreholes in the exposed earth wall connecting the exposed ends of some or all of the earth anchors to form a substantially continuous linear or mesh-like tensile member over the exposed face of the earth wall and then spraying the wall with concrete to encapsulate the reinforcing structure. The excavation and shoring wall construction is carried out in successive phases to reach a required depth of excavation.

Description: STABILIZATION OF EXCAVATIONS This invention is concerned with stabilization and support of earth walls in excavations and particularly, although not exclusively to earth excavations associated with the construction of high rise buildings. In the construction of high rise buildings it is commonplace to excavate a cavity in the earth up to thirty meters deep to facilitate construction of underground car parks, storage rooms and the like. The excavated cavity is generally rectangular in plan and has vertical walls and a flat horizontal floor. To maximise the available site area the walls of the excavationare formed as close as possible to the site perimetre making allowance where necessary for adjoining buildings, footpaths, roadways etc. When the excavation is complete, foundation piles may be then sunk and subsequently basement floors and support columns formed. A typical earth profile through which the excavation proceeds may comprise, a surface layer of fill, a subsurface layer of residual soil (usually a clay formation) a next layer of extremely weathered shale bedrock and finally a layer of highly weathered bedrock. Each of these earth layers is quite unstable and in order to raaintain safe working conditions within the excavated cavity it is usually necessary to provide shoring of the vertical walls to prevent collapse. In unstable earth formations such as the profile described above, collapse can occur due to slippage along a theoretical fault or shear plane extending upwardly and rearwardly from the cavity wall into the earth formation at about 45°. In hard rock formations collapse may be gradual due to frittering of the rock face or sudden with large masses of rock falling from the vertical cavity face. In an endeavour to provide support for the vertical walls of such earth excavations, temporary and permanent shoring systems have been proposed. A commonly used temporary shoring system comprises a plurality of spaced upright steel columns secured to the earth wall by means of earth anchors. Timber shuttering is then placed between the steel columns to provide a barrier against earth collapse. The main difficulty associated with this type of system is that the excavation must be completed or substantially completed before the upright steel columns can be placed against the earth face for anchoring thereinto. Collapse of walls can occur during the course of the excavation. Another difficulty is associated with the anchoring of the upright steel columns (known as "soldiers"). The columns are usually anchored towards the upper end and middle region to the cavity wall by earth anchors comprising a 7-10 metre length of 10mm steel cable or rod extending into a borehole in the earth formation downwardly at an angle of 30-45 to the horizontal. One end of the rod or cable is anchored in the end of the.borehole by a cementitious or chemical grout and the other end is secured to the steel column by an appropriate fixing means such as a clamp, post-tensioning collets etc. As the ability of this shoring system to withstand collapse is effectively a function of the tensile strength of the steel rod or cable and integrity of its anchoring, the rod or cable anchor is of necessity quite long in order to be anchored effectively. When using anchors of 7-10 metres in length considerable difficulties are encountered when there is a basement structure in an adjoining building which may necessitate a reduction in size of the excavation. Where there is no adjacent structure limiting the anchor depth, there remains the~problem of removal of the anchors in the event that the adjacent building site might one day be excavated. Yet another problem with the temporary shoring systems described above is that they are costly due to the high degree of labour involved in installation and removal as well as transportation, handling and storage costs between jobs. Many of the problems of such prior art shoring systems are overcome by permanent shoring. One such system comprises drilling 0.5 metre diameter shafts to the required excavation depth around the perimeter of the excavation. The shafts are drilled in avertically overlapping fashion such that the interior of one shaft communicates with the interior of each adjacent shaft. When the shafts are drilled concrete is poured down the shafts together with reinforcing steel to form a contiguous reinforced concrete wall approximately 0.5 metres thick. The region within the contiguous wall may be then safely excavated without fear of wall collapse. Although very effective in acting as a shoring for excavations, such structures are extremely expensive due to drilling costs and material costs. Generally speaking such permanent shoring systems are used only where a particular construction requires such a structure or where site conditions mitigate against temporary systems hereinbefore described. A further difficulty with permanent contiguous wall structures of the type described is that adverse weather or earth conditions can seriously delay progress due to filling of the shafts with rain water or subterranean water which may lead to partial collapse of the shaft walls- It is an aim of the present invention to overcome or alleviate the difficulties associated with prior art shoring systems for open excavation. According to one aspect of the invention there may be provided a method and/or structure for shoring the walls of open earth excavations in which the walls of the excavation have applied thereto a pumpable cement itious composition to cover at least a substantial portion of the exposed earth surface. If required the cementitious layer formed on the excavation wall may include reinforcing in the form of steel glass or plastics fibres, fine or coarse steel mesh, tensile elements in the form of elongate members anchored adjacent the wall surface or a plurality of tensile elements in linear or mesh-like array interconnected to form substantially continuous tensile elements, or any combination of the above. The shoring system may include earth anchors in the form of tensionable rock bolts, tensile members anchorable by mechanical means or by cementitious or chemical grouting. The cementitious layer may be of any suitable thickness between 25mm and 250mm and may be of substantially even thickness over the face of the excavation or it may be selectively varied in thickness. The shoring structure may include a region of increased wall thickness adjacent the upper perimeter. Preferable the shoring structure is formed progressively as the depth of the excavation increases. According to another aspect of the invention there may be provided a method and/or structure for shoring the walls of open earth excavations wherein a plurality of spaced upright boreholes are formed adjacent the perimeter of one or more walls of an excavation and further boreholes adjacent the spaced upright boreholes are formed at an angle of between 20 and 70 thereto extending downwardly and away from the wall or walls of the excavation. Tensile members in the form of wire cable or rod are anchored in the boreholes by any suitable means such as grouting and the free ends of the tensile members protruding from adjacent boreholeβ are connected to form an inverted "V" shaped member. If required the inverted "V M shaped members may be interconnected by one or more tensile members extending substantially parallel to the face of the excavation wall to form an integral tensile structure. Various aspects of preferred embodiments of the invention will now be described with reference to the accompanying drawings in which:- FIG 1 shows a cross section through an excavation wall; FIG 2 is a front elevation of the structure shown in FIG 1; FIG 3 shows a typical joint in the mesh array of FIG2; FIG 4 shown a typical joint in the linear array of FIG 2; FIG 5 shows a front elevation of a structure embodying an upper rim reinforcement; FIG 6 shows a cross section of the structure of FIG 5; FIG 7 shows a detailed view of the rim reinforcement of FIG 5; FIG 8 shows a cross section through the structure Of FIG 7; FIG 9 E j hows cross sectional detail ^ of a drainage configuration; and FIG 10 shows an alternative embodiment of the invention. In FIGS 1 and 2 there are shown respectively a cross sectional and a plan view of an earth formation 1 in which an excavation 2 is formed. The earth formation comprises a layer of fill 3, a layer of residual soil 4, a layer of extremely weathered bedrock 5 and a layer of highly weathered bedrock 6 in which the floor 7 of the excavated cavity is formed. Extending through the face 8 of the excavation are a plurality of boreholes extendinq downwardly at an angle of about 45 to the horizontal. Located within the boreholes are anchor members in the form of an elongate U-shape. The insertable portion or shank 8 is full column grouted in the borehole with a cementitious grout and a free end 9 in the form of a closed loop is bent to lie parallel and adjacent to the face 10 of the excavation. To ensure adequate anchoring the upper anchors 11 are of a length Csay, 7 metres) sufficient to be anchored into at least the first layer of bedrock as the surface fill and residual soil layers are extremely unstable. The remaining anchors 12 are shorter in length (say, 3 metres) . As the upper layers (3 and 4) of the earth formation are more prone to collapse, that region is supported by an interconnecting mesh-like array of anchors 13 as shown in FIG 2. The mesh-like array is formed by inserting two U-shaped elements in each borehole and bending the free looped ends at 90° to each other such that the looped ends overlay adjacent boreholes. Subsequent U- shaped members are then inserted into the adjacent boreholes through the closed loop of the preceding members. In the lower regions of wall 10 where the integrity of the earth formation is greater, linear arrays are found to provide sufficient support strength. Depending on the nature of the earth formation an additional linear array 15 may be inserted or the entire face 10 may be covered with a mesh- like array. The entire surface of the wall 10 is then sprayed with a pumpable cementitious composition to a thickness of say 50mm. In the region of the upper rim of the excavation the wall thickness of the pumpable cementitious composition (sprayable concrete known as "Sprayσrete") is increased to form a continuous ring beam 16 around the perimeter of the excavation. The ring beam 16 may be formed as simply an increased thickness of Sprayerete or it may be formed in boxing as an inwardly projecting beam 16 as shown or an outwardly projecting beam l>7.as showninphantom. The ring beam structure suitably incorporates steel reinforcing and is particularly advantageous in enhancing structural integrity of the shoring system where large inwardly directed forces are- encountered with hydraulic upper earth layers. The preferred sequence of events in constructing a shoring system for an open earth, excavation are as follows:- 1. Excavate to a first stage depth of say 1-2 metres below the surface level. 2. Drill boreholes in the excavation wall at approximately 45 to the horizontal to penetrate a minimum distance of 3 metres into the upper bedrock layer. The boreholes may be skewed at say 10° to the wall face and spaced at 1.5 metre centres. 3. The shanks of the anchor members are then inserted into the boreholes to the required depth and grout is introduced into the borehole to fill it completely. 4. Reinforcement and/or boxing is then fixed into place to form the upper ring beam. 5. Spraycrete is then applied to the excavated wall to form the upper ring beam structure. 6. Excavate to second stage depth, a further 1-2 metres. 7. Drill boreholes, insert and grout remainder of anchors in a mesh-like array as hereinbefore described. 8. Spray exposed wall face and anchor members with Spraycrete to a thickness of 50mm. 9. Excavate to third stage depth, drill boreholes, insert and grout anchor members in a linear array and spray with Spraycrete to 50mm thickness. 10. Repeat step 9 until desired excavation depth is achieved. FIG 3 shows a typical intersection of anchor members for the mesh-like array 13 of FIG 2. FIG 4 shows a typical intersection of anchor members for the linear array 14 of FIG 2. FIG 5 shows an elevational view of a shoring structure with a ring beam structure 18 shown in phantom. The ring be.am structure 18 is stepped to take into account a sloping natural surface level 19. FIG 6 shows a cross sectional view of the structure of FIG 5. It is a particular advantage of the present invention that the shoring structure may form a permanent structure with suitable support. In FIG 6 a basement wall 20 of a building structure may be erected closely to the shoring structure. The cavity 21 therebetween may be filled with aggregate or other porous material 22 to provide moisture drainage into a sump or the like 23. Instead of filling cavity 21 with aggregate, suitable props may be placed between wall 20 and the shoring wall to provide an air gap for ventilation ducts etc. FIG 7 shows in part an alternative form of shoring construction. Ring beam 18 is formed adjacent the upper perimeter of the excavation as previously described. Instead of spraying the entire remaining surface of the excavation, including the exposed portions of anchor members, only the exposed portions of the anchor members are sprayed with Spraycrete. In this manner the structure resembles an array of steel reinforced concrete beams 24 intersecting to form a mesh-like array and/or a plurality of elongate separate reinforced concrete beams. Such a structure may be used in conjunction with a fine wire mesh to restrain earth fragments in the regions 25 between the "reinforced concrete beams". Alternatively the regions 25 between the "reinforced concrete beams" could employ a layer of Spraycrete to a much lesser thickness than in the region of the anchor members. FIG 8 shows a detailed cross section of a "ring beam" structure. Prior to formation of the ring beam 18, reinforcing steel 26 and a utilities duct or the like 27 are placed within the region to be encapsulated. In this manner the shoring system may be used as a permanent internal wall of the basement of the subsequent structure. A bracket 28 is provided to receive precast wall panels or the like. FIG 8 also shows the manner of insertion and interconnection of the anchor members. Anchor member 29 is inserted into borehole 30 through the closed loop of adjacent anchor member 31. The free end of anchor member 21 is then bent from the position shown in phantom to lie against the wall 32 of the excavation and overlie an adjacent borehole (not shown) . FIG 9 shows an arrangement including a drainage system for moisture accumulating behind the shoring system. A layer of porous fabric 33 is placed at suitable positions on the face 34 of the excavation behind the anchor members 35. If required a non-porous or semi-permeable membrane film 36 can then be placed on the outer surface of fabric 33. A length of tubing 37 is situated in a cavity 38 in the face 34 of the excavation. The inner end of the tubing 37 is slotted to receive portion 39 of the fabric 33 which portion acts as a wick to direct water into the tube 37. After placing drainage tubes at appropriate places the shoring is sprayed with spraycrete as previously described. FIG 10 illustrates yet another alternative embodiment of the invention. Prior to commencing excava'tion, boreholes 50 are drilled into the earth formation to the desired excavation depth. The boreholes 50 are positioned say 1-3 metres from the proposed excavation wall 51 and are spaced at suitable distances of say 1-4 metres from each other. The boreholes may have a diameter of say 100mm to 500mm. Further boreholes 52 are then drilled adjacent the openings of boreholes 50 rearwardly and downwardly as shown to intersect the theoretical fault or slip line 53 for the earth formation and extend say 1-3 metres therebeyond into a relatively stable bedrock layer 54. Anchors 55 and 55a respectively in the form of steel rod or cable are then inserted into boreholes 50 and 52 and anchored by grouting. Preferably the anchor in borehole 50 is full column grouted and preferably the anchor 55a in borehole 52 is end grouted to say 2 etrπs. The anchors 55 and 55a may comprise a single length of cable or rod or they may be separate and joined at their intersection 56 by clamps welding or any other suitable means. In this manner a contiguous inverted "V"-shaped anchor is formed. Adjacent intersections 56 may be joined by one or more tensile members to form a substantially continuous tensile member extending along the entire face of wall 51. In this manner any localized earth forces directed inwardly towards the excavation may be dissipated as a tensile load along the substantially continuous tensile member. Once the inverted "V-shaped anchors have been formed excavation may proceed with the resultant excavation walls being substantially supported against collapse. Depending upon the nature of the earth formation, the excavation may be completed to the proposed base or floor level 57 and if required the face of wall 51 may be sprayed with Spraycrete, with or without reinforcing in the form of included fibres or mesh or other tensile.members anchored to the wall 51. The anchor member 55a in rearwardly extending borehole 52 is preferably grouted at its remote end only and is disconnectable from anchor member 55 in borehole 50. In this manner an adjacent excavation at some future time is not impeded as the anchor 55a is minable during excavation without damage to excavation equipment or damage to the shoring system. If required anchor members 55. 55d may be tensioned to stress the shoring structure. Although the present invention has been described with reference to certian specific shoring systems and methods of construction thereof, it will be clear to a skilled adressee that there are many modifications or variations possible without departing from the spirit and scope of the invention. For example in its simpliest form the invention may comprise the stepwise spraying of and exposed excavation wall with a pumpable cementitious composition as excavation proceeds through a series of stages to the required depth. In other forms the invention may include any suitable form of earth anchors and/or reinforcing means or any suitable combinations thereof to suit the particular 5 nature of the earth formation being excavated. In the various aspects of the invention hereinbefore described it will be clear to a skilled addressee that for permanent shoring structures, the earth anchors and/or tensile elements should be protected against 10 corrosion. In this regard the anchors and tensile elements may be coated with a galvanized finish or an appropriate anti-corrosive finish such as paint or the like. To further assist in corrosion prevention, the earth anchors are preferably centred in their respective τ_5 ' boreholes to ensure complete encapsulation by the grouting material. The anchors may be centred by conventional centreing devices in the form of concentric rings attached by radiating spokes. Alternatively the insertable shank portions of the anchors may include a plurality of spaced 20 "kinks" or deformations radiating away from the central axis of the shank in two or more directions eg. at 180 degrees, 120 degrees or 90 degrees with respect to adjacent deformations. The extent of the deformations is such that the insertable shank portion is located generally centrally * "" ** of a borehole apart from the small regions of the deformations which lie generally against the surface of the borehole. In yet another embodiment of the invention the support structure for the excavated wall may comprise elongate anchors in the form of rods, cables or even rock bolts anchored in their respective boreholes by grouting. The exposed ends of some or all of the anchors may be connected by tensile elements to form a substantially continuous tensile member in the form of a linear or mesh array. The linear array for example may be formed by bolting or welding a steel rod or cable to the exposed ends of the anchors. A mesh array may be formed by attaching rod or cable to the exposed anchor ends in two dimensions. Alternatively a preformed steel mesh may be attached to the exposed anchor ends in panels either by bolting, welding or the like to form a plurality substantially continuous tensile members over the exposed surface of the excavation. The invention provides distinct advantages over prior art excavation shoring systems in relation to cost, speed of installation, flexibility of construction in provision of either temporary or permanent structures and control of subterranean water seepage.

Claims: The claims defining the invention are as follows: 1. A method for shoring substantially upright walls in open earth excavations wherein excavation is carried out in successive stages to a required depth to form a floor in said excavation, each stage of said excavation including the steps of - forming spaced boreholes to predetermined depths over a wall face exposed by an excavation stage; inserting anchor members into respective boreholes and anchoring said anchor members in said boreholes with a grouting composition; connecting exposed ends of at least certain of said anchor members with tensile elements to form at least one substantially continuous tensile member extending over the surface of said exposed wall face; spraying a cementitious material over said wall face to encapsulate said at least one substantially continuous tensile member, the steps of each said stage of excavation being completed before a successive stage of excavation is commenced. 2. A method as claimed in claim 1 wherein said earth anchors and said tensile elements comprise rod-like linear steel members. 3. A method as claimed in claim 1 wherein said earth anchors comprise rod-like linear steel members and said tensile elements comprise a steel mesh. 4. A method as claimed in claim 1 wherein said earth anchors comprise elongate U-shaped members, said U-shaped members including an anchor portion for anchoring in a borehole and a free end in the form of a closed loop lying adjacent the surface of an exposed wall surface, said free end comprising a tensile element for connection with an earth anchor in an adjacent borehole. 5. A method as claimed in claim 4 wherein respective tensile elements of said earth anchors are connected to form a substantially continuous linear tensile member. 6. A method as claimed in claim 4 wherein respective tensile elements of said earth anchors are interconnected to form a mesh-like substantially continuous tensile member. 7. A method as claimed in any preceding claim wherein a reinforced concrete ring beam is formed about the ' _> periphery of the mouth of said excavation, said ring beam being formed integrally with a wall support structure formed during a successive excavation stage. 8. A shoring structure for shoring the walls of an open earth excavation comprising; a plurality of earth anchors anchored into boreholes in an exposed face of said excavation, some or: all of said earth anchors having exposed ends connected by tensile elements to form at least one substantially continuous tensile member extending over said exposed face; and a layer of cementitious material sprayed over said exposed face to encapsulate said at least one substantially continuous tensile element. 9. A structure as claimed in claim 8 wherein said at least one substantially continuous tensile element extends linearly over the exposed face of said excavation. 10. A structure as claimed in claim 9 wherein said substantially continuous tensile element extends as a mesh like array over the surface of said excavation. 11. A structure as claimed in any one of claims 8-10 wherein said earth anchors comprise rod-like steel members. 12. A structure as claimed in claim 11 wherein said substantially continuous tensile members comprise either linear rod-like steel members or steel mesh attached to the free ends of said anchor members or a combination of both linear and mesh members. 13. A structure as claimed in any one of claims 8-12 wherein a reinforced concrete ring beam is formed about the periphery of the mouth of said excavation. 14. A structure as claimed in any preceding claim wherein said sprayed cementitious material includes a fibrous reinforcing means.

Inventors: TUCKER MICHAEL CHARLES, TUCKER, MICHAEL, CHARLES
Applicants: GEARHART AUSTRALIA, TUCKER MICHAEL C, GEARHART AUSTRALIA LIMITED, TUCKER, MICHAEL, CHARLES
Application year: 1986
Main classification: E02D17
IPC codes: E02D17, E02D29