EP1944415A1

EP1944415A1 - Ground surface access assemblies

Info

Publication number: EP1944415A1
Application number: EP07254813A
Authority: EP
Inventors: David Henry Edmonds; Brian Mills
Original assignee: Wrekin Welding and Fabrication Engineering Ltd; Atlanta Impex Ltd
Current assignee: Wrekin Welding and Fabrication Engineering Ltd; Atlanta Impex Ltd
Priority date: 2006-12-12
Filing date: 2007-12-12
Publication date: 2008-07-16
Also published as: GB0624752D0; GB2444769A

Abstract

A ground surface access assembly 10 includes a base frame 12 defining a base frame opening 14 and a cover 30, which is mountable in a first configuration in use to the base frame 12 in the base frame opening 14. The assembly 10 includes a raising frame 52 defining a raising frame opening 56. The raising frame 52 is mountable in a second configuration in use to the base frame 12, the cover 30 being mountable in the second configuration to the raising frame 52 in the raising frame opening 56.

Description

The present invention relates to ground surface access assemblies.
Ground surface access assemblies such as manhole covers generally comprise a frame defining an opening and a cover which is mountable to the frame in the opening. In use, the opening is positioned over a hole in the ground such as a manhole or drain, the cover being removably mountable to the frame to permit access to the hole. Such access assemblies are provided for a range of duties ranging from relatively light duties to heavy duty applications in which the integrity of the access assembly in use is important. Access assemblies for installation in roads, highways and other public areas have to comply to regulatory standards such as British and European Standards.
A number of problems arise in the use of such access covers. To meet the standards required for heavy duty applications, the access covers and frames are formed of material such as ductile iron. Concern regarding the health and safety issues involved in lifting relatively heavy covers have prompted designers to consider providing covers which are hingedly mounted to frames. However the regulatory standards specify a depth of insertion of the cover into the frame which often results in a hinged cover fouling the frame. For example, one regulatory standard requires the depth of insertion of the cover into the frame to be 50mm.
Another problem related to the use of ground surface access covers arises during installation. Frames are typically mounted to be level with the finished height of the roadway, but frequently the topmost, or wearing course of the roadway is not laid until construction work on a site has been completed. The frames are therefore vulnerable to damage as they project above the level of the unfinished roadway during the construction phase of a project. They also constitute a hazard to pedestrians, and do not function as drains in diverting surface water.
According to a first aspect of the present invention, there is provided a ground surface access assembly, the assembly including a base frame defining a base frame opening, and a cover, which is mountable in a first configuration in use to the base frame in the base frame opening, a raising frame defining a raising frame opening, the raising frame being mountable in a second configuration in use to the base frame, the cover being mountable in the second configuration to the raising frame in the raising frame opening.
Possibly, the ground surface access assembly includes packing means to reduce the possible amount of movement of the cover when mounted to the base frame. Possibly the packing means include one or a plurality of packing members, which may locate between the cover and the base frame.
Possibly the cover includes a pair of cover members. Possibly the packing means includes an interlocatable packing member, which in the first configuration may locate between the two covers.
Possibly the base frame opening is rectangular in plan, and the interlocatable packing member may extend from one corner of the base frame to a diagonally opposite corner.
Possibly the assembly includes locating means for locating the interlocatable packing member to the base frame. Possibly the locating means includes a recess defined by a mounting part located at each of the diagonally opposed corners of the base frame. Possibly the interlocatable packing member includes a locating formation or formations which in the first configuration are receivable within the locating recesses.
Possibly the assembly includes securing means, for securing the packing member in position between the cover members.
Possibly the assembly includes hinge means for hingedly mounting the cover to the raising frame. Possibly the assembly is arranged so that the cover is pivotally mountable between an open and closed condition, and may be arranged so that in use as the cover moves between the open and closed condition, the position of the pivot axis about which the cover pivots varies.
Possibly, the assembly is arranged so that as the cover moves away from the closed condition, the cover moves as a whole out of the raising frame opening. Possibly, as a force is applied to move the free side of the cover from the closed condition out of the raising frame opening, the force also moves the hinged side out of the raising frame opening.
Possibly, the assembly includes lifting means, for lifting the cover out of the raising frame opening. Possibly the lifting means include a plurality of lifting surfaces.
According to a second aspect of the present invention there is provided a ground surface access assembly, the ground surface access assembly including a frame defining an opening, a cover mountable to the frame in the opening, hinge means for hingedly mounting the cover to the frame, the assembly being arranged so that the cover is pivotally movable between an open and closed condition and being arranged so that in use as the cover moves between the open and closed condition, the position of the pivot axis about which the cover pivots varies.
Possibly the assembly is arranged so that as the cover moves away from the closed condition, the cover moves as a whole out of the opening. Possibly, as a force is applied to move the free side of the cover from the closed condition out of the opening, the hinged side is also moved out of the opening by the force.
Possibly the assembly includes lifting means for lifting the cover out of the raising frame opening. The lifting means may include a plurality of lifting surfaces:

According to a third aspect of the present invention there is provided a method of installing a ground surface access assembly, the method including the steps of installing the ground surface access assembly in a first configuration so that an upper most part of the assembly is at a first level, and then subsequently installing the ground surface access assembly in a second configuration in which the uppermost part of the ground surface access assembly is at a second level, which is higher than the first level.

Possibly the ground surface access assembly is as described in any of the preceding statements.
Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:-

Fig. 1 is a perspective view of a base frame;
Fig. 2 is a perspective view from the underside in use of a cover member;
Fig. 3 is another perspective view from the underside in use of the cover member;
Fig. 4 is a perspective view of an edge packing member;
Fig. 5 is a plan view of a ground surface access assembly in a first configuration;
Fig. 6 is a perspective view of a raising frame;
Fig. 7 is a perspective view of the ground engaging access assembly in a second configuration;
Fig. 8 is a perspective view of another frame;
Fig. 9 is a perspective view from the underside in use of an interlocating packing member;
Fig. 10 is a perspective view from the underside in use of the interlocating packing member of Fig. 9 assembled to the base frame of Fig. 8;
Fig. 11 is a perspective view from the underside in use of the interlocating packing member of Fig. 9 and a pair of cover members in an assembled condition;
Fig. 12 is a perspective view of another ground surface access assembly in a first configuration;
Fig. 13 is a perspective view of another raising frame;
Fig. 14 is a perspective view of the raising frame of Fig. 13 assembled to the base frame of Fig. 8;
Fig. 15 is a perspective view of another pair of cover members;
Fig. 16 is a perspective view of a ground surface access assembly in a second configuration;
Fig. 17 is a perspective view of a frame of another ground surface access assembly;
Fig. 18 is a perspective view of part of the frame of Fig. 17;
Fig. 19 is a side view of a ground surface access assembly in an open condition; and
Fig. 20A to 20E are a series of views showing the movement of part of a cover from a closed condition to an open condition relative to part of a frame.

Figs. 1 to 7 show a first ground surface access assembly. The first ground surface access assembly 10 includes a base frame 12 defining a base frame opening 14, which is substantially square in plan. The base frame 12 includes side walls 16 which define the base frame opening 14. A flange 18 extends outwardly from the lowermost parts in use of three of the side walls 16.
The base frame 12 includes a plurality of corner mounting projections 20, which extend inwardly into the base frame opening 14 from each corner. Each corner mounting projection 20 includes locating means in the form of one or more locating projections 22, which project upwardly in use from an upper seating surface 24 of the corner mounting projection 20.
The base frame 12 includes securing means in the form of a plurality of securing tabs 26 which project inwardly from the side walls 16 into the base frame opening 14, each securing tab 26 defining a securing hole 28. Two opposed corner mounting projections 20 also define securing holes 28.
The ground surface access assembly 10 includes a pair of substantially identical cover members 30 as shown in Figs. 2 and 3. Each cover member 30 includes a substantially planar surface part 32 which defines a plurality of apertures 36. A plurality of reinforcement parts 34 extend downwardly in use from the underside of the surface part 32. In plan, the surface part 32 is triangular.
Each cover member 30 includes a corner mounting 38 which extends downwardly in use from the surface part 32 at each corner of the triangle.
Each cover member 30 includes hinge means in the form of an interlocating hinge member 40 which extends downwardly in use from the underside of the surface part 32 on one of the sides. The interlocating hinge member 40 defines a passage in the form of a curved slot 42.
As shown in Fig. 3, each cover member 30 includes interlocking means in the form of a pair of in use horizontally extending tabs 48A, 48B. The interlocking means include a pin 44 which extends from one of the tabs 48A, the other tab 48B defining a pin receiving hole 46. In use, pairs of cover members 30 interlock, the pin 44 of one cover member 30 locating in the pin receiving hole 46 of the other cover member 30.
The ground surface access assembly 10 includes packing means in the form of an edge packing member 50 as shown in Fig. 4, the edge packing member 50 being in the form of a substantially square frame having plain walls.
Fig. 5 shows the ground surface access assembly 10 in a first configuration. In this first configuration, two cover members 30 are assembled together as previously described. The cover members 30 are mounted to the corner mounting projections 20 of the base frame 12 within the base frame opening 14, the corner mountings 38 of each of the cover members 30 locating between the locating projections 22 and the side walls 16 on the seating surfaces 24.
With the cover members 30 mounted to the base frame 12, a gap is defined between the outer edges of the cover members 30 and the side walls 16. The edge packing member 50 is inserted into this gap, between the outer edges of the cover members 30 and the side walls 16, to reduce the possible amount of movement of the cover members 30 within the base frame 12 in use.
Alternatively, any suitable number of packing members such as shims could be used to fill the gap.
In the first configuration, the surface part 32 of the cover members 30 is substantially at the same level as the uppermost parts of the side walls 16.
Fig. 6 shows a raising frame 52, the raising frame 52 including four side walls 54 defining a raising frame opening 56 which is substantially square in plan.
The raising frame 52 includes four corner mounting projections 60 which are similar to the corner mounting projections 20 of the base frame 12, each corner mounting projection 60 having locating projections 62 and seating surfaces 64.
The raising frame includes securing means in the form of a plurality of securing tabs 66 which are similar to the securing tabs 26 of the base frame 12 and extend from the side walls 54 into the raising frame opening 56, each securing tab 66 defining a securing hole 68.
The raising frame 52 includes hinge means in the form of two pairs of hinge yoke members 70 which extend into the raising frame opening 56 from opposed side walls 54. Each hinge yoke member 70 defines a through hole 72 in which a hinge pin (not shown) is receivable.
Fig. 7 shows the ground surface access assembly 10 in a second configuration. In the second configuration, the raising frame 52 is mounted to the base frame 12, the corners of the raising frame 52 locating on the corner mounting projections 20 of the base frame 12, the securing holes 68 of the raising frame 52 corresponding in position with the securing holes 28 of the base frame 12 so that fasteners such as bolts may be located through the securing holes 28, 68 to fasten the raising frame 52 to the base frame 12.
A pair of cover members 30 are assembled as previously described and the interlocating hinge member 40 of one of the cover members 30 located in the gap 71 defined between each of the members of the pairs of hinge yoke members 70 of the raising frame 52. A hinge pin (not shown) is located through the holes 72 of the hinge yoke members 70 and the curved slot 42 of the interlocating hinge member 40. The hinge pin could be in the form of a fastener such as a bolt.
The cover members 30 are located so that the corner mountings 38 of each of the cover members 30 locate between the locating projections 62 and the side walls 54 of the raising frame 52 on the seating surfaces 64.
As shown in Fig. 7, the raising frame 52 raises the level of the surface parts 32 of the cover members 30 above the uppermost parts of the side walls 16 of the base frame 12. On a construction site, the ground surface access assembly 10 can be installed in the first configuration, and the base course of a roadway installed up to and level with the uppermost part of the side walls 16. When the construction phase draws to a close, the wearing course will be installed to bring the roadway up to its final height. The ground surface access assembly can be installed in its second configuration in which the uppermost parts of the side walls 16 are at a higher level.
In the second configuration the cover members 30, are hingedly mounted to the raising frame 52, and are movable between a closed condition and an open condition. The assembly 10 is arranged so that as the cover members 30 move between the open and closed conditions, the position of the pivot axis about which the cover members 30 pivot varies, permitting the cover as a whole to be moved out of the opening. The hinged side of the cover moves out of the opening as a lifting force is applied to the free side of the cover, thus permitting a relatively deep section, heavy duty cover member complying with regulatory standards to be hingedly mounted. The features and operation of the hinge means are described in more detail below.
The example shown in Figs. 1 to 7 thus provides a ground surface access assembly which permits the raising of the road level without requiring reinstallation of the base frame 12, while providing a hingedly mounted cover, thus providing health and safety benefits.
Figs. 8 to 16 show another ground surface access assembly 110. Many features of this ground surface assembly are similar to those described previously, and the same reference numerals have therefore been used for similar features which will not be described in detail again for the sake of brevity.
Fig. 8 shows a base frame 112, the base frame 112 including side walls 16, a flange 18 extending outwardly from an in use lowermost part of the side walls 16, the side walls 16 defining a base frame opening 14. The base frame opening 14 is substantially square in plan.
The base frame 112 includes corner mounting projections 120 extending from the side walls 16 into the base frame opening 14. Two diagonally opposed corner mounting projections 120A include locating means in the form of a recess or notch 80.
The assembly includes securing means in the form of a plurality of securing tabs 26 which extend from the side walls 16 into the base frame opening 14, each of the securing tabs 26 defining a securing hole 28.
The ground surface access assembly 110 includes packing means in the form of an interlocating packing member 82, which is shown in Fig. 9. The interlocating packing member 82 includes a planar surface part 84 and a rib 86 extending downwardly in use from the underside of the planar surface part 84. The rib 86 defines a pair of securing holes 88. The interlocating packing member 82 includes downwardly directed seating surfaces 94 located at or towards each end of the interlocating packing member 82.
Figs. 10, 11 and 12 show the ground surface access assembly 110 in different stages of assembly. Fig. 10 shows the interlocating packing member 82 mounted to the base frame 112. In an assembled condition, the interlocating packing member 82 extends diagonally across the base frame opening 14 from one of the notched corner mounting projections 120A to the other notched corner mounting projection 120A. In the assembled condition, the seating surfaces 94 of the interlocating packing member 82 locate on the corner mounting projections 120A, each end of the rib 86 locating in one of the notches 80.
The ground surface access assembly 110 includes a pair of substantially identical cover members 30, which are shown in Fig. 11.
Each of the cover members 130 includes a substantially planar surface part 32, a plurality of reinforcement parts 34 extending downwardly in use from the surface part 32. In contrast to the cover members 30 previously described, the cover members 130 shown by way of example in this embodiment have a substantially solid surface part 32.
The surface part 32 is substantially triangular in plan, and each cover member 130 includes a corner mounting 138 at each corner. A pair of securing tabs 148 extend downwardly from the surface part 32, spaced along the edge which forms the hypotenuse of the triangle, each securing tab 148 defining a securing hole 149.
In assembling the ground surface access assembly 110 to a first configuration, the covers 130 and interlocating packing member 82 are firstly assembled. The securing holes 149 of the cover members 130 and the securing holes 88 of the interlocating packing member 82 are aligned and a fastener such as a bolt 78 located through the aligned holes 88, 149. A nut could be engaged with the bolt 78 to secure the bolt 78, but is not tightened, so that each cover 130 has some degree of movement relative to the other cover 130.
The assembled cover comprising the two cover members 130 and the interlocating packing member 82 is then mounted within the base frame opening 14 so that the notches 80 of the notched corner mounting projections 120A receive the ends of the rib 86. The assembled ground surface access assembly 110 in the first configuration is shown in Fig. 12. In this first configuration, the uppermost parts of the side walls 16 are substantially level with the surface parts 32 of the cover members 130.
Fig. 13 shows a raising frame 152, which includes side walls 54 defining a raising frame opening 56 which is substantially square in plan. The raising frame 152 includes corner mounting projections 160 projecting inwardly into the raising frame opening 56 at the corners. Two opposed corner mountings 160B include locating projections 96 which project inwardly into the raising frame opening 56, the side walls 54 including flattened portions 90 in the vicinity of the locating projections 96.
The raising frame 152 includes securing means in the form of a plurality of securing tabs 66 which extend inwardly into the raising frame opening 56 from the side walls 54, each securing tab 66 defining a securing hole 68.
In use, the ground surface access assembly 110 is assembled to a second configuration as follows. The raising frame 152 is mounted within the base frame 112 as shown in Fig. 14, the securing holes 68 of the raising frame 152 corresponding with the securing holes 28 of the base frame 112, fasteners such as bolts being located through the corresponding holes 28, 68 to fasten the raising frame 152 to the base frame 112.
Fig. 15 shows two covers members 130 which are assembled together, a bolt 78 being located through each one of the pair of corresponding securing holes 149 defined by the securing tabs 148.
Two of the corner mountings 138A located at the ends of the side which forms the hypotenuse of the triangle define recesses which in the assembled condition with the other cover member 130 together form a channel 98, the corners 100 of the surface part 32 being flattened adjacent to the corner mountings 138A.
The assembled cover members 130 are mounted to the raising frame 152 within the raising frame opening 56 to form the ground surface access assembly in a second configuration as shown in Fig. 16. In this configuration, the assembled cover members 130 must be mounted correctly within the raising frame opening 56, so that the flattened corners 100 of the covers 130 locate within the flattened side wall portions 90 of the raising frame 52, and the channels 98 of the assembled cover members 130 receive the locating projections 96 of the raising frame 52. The features of the flattened corners 90 and portions 100, the locating projections 96 and the channel 98 ensure that the assembled cover members can only be fitted within the raising frame opening 56 in two out of a possible four orientations.
As previously described, the raising frame 152 enables the finished level of the surface parts 32 of the cover members 130 to be raised to accommodate an increase in road level due to resurfacing or the application of a wearing course. The interlocating packing member 82 has the advantage of being a single extra part, which is secured in position between the cover members 130.
Figs. 17 to 19 show another ground surface access assembly 210, many features of which are similar to those previously described. Where features are similar, the same reference numerals have been used and the features will not be described in detail again for the sake of brevity.
The ground surface access assembly 210 includes a base frame 212, the base frame 212 including side walls 16, a flange 18 extending from a lowermost part of three of the side walls 16, the side walls 16 defining a base frame opening 14 which is square in plan.
The base frame 212 includes corner mounting projections 60 extending inwardly from the side walls 16 into the base frame opening 14. Each of the corner mounting projections 60 includes one or more locating projections 62 projecting from seating surfaces 64.
The base frame 212 includes hinge means, the hinge means including two pairs of hinged yoke members 70, one pair projecting from each of two opposed side walls inwardly into the base frame opening 14. Each hinge yoke member defines a hinge pin receiving hole 72.
The corner mounting projections 60 and hinge yoke members 70 are substantially similar to those previously described in relation to the raising frame 52 as shown in Fig. 6.
The ground surface access assembly 210 could include a pair of cover members 30 which are substantially the same as the cover members 30 shown in Figs. 2, 3 and 7, and are mounted to the hinge yoke members in a similar manner to that previously described. Referring to Figs. 2 and 18, a pair of cover members 30 is linked together, and the interlocating hinge member 40 of one of the cover members 30 is inserted into the gap 71 between one of the pairs of hinged yoke members 70 so that the curved slot 42 of the interlocating hinge member 40 corresponds in alignment with the holes 72 of the hinge yoke members 70. A fastener, which could be in the form of a bolt 74 is then passed through the holes 70 and curved slot 42 to retain the cover members 30 in position.
In one example, the cover members 30 could be required for regulatory purposes to have a relatively deep section thickness, such that hinged mounting of the cover members 30 results in fouling of the reinforcement and corner mountings 38 of the cover members on the side walls 16. In the arrangement of the present invention, the corner mounting projections 60, corner mountings 38, the interlocating hinge 40 and the hinge yoke members 70 are arranged to permit the cover members 30 to move from a closed condition to an open condition freely without fouling. As shown in Fig. 19, the free edge of the cover members 30 describes a track 104 in moving from the closed condition to the open condition which is not a simple arc such as would result from movement around a fixed pivot axis. An initial, substantially vertical part of the track, 104A, shows that the initial movement from the closed condition is substantially vertical, permitting the section depth of the cover 30 to move clear of the side walls 16. This is achieved as follows.
Referring to Fig. 2, each of the corner mountings 38 include lifting means in the form of forward facing lifting faces 101 and laterally facing lifting faces 103, the forward facing lifting faces 101 and laterally facing lifting faces 103 forming angled chamfers to the corner mountings 38.
The locating projections 62 of the corner mounting projections 60 of the base frame 212 include lilfting means in the form of corresponding rearwardly and laterally facing lifting faces 105, 107, which again are in the form of angled chamfers to the locating projections 62. The angle of the chamfers could be approximately 45°.
Figs. 20A to 20E show the movement of one of the cover members 30 from the closed to the open condition. In these figures, which are schematic only, the outline of the hinge bolt 74 and curved slot 42 has been superimposed on one of the cover corner mountings 38 and frame corner mounting projections 60.
Initially, the cover corner mounting 38 is seated on the seating surface 64, and the forward lifting face 101 abuts the rearward lifting face 105. In this position, the frame lateral lifting faces 103 also abut the cover lateral lifting faces 107. The cover lateral lifting faces 107 are a tight fit to the frame lateral lifting faces 103, such that the cover mounting projections 38 are slightly deformed, being formed of a resilient material.
The resilient deformation of the cover corner mountings 38 relative to the frame corner mountings 60 could be produced by the self weight of the cover members 30, or by forces imposed in use, for example, by the effect of traffic over a period of time.
To move the cover members 30 from the closed condition to the open condition, a user applies an upward force at or towards the free edge of the cover members 30, that is the edge away from the hinged side. As the lifting force is applied, the cover forward lifting faces 101 move upwardly and forwardly over the frame rearward lifting faces 105. At the same time, the resilient property of the cover material biases the cover lateral lifting faces 103 upwardly from the lateral lifting faces of the frame 107, so that the cover members 30 as a whole lift upwardly away from the frame 212 out of the opening 14. During this movement, as shown in Fig. 20B, the pivot axis 107 of the movement is located approximately at the top inside edge of the side walls 16, where the side walls 16 contact the cover mountings 38.
Thus the resilient deformation of the cover corner mountings 38 provides biasing means to bias the movement of the cover members upwardly out of the frame 212. This biasing means is insufficient to unseat the cover members 30 from the frame 212, but contributes to the ease of opening and the initial lift during opening.
The opening movement continues as shown in Fig. 20C, there being two points of contact between the cover member 30 and the frame 212, the two points of contact being between the corners of the cover member 30, and the side walls 16 and the seating surface 64. During this part of the movement, the pivot axis 107 moves downwardly towards the hinge bolt 74.
In Fig. 20D, the cover 30 has rotated through substantially 90° and must be lifted slightly so that the cover corner mounting 38 clears the rearward facing lifting face 105. On clearing the lifting face 105, the cover member 30 then pivots on top of the rearwardly facing lifting face 105.
Finally, in the fully open condition as shown in Fig. 20E, the cover member 30 has three points of contact with the frame 22, the three points of contact being with the side wall 16, the recessed seating surface 64 and the bolt 74. The opening angle of the cover is approximately 103.5°. i.e. over top centre, providing stability. This opening angle, and the three point contact provide stability for the open covers.
It will be noted from a consideration of Fig. 18 that not all of the corner mounting projections 60 of the base frame 212 have rearward lifting faces 105.
The invention thus provides hinge means which permit a relatively deep section cover to be hingedly mounted within a frame and to open without fouling the frame. The cover is relatively easy to open and is safe. The hinge parts are internal to the frame, so that in the case of the rising frame the hinge parts do not foul the base frame, and in the case of the base frame the hinge parts are not covered by material in which the base frame is bedded. The hinge pins remain accessible in use. The provision of two sets of hinge parts permits covers to be reversed for use on islands, one way streets, dual carriageways etc since for safety the hinged side must be the side closest to oncoming traffic.
Various other modifications could be made without departing from the scope of the invention. The invention could include any combination of the features described above. The cover members, base frame, raising frame and packing members could be of any suitable size and shape. For example, the frame openings could be circular or rectangular. The cover members could be solid or define apertures. The cover members could be of different section thickness. The cover members, frames and other components could be of any suitable material. For example the cover members could be formed of plastics material or steel. Any suitable fastening means could be used.
Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

21. A method of installing a ground surface access assembly (10, 110), the method including the steps of installing the ground surface access assembly in a first configuration so that an upper most part (32) of the assembly is at a first level, and then subsequently installing the ground surface access assembly in a second configuration in which the uppermost part of the ground surface access assembly is at a second level, which is higher than the first level.
22. A method according to claim 21, in which the ground surface access assembly is as defined in any of claims 1 to 15.