WO2010003214A1

WO2010003214A1 - Method of reducing stress on tunnel rock face under heavy overburden conditions

Info

Publication number: WO2010003214A1
Application number: PCT/CA2009/000108
Authority: WO
Inventors: Michael P. Mcnally
Original assignee: Mcnally Michael P
Priority date: 2008-07-07
Filing date: 2009-01-30
Publication date: 2010-01-14

Abstract

A method of providing an overall improvement to a tunneling operation is disclosed. A standard TBM has the extension fingers removed from the leading part of the TBM (the stabilizer). A series of steel angles are welded onto the exterior surface of the stabilizer to provide a series of receptacles axially extending along the stabilizer. A plurality of roof support members which have been previously inserted in the receptacles so formed on the stabilizer, are fed out of the receptacles as the TBM moves ahead. At predetermined intervals ribs are mounted on these roof support members. The rib members are installed in the tunnel roof as soon as possible behind the stabilizer.

Description

METHOD OF REDUCING STRESS ON TUNNEL ROCK FACE

UNDER HEAVY OVERBURDEN CONDITIONS

BACKGROUND OF THE INVENTION

This invention relates to a method of boring a tunnel in rock where substantial overburden exists. In prior art methods of tunnel building, problems arise during boring due to failure of the rock at the tunnel face. This occurs when stressful conditions are encountered during the operation of a tunnel boring machine (TBM).

The condition known as tunnel face distabilization occurs when the tunnel is being bored through a largely homogenous rock mass loaded with sizable overburden. Such situations occur where a tunnel is being driven or bored deep in the earth (where rocky conditions occur) or through a mountain where a large overhead burden exists. Such conditions lead to the fracture and breaking up of the rock face ahead of the TBM so that instead of presenting a circular disc shaped rock face to the TBM, the tunnel face appears to be jagged and cracked, with the TBM having to disgorge large pieces of rock which have broken off from the tunnel rock face and usually are too big to pass through the TBM mucking buckets. These chunks of rock are rotated in front of the boring head of the TBM thus tending to break off more rock from the unstable tunnel face. This only tends to magnify the problem as the TBM knocks more rock loose at the exposed tunnel face, which abrades the TBM head, cutters and other components critical to the operation of the TBM.

It is not known exactly why the tunnel face is so unstable under the above conditions but it is believed to be due to the heavy stresses that are ever present in the rock but are relieved when a tunnel is bored through the rock. The rock surrounding the tunnel being driven tends to relax when the stresses due to the overburden are relieved by the excavation process. As a result the tunnel walls relax inwardly as the tunnel advances. This relaxing of the walls of the tunnel cannot occur at the tunnel face where the TBM is excavating (the face) because the disc of rock acts as a compressive diaphragm that resists the movement of the adjacent walls. As the face resists the movement, it is subjected to large stresses. These stresses can exceed the rock's compressive strength and cause the face to disintegrate yielding the above described problems in driving the tunnel.

If the geometry of the tunnel being bored is greatly disturbed, the action of the gripping devices in the TBM will likely be impaired and with the production of the tunnel being impaired. Loose rock clinging to the walls of the tunnel may indeed impair the action of the gripper mechanisms in their effort to propel the TBM forward. The release of pressure at the tunnel surface often caused the tunnel roof to become unstable even though the tunnel had been driven through a homogenous rock mass. The roof collapse may impede progress of building the tunnel but the instability of the rock face at the point of boring tends to cause the greatest problems to the operation of building a tunnel.

Most TBM' s are tailor made to perform a specific task when confronted by situations that are known and expected. For the TBM 's designed to bore through rock, usually the rotating head is provided with a series of sharpened rollers (called cutter wheels) which are forced with great force against the tunnel face. The sharpened rollers which are located at various radial distances from the center of the rotating wheel, tend to spall the rock from the tunnel face as the head rotates.

As the tunnel depth increases, the phenomenon known as tunnel face instability occurs. Instead of spalling the rock from the tunnel face, the face cracks, with good sized rock chunks breaking off from the tunnel face, and these rock chunks are forced to rotate with the rotating head thus interfering with the proper operation of the TBM. In addition to causing the slowdown in the speed of boring the tunnel, these vagabond rocks cause the dimensions of the tunnel being driven to become non- circular causing instability. The science of driving tunnels through homogenous rock strata at great depths is fraught with problems not experienced at shallower depths. The problem of tunnel face instability is ever present; the problem of break away rock chunks from the rock face leads to damage to the TBM' s cutter wheels, its mucking buckets and lastly the finished tunnel may not be round and difficult to support.

The solution to the above problem seems to be the prompt provision of a tight tunnel roof supporting structure which is supplied to the tunnel before the roof has a chance to completely relax. Provision of a prompt, tight support appears to limit the inward movement of the tunnel walls near the face and thus reduce the loads and stresses which would otherwise tend to fracture the face.

It is believed that this phenomenon will be curtailed by the suitable installation of a roof for the tunnel which is installed as promptly as possible after the tunnel is bored.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 represents an elevational view of a tunnel being bored in rock having a substantial overburden. Figure 2 shows a modification made to a TBM according to this invention. Figure 3 shows an enlargement of the ends of receptacles shown in Figure 2.

Figure 4 shows a tunnel roof being constructed according to this invention. Figure 5 is an illustration of the rib and rock bolts used according to this invention.

Figure 6 is a cross sectional view of a tunnel showing the factors necessary for optimization of tunnel roof placement process.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, Figure 1 is a view partially in section to an elevational view of a tunnel 10 being formed in a solid rock strata. The rock strata from which the tunnel boring machine (TBM) 1 1 tends to be homogenous. The TBM 11 in this instance is modified somewhat from prior art models to enable this invention to work. As shown, the TBM 11 has a frame 12 (or main beam) to which most components of TBM 11 are mounted. TBM 11 has mounted thereon a standard rotating boring head 14 on which are mounted a series of cutting wheels 16 mounted at a variety of radial distances from the center of the rotating head 14. Cutting wheels 16 are especially shaped to spall the rock at the point of contact with the tunnel rock face as wheel 14 turns. Each of the wheels 16 is journalled in suitable bearings (not shown) and has a typical profile to encourage the spalling of the rock face presented to the TBM 11. Rotating head 14 is shown also with bucket devices 18 which are provided to pick up and deliver the rock debris spalled from the tunnel face and deliver it to the conveyor 30 provided on a standard TBM. A cylindrically shaped device 20 (in this instance called a stabilizer) located in close proximity to and just following the rotating head 14 of TBM 11. In this instance the stabilizer 20 is made to be in sections so that it is adjustable in diameter to press against the tunnel walls 10 with a predetermined force. In most prior art TBM's the stabilizer will be provided with a series of trailing fingers mounted at the rear of stabilizer 20. The trailing fingers are generally of an equivalent length to the axial length of the stabilizer 20. In order for this invention to accomplish its result it is important that any trailing fingers be removed so that access to the tunnel 10 may be gained immediately following the passage of stabilizer 20 in the tunnel 10.

The stabilization device 20 is attached to the machine frame 12 and the drive motors for driving the rotating head 14 of TBM 11 are usually mounted therein. Stabilization device 20 also contains the bearings for the rotating head 14 which are mounted therein. The propulsion cylinders for the TBM 11 are shown as 32 on TBM 11.

Stabilizing device 20 is composed of a plurality of shells of which top shell 22 is shown and lower shell 24 is shown. The shells have the same contour as the tunnel wall profile and the sliding joints 26 allows enough movement between shells 22 and 24 to accommodate the tunnel diameter existing at the location of the stabilizing member 20.

The invention will now be described in detail. It will be seen that TBM 11 is somewhat modified by welding a series of longitudinally extending members 28 to the exterior surface of stabilizing shell 22. This will form a series of longitudinally extending receptacles 32 (see Figure 3) therein.

Members 34 (see Figure 4) are inserted into each of the receptacles 32 and are gradually fed out of the receptacles 32 to the tunnel roof as the TBM 11 moves ahead. The joints formed by members 34 should never be aligned, they should be staggered.

Lastly a rib member 36 is installed on the tunnel wall 10 directly and promptly behind stabilizer 20. It is believed that the installation of rib 36 is critical to the invention. Here rock bolts 38 are shown holding the rib 36 in place. Rib 36 may comprise a complete circular hoop if desired, the only stipulation being that the devices 36 and 38 be installed promptly behind stabilizer 20 as close as possible to the stabilizer 20 during the tunnel boring process. It cannot be overemphasized that the installation of ribs 36 and roof bolts 38 should be installed as quickly as possible and tightened to prevent complete roof relaxation.

Members 34 may have a rectangular cross section, but almost any shape will function well. The composition of members 34 may be wood, metal, or plastic depending on the strength of the material required for tunnel roof support. It is usually preferred that members 34 be steel straps of a length about equal to the axial length of the stabilizer 20.

The rib members 36 are usually the cross sectional shape of an I- beam or they may be channels of an arcuate shape to match the curvature of the tunnel being driven or they may be circular so as to extend all the way around the tunnel to conform to the tunnel diameter. These members 36 must be of sufficient strength so as to prevent any distortion of the roof profile of the tunnel.

Referring now to Figure 6, a cross sectional view of the tunnel 10 is showing the region where the tunneling machine 11 has just departed and left a short part of the tunnel surface exposed and where it is desired to install members 34 at the earliest possible moment at the tunnel roof which TBM 11 has just exposed. Here rock bolts 38 are inserted through the apertures provided in rib members 36 (see Figure 5) to secure the members 34 against the surface of tunnel 10.

It is desired to make the distance oc as small as possible in order to achieve maximum tunnel face and roof stability. This distance may be kept to a minimum if:

1. The depth of storage tubes 28 of TBM 11 is maximized. Of course with increasing depth of members 28 comes the increase in wall thickness to withstand the crushing force necessary to restrain the stabilization head 20 from wobbling against the tunnel surface. Therefore there is a practical limit as to how deep tubes 28 may be.

2. Slat thickness is kept to a minimum. Members 34 may be more easily inserted in the receptacles 28 if member 34 are made to be very thin. This diminishment in thickness leads to a condition where members 34 will not be able to carry the tunnel roof. Members 34 must have a reasonable thickness to withstand the load to be carried by members 34.

3. The rib depth is minimized. Here the tunnel ribs 36 are shown as a modified "C" channel shape having a substantial web depth

40. If the depth of ribs 36 could be eliminated or made very shallow, the ribs 36 would be merely flat straps, having little or no circumferential strength, so that the round tunnel geometry might possibly be lost! In order to permit the tunnel to maintain its roundness, there must be a certain minimum depth 40 provided on ribs 36.

So in order to be able to insert the roof supporting members into the receptacles 28 at the earliest possible moment the rib 34 thickness must be kept to a minimum. Likewise the storage tube 28 depth must be maximized. Lastly the rib depth 40, i.e. the maximum distance the rib 36 intrudes into the excavated tunnel 10 must be minimized.

When these three arbitrary values are optimized, it will be found that the insertion gap oc will be minimized.

Thus the prompt insertion of the tunnel roof will improve the stability of the rock face upon which the head 18 of TBM 11 contacts. Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that the modifications and embodiments are intended to be included within the scope of the dependent claims.

Claims

What is claimed:

1. A method of minimizing stress on a rock face of a tunnel during a tunnel boring operation comprising: providing a TBM for constructing said tunnel, said

TBM having a stabilizer in the shape of a hollow cylinder having an exterior surface which contacts said tunnel immediately behind the boring head, and forming a plurality of axially extending receptacles of a first predetermined size on the exterior surface of said stabilizer which open rearwardly of said stabilizer, and inserting elongated roof supporting members into said receptacles as soon as feasibly possible, and gradually feeding said roof supporting members from said receptacles as said TBM moves forward, and attaching rib members to said tunnel roof at third predetermined intervals to secure said supporting members to said tunnel roof as soon as possible following passage of said TBM, and tightening said rib members against the roof supporting members.

2. A method as claimed in claim 1 wherein said receptacles are formed on said stabilizer where said stabilizer contacts said tunnel roof.

3. A method as claimed in claim 2 wherein said receptacles extend substantially the length of said stabilizer.

4. A method as claimed in claim 3 wherein said receptacles are rectangular in cross section.

5. A method as claimed in claim 4 wherein said receptacles are formed by welding axially extending elongated steel members onto the surface of said stabilizer.

6. A method as claimed in claim 5 wherein said receptacles are formed by welding a plurality of steel angle members in abutting relationship to each other onto said stabilizer.

7. A method as claimed in claim 6 wherein said stabilizer is made in sections so as to be adjustable in diameter.

8. A method as claimed in claim 7 wherein said stabilizer has an upper and lower section, and the receptacles are formed on the upper section of said stabilizer.

9. A method as claimed in claim 1 wherein said ribs are contoured to have the same contour as said tunnel.

10. A method of stabilizing a tunnel face for a tunnel being bored in a location where the integrity of the tunnel face is a problem including, providing a modified TBM in which parts of the TBM are removed from the TBM to gain immediate access to the tunnel roof as soon as the TBM has moved forward a first predetermined distance, and welding a series of longitudinally extending members to a part of said TBM so as to provide longitudinally extending receptacles on said part of said TBM, and inserting suitable roof reinforcing members into said receptacles as soon as possible and feeding said roof reinforcing members out of said receptacles as the TBM moves forward, and installing rib members at predetermined locations on said tunnel roof to engage and secure said roof reinforcing members to said tunnel roof, as soon as possible after said TBM has traversed a predetermined distance.

11. A method of minimizing the forces on a tunnel face of a tunnel which must pass through a rocky strata having substantial overburden; a) providing a modified TBM to bore said tunnel, said TBM having a rotating wheel for spalling rock at the face of said tunnel, said wheel being provided with rock spalling wheels mounted

5 thereon, b) a cylindrically shaped stabilizing member for said TBM located behind and in close proximity to said rotating wheel of said TBM; said stabilizing member having a series of elongated hollow io receptacles formed thereon extending in an axial direction along and contacting said tunnel roof, c) feeding elongated members of a predetermined shape and composition from said elongated receptacles as said TBM moves forward, and is d) securing said elongated members to the tunnel roof at as close as possible proximity to said stabilizing member.

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