Shani Wallis, TunnelTalk

Using fullface TBMs for flat floor tunnels presents a problem in that the initial cut is the wrong shape. Until recently, this was not a serious concern since fullface TBM excavation could not complete ecomomically with drill+blast. But the number of used TBMs now available, together with the faster and more competent tunnels achieved by TBMs as well as the growing restriction of explosives, is seeing a growing use of fullface TBMs to excavate tunnels that ultimately require a flat invert and therefore need modification.

At the Selby coal mine development for British Coal in the UK, the invert trough of the circular TBM-excavated drift linking the five mines with the central handling point at Gascoigne wood is being backfilled with the tunnel muck to provide a roadway. For the four-lane twin-tube road tunnel at Bergen, Norway, and the Rogers Pass railway tunnel in Canada, the fullface circular TBM-driven tubes were modified by a secondary drill+blast operation to provide the necessary filat invert. More recently at Neuchatel in Switzerland, roadheader booms fitted to the body of TBM are cutting out the corners of the circular profile as a one-pass operation. While each example is no doubt achieving the ultimate requirement, the methods are not ideal. Backfilling wastes the effort and expense of excavating the space in the first place, a second drill+blast operation is time consuming and disturbs the integrity of the initial circular profile, and incorporating roadheader booms could be considered the unhappy marriage of two incompatible techniques depending on the ground conditions.

But for contractors and clients who want to employ the advantages of the modern fullface TBM for flat-inverted Tunnels, more economical and effective methods of providing them are needed.

The advancing Mobile Miner leaves a square tunnel behind

While attention has been focused on this challenge, there is as yet only one TBM in existence which cuts a fullface flat-inverted tunnel in one pass. It is the Mobile Miner developed by Robbins and the Mt lsa copper and silver-lead-zinc mine in Queensland, Australia, one of the largest underground mining operations in the world.

Mt Isa Mines (MIM) had for many years recognised the need for a continuous mining machine to cut its hard rock economically but the rock proved too hard for roadheaders during trials in 1979 and fullface TBMs, while considered, were never tested because of their lack of flexibility in a mining situation.

In late 1983 Robbins and MIM agreed to develop a new machine as a joint venture that would cut hard rock and provide the necessary flexibility. Robbins and MIM have a longstanding, good working relationship which began with the introduction of the first Robbins raiseborer to the mine in April 1969.

The result is the Mobile Miner MM 120- 3001 which actually cuts a rectangular hole. The cutting head is based on the rotating wheel principle as is the fullface TBM, but instead of the head being flat against the tunnel face with all the disc cutters in contact with the rock as the head rotates, the discs are mounted on the circumference of a cutter wheel which attacks the rock face in a drum-like fashion with only half of the discs in contact with the face as it turns. This relatively thin wheel then slews from side to side across the face from its central pivot on the body of the machine cutting a tunnel with a flat invert, a flat roof and slightly curved walls (Figs 1 and 3).

The cutterhead of the tracked machine attacks the face side-on

The cutterhead, with its 28, 17in (432mm) diameter discs designed to with-stand continuous loads of 10 to 20 tonne/disc, creates the profile by sumping in at one wall by about 100mm, then swinging across the face to the opposite wall where the process is repeated. These cutters are arranged in a stepped configuration so that each cutter follows the groove cut by the previous cutter. Combined cutterhead rotation, at a fixed 13.5 rev/min, and sweeping action causes each cutter to follow a spiral path from the top to the bottom of the face. Four floor jacks and two roof grippers hold the machine steady while cutting. There are no side grippers.

Thrust cylinders with a 760mm stroke advance the cutterhead carriage relative to the braced crawler frame. Grippers and jacks are then released and the crawler frame advances for the next stroke.

The works
The head contains two large, tapered roller bearings which accept and transmit the cutting forces through a dead shaft arrangement to the boom assembly. Speed reduction gearing comprising two bevel gear sets driving into two double compound planetary gear trains are contained in sealed cavities with the cutterhead. Each gear reducer set is driven by a 150kw water-cooled electric motor.

Fig 1.Plan and side elevation of the Mobile Miner and its ventilation system

The boom assembly is a plate girder structure to which the cutterhead is attached. The rear end of the structure is connected to the carriage assembly by two vertical axis pins. Two cylinders either side of the pins are attached to the boom and carriage assemblies providing the force required to swing the cutterbead in the horizontal plane.

The cutting forces are transmitted by the boom through the pins and cylinders to the carriage assembly. The machine's hydraulic system consists of four hydraulic pumps driven by two electric motors.

After fabrication and testing of the Mobile Miner prototype in Seattle, it was shipped to Mt Isa in July '84 where it was set to work excavating the S60 conveyor decline in Sept '84. Excavation of the R61 service drive had already started using drill + blast (Fig 2).

Prior to accepting the plan to develop the Mobile Miner, MIM's intention was to use a fullface TBM, despite the technique's basic dis-advantages, to drive two decline drifts, one to house the conveyor and the other as a service drive, the invert of the two circular tunnels bring back filled to form a flat floor. With the Mobile Miher a single 3.7m high x 6.5m wide decline could be excavated which would not only be wide enough to hold the conveyor and the service drives together but also provide the necessary flat invert in one operation (Fig 3).

The Mobile Miner would also have the following additional advantages over a fullface TBM: faster, less expensive assembly and set up; greater mobility; ability to negotiate small radius curves; excavation of variable width tunnels; and easier access to the cutterhead and face.

The hard and the soft
The geology along the S60 decline comprises mainly heavily altered sandstone and basic volcanic rocks with 70% of the drive in extremely hard and brittle quartzites with a compressive strength of 110 to 270MPa with peaks of up to 430 MPa and 300% in soft, ductile chlorite schists or greenstones. Apart from one virtually continuous 60m zone, the greenstone occurs as narrow bands and irregular patches surrounded by quartzite. There are numerous steeply dipping fractures many of which are open and show minor water flows. The quartzites also have widely spaced joints and many irregular fractures, the RQD averaging 60-70%.

Having decided to employ this prototype, MIM recognised the risk elements as being the ability of the machine to cut the hard, abrasive quartzites, cutter costs, dust control and the mechanical availability of the machine.

Once launched, the machine quickly demonstrated its ability to cut the rock, a 760mm stroke taking 70 to 90 min in the quartzite and as little as 45 min in the greenstone. In operation, muck is discharged from the machine's conveyor into a 10 tonne mobile surge bin with its own chain conveyor. Diesel LHD trucks then fill a train of 12X7.5m³ Granby skips pulled by a 20 tonne diesel loco to the central shaft area. Tramming, setting up and alignment of the Mobile Miner was reduced to less than 10 min with an experienced operator.

The normal operating crew for the machine comprises two machine operators, two mucking unit operators and a loco driver. Three of the men in the five man crew are trained operators.

Cutter performance was normally 12 to 16m/set of cutters from one to 11 with gauge cutters being Changed less free quently. In the softer greenstone cutter life improved to a maximum of 61m/set. With easier access and mote space in the cutterhead area, cutter inspection and changes are simpler than on fullface TBMs. It takes two men about 15-20 min to change each cutter.

Water sprays, a dust shield and dust extractors are the three basic components of the dust control system. Four water sprays fitted in front of the dust shield spray a fine mist of water directly onto the face. Two 610mm 4.2m³/s capacity Engart dust extractors draw dust contaminated air from the cutting area through the scrubbers (Fig 1).

Mechanical muddles
Being a prototype, many of the designs encountered we are inevitably due to mechanical failures.

Differential shrinkage of the bali in the ball-and-cup swing bearings Which transmit reaction forces from the boom to the boom carriage, together with high interference fit, caused line contact rather than spherical contract. It was replaced by a brush and journal bearing arrangement.

The nature of the cutting action produces cyclic loading on the structure which, with some underdesigned components to minimise machine weight, caused several structural problems.

Failure of the pedestals bolted to the cutter wheel in which the disc housings are welded has been overcome by the addition of shear bars.

The roof support structure, incorporated initially for machine and personnel protection, had to be replaced by a simpler more robust support for the more rigorous boom carriage stabilisation function.

Maintenance problems occurred in the number of reduction stages on the cutter wheel drive train, particularly the right angle drives which transfer the rotation axis from along the longitudinal axis the machine to the transverse axis. Early failures of the input pinion side of the gearbox were attributed to loss of preloading on the bearings, contamination of the lubrication oil and lack of bearing lubrication.

Fig 2. Having completed Mt !sa's S60 decline, the machine is now finishing the R61 drive

These problems were overcome in three ways. The original locking nuts and tabs preloading arrangement was changed to incorporate a spaced with positive lockup arrangement. The seals at the pinion end were upgraded and an air purge incorporated. An additional lubrication line was also fitted to provide positive lubrication to all bearings in the gearbox.

Fine silica particles produced by the cutting action, mixed with the water of the dust suppression system, creates a highly corrosive and infiltrating silica slurry. Silica infiltration of the lubrication system, primarily through the seals around rotating parts, posed problems and meant frequent oil changes, high filter cartridge costs and accelerated corrosion of machine components causing excessive machine downtime. Air purges have been incorporated into the cutter wheel drive motor's bearings and on the right angle drive input pinion seals to inhibit ingress. The cutter wheel cavity has also been placed under a slight positive pressure to combat the problem.

Despite these problems, the Mobile Miner completed the 1.15km long decline in 20 months finishing in Oct '86, all necessary modifications being carried out underground. The machine has since been dismantled and withdrawn to the surface for major overhaul and cutterhead replacement before starting its next task since MIM is committed to the continued use of the Mobile Miner not only because it has invested a great deal in its development and refinement but also because it believes in the technique and is convinced that it can be used for cost effective future development of the mine.

In July '87 the machine was reassembled underground to continue the narrower 5.3m wide R61 service drive started as a drill + blast operation. The first 90m of the drive was in soft ductile greenstone which caused problems and limited drill + blast progress. The machine is in quartzite again and progress in the narrower horizontal drive is considerably faster with greatly reduced maintenance downtime as compared with the first drive.

However, despite the promising performance of the Mobile Miner at Mt Isa, MIM's commitment and confidence in the viability of the technique, and the considerable interest shown in the technique's development and potential, it has hot as yet been adopted by other milling operators nor for similar jobs in the civil engineering industry.

British Coal (BC) for example says that it is aware of the developments of the Mobile Miner at Mt Isa and that while a large part of its tunnelling requirements are met by boom-type road headers driving in medium and soft strata, it has a number of drivages in hard strata which employ dtill+ blast. BC continually reviews new equipment applied in the mining field internationally, and as the Mobile Miner has been developed for driving square cross-section entries in very hard and abrasive strata as an alternative to drill + blast, the technique comes into this category. British Coal said that it would need to be convinced of the cost-effectiveness and reliability of such a technique compared with current practices before considering its application in its mines.

Fig 3. Cross section of the square tunnel

Decisions on the application of the Robbins TBM at Selby were made in 1980 and the machine itself commenced cutting late in 1981, much too early for serious consideration to be given to the Mobile Miner. The choice of a TBM to drive a circular tunnel a planned distance of 15km through carboniferous strata was in keeping with European practice at the time. Also, British Coal was not interested in using the Selby spine roads as a testing ground for untried technology. More development is required before the Mobile Miner could tackle a large civil project such as a road tunnel, but a possible short term Civil application could be the rehabilitation and upgrading of unlined drill + blast water tunnels through hard rock.

The visit to Mt Isa by Tunnels & Tunnelling coincided with that of a representative of the Norwegian hydro-electricity authority which is investigating the use of a Mobile Miner to rework long hydroelectric waterway tunnels driven originally by drill + blast. A Mobile Miner could remine the unlined tunnels smoothing out the irregular drill + blast surfaces thus vastly improving their hydraulic properties and structural integrity.

Meanwhile, the continued research and development of the concept by Robbins has resulted in several improvements. Future machines will incorporate design features to react forces to the rock as early as possible during the swing cycle, and will provide more flexibility in developing short radius turns. Robbins is also looking at the use of a programmable logic controller (PLC) system to permit automatic, or at least semi-automatic, operation of the machine. For work in softer rocks such as sandstone, a 'ranging' head to cut an arched tunnel may be available if such a profile is called for.

Robbins feels that such developments would make the machine an attractive option for a wide range of mining and civil applications.

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