Mixshield changes mode at Grauholz Sep 1990

Shani Wallis, TunnelTalk
Many tunnelling conventions are being challenged at Grauholz, the rail tunnel project near Bern in Switzerland. Here the Swiss are using the largest Mixshield to date (1990), at 11.6m o.d., and will convert it from a closed bentonite slurry shield to a non-pressurised disc cutting machine, and back again, during the course of its 5.5km drive. Several other innovations are being introduced for the first time.

The Swiss railway authority has traditionally built double-track, single-tube rail tunnels and Grauholz, the newest, is no different. Large, open faced Memco Big John backhoe shields of up to 12.2m in diameter have been used in the past, and, the same was planned again at Grauholz, with a dewatering system taking care of the high groundwater table at the beginning and end of the tunnel alignment (Fig 1).

11.6m diameter Mixshield must convert from a slurry shield to the open mode and back again to complete the 5.5km long Grauholz double-track railway tunnel
11.6m diameter Mixshield must convert from a slurry shield to the open mode and back again to complete the 5.5km long Grauholz double-track railway tunnel

In 1988, a consortium of four contractors, led by Marti of Switzerland with Frutiger, Wayss & Freytag, Schafir & Mugglin, and Kopp, won the Grauholz contract by submitting the lowest conforming bid for both ends of the tunnel. The plan by the client and its consulting engineer Balzari & Schudel, was to award two separate contracts working from each portal with only one tunnelling machine. Having won both contracts, with a combined total of SFr250 million, the Marti-led JV submitted the convincing alternative of combining the two into a single cost-saving contract of SFr210 million, using only one site installation and working from one portal.

Working with an open faced tunnelling shield in the central 2km of sandstone would not be a problem but working with an open face in the waterbearing glacial moraine at either end, under a maximum water head of 40m, with only a dewatering system to stabilise the situation, was more worrying - particularly as there was less than a tunnel diameter of overburden in some parts at each portal.

Marti wanted to use a Mixshield soft ground TBM in the waterbearing ground to avoid having to use dewatering, which was a major environmental consideration since the groundwater is an important source of local drinking water. The soft ground bentonite slurry system Mixshield would therefore have to convert after the first 1.5km to an open faced non-pressurised shield for the hard, dry sandstone section, and back again to the slurry system for the final 2km, also in glacial moraine and containing large boulders (Fig 1).

Fig 1. TBM will convert to open mode for the central sandstone section
Fig 1. TBM will convert to open mode for the central sandstone section

By accepting the Mixshield, the JV partners committed themselves to ordering and using one of the largest soft ground pressurised tunnelling shields ever, since it was not able to convince the client to excavate two smaller diameter, single-track tunnels instead of the one, large diameter double track system.

In May 1988, the JV placed a SFr14m order with Herrenknecht to supply the 11.6m diameter machine. It also included the German company Wayss & Freytag in the JV with an 11% stake to bring its extensive soft ground TBM expertise to the project.

The large machine is a major technological advance in soft ground TBM tunnelling. Based on the familiar bentonite slurry TBM technique, the 1,300 tonne, 10.6m long machine is an articulated shield with a 1,800 tonne, 230m long back-up train. It has an installed power capacity of 2,000kW with the 16,000kV power input being transformed to 380V by the two 2,000kVA transformers on the back-up.

The 11.6m o.d. cutting wheel rotates at up to 2.8 rev/min and is equipped with 65 x 17in disc cutters supplied by ICC Palmieri of Italy, plus 92 soft ground teeth. It has a torque of 1,200 metre tonne and is powered by 12 electro-hydraulic drive motors with a total capacity of 1,250kW. These motors are situated in a bank on the first backup sledge and connected to the 5.6m o.d. main bearing, supplied by SKF of Sweden through RKS, its French manufacturing subsidiary. The three-row bearing, milled in single units, weighs 35 tonne and is housed in the largest component of the machine, weighing 160 tonne.

Fig 2. Mixshield incorporates a large stone crusher and a primary bentonite separation plant on the back-up
Fig 2. Mixshield incorporates a large stone crusher and a primary bentonite separation plant on the back-up

The shield is propelled at all times off the concrete segmental lining via 48 hydraulic thrust cylinders coupled in 24 pairs. These have a stroke of 2.5m and the maximum installed thrust capacity is 10,000 tonne. A further bank of 12 hydraulic cylinders with a 0.65m stroke and a thrust of 1800 tonne controls the cutting wheel for steering purposes.

Integrated stone crusher

Four more hydraulic cylinders operate the central crusher. This is the first time a crusher of this size has been installed in a Mixshield from the design stage. The rams operate on separate horizontal sequences to pulverise boulders of up to 1.2m in dimension. The upper two rams crush them into 40cm-50cm pieces which are then reduced to 150mm by the lower two. All muck is channelled through the crusher, which will operate at all times. Particles then fall to the bottom of the shield for removal through the 350mm diameter slurry exhaust pipe system. When converted to the open excavation mode, the lower two crusher rams will be removed to allow extension of a belt conveyor through the bulkhead. The upper two rams will remain in operation to crush large blocks of sandstone.

Vacuum segment erector lifts 10 tonne segments into position
Vacuum segment erector lifts 10 tonne segments into position

In the slurry mode, laden bentonite is transported at a rate of 1,300m3/h back to a primary separation plant on the back-up (Fig 4). Here bentonite is purified to 40µ and recycled to the face. Two-thirds of the separation cycle is carried out in the back-up plant, with the extracted muck transported out of the tunnel via a continuous conveyor for disposal. All muck removal from the TBM, even in the dry open mode, is via continuous conveyor. This continuous single conveyor, supplied by REI of France, has a capacity of 550tonne/hr at a speed of 3m/sec and will be extended in 200m lengths to its full distance of about 6km.

Some 150m3/hr, or one third, of the used bentonite is pumped out of the tunnel to the secondary purification plant which cleans the bentonite down to 10µ via a centrifuge and a series of settling tanks and flocculation processes. The slurry is completely cleaned even of bentonite particles when passed through the filter presses. Fresh bentonite is then added to this cleaned solution.

The bentonite purification system was designed and supplied by Sotres of France. The primary system on the TBM back-up has a recycling capacity of 1,300m3/hr while the secondary system has a 200m3/hr capacity. There is a 60m3 reservoir of fresh bentonite on the TBM back-up and a further 200m3 reservoir at the surface installation. In the open tunnelling mode, the bentonite system will be disconnected and connected again for the final soft ground section at the far end of the tunnel drive.

Six segments and a key in each 1.8m-wide ring of the bolted lining
Six segments and a key in each 1.8m-wide ring of the bolted lining

As the TBM advances, it erects the precast concrete segmental lining and the invert is backfilled to provide a flat roadway. The segmental lining is the final finish of the tunnel and the backfill, with the drainage system also installed as the TBM advances, will provide the permanent foundation for the rail track ballast.

The segments, designed by Wayss+Freytag, are fabricated on site. Some 24 segments are produced/day using two steam curing carousels. Segment production starts with the cutting and welding of 80kg/m3 rebar reinforcement cages and ends with the fitting of the Phoenix waterproofing sealing gaskets. Being the primary and final lining, the segments are being produced to tight tolerances of only 1mm. The six 1.8m wide x 6m long x 40cm thick precast concrete segments of each ring, weighing 10 tonne maximum, and the key, are cast in Sacma moulds and are checked at several stages for concrete quality and fabrication tolerances.

Segment handling is by vacuum with a vacuum pressure of -0.9 bar, holding a 10 tonne segment for three quarters of an hour during a power failure before power was restored. A ring of segments is transported into the tunnel by truck where they are lifted and transported forwards by crane before being turned and placed on the overhead segment-feeding magazine. The entire back-up system, including the segment magazine, is designed and manufactured by Rowa of Switzerland. With the segments too heavy to track forwards on a rack-and-pinion system, the magazine itself moves back and forth, transferring the segments to the Herrenknecht-designed vacuum segment erector.

Operators’ control cabin on the Mixshield
Operators’ control cabin on the Mixshield

With the segment in position over the erector, two locating pins extend into holes cast into the inner surface of the segment and the vacuum seal is engaged. The erector, with its 360° rotation, then swings the segment into position. Here again, emphasis is on build tolerances of millimetres. Such precision, although it does involve extra time and labour, is not only required for the correct functioning of the waterproofing seals but also to avoid fine hairline cracks which would occur if the TBM applied forward thrust on misaligned segments. Such cracks would affect the quality of the lining, which is designed to be watertight. Once back-grouting the bolts in the rings, which in this case are not required for the final integrity of the lining, are removed, to save on material costs and avoid any possibility of corrosion setting in.

To withstand the maximum 40m waterhead, the TBM is fitted with a Phoenix rubber tail seal. This is fabricated in 60cm sections which are bolted to the tail skin, the bolts passing through the rubber lip. Annular back grout is injected through the segments hard up against the tail seal. This is important for the prevention of settlement. There is also an emergency inflatable seal behind the rubber seal. As no tunnel excavation takes place at the weekends, it is essential to clean the grout lines thoroughly before stopping.

Work on site at Grauholz started with the excavation of a large open cut launch area. Diaphragm walls of 20m deep were installed to control the ground water, which proved very difficult to control, and a 800 litre/min dewatering system has had to be installed. Preliminary work also included the building of a 480m section of arched in-situ cast concrete cut-and-cover tunnel. Some SFr60 million has been spent in site and project mobilisation, including procurement of the TBM.

The TBM started to arrive from the Herrenknecht factory in Schwanau by road in May 1989 and was assembled in 15 weeks. It started boring in August 1989 and to June 1990 had advanced some 400m, achieving a best advance of 14.46m in one 20hr working day. During this first 400m, the TBM passed just 9m under a local public road. Settlement recorded on the road measured about 1.5cm, which is remarkable considering the size of the TBM and the shallow, less than one TBM diameter, overburden.

Cut-and-cover at the east portal
Cut-and-cover at the east portal

According to the JV programme, the 5.5km tunnel is expected to be completed in 38 months working a 5 day/wk excavation schedule. To achieve this, the TBM must advance at an average rate of 10m per the two 10h shifts/day in the open mode in the sandstone and 7m/day in the closed Mixshield mode.

Unfortunately, after a positive start, the TBM came to a halt in mid-June due to a face collapse under very shallow overburden. Launch of the TBM was determined by the road which had to be underpassed. There was one face collapse in the learning curve stages of the drive. Tunnelling then progressed satisfactorily under the road and a small hill. The overburden then dropped away again on the far side and it is here that the second collapse occurred.

A possible cause may be that the high TBM pressure needed to hold the face of cobbles and gravel with a minimal content of fines may have threatened a blowout under the shallow 7m overburden. By reducing the pressure to avoid a blowout, the face collapsed. The gravelly material fell into the cutterhead and managed to somehow jam the cutting wheel. Even with maximum power and extremely high torque, the cutting wheel would not move.

After building an access road to the area, a sheet piled shaft was sunk in an arc around the cutterhead and the material removed to free the wheel. This took 10 weeks and tunnelling was expected to resume at the end of September. The overburden will continue to increase from here and the next critical point should be deciding exactly when to convert from soft ground slurry tunnelling to open faced excavation. The predicted sandstone profile is not regular and the conversion is not planned to take place until there is a substantial overburden of sandstone.

Early tunnelling experience has also indicated that the ground contains a higher quartz content than anticipated. This is causing higher wear on the discs and cutting tools. The cutting wheel has been equipped with disc cutters from the start of the drive and in the soft ground there is the added problem of the discs being unable to rotate in the soft, waterbearing ground which causes flat spot wearing of the discs. Disc wear in the first 400m of the drive has been double that expected. The sandstone is now also expected to contain a higher quartz content and anticipated cutter wear has been re-estimated.

A high content of fines in much of the waterbearing ground has also called for substantial upgrading of the bentonite slurry purification system. An extra centrifuge and several extra settling tanks have had to be included in the surface separation plant and the maximum amount of bentonite is having to be transferred to the surface for secondary separation to maintain adequate quality of the recycled bentonite. Bentonite consumption over the first 400m of the drive is higher than expected at about 20kg/m3 of excavated material.

Fig 3. The new tunnel provides a high-speed link into the Bern station
Fig 3. The new tunnel provides a high-speed link into the Bern station

A sophisticated computer system, designed and supplied by Kern of Switzerland, is recording every operation of the TBM. Every change in hydraulic pressure and response of the machine to the prevailing ground condition is being recorded for future back-analysis to assist in the development of even more sophisticated tunnelling systems. The TBM is guided on its 1,040m minimum radius S bend alignment by an electronic system developed by Stoliska of Austria and Kern of Switzerland.

When the TBM completes its 5.5km long drive at the west portal, the new tunnelled alignment is required as part of Switzerland's Project 2000 railway strategy which is designed to provide an alternative to increasing motorway congestion, particularly of the trans-Swiss European Community freight transportation. The new line will become part of new 200km/hr passenger and freight train services across Switzerland both in the north-south and east-west direction. The old line will remain open for local slower speed services (Fig 3). The new high speed tunnelled route was programmed to come into operation by 1994-95.

As part of Project 2000, Switzerland plans many hundreds of new high-speed rail networks with as much as 300km travelling through tunnel. Some of that tunnelled length is encompassed in the proposed 50km-long base line tunnel through the Swiss Alps to increase the capacity of the existing St Gottard Tunnel. Another new tunnel for Lötschberg is envisaged to increase the capacity of the Bern-Lötschberg-Line. Switzerland has embarked on an equally ambitious programme of motorway network expansion, again including many kilometres of tunnel. If Grauholz is an indication of things to come, it has the money and the courage to enagage the most advanced and sophisticated tunnelling systems developed to date, even if this means going beyond existing tunnelling conventions.

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