Excavation in mixed ground conditions is always a challenge, but beneath a dense urban environment the stakes become even higher. At India’s Mumbai Metro, two 6.65m Crossover (XRE) TBMs are successfully boring parallel 2.8km tunnels below the city in basalt rock with transition zones of shale, tuff and breccia. They have made intermediate breakthroughs at the 1.2km mark and overcome rock strengths up to 125MPa UCS with significant water ingress, all just one year after factory acceptance, shipping, site assembly and launch. The machines, which incorporate features of both hard rock single shield and EPB TBMs, are optimised for abrasive rock geology using a robust cutterhead mounted with disc cutters and a reinforced screw conveyor at the centerline. The machines can also operate in closed or semi-closed mode using features designed to advance in soft ground with water inflows: dual ratio gearboxes to adjust cutterhead speed and torque to the geology, screw conveyors with bulkhead gates and discharge gates, ground conditioning with foam and polymers, and probe drills for pre-excavation grouting.

Mumbai’s Metro Construction

In 2014, the first line of Mumbai’s metro system opened to curb the growth of road traffic. Seven additional lines totalling 235km are under construction with overall project completion expected in 2025. The 33.5km of underground metro Line 3 will extend from the south end of Mumbai in the business district of Cuffe Parade to the SEEPZ district and will serve the Chhatrapati Shivaji international airport. Line 3 is expected to initially decrease road traffic in the area by 35%, reducing fuel consumption by 460,000L/day and slashing journey times.

The Line 3 project is divided into seven packages. Package 1 was awarded to the joint venture between Larsen & Toubro and the Shanghai Tunnel Engineering Company (L&T – STEC) and consists of two 2.8km parallel tunnels between the Cuffe Parade station and the Hutatma Chowk station. Two 6.65m Robbins hybrid or Crossover TBMs began boring in August 2018 and November 2018, respectively. Robbins also supplied two slurry TBMs for package three that are boring the 3.5km section of tunnels between the stations of Mumbai Central and Worli.

Mixed Ground Conditions

The geology of package 1 consists of fresh greyish basalt, soft volcanic tuffs, shale and breccias which are consolidated rocks of angular fragments of disintegrated volcanic rock. The tunnels transition between these rock formations several times:

  • Full tunnel face of fresh-to-slightly weathered Basalt with UCS up to 125 MPa;
  • Full tunnel face of weak-to-completely weathered shale and breccia of 10-15MPa UCS, and
  • Mixed tunnel face while transitioning between a fresh basalt layer and a weak layer of shale and Breccia.

The TBMs on Line 3 excavate with only 15-20m of cover above the tunnel. At the surface, some structures such as the Mittal towers and the historic Bhikha Behram Well have been instrumented to monitor vibrations, movements and potential settlement.

The geology and the presence of ground water required the use of TBMs with the ability to excavate effectively in continuously changing conditions. As expected, the TBMs have encountered significant amounts of groundwater with up to 300L/min between Cuffe Parade and Vidhan Bhavan. Water pressure of up to two bar is foreseen.

Adaptable Cutterhead

The two-speed gearboxes installed on each of the eight drive motors of the two machines allow a quick adaptation of cutterhead torque and speed to the type of ground, whether hard rock, soft ground or mixed face.

The cutterhead design is optimised for hard rock with six peripheral bi-directional muck buckets. In extreme soft and running ground conditions, the opening ratio of the cutterhead can be increased by removing part of the face plates and installing soft ground cutting tools.

The roll angle of each TBM is maintained by alternating the direction of rotation of the cutterhead at each TBM stroke. This allows for a simpler thrust system, eliminating the need for a skew ring.

As on an EPB machine, disc cutter maintenance is made possible by a man-lock for hyperbaric interventions inside the cutterhead chamber of up to three bar. All cutters are replaceable from the cutterhead chamber.

Centre Screw Conveyor

Dedicated hard-rock TBMs generally use a belt conveyor located in the centre of the machine as a means to convey muck out of the cutterhead chamber. Pure EPB TBMs on the other hand use a screw conveyor most often located at the bottom of the excavation chamber. By using a screw conveyor in the centre of the machine, the Crossover TBMs used in Mumbai offer the advantages of both designs.

Ground conditioning, mapping and consolidation The TBMs are equipped with a foam plant, six injection nozzles on the cutterhead, and one at the inlet of the screw conveyor. Foam has several uses on a TBM:

  • Controls dust in the excavation chamber in rock mode;
  • Reduces friction and wear in soft-ground mode when the cutterhead chamber is full of material, and
  • Reduces the permeability of the ground in order to reduce ground water inflow.

A probe drill can be installed on the erector ring for ground mapping ahead of the excavation. Drilling is undertaken through 12 ports located all around the TBM shield at a seven-degree angle from the TBM axis, and through six ports straight forward through the cutterhead. Probe holes can be used in an umbrella pattern for pre-excavation grouting to consolidate and seal the ground in front of the cutterhead for higher boring advance rates. Several lubrication ports are installed radially along each TBM shield for injecting bentonite to reduce friction with the tunnel wall in sticky conditions or squeezing ground.

Site Assembly, Training and Launch

Although Larsen & Toubro (L&T) had extensive experience in both traditional and mechanised tunnelling methodology, they had not had any hands-on experience with the design features of the crossover machine. As part of the machine supply agreement, Robbins provided a team of key personnel to train and familiarise L&T’s team in all aspects of the Crossover machine’s design, technical and operational features, including during boring operations.

Initial Drive Machine Configuration

Excavation of the launch shaft had suffered delays due to working hour restrictions as the site was located close to several high-rise residential apartment blocks. It was clear that launching a completely assembled machine and backup system would not be possible.

Several assembly/launch configurations were prepared hoping that space would be available for launch with a minimum of machine, bridge gantry and gantry 1. This setup would allow the segment handling system to be fully operational and facilitate the installation of a shortened conveyor system for mucking. However, at the time of launch the available space dictated that the machine was launched in a worst-case scenario, without any gantries at all. Electrical and hydraulic umbilicals had been procured to enable the initial drive of 105m to be completed with the machine only. With this configuration, production operations would be hindered due to mucking directly from the screw conveyor into a relatively small skip rather than a muck car, and segment handling would be slow without the segment handling system. With this in mind, discussions took place to assess the benefits of carrying out a phased initial drive as follows:

Although the phased assembly sequence would substantially reduce the time taken to complete the initial drive for TBM 1, the downside would be delays to the assembly of TBM 2. There was insufficient space available to mobilise a dedicated crane for each machine, hence assembly of TBM 2 would be disrupted when the crane was being used for each phase of the TBM 1 assembly operations. Finally, it was decided to complete the whole of the initial TBM 1 drive using the machine only. Shaft excavation would continue as a parallel activity, which would shorten the overall length of boring required to enable lowering of all gantries.

Initial Drive Production Rates

A couple of days before TBM 1 commenced boring, assembly operations on TBM 2 commenced. This should be considered when assessing the production rates during the initial drive. Other major factors that have already been mentioned are mucking with a single skip, which had to be transported to the shaft, lifted to the surface and tipped before being returned to the discharge point of the screw conveyor, and a makeshift segment handling system.

TBM 1 completed 70m/42 rings of boring in 33 working days, giving an average production rate of 2.12m per day. Only 70m was required for the initial drive of TBM 1 because the shaft excavation was extended during this time, which provided the space required for the back-up gantries. The initial drive of TBM 2 consisted of 97m of boring and, using the same configuration as TBM 1, (machine only, fed by umbilicals) was completed in 37 days with an average production rate of 2.6m/day. Both of the initial drives were completed on time. The geology along the alignment of both initial drives consisted of basalt ranging from weathering grade I to weathering grade III. Water ingress was not significant, hence the machines were operated in open hard-rock mode.

Accurate Borehole Info

One of the biggest concerns along the alignment of the package 1 tunnels between Cuffe Parade and Vidhan Bhavan station was that the drives ran in close proximity to the coast. At one point, TBM 1 would be little more than 25m from the coastline, with the invert level of the tunnel running approximately 22m below mean sea level.

Boreholes had identified the geology as basalt with varying weathering grades up to grade IV. Lugeon values taken from the bore holes indicated low to moderate hydraulic conductivity with low to moderate rock mass discontinuities.

Although the geology at the borehole locations looked reasonably good, the geology between the boreholes was cause for concern.

Due to the relatively short length of the screw conveyor there was a risk that if the machine encountered geology with high water pressure and low fines content there would be difficulties maintaining a plug in the screw conveyor. To mitigate this risk, the machine was designed with an option to operate in sequential mucking mode if required.

Fortunately, the borehole information proved to be relatively accurate. Although the ground was more fractured than expected, the machine was operated in closed mode through parts of this section of the alignment, and the sequential operation feature was not required. Screw conveyor speeds as low as two RPM were used in order to maintain a plug in the screw and maintain earth pressure.

The machine operating parameters while boring in the competent rock are detailed in Table 1.

When the machine encountered the fractured ground, the drive-motor safe sets were blown (which prevents damage to the motors), so the low speed–high torque gears were engaged, and the machine operating parameters were changed accordingly (see Table 2). Mapei Polyfoamer FP/LL, anionic surfactant/ lubricating polymer was injected into the chamber and cutterhead to reduce friction and prevent clocking of the cutterhead and muck buckets. The machine traversed this section of the alignment without any major problems.

Production Rates

The bar chart in Figure 1 shows the monthly production rates of TBM 1.

These are represented throughout the whole of the first drive and the initial 350m of the second drive.

It should be noted that the machine was not launched until 20 August 2018 and production rates were affected up until the end of October 2018 by the:

  • Short start-up;
  • Stoppage to complete the machine back-up gantry assembly;
  • Removal of the temporary rings/thrust frame, and
  • The shaft setup.

The machine completed the first drive in April 2019. May and June were taken up dragging the machine through the station box and setting up for re-launch. Taking aside the months that were affected by short start-up, shaft set-up, and assembly of the backup gantries, the average production rate for the first drive was 195m/month. The machine was relaunched for the second drive complete with all gantries on 8 July 2019 and by the end of August had bored 351m.

Figure 2 shows the monthly production rates of TBM 2 throughout the whole of the first drive and the initial 230m of the second drive. During the months of November and December 2018, production rates for TBM 2 were affected by the same short start-up procedures as TBM 1.

Production Restrictions

Both machines achieved impressive production rates for TBMs with closed-mode capability, boring in rock, however their performance could have been better. Substantial bottleneck delays were suffered in muck-shifting from site. Site access was restricted and even prohibited during the twice-daily peak traffic periods. Each round trip to the muck disposal area was 50km, which in Mumbai traffic could take over three hours. The maximum capacity of the muck storage bins for each machine could not accommodate more than 4.5m of boring before the whole operation became muck-bound. Without these muckshifting delays, TBM production rates would have peaked at well over 300m/month.

Cutter Consumption

A major advantage of the centre screw and peripheral muck buckets is a substantial reduction in wear to cutter tools compared to an EPB machine boring in rock, especially for an EPB machine in open mode due to the inefficiency of the standard inclined screw conveyor. Also, the excavated material in the lower chamber/face of an EPB is constantly subjecting the cutting tools and cutterhead to a re-grind wear action.

Table 3 shows the cutter consumption related to normal cutter wear of both machines after completing a combined 1,890m of boring. The actual number of cutters consumed was 129, but 20 cutters failed during two separate wipe-out events. Because wipe-out failures are not considered normal wear and are avoidable, only the cutters that initially failed during these two events have been included in Table 3.

An average of almost 600m3 of rock cut per cutter is an impressive statistic when assessing cutter costs. Operationally, 17 linear meters of boring per cutter reduced the frequency of interventions to an average 54m of boring.

Construction Update

The two TBMs broke through at Vidhan Bhavan, the first of two intermediate stations, in April 2019 and resumed boring towards Churchgate station in July 2019.

The machines broke through on October 3 and October 22, 2019 when they completed 499m of tunnelling between Vidhan Bhavan and Churchgate stations. The machines are now excavating their third, 700m-long drive (started January 25 and Feb 15, 2020 respectively).

Conclusions

Right from the very early stages in the design phase of metro projects around the world, design consultants have included a specification clause stipulating that the tunnel boring machines must be capable of boring in closed mode and be able to maintain face pressure. This of course is to mitigate the risk of ground settlement associated with open-face machines when they encounter unstable geology. However, the clause is also related to preventing depletion of the water table on many projects. After studying this clause in the tender documents, contractors invariably start to assess the advantages and disadvantages of slurry machines versus EPB machines. The problem is that neither slurry nor EPB machines perform particularly well when boring in rock.

As already mentioned, EPB machines suffer from excessive wear to cutter tools when boring in rock, as does the screw conveyor. Inefficient mucking and the time lost carrying out interventions due to high wear are major disadvantages. Another issue with EPB machines is the heat generated by friction of the material moving around in the chamber/ cutterhead: this can lead to substantial delays waiting for temperatures to reduce low enough to allow personnel to enter the chamber and carry out interventions as safely as possible.

––Slurry machines suffer from fewer issues with wear to the cutterhead and cutter tools. Heat is not an issue as the circulation of slurry acts as an efficient heat-transfer mechanism; however, production rates are restricted by the capacity of the slurry transportation and separation plant. Both the cost and the size of site footprints are major issues with slurry transportation and separation plants. Production data from projects worldwide show that the efficiency of rock machines boring in rock is far superior to either slurry machines or EPB machines.

The term ‘hybrid machine’ has been used in the industry for some time now. However, this term has often been applied to machines that require lengthy operations to replace the screw conveyer with a belt conveyor to allow efficient performance in different ground conditions.

There is now a generation of crossover machines that are truly hybrid: machines that can change from rock mode to soft-ground mode almost instantly.

‘Rock TBM’ production rates can now be achieved while still meeting the tender/ design criteria of machines operating in closed mode.