Crossover TBM for hard rock Mumbai Metro drives 12 Mar 2020

Two 6.65m diameter Crossover TBMs completing the 2.8km twin running tunnels for Package 1 for the Mumbai Metro Line 3 project. The hybrid-type rock/soft ground single shield machines have completed the first two of their three drives through challenging mixed ground conditions of basalt rock with transition zones of shale, tuff, and breccia, taking about 90 production days to complete the 499m and 482m drives from Vidhan Bhavan Station to Churchgate Station (Fig 1).

Fig 1. Location of the Crossover TBM drives for Mumbai Metro Line 3

Package 1 of the Line 3 project between Cuffe Parade Station and Hutatma Chowk Station was awarded to the Larsen & Toubro and the Shanghai Tunnel Engineering Company (L&T-STEC) JV. After delivery of the two 6.65m Robbins hybrid Crossover TBMs the first started boring in August 2018 and the second in November 2018 and broke through into Vidhan Bhavan Station in April 2019 and Churchgate Station in October 2019.

The two TBMS are working beneath a cover of 15m-20m with some structures on the surface, such as the Mittal Towers and the historic Bhikha Behram Well, have been instrumented to monitor vibrations, movements, and potential settlement.

The geology on the alignment consists of fresh greyish basalt, soft volcanic tuffs, shale, and breccias, which are consolidated rocks of angular fragments of disintegrated volcanic rock, and the tunnels transition between rock formations several times presenting:

• Full faces of fresh to slightly weathered basalt with a UCS of up to 125MPa;
• Full faces of weak to completely weathered shale and breccia with a UCS in the 10MPa to 15MPa; and
• Mixed faces while transitioning between a fresh basalt layer and a weak layer of shale and breccia.

Deposits of the geological profile of Package 1

As expected, significant amounts of groundwater with up to 300 litres/min have been encountered between Cuffe Parade and Vidhan Bhavan. Water pressures of up to 2 bar are foreseen. The expected geology and the presence of ground water required the use of a TBM with the ability to excavate effectively in continuously changing conditions.

Optimised for abrasive rock geology using a robust cutterhead mounted with disc cutters and a reinforced screw conveyor at the centerline, the hybrid XRE Crossover machines can operate in open, closed and semi-closed mode using features designed to advance in soft ground with water inflows. Dual ratio gearboxes adjust cutterhead speed and torque to the geological conditions encountered and the , screw conveyors and their bulkhead gates and discharge gates control muck extraction from the excavation chamber, while conditioning with foam and polymers and probe drills for pre-excavation grouting assist TBM operations.

Adaptable cutterhead

Design of the Crossover XRE machines

The two-speed gearboxes installed on the eight drive motors on the TBMs allow for a quick adaptation of the cutterhead torque and speed to the type of ground. The high speed ratio is used in full faces of hard rock, with the high torque ratio, which increases the available torque by a factor of 2.5, allow for operation in soft or mixed face conditions. The extra torque is required to start the cutterhead should there be a face collapse, or to advance when the cutterhead is full of excavated material.

The cutterhead design is optimised for hard rock with six peripheral bidirectional 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 the TBM is maintained by alternating the direction of cutterhead rotation at each TBM stroke. This allows for a simpler thrust system, eliminating the need for a skew ring. As in an EPB machine, maintenance intervention into the excavation chamber is via a manlock with hyperbaric interventions at up to 3 bar when necessary in high water ingress conditions. All cutters are replaceable from the cutterhead chamber.

Central screw conveyor

A screw conveyor in the centre of the XRE Crossover TBM offer several advantages over both traditional hard rock and EPBM designs:

• Full mucking of the cutterhead chamber in rock mode is possible via six loading plates that continuously scoop and deposit excavated material from the invert into the screw conveyor inlet. This design allows for minimal wear by keeping the amount of rocks inside the chamber to the minimum.
• There is the ability to work in pressurised modes to control ground water, with the cutterhead chamber full of material and using the screw conveyor as a plug to maintain pressure at the excavation face, vital for avoiding ground settlement at the surface.

Due to the geology being primarily rock, extra wear protection was added to the screw conveyor and the conveyor includes a replaceable casing liner, replaceable flight plates, and extra access doors in the casing for maintenance and replacement of the wear plates.

Table 2: XRE Crossover TBM Parameters in competent rock Cutterhead speed 5 to 6 rpm Cutterhead torque 1800 to 2400 kNm Main thrust force 10,000 to 11,000 kN Advance rate 40 to 50mm/minute	Table 3: XRE Crossover TBM Parameters in fractured rock Cutterhead speed 2 to 3.5 rpm Cutterhead torque 4000 to 6000kNm Main thrust force 13,000 to 16,000 kN Advance rate 15 to 35 mm/minute
Table 4: Cutter consumption Total linear meters bored 1,890m Volume of rock cut 65,652m3 Cutters/normal wear 111 Linear meters bored/cutter 17.03m3 Volume of rock cut/cutter 591.5m
TBM 1 progress for the first drive and start of the second	TBM 2 progress for the first drive and start of second

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 EPBM boring in rock. This is especially the case when an EPBM is operating in open mode, due to the inefficiency of the standard inclined screw conveyor. The amount of excavated material in the lower chamber of an EPBM feeding the screw also subjects the cutting tools and cutterhead constantly to a re-grind wear action. An average of almost 600m3 of rock cut per cutter is an impressive statistic when assessing basic cutter costs. From an operational point of view, 17 linear meters of boring per cutter reduced the frequency of interventions to an average 54m of boring.

Ground conditioning, mapping, and consolidation

The TBMs are equipped with a foam plant feeding six injection nozzles on the cutterhead and one at the inlet of the screw conveyor. Foam fulfils several tasks to:

• Control dust in the excavation chamber in rock mode
• Reduce friction and wear in soft ground mode when the cutterhead chamber is full of material, and
• Reduce the permeability of the ground to reduce ground water inflow.

A probe drill can be installed on the erector ring for ground mapping ahead of the excavation. Drilling is through 12 ports around the TBM shield at a 7-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 to permit higher boring rates. Several lubrication ports are installed radially along the TBM shield for injection of bentonite to reduce friction in sticky conditions or squeezing ground.

The term hybrid machine has been used in the industry for some time. However, the term has often been applied to machines that require lengthy operations to change the screw conveyer for a belt conveyor to perform efficiently in different ground conditions. We now have a generation of machines that are truly hybrid, which can change from rock mode to soft ground mode almost instantly.