A Φ9.03m gripper TBM named Caiyun Hao (literally meaning rosy cloud) for a 13km drive just finished FAT at Kunming in early August. The new railway network, the Gaoligong Mountain Rail Tunnel, which is over 30 kilometers long will link the southwestern province of Yunnan to Myanmar. This grand infrastructure project was approved by the National Development and Reform Commission. Trains in the soon to be constructed railway can run at a maximum speed of 250 kilometers per hour in certain sections while speed limits in other sections will only be at 180 kilometers per hour.

The mountainous tunnel drive was awarded to China Railway Tunnel Group (CRTG). The longitudinal slope of the TBM drive follows a Herringbone-shaped alignment. The maximum gradient at the TBM-launch section is 23.5% and -9% at the TBM-reception section. Engineering challenges posed by this project are similar in magnitude to those seen in the construction of railways in Tibet's permafrost. The project faces many challenges, chief among them are:

Squeezing ground
The anticipated geology consists schist, slate, phyllite and altered rock. Such complex geology puts TBM at high risk of being trapped or struck. 

Fault zones
Geological surveys showed the presence of major fault zones associated with water under pressure along the tunnel length. The difficult conditions might result in fallout or collapse in front of the face. 

High temperature
Part of the mountainous tunnel lies in the Mediterranean-South Asian geothermal anomaly area. Extreme conditions like high-temperature, high pressure hot water and hot dry rock can be a major challenge to TBM boring.

Water inflow
Sudden water or slurry inflow is likely to occur in fault zones associated with water. The working face might collapse to bury the cutterhead or even the overall machine. In some cases, the machine would be flooded. 

Long distance drive without intermediate maintenance shafts
As aforementioned, the TBM drive is about 13km without any maintenance shafts between the TBM-launch and –reception. Also, according to geological survey, the quartz content in the rock is as much as 40%. The long, large diameter drive, plus the presence of quartz may result in high wear of the cutting tools. 

China Railway (CR) attaches great importance to this project by establishing a major Scientific Research Program "Research and Application of New Type of TBM in Difficult Ground". The program is led by CRTG together with China Railway Eryuan Engineering Group Co., Ltd and China Railway Engineering Equipment Co., Ltd. which was responsible for manufacture. The supplied customized TBM is designed specially to deal with the above challenges.

Steps to mitigate abrasion of cutting tools
Non-linear arrangement of cutting tools ensures even stress of cutterhead to avoid stress concentration, which extends the service life of cutterehead. This design also minimizes the disturbance during excavation in rocks, which as a result reduces cutter abrasion. Sufficient number of muck chute not only guarantees efficient rock breaking but also protects cutting tools from high wear.

Steps to tackle squeezing ground
To prevent TBM from getting stuck, thickened C block is mounted at cutter shaft to achieve mechanized over-boring. The bottom shield is capable of moving up and down to expand excavation diameter on the crown while maintaining the same at bottom, which facilitates control over TBM attitude. The machine is also equipped with high torque to overcome the loading on the cutterhead.

Table 1: Main Drive Specification
Power: 12×350=4200kW
Rated torque: 11797kNm
Breakout torque: 17695KNm

Advance grouting is adopted for consolidation. Probe drill is mounted at the lower part of main shield platform. It can be lifted to connect with the rail on both sides for advance probe drilling.

Steps to overcome fault zones
3D seismic technique is applied to TBM, 5 groups of three-component seismometer (10 in total) 15 to 35m in advance of the face, 3 groups of hydraulic earthquake hypocenter (6 in total) 50 to 60m in advance of the face. The detection points of seismometer is located on main beam and equipment bridge, which has no impact on construction and requires little personnel activity. The detection point of hypocenter is on equipment bridge and Gantry 1, which shall not affect the construction.

Customized cutterhead design for fault zones
The cutterehead penetration into rocks is kept short to reduce disturbance to wall rock. Proper muck chute dimension (both in axial and radial direction) prevents inrush of massive broken blocks to clog or trap the belt conveyor. Reasonable cutter extension keeps disturbance of cutterhead face to ground as little as possible.

Auxiliary construction method: the machine is equipped with prefabricated rebar, wire mesh, ring beam erector, anchor drill, pre-concrete spraying and L2 zone concrete spraying system.

Pre-shotcrete manipulator
The steel arch erector has gear ring and pre-mixed spraying mount. The pre-mixed spraying reach is about 2m in axial direction on about 270-degree basis.]

Emergency damp spraying, which can be used in a flexible way to control collapse scope and workload of muck clearing and meanwhile increase the passing ability of gripper shoes.

Steps to cope with high temperature 
The solution to this problem is supply of proper ventilation and cooling system. For example, the cutterhead drive is water cooled. The machine is equipped with cooling system to lower temperature in the working area. Locations and interfaces to install refrigeration unit are reserved. The machine is also designed to allow the transporting of ice cubes into the tunnel.

Steps to counter water and slurry inflow
The protection class of key electric component is IP 67. The capacity of dewatering system is as high as 500m³/h. In addition, more than 20 electrodes are installed on shield and cutterhead to detect the presence of water within 30 to 40 m ahead of the face. 

Advance probe drill-advance water detection and advance grouting off
The probe drill at the main beam has an advance borehole with flare opening ratio of 7-degree. The probe is drilled on a 360-degree basis. The crew can grout off water inflows and consolidate the wall rock in front of the face with the support of other equipment.

Major specification
Excavation diameter: φ9030mm
Drive power: 12×350=4200kW
Rotational speed: 0～6.5rpm
Rated torque: 11797kNm
Breakout torque: 17695kNm
Thrust force: 25133kN @ 320Bar
Advance speed: 100mm/min
Thrust stroke: 1800mm
Max. ground pressure of gripper shoe: ≤3.92MPa
Max. ground pressure of bottom shield:<1MPa
Mucking capacity: 1030 t/hr