Date: 2019-09-26
A new horseshoe-shaped TBM, developed in China and manufactured by manufacturer CREG, was used to build the 3.4km long Baicheng Tunnel through loess on the Menghua railway line. Receiving the 2018 ITA Award for the best technical innovation, the new EPBM replaced the NATM drive originally planned for the project, leading to a 10% to 15% cost saving on the excavation phase. With about 560km of road and rail tunnels scheduled to be excavated in loess during the coming five years in China, the prospects look positive for the new TBM concept.
The Baicheng Tunnel is located in Shaanxi Province in central China, bordering the Inner Mongolian Desert and the north China loess plateau. The prevailing geology includes quaternary to sandy new loess and soft rock. The 3.4km long double track railway tunnel is part of a rail link between Mengxi and Huazhong, in which coal is transported from the north to central China. As the sole means of transportation, it is set to replace the previous system, which involved switching from rail to ship. The overburden of the tunnel varies from 7m to 81m. About 2.7km of the TBM drive was undertaken in rock class V and 305m in rock class VI with an additional 300m long section constructed by cut and cover.
The almost 11m high x 12m wide x 118m long horseshoe-shaped EPBM was developed specially for the Baicheng project by the Chinese company Mengxi Huacheng Railway Co, China Tiesiju Civil Engineering Group, China Railway Engineering Equipment Group (CREG) and China Railway Design Corp. The shield body possesses the geometry of an upside-down U, or horseshoe shape. The shield mainly consists of nine smaller cutterheads with diameters of between 1.1m and 4.9m, plus two screw conveyors and two agitators in the lower part part of the shield and cutting shoes primarily set up in the lateral areas of the shield. In July 2016, the horseshoe-shaped EPBM was assembled and tested at the CREG factory in China before being shipped to the construction site and commencing excavation in October 2016. The breakthrough was celebrated in January 2018 and the tunnel is now operational.
The new horseshoe-shaped TBM was developed, it was explained, to enable the tunnel profile to be excavated more accurately than by means of the originally adopted NATM method. This resulted in a significant saving potential in the volume of material excavated and the shotcrete used to backfill any excavation overbreak. Backfill of the invert to create a level rail deck was also reduced. According to the contractor, in spite of the use of backfill material, about 37% less steel, 42% less concrete and 27% fewer ground anchors were deployed and a 15% smaller area was excavated compared to the open face NATM excavation alternative. The amount of energy consumed on site was also substantially reduced (Fig 1).
The TBM required a launching frame at the entrance portal, which had to be adjusted for its U-profile shape. A wide concrete trough was constructed in the lower section and a steel ring set up as an abutment. The EPBM is limited to operating in soft soils or low rock strengths. At the working face, the cutter wheel concept was adapted for the special geometry in that the individual large, round cutterhead was replaced by multiple smaller, partially overlapping, cutterheads. Blind gaps and small corners occur where the cutterheads fail to cover the working face. The cutting shoes, mounted on the front shield, remove the ensuing material. Each of the nine cutterheads has its own drive unit, which can be controlled individually to rotate either in the same direction or separately in different directions. All cutterheads can be synchronised with relative ease.
The excavation concept is geared, correspondingly, to remove soft materials. In the lower section of the machine there is a larger area available on account of the greater space occupied by the horseshoe shape compared to a round TBM. At the working face, the extracted spoil falls downwards and is carried away by two screw conveyors. Two agitators are set up between the screws, ensuring that the material does not accumulate. The spoil is carried to the screws, which are mounted towards the outside to avoid plug formation. The screw conveyors are driven by means of a common hydraulic aggregate. The earth balance is controlled by the material carried through the screws. It is essential that the screws run in synch and that the material is removed uniformly from the cutterhead area. The spoil was carried away by a continuous belt conveyor, which was covered outside the tunnel so that freezing of the soil and, in turn, blockages were avoided when extremely low temperatures were experienced.
The segments and rings of the precast segmental lining was produced to correspond with the horseshoe shape of the TBM and tunnel profile. With varying radius around the lining ring, a vacuum erector could not be used for installing the individual segments. A mechanised gripper system was therefor developed instead.
The potential of shield roll is claimed as considerably less for a horseshoe-shaped TBM than for a circular shield. To further counteract roll, each individual cutterhead can rotate in either direction during the drive so that the entire TBM can be controlled more easily. The occurrence of a deflecting force that would lead to the shield rolling is considered practically impossible. A navigation system continuously plots the direction of the machine while controlling rolling and at the same time enabling any deviations to be identified and corrected.
Foam nozzles are installed in the shield, as for any other EPBM. These are located in the plenum bulkhead. For the horseshoe-shaped TBM, foam lubrication of the shield is of particular importance as only a small overcut, or no overcut at all, is created. The danger of clogging without lubrication is therefore significant. The shield is always in contact with the soil and bentonite must be added, especially in those areas where the overcut is limited and the cutting shoes are in operation. This lubricates the shield so that it can advance easily through the soft ground. Owing to geological conditions on the Baicheng project, the TBM operated continuously in closed EPB mode with a manlock allowing access to the pressurised excavation chamber for maintenance and repairs.
Providing the geological conditions are favourable, the horseshoe-shaped TBM can achieve savings on the volume of excavated material, the construction materials and construction time, and the future appears positive for this particular type EPBM excavation technique.
For the original article, please visit https://www.tunneltalk.com/China-26Sep2019-Horseshoe-shaped-TBM-for-the-Baicheng-Tunnel.php
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