First Shield Machine for horseshoe-shaped Cross Sections will be used to bore the Baicheng tunnel of the Mengxi railway project, connecting the west area of Inner Mongolia and Central China. The single tube Baicheng railway tunnel with a maximum depth of 81 m is 3345 m long. The whole alignment of the tunnel runs in a straight line, of which the maximum track gradient is 5 ‰. The Baicheng tunnel is located in the north of the Chinese Shaanxi Province, where in the crossing strata silty sand, sandy loess and fine sand are distributed. U-shape Tunnel Boring Machine for the Baicheng Tunnel The cutting cross section of the U-shape TBM is 10.95 m high and 11.90 m wide. Compared to a circular TBM the design and manufacture of the horseshoe shape cutter head is much more challenging due to the complex machine structure at the cutter head and the approach to cut the ground with several partly overlapping cutting wheels. The U-shape machine adopts a tapered shield and two symmetric screw conveyors for muck transfer. The cutter head is composed of seven small cutting wheels which can rotate either simultaneously or independently in different directions. In the event of rolling, several cutter heads would rotate in the same direction in order to generate a counter torque for the shield machine to rectify the deviation. When the machine is applied to soft soil tunnelling the tunnel space utilization could be improved up to 20 % compared to circular cutting. The U-shaped TBMs are very suitable for traffic tunnels in soft ground conditions. CREG improves the multi-cutting-wheel technology which is also used for the rectangular pipe jacking machines step by step and extends the capability of this technology to harder ground conditions.
Rectangular Pipe Jacking Machine
EPB rectangular pipe jacking machine consist of cutter head, drive system, shield structure, screw conveyor, back-up gantries and thrust systems. The rectangular excavation shape is implemented by the arrangement of multiple cutter heads and rectangular shield structure. The shield includes front shield, which is connected by articulation cylinder to the rear shield. In the front shield the drive system of each cutter head and the conveying system consisting of one or two screw conveyors is located. The screw is conveying the muck and controlling the earth pressure as common in the EPB technique. The Back-up gantries are equipped with of soil conditioning systems like bentonite and foam plants, ventilation fan, high pressure water supply system, muck conveyor etc. The thrust frame consists of erected plate, support of thrust cylinder, support frame. Intermediate jacking stations can be installed to push sections of the concrete pipe forward. The ground is excavated by several cutter heads which can be hydraulic or electrically driven. During excavation bentonite and foam can be injected in the cutter head chamber to control the earth pressure and the material flow. The cutter heads can rotate in parallel, but to increase the mixing of ground and to improve the muck flow each cutter head can be rotated individually and counter rotate. During excavation the thrust system at launching shaft push the pipe and the machine forward. After pushing one segment forward the thrust cylinders are re-tracked and the next concrete ring can be placed into the thrust frame.
Tunnel boring by mechanically enlarging a pilot heading to a tunnel of bigger diameter is a WIRTH developed technology which roots go back into the 1970s. Application of this technique has its advantages. The project aided geological research, as samples were taken from the excavated material from the pilot tunnels. The big diameter of the tunnels improved environmental conditions underground with increased ventilation. The design of the machine made the tunnel more accessible for installing roof supports close to the tunnel face, as well as free access to the face itself, something no other TBM has been able to provide. The reamer technology has been successfully used in projects all around the world like e.g. Japan, Korea, Spain, and Switzerland.
Reamer TBM combined with undercutting method:
The enlargement machine’s cutter head for undercutting also consists of a basic frame and up to six boring arms depending on the boring diameter. The cutter head turns on the tensioned inner Kelly running on bearings in the pilot heading and in the large tunnel cross-section. The cutter rollers are offset both axially as well as radially to the tunnel axis and arranged on radially movable cradles. When the cutter head turns and the cradles move radially at the same time, each roller describes a spiral-shaped path around the tunnel axis. A step-formed face is created through the outer rollers advancing so that each cutter rollers shears off the rock against a free area (back-cutting principle). At the start of boring (axial excavation section per radial stroke of the cradles), the inner cutter rollers e.g. start in the pilot bore and the rollers arranged further towards the outside in the stage previously bored by the inner roller. The advantages of this excavation method compared to conventional TBMs are:
- less energy required for the excavation because the cutter rollers operate against the tensile strength of the rock, which is substantially lower than the compressive strength
- shorter cutter head design, thus doing away with thrusting forces in the tunnel direction, making rock supporting close to the face possible local overcutting of the circular cross-section possible
- material masses are saved through optimal adaptation of the excavation; rock supporting possible between the cutter arms when the machine is at a standstill
- lower dust development as the material is destroyed to a lesser degree
- smooth rock excavation at the face/pilot heading as there is no load on the rock parallel to the pilot heading.