The under-river tunnel project of Yuan River in Changde is a twin-tube tunnel in the Chinese inland city of Changde. The tunnel connects two sides of the Yuan River, and with its two-way and four-lane, it will promote harmonious development of regions along the river. The tunnel excavation was completed in July 2019. 3360 meters of the twin-tube tunnel (each of 1680 meters long) with an outer diameter of 11.3 meters and an excavation diameter of 11.71 meters were driven by a CRCHI independently developed slurry pressure balance shield machine (SPB), which is the first set super large-diameter SPB with cutter replacement under normal pressure.
As for the tunnel’s vertical section, the maximum longitudinal gradient is 3.54%, the minimum turning radius is 680 meters, the maximum overburden is 18. 2 meters, the minimum overburden is 11.3 meters, the maximum water-earth pressure in the river is 4.5 bar, and the utmost water-earth pressure is 5.5 bar.
Main geology is round gravel soil. Pebble strata is distributed in an area of 400 meters long at the northern part. A small amount of silt interlayer is distributed in the river’s bottom. Soil dig-ability grades are Grade I~III, surrounding rock classification is Grade V. The gravel-cobble stratum, among others, is highly permeable, its permeability coefficient being 120m/d. The maximum particle size of the pebble is 45cm, the range of point loading strength is 0.51~5.21MPa and the uniaxial compressive strength is 73.57 Mpa.
Project challenges are large excavation diameter, complicated hydrogeological conditions and confined water. Due to characteristics of high strength, high permeability and high-water pressure, ensuring stable and safe excavation is a key point for this project.
When the SPB is driven in the gravel-cobble stratum with maximum permeability coefficient of 120m/d for a long distance, underground water will constantly enter the excavation chamber to cause slurry fail to reach the standard. Therefore, muck is difficult to discharge and will be accumulated at the bottom of the excavation chamber. This will increase the wear to the cutting tools and slurry pipes and the frequency of cutter replacement. So, driving in the pebble stratum in a smooth way is a big challenge.
Driving in the clay may cause formation of mud cakes, which will reduce tunneling efficiency. So, preventing formation of mud cakes during excavation is another big challenge.
TBM type and parameters
TBM type: ZTS11710 SPB with cutter replacement under normal pressure
Excavation diameter: 11750mm
Length of shield+back-up gantries: 132 meters
Weight of shield+back-up gantries: 3000 tons
Installed power: 6100 kW
Maximum advance rate: 60mm/min
The mega 12m-level SPB with cutter replacement under normal pressure is applicable in high water pressure and long-distance tunneling and available for high requirement on ground settlement.
The self-developed technology of cutter replacement under normal pressure effectively reduces the risk of workers during cutter change by workers, making the replacement safer and quicker.
Application of a high precision gas-liquid pressure balance control technology guarantees that the pressure is always kept balanced between the tunnel face and the excavation chamber. A big-power slurry circulation flushing system ensures high-efficiency excavation and delivery so as to operate easily and adjust parameters.
Alloy-toothed crushers at the slurry outlet are hydraulic-driven and have high crushing efficiency.
The SPB uses CRCHI integrated control systems and has stable and reliable performance. The successful development and application of cutter change under normal pressure technology is a big leap for Chinese TBM development technology, filling in the domestic gap in this field. Besides, application of box culvert synchronous installation technology allows for the assembly of precast box culverts parallel to tunneling, effectively shortening the construction period.
- Maximum daily advance rate is 22m/day
- Maximum monthly advance rate is 362m/month.