PROJECT | CHINA - DALIANGSHAN NO 1 HIGHWAY TUNNEL


rebound by the geophones installed in the rock of


the tunnel wall behind the TBM; they help researchers to analyse the stratum reflection energy spectrogram and velocity distribution, which enables prediction and mapping of geological hazards in front of the face up to 120m in hard rock, and 100m in soft rock.


EXCAVATION As the TBM advanced, use of the HSP217T horizontal seismic prediction system was found to give high accuracy in predicting the major geological features as encountered. As it happened, the greater issues were in the karstic limestone and fractured basalt, and less so in the mudstone which was expected to present squeezing behaviour but proved favourable for soft-rock TBM excavation. The TBM bore went well, better than expected, and


continued further than originally planned - driving an extra 2km when the planned 5km-long drill & blast advance from the opposite portal hit bad ground and came to a stop after 3km. Reaching best advance rates of 49.12m/day and


723m/month, the TBM completed its 12km-long drive in 36 months, in September 2023. Excavation support was provided systematically with steel ribs on 900mm centres and primary shotcrete. Constructing a pilot tunnel ahead of the main tubes


however, and supporting groundwater drainage, didn’t eliminate the need for PEG. Control of water inflow was required on the project for environmental reasons to sustain the nature reserve on the surface, as well as to improve conditions for the excavation process and ensure


the long-term durability of the structure in its design life. As it happened, the TBM drive, and the main drill & blast advances for the main tubes, encountered two identified fault zones in which estimated water inflow rates were significant. During the site visit in May 2024, just a total of 5.2km


of the main tubes remained to be excavated by drill & blast (approximately 2.6km in each tube), with excavation in both directions from a cross passage being used for vehicle access and mucking out through the pilot tunnel. At the same time, several final lining operations


were advancing, leading with gantries used to install the waterproofing membranes in the main tubes and the pilot tunnel, and also formwork shutters for in-situ concrete, and also advancing the reinforced final lining in all three tubes.


LOOKING AHEAD As an auxiliary construction support drive, the pilot tunnel is finished with a transverse ventilation ceiling; in the event of a fire emergency in the main tunnels, smoke will be drawn into the ventilation ceiling duct of the pilot tunnel while the longitudinal ventilation fans in the main traffic tubes will keep the main tunnels fed with fresh air. In another effort to maximise the value and return


of the pilot tunnel investment, the Lexi Expressway Company is working with the regional electricity provider to negotiate a deal to carry high voltage cables through the tunnel rather than constructing pylons across the inaccessible terrain of the Daliangshan mountains. Wind farms for electricity generation are being actively developed in the region. Upon completion of the Daliangshan No 1 tunnel, the


future of the design will be examined. “Maybe next time, we will have three TBMs - pilot


and two main tubes,” said Director Lan. Through our interpreter, he explained: “The problem is, however, that a TBM profile to accommodate three traffic lanes would have to be about 13m in diameter with an excavated area of 132m2


, while a drill & blast profile at about 12.5m-wide has an area of 105m2 and that, other than


having a utility conduit in the invert, the lower part of the circular profile must be back-filled to create the road deck. This appears to be a waste, but this experience and others prove that TBM excavation is four times faster than drill & blast,” he added. “So, while this is a first experience with the design


of a central pilot tunnel for an express highway the experience has been positive and will be applied again,” he said. “There are three new long express highway tunnel projects in planning in Sichuan and the method has been adopted for one of them and is being considered for the other two, and more.” Given the Daliangshan No 1 tunnel experience, the


Above: Project Director Lan leads a tour of the project during the site visit in May 2024 34 | September 2024


adoption of separate pilot drives and the introduction of TBM excavation to the field of express highway tunnel construction through the mountains has well and truly arrived.


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