ALL IMAGES COURTESY ROBBINS

Two brief stories of technological success, from

Texas and Shaanxi, respectively.

Both constructing hydraulic tunnels – one for a drainage tunnel, the other to build a long water diversion tunnel.

In the US southwest, the Mill Creek drainage project saw the execution of a pre-planned in-tunnel diameter conversion of the hard rock TBM tunnelling system – without benefit of a shaft, chamber, cavern or pre-excavated portal – draw more plaudits. This time acknowledgement of the technological milestone success came from the International Tunnelling and Underground Space Association (ITA) in its annual awards.

In central China, on the 17.5km long Yin Han Ji Wei water diversion tunnel in Shaanxi province, the McNally slat support system once more proved its effectiveness for difficult ground when the TBM excavation was prepared for high overburden, rock stresses and numerous rock bursts.

IN-TUNNEL TBM DIAMETER CONVERSION

At its most recent annual awards, the International Tunnelling and Underground Space Association (ITA) announced the 2022 winner of the Technical Innovation of the Year to be Robbins for its ‘Unprecedented intunnel diameter conversion of the largest hard rock TBM in the USA” on the Mill Creek/Peaks Branch/State- Thomas Drainage Relief Tunnel, in Dallas, Texas.

The task for the Main Beam TBM, a re-built machine named ‘Big Tex’ for the job, was to bore an 8km-long large diameter drainage sewer through chalk and shale.

Some 2.8km into its drive, the 11.6m-diameter TBM diameter was halted, as planned, and its diameter reduced to 9.9m through a series of sequential steps, the machine having been designed for that purpose. The TBM’s supporting tunnelling system (trailing gear, continuous conveyor system) was also adjusted.

Robbins field service manager, Evan Brinkerhoff, said in a statement shortly after the machine holed through in mid-2022 to complete its drive: “We started with a smaller machine, then we made a kit or second skin that was slightly larger. And then, when the time came, we just took that skin off.”

It was that simple, he says, but the sequential process in-tunnel took about four months to perform, in early 2021.

The challenge arose from the drainage system design requiring different tunnel sizes, and cross-sections envisaged, so as to handle different flows rates on the upstream and downstream side of a branch tunnel feeding in on the network. Upstream, a circular crosssection was proposed, downstream a horseshoeshaped section over the final 2.8km.

TBM boring was to be used throughout but the downstream end would be widened to the horseshoeshaped section by roadheader, creating a flat invert. Different formworks would be needed for the downstream and upstream lengths of tunnel. But evaluations, including of time and cost, dropped the two-stage tunnelling plan in favour of using a single TBM for the entire length – but not a single diameter.

The larger drainage tunnel flow rate for the downstream section would be achieved with a larger diameter section, excavated during the initial phase of tunnelling. The upstream section would be bored after the in-tunnel changes of the TBM and tunnelling support system.

The ground was not ideal where a local backup of the machine was planned to facilitate the changes, and so with extra rock support to the crown only a miniscule backup could be done and was sufficient. A 52m deep drop shaft close by, reached via a short adit opened up for the purpose, would be the access route for personnel to execute the works.

Four large pieces of the cutterhead, the heaviest up to 15 000kg, were removed and taken to the surface, re-fitted for the smaller diameter requirement and then taken back underground. While refurbishment was performed on the surface, the changes were made to the rest of the TBM, trailing gear and conveyor.

By the time of the in-tunnel works, the geology in the downstream section had been of low rock strength and low abrasivity that few discs on the cutterhead needed changing. But the opportunity of the planned stoppage was taken to swap them all out – there was full dress change of the cutters. Afterward, on the narrower upstream drive, only 14 cutter changes were needed.

Breakthrough was achieved in the middle of 2022. Final lining is being constructed. The project is due for completion by early 2025. The drainage tunnel is designed to handle flood events of 1-in-100-year return period in east Dallas.

MCNALLY SLATS SUPPORT DELIVERY

Tunnel boring on the Yin Han Ji Wei water diversion tunnel was a tough challenge, as expected, and the 8m-diameter Robbins Main Beam TBM on the project came fitted with the McNally support system. Well proven of many projects with difficult rock conditions

In 2015, based on then recent experience of TBM success among many prior tunnelling projects in the Qinling Mountains of the region, a TBM once again began to bore in the region, this time for the Yin Han Ji Wei water diversion tunnel. The Robbins machine was chosen after the manufacturer had prior success in the mountains with two larger diameter Main Beams, for the West Qinling Rail Tunnels project.

Suitability of TBM tunnelling also had reference to a project on the other side of the world, in Peru, on the Olmos water tunnel project in the Andes mountains where a 5.3m-diameter Main Beam handled high overburden and rock burst conditions, as anticipated would be the case in China. At Olmos, the McNally support system was used as part of the solution. The McNally system was also used for in part of the Liaoning NOW water transfer project, in China, where an 8.53m-diameter Main Beam was used on Lot T5. It would also be specified from the outset for the Yin Han Ji Wei water diversion tunnel.

The diversion tunnel is one of three in a larger programme of tunnelling for a water transfer and hydropower scheme in the Hanjiang and Weihe catchments. While most of the tunnelling will be by drill and blast, the method was expected to face more challenges with multiple headings and adits under high overburden. The decision was taken to use a TBM for the most challenging sections of the diversion tunnel.

Project owner is Hanjiang-to-Weihe River Valley Water Diversion Project Construction Co, and the contractor for the project was China Railway Tunnel Group (CRTG). In addition to the high overburden and stresses in hard granite rock along the alignment, they also saw risk of groundwater inflows to the tunnel works.

The cutterhead of the Robbins Main Beam had 8 x 17” centre cutters and 43 x 20” single cutters, and the machine has maximum thrust and torque of 21,087kN and 14,614kNm, respectively. It had main drive power of 3300kW. The machine was supported by a continuous conveyor. TBM boring for the tunnels was executed in two drives – 9.9km and 7.6km lengths.

From early on the challenges expected did present themselves – hard and abrasive rock, high cutter wear and consumption, and inflows. Rock bursts were numerous and support needed to be installed. UCS rock strength ranged from 107MPa – 309MPa, averaging close to 200MPa, and the quartz content was high. The difficulties were more than expected per geological indications from the bid documents, although site investigation in mountains is limited, as is typical – although the other TBM local projects, and their successes despite challenges – gave some information and context.

The McNally system was important to help deal with the challenges, including preventing rocks falls on to the machine and increasing worker safety.

McNally System

The McNally system was originally developed, designed and patented by C&M McNally almost 20 years ago and early on used wooden slats, being employed on dozens of projects in Canada (especially Ontario), says Robbins. The TBM manufacturer obtained exclusive licence of use on its machines and has evolved the system to steel slats, and the significance of its value to TBM tunnelling was highlighted by the Olmos experience.

At Olmos, where squeezing ground and numerous rock bursting events troubled the tunnelling works, modification to the machines included in-tunnel welding of wear plates plus removing of roof shield fingers behind the cutterhead support in favour of, instead, a system of pockets for continuous installation of steel slats, then still a relatively novel ground support system for TBM tunnelling.

A major benefit is the provision of customisable continuous support running in parallel and close together along the roof area of the tunnels, mainly, but can be extended along the sidewalls. Broken rock stays in location, which helps also to keep the natural rock arch. The strength and size of slats can be adjusted to suit varied tunnelling, overburden and rock stress conditions. They can also be become part of the final lining of the tunnel, as in Olmos.

Diversion Tunnel challenges in China

The McNally slats made a difference to progress of tunnelling at Yin Han Ji Wei diversion tunnel, along with many other important systems.

But even for them there were limits in the environment where there were some excessive stress deformation and especially high energy rock bursts, requiring much extra excavation support – shotcrete, rockbolts, mesh, and steel arches.

Stress relief holes were also drilled in a 120° arc across the crown.

The rock bursts caused frequent damage to the machine. Following passage of the TBM, in some sections of tunnel insitu concrete lining was constructed for long-term durability.

Groundwater inflows only added to the difficulties and there was a major flood in 2016. Probing ahead of the face helped to assess risk of rock burst (though not so much timing or severity) and of inflows, and when grouting would be necessary. High temperatures underground and severe humidity were further extreme challenges. Improved cooling and ventilations systems were added.

A major review by experts from China and Robbins, in 2016, concluded that a reasonable monthly progress rate should be only up to 240m, or half that contracted. The two drives were completed in late 2018 and early 2022, respectively.