Robbins TBM equipped for high pressure water control 03 Aug 2017

Like many in the tunnelling industry this year, Lok Home, President of The Robbins Company, found himself reflecting on news that Elon Musk was showing interest in excavation technologies along with his other lean technological approaches that grab the headlines for rocketry, electric vehicles and a HyperLoop transport system.

Pre-excavation grouting must seal against 30 bar pressure

Reflecting on Musk’s plan to come into tunnelling, as a facilitation step to create underground space, Home welcomed the entry of more visionaries into the industry, commenting that advance rates in tunnelling could be “significantly improved” with the injection of development money into the sector. Such funding however is scarce, with most tunnel contracts going to the lowest bidders, and they in turn seeking the lowest price equipment. “That is further discouragement to development,” he noted.

Going further Home stated that nearly all tunnels are heavily specified to reduce risk for the project owners and adding a further barrier to new underground development. “As a result, the industry has been slow to improve advance rates, but with Musk bringing the issue into the spotlight, perhaps things will change.”

While noting the types of challenges discussed, innovations are taking place in the industry, such as multifold steps with chemical grouting and grouting technology and developments for automating cutter and bit changes as much as possible, said Home. One such instance is the Delaware Aqueduct repair project under the Hudson River in New York where both enhanced grouting and probe-drilling systems are being employed on the single shield TBM to deal with high water pressure challenges.

The industry is working on and more work is needed to improve the integration of grouting cycles with excavation. Home said, “I do not see the future of rock tunnelling under high water pressure being left to divers to change cutters or repair cutterheads. We all know it is not cost effective to send divers to work in confined spaces and under pressures of up to 10 bar.”

Alignment of the Delaware Aqueduct bypass

Delaware Aqueduct repair project by the New York Department of Environment Protection will fix a leakage problem in a 1940s water supply tunnel under the river using a TBM to excavate a 3.8km long bypass tunnel. The contract was awarded to the Kiewit/Shea JV and McMillen Jacobs is the project consultant.

The alignment of the new bypass tunnel passes some 183m (600ft) below the Hudson River through mainly shale and limestone geology with the groundwater head ranging from 213m to 267m under the highest cover (Fig 1).

The 6.8m diameter Robbins TBM procured for the project is designed to perform probing and pre-excavation grouting (PEG) under potential groundwater pressures of up to 30 bar based on a 20 bar hydrostatic pressure x a 1.5 factor of safety. During development, the client, consultant and contractor were “heavily involved” in the specification and design of the machine, an approach seen as critical to ensure the best chance of project success.

Factory acceptance of the TBM was in February 2017 and the machine is assembled on site ready for launch.

Home has described the Delaware Aqueduct repair project as “a flagship project for what I hope will become more common practice in the industry” and towards a more reasonable bid environment seeking procurement of specialised TBMs with greater whole life cost benefits and within which the machine and excavation systems are considered an asset under development, rather than low cost regimes continuing to look for the cheapest possible options.

TBM ready for drive under the Hudson River

For the Delaware Aqueduct project, the Kiewit/Shea JV was willing, said Home, to “move forward with several new developments”, including a chemical grouting approach to handle high water pressure conditions and to allow both TBM operation and cutter changes at atmospheric pressure. “These technologies have the potential to reduce downtime and increase safety.”

Home believes such approaches present a “significant step forward for our industry” in using grouting systems capable of working under high water pressure, rather than requiring EPB or slurry machines. He adds there have been “halfway attempts” towards a grouting solution of combining the procedure with pressurised TBM excavations, such as on the Arrowhead and Lake Mead Intake No 3 water tunnel projects in the US, “but these have come at high cost and sometimes long delays.”

Home and a Robbins Project Manager Martino Scialpi discussed the approaches being undertaken at Delaware in more detail with TunnelTalk.

TBM designed to meet high demands

“The concept of totally relying on grout to hold out water in the TBM area is relatively new,” said Home. “Particularly advanced is the concept that the TBM is designed to hold 30 bar pressure while the grout is injected and for the pressure to drop to zero by releasing water before boring commences.”

He added: “In essence, both Arrowhead and the Hallandsås railway project inSweden ended up essentially relying on grouting and ground freezing to control water ingress but had a pressurised TBM as a sort of back-up. Neither of these projects was particularly successful with regards to coming even close to their original schedule for completion.”

Multiple-steps to seal TBM against high water inflows

In terms of the concept being deployed at Delaware, Home said that more generally it should be possible to use pre-excavation grouting for projects facing design plus safety-factor water pressures of more than 30 bar and that, a single shield TBM would most probably be used. Robbins has patents pending related to the technology said Home. “We will see the concepts and new designs put to use at the Delaware Aqueduct project and will plan to expand on this technology from there.”

The pre-excavation grouting system is designed to grout two holes at the same time with double-acting piston pumps capable of 69 bar (1,000psi) maximum pressure and 190 litres/min (50 gallons/min) maximum pumping capacity. The TBM is equipped with two grout mixing and pumping plants.

Drilling and grout injection can be achieved from16 different positions with an additional 14 drill collars built into the rear shield body. Scialpi believes this to be an industry record for a hard rock shielded machine. The two drills installed permanently in the machine shield are able to operate at the same time and each through a full 360 degree rotation. An extra potable drill can be mounted on the erector if needed.

The compact drills are equipped with water powered, down-the-hole hammers to achieve minimum deviation drill holes of 61.5m (200ft) and longer.

Scialpi tells TunnelTalk this is “a much more efficient alternative” to top hammer drills. “As far as I know, besides some testing on other jobs such as La Maddalena tunnel, in Italy, the Delaware project will be the first TBM to rely extensively on this technology.”

The PEG system is one of two grouting systems on the TBM, the other being a two-component system to backfill the annulus behind the segmental concrete rings. For safety in the high head environment, the TBM has the ability to backfill the annular gap through the tailskin of the shield rather than through the segmental lining.

Erector and aft drilling systems

Discussing the approach to the project, Scialpi told TunnelTalk: “This is a drilling and grouting project, as much as it is a TBM boring project.” Systematic probe drilling and inflow measurement is mandatory along the full length of the bypass tunnel and the aim is to have the drilling and the grouting tasks integrated as routine activities and while water inflows are “continuously negotiated”, employing the powerful sealing capability of the TBM for when the machine is stationary for grouting cycles or in an emergency situation, and using an installed high water pumping capability of up to 9,500 litre/min or 2,500 gallon/min to control water ingress until the grouting processes can take over the water control management.

When presented with high ground water inflows a rapid five-step sequence would be performed to seal the single shield and activate the TBM’s powerful pumping capability. The five steps are: close knife gates over the muck chute; retract the conveyor frame; retract the belting out of the excavation chamber; retract the bulkhead sealing plate; close the stabiliser doors.

The seals for the main bearing, bulkhead, articulation and tail, are all designed to hold up to 30 bar static pressure, explained Scialpi. This he believes is the highest rating ever achieved for a TBM.

A new main bearing sealing system has been engineered with multiple rows of traditional lip type seals, which are flushed and lubricated with grease, and emergency inflatable seals that are not in contact with moving parts of the sealing system during boring.

These high specification features are presented in greater detail in a technical paper presented and published in the proceedings of the 2017 World Tunnel Congress. All will be put to the test when the TBM is launched into its 3.8km long Delaware Aqueduct report tunnel drive in the coming weeks.