One TBM for two-diameter Mill Creek alignment 24 Oct 2019

Jonathan Rowland for TunnelTalk

A Robbins TBM designed to convert from 37.5ft to 32.5ft (11.6m to 9.9m) diameter midway through excavation will be used to excavate the 5 mile (8km) Mill Creek drainage relief project in Dallas, Texas, USA. The change-over will be implemented underground and allows the project to utilise one machine for the entire alignment, lowering project costs and reducing the construction period.

Fig 1. The 5 mile route with five intakes and an outfall
Fig 1. The 5 mile route with five intakes and an outfall

The upstream 3.24 miles (5.2km) are designed with a circular cross section for a peak flow of 15,000ft3/sec (42 m3/sec). The downstream 1.76 miles (2.8km), running between the outfall shaft to the East Peaks Branch Intake (Fig 1), was designed originally with a horseshoe cross section to allow a higher peak flow of 20,000ft3/sec (565m3/sec). The horseshoe section was to have been excavated by the TBM initially and expanded by roadheader to create a flat invert. Two different sets of formwork would also have been required to cast the final lining for both the round and horseshoe profiles.

This process would have been time consuming and costly. As a result, a TBM was developed that could transition from the larger to the smaller diameter underground, allowing one machine to bore the two required cross-sections. To achieve this, the cutterhead has been adapted so that the spacers, marked in blue and orange in Fig 2, can be removed and the bucket lips adjusted to convert to the smaller diameter.

Fig 2. Arrangement for changing the cutterhead diameter
Fig 2. Arrangement for changing the cutterhead diameter

To make the conversion, the TBM will complete the 37.5ft diameter (11.6m) alignment then back up about 75ft (21.3m) to the transition area (Fig 3). The outer wraps on the gripper shoes, cutterhead support side stabilisers, and the upper shield sections will be removed, followed by removing the outer cutterhead segments and cutterhead spacers. The outer segments are then re-bolted directly onto the centre section, creating the 32.5ft (9.9m) dimeter cutterhead (Fig 2).

The outer cutterhead segments and cutterhead spacers will be retrieved to the surface through a 20ft (6.1m) intake lateral located in the transition area (Fig 4). Lifting points have been pre-designed and the parts made in a modular fashion, so that simple lifting gear and jacks can be used to make the conversion within the confines of the tunnel. The TBM will then continue the excavation, beginning with a 2% uphill bore until the crown is aligned (Fig 4). The crown will then be smoothed after the TBM has advanced.

At depths between 100ft and 150ft (31m - 46m), the route passes through Austin Chalk with an unconfined compressive strength (UCS) of 1,800psi to 4,450psi (12.4MPa to 30.7MPa) with the potential of encountering natural gasses (Fig 5). As a result, probe drilling is mandatory.

Fig 3. TBM disassembly and relaunch chamber
Fig 3. TBM disassembly and relaunch chamber

Ground support, comprising eight 13ft (3.9m) long rock bolts on 5ft (1.5m) centres with wire mesh and channel straps, is expected to be required in some areas and the drainage infrastructure will be finished with a 15in (380mm) thick cast-in-place concrete lining. MSP Structures is suppling a telescopic shutter for the final lining, which is also capable of changing in diameter in the transition area to cast the lining in both cross sections.

The project provides flood protection for east Dallas, reducing the risk of major flooding that regularly hit the region, most recently in 2012 and 2015. The need for additional drainage was first identified in studies conducted between the 1970s and 2000s by project designers Halff Associates with tunnel design subcontracted to Jenny Engineering, now part of COWI North America. Initial design work for the current project began in 2009, when the option of a deep drainage tunnel that combined sewerage overflow from several locations was first decided upon as a cost-effective and quicker option. Almost a decade then followed, including various delays to the award of the construction contractor, before the US$206 million contract was awarded to the Southland Contracting-Mole Constructors JV and given notice to proceed in Spring 2018 with Black & Veatch engaged as Construction Manager.

Fig 4. Transition zone from larger to smaller diameter
Fig 4. Transition zone from larger to smaller diameter

Construction began on the outfall shaft, and the launch shaft for the TBM, in Summer 2018, and is now complete, along with the 40ft x 40ft x 250 ft (12.2m x 12.2m x 76.2m) horseshoe starter tunnel. The TBM arrived earlier this year and is currently being assembled, a process that is slightly behind schedule with excavation due to have begun but now scheduled to start in early 2020.

Based on previous work through the same geology, the project team expects TBM excavation to progress rapidly at about 80ft/day (25m/day), allowing the project to be completed on time in 2023. Muck haulage will be by a continuous conveyor system also supplied by Robbins and designed with special idlers to allow the conveyor, to bank around the 60% radius curves on the route (Fig 1). To mitigate delays to the start of excavation, a specific muck removal shaft has been sunk to allow round-the-clock hauling operations. In total, more than 1.5 million yd3 (1.15 million m3) of material will be excavated over the course of the project. To maintain the anticipated rate of advance, up to 200 truck/day will be needed to take muck away from site during the excavation phase.

Fig 5. One tunnel, two diameters, through the Austin Chalk
Fig 5. One tunnel, two diameters, through the Austin Chalk

In addition to the outfall and muck removal shafts, there are five intake shafts linking to the main tunnel and a shaft with three submersible pumps. The shafts vary in diameter from 14ft (4.3m) for the intake shafts to 20 feet (60.1m) for the pumping shaft, and 47ft (14.3m) for the outfall shaft. With a ban on blasting within the city confines, shaft excavation is by excavators and hydraulic breakers. All intake shafts are offline with short linking tunnels or laterals to be constructed by roadheader and excavator. The 170ft (51.8m) deep pump station is located at the point where the diameter the tunnel alignment changes and will house three 10,000-12,000 gallon/day (37,850-45,425 litre/day) submersible pumps to dewater the tunnel during maintenance. During normal operation, the system is designed to be gravity driven, functioning as an inverted syphon.

To research the details of the article, TunnelTalk spoke with Milton Brooks, Senior Project Manager for the City of Dallas; Paul Smith, Construction Manager for Black & Veatch; Nick Jencopale, Project Manager for the Southland Mole JV; Todd Woodson, Senior Project Manager at Halff Associates; and Dennis Ofiara, Chief Engineer, The Robbins Company.


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