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January 2017

Securing water supplies to Fort Collins, Colorado, USA

The VMT name is often associated with major tunnelling works, with long and often difficult drives. This is not however always the case. VMT recently supplied a TUnIS Navigation TBMLaser System to the Michigan Ditch Tunnel Project which was to secure water supplies to the City of Fort Collins, Colorado, USA.

Fort Collins utilises two main water sources one of which uses the Michigan Ditch, a 5 mile (8 km) long pipeline and open channel water course connecting the city to water sources in the high mountains. The water course passes through an area known as ‘The Mudslide’ where springtime ground movements often led to the need to repair the Michigan Ditch.

VMT in the tunnel againIn September 2014 however significant movements showed that a more permanent solution to the ongoing problems was needed. It was decided that, given the value of the water source to the town, a tunnelled option running beneath the problematic area was the right solution. The project was awarded to BT Construction.

The Rib & Lag design tunnel, located in a remote area at some 3050 m (10,000 ft) above sea level, was planned to be just 750 ft (229 m) long but required a route that would have to incorporate a tight curve with a radius of just 600 ft (183 m), as well as tight space within the 2.5 m (98 in) o.d. Akkerman 720 Series II TBM which left only a small window for the laser navigation system operation. What added to the difficulties of the relatively short drive was that it would need to excavate in mixed ground conditions comprising up to 15,000 psi compressive strength pegmatite rock, and fractured rock with clay seams. Akkerman provided the unit with a custom mixed-ground disc cutterhead.

The TBM did not come with a navigation system that could guide the TBM in such circumstances so VMT was approached to supply a suitable system.

TBM punches out of the mountainWith a planned advance of 6 m (20 ft) per day, excavation started in June 2016 with the final holing occurring precisely on target in September 2016. The difficult and varying ground conditions meant that ultimately an average of 2.5 m (8 ft) per day advance was achieved. Even then the tunnel was finished before the first snowfall of the season. The project team members credited not only the TBM operators, but also the VMT navigation system for the success of the project.