Down Deep in Durango

After decades of debate and hard work, completion of the Animas La Plata main reservoir in Colorado was down to just two intake lines.

Project deadlines stipulated that crews would begin filling the man-made lake bed by the end of April 2009. Contractor BTrenchless needed an effective and swift construction method for the rock tunnels through mudstone. The contractor decided on a hard rock trenchless boring machine, called the Robbins Motorized Small Boring Unit, to complete the job within the tight schedule.

“Not only is there a tight deadline, but the rock tunnels are connected to an intake structure. We’re making two mid air breakthroughs into a vertical shaft that require crane lifts,” said Tina Connett, project manager for BTrenchless. The design will minimize the time required to construct the intake lines.

Dating back to a 1968 decree, the Animas La Plata project is no short term fix. The $570 million project is being built to fulfill 1868 water rights obtained by the Ute Indian tribe, located on lands near Durango, CO.

“This project is a major milestone highlighting 50 years of work to get water into this area, which is very dry. Once complete, it will distribute water through 45 miles of pipelines down to the New Mexico border,” said Reid Tanner, project manager for EPC Construction (general contractor for the Animas La Plata Intake Structures).

Water stored in the Ridges Basin Dam on the Animas River will form a reservoir, called Lake Nighthorse, which will store 120,000 acre feet of water for use in surrounding areas, and will also serve as a recreational and fishing area. Filling of Lake Nighthorse began in spring 2009 and is expected to take up to two years to complete. The system is scheduled to go online by 2012.

The Bureau of Reclamation (BOR) first contracted the La Plata West Water Authority (LPWWA) to create a distribution system from the new lake to arid areas of La Plata County. Initial site work began in 2002, including construction of an inlet conduit, pumping plant and a dam in Ridges Basin.

Work begins

In 2008, work began on an intake structure consisting of a 118-foot deep intake shaft and two intake tunnels which would allow for dispersal of water from the reservoir. The shaft was completed by Anderson Drilling of Denver, CO, using a down drill auger. Corrugated steel was then set into the shaft and grouted in place, forming a 15-foot diameter wet well.

The LPWWA contracted EPC Corporation of Glendale, AZ, to design and construct other intake structures. The system included two tunnels, an upper and a lower, to convey water from the lake bed into the deep shaft. EPC completed 1,000 feet of open-cut pipeline installation in soft soils; however, core sample tests of the area revealed sections of rock. The moderately hard mudstone geology, ranging between 5,000 and 16,000 psi, required a method other than conventional open cut.

EPC sub contracted BTrenchless, a division of BT Construction Inc., Henderson, CO, for the remaining sections of the two tunnels – a lower intake of 370 feet and an upper intake of 130 feet. Once finished, the tunnels will transfer water from the reservoir to the 118-foot deep shaft, where it will be pumped into underground lines for distribution.

The contractors decided on a Robbins 48 inch diameter Motorized Small Boring Unit (SBU M) and Auger Boring Machine (ABM) setup for the two tunnels. “The length of the lower tunnel required accuracy only a tunneling machine could provide. We also needed disc cutters to provide the ability to cut the anticipated mudstone,” said Connett.

EPC had worked with tunneling machines in the past and determined they would be a good fit for the project. “We looked at a variety of methods, but felt that the Robbins machine was likely to do the best job given the conditions,” said Tanner.

Heavy lifting

Both tunnels required unique planning for breakthroughs, as the machine would hole through into the vertical intake shaft each time without support. A plan was developed to secure the SBU M and lift it out of the shaft after each bore using a hydraulic crane.

To start the bore, EPC drilled and blasted a 44-foot long launch pit for the 48 inch motorized SBU and auger boring machine. Excavation of the lower tunnel began in late February 2009.

The SBU M progressed well despite isolated difficulties including pockets of groundwater. “We did encounter groundwater that slowed our advance, but the Robbins team was a very good resource,” said Mark Wellensiek, BTrenchless project superintendent.

The machine broke through the 370 foot lower tunnel in early April 2009, within line and grade parameters. The unique hole through was 18 feet above the shaft floor, requiring the crane to perform a mid air lift of the 22,000 pound SBU M to bring it to the surface. A crane supported man basket was used inside the shaft to allow the crew to prepare the shaft wall for the break through, release the SBU M from its steel casing, and secure the machine to the crane for lifting.

After the breakthrough, BTrenchless grouted the exterior annular space around the 48 inch steel casing and then skidded the 36 inch diameter PVC carrier pipe into the tunnel. The interior annular space was then grouted as the final step to complete the tunnels.

As of early April 2009, the SBU M was on the surface and being readied to bore the 130-foot long upper tunnel. The second tunnel is at an offset angle to the lower and will break through into the shaft approximately 60 feet from the top of the shaft. BTrenchless anticipates the second launch will occur in mid April.

BTrenchless, a division of BT Construction, Inc. located in Henderson, CO, has been in business for more than 25 years. During this timeBTrenchless has completed numerous projects in Colorado and the Western United States includingmicrotunneling, tunnel boring, pipe jacking, guided auger boring, auger boring, pipe ramming, horizontal directional drilling, pipe bursting, sliplining and potholing.

EPC Corporation of Glendale, AZ, is a construction company offering general contracting, design/build, and construction management services. EPC specializes in commercial/industrial building, heavy/highway construction, utility construction, industrial maintenance, water/wastewater plants, correctional institutions, and power plants.

The LPWWA’s project is being managed by Bikis Water Consultants LLC. The approximately $6 million project to distribute water from the new lake has been funded by the following sponsor entities: Animas La Plata Water Conservancy District, Southwestern Water Conservation District, Colorado Water Conservation Board Ute Mountain Ute Tribe, Southern Ute Indian Tribe, and the Colorado Water Resource and Power Development Authority.

BTrenchless, (303) 286 0202
EPC Corporation, (623) 934-0225,
Robbins TBM, (440) 248-3303,

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