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Down Deep in Durango
Breaks Rock In Southern Colorado To Complete Reservoir Project
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.
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.