Standley Lake Improvement: Shafts & Tunnels

Standley Lake Improvement: Shafts & Tunnels

Westminster, Colorado

The Standley Lake Dam Improvement Project included the complete replacement of the outlet works, a new spillway with roller compacted concrete drop structures, enlargement of the toe berm, valve house construction, and the abandonment of the existing outlet works.

Standley Lake Dam Improvement: Shafts & Tunnels

The underground portion of the Standley Lake Rehabilitation was comprised of two intakes, a shaft, and an outlet tunnel. The 100-foot deep, 35-foot diameter shaft, located on the north abutment of the Standley Lake Dam, had to be watertight. Primary support for the shaft during excavation consisted of shotcrete, rock dowels, and steel ribs. The final support was a 27-inch thick reinforced concrete lining. From the shaft, two 72-inch steel intake pipes were microtunneled out into the lake at lengths of 1,246 and 636 feet.

In both cases, the MTBM required an underwater retrieval. The outlet tunnel, 962 feet long and 11.5 feet in diameter, was constructed in swelling claystone that quickly slaked into mud when exposed to air and water. The tunnel was driven with a roadheader from a downstream portal to the valve shaft. Ground support consisted of shotcrete and lattice girders. Upon tunnel completion, 104-inch diameter steel pipe was placed in the tunnel and the annular space was filled with cellular concrete grout.

Deere & Ault’s staff prepared the Geotechnical Baseline Report, designed the microtunnel lake taps, valve shaft, and outlet tunnel, and provided construction engineering for the shaft and all underground work.

Clients:  FRICO, Cities of Westminster, Northglenn, and Thornton

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