Standley Lake Dam Improvement: Shaft and 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.
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
