Franson Civil Engineers was retained by Gunnison Irrigation Company to prepare designs and provide construction engineering services for Dairy Dam. The dam is a zoned earthfill embankment with a 1,200-foot crest length and a maximum height of 35 feet. Dairy Dam provides water storage and pressure for an irrigation system while improving water management between two of the owner’s canals. Unique project elements included:
- Core Blanket: Soil tests indicated proposed core materials were dispersive, which tend to wash away upon contact with water. Our solution was to mix native materials with bentonite to create a core blanket, which form a barrier between the core and foundation.
- Cutoff Trench: The dam has highly permeable bedrock at the abutments. Typical fixes include a grout curtain or excavating a keyway through permeable zones. These options would have been expensive and time-consuming. A cutoff trench provided an effective and less expensive alternative. A large trenching machine cut a trench across the entire dam axis, which was then filled with an impermeable material.
- Plastic Concrete: Plastic concrete is concrete with bentonite added so the concrete will maintain its plasticity and “heal” if cracks form during any stresses. This material was used to fill the cutoff trench. The procedure is commonly used in Europe and is becoming more widespread in the United States.
- Geogrid and Riprap: Local rock sources did not meet durability requirements for riprap. A local gravel pit had an extensive supply of strong 3-inch to 6-inch waste rock. However, the rock was too small to effectively withstand wave action on the upstream face. We designed the use of an 8-inch deep HDPE geogrid that could be used on the upstream face of the dam to contain rock materials during wave action. This provided economical protection for the upstream face of the dam.