Engineering Geologic Assessment Memorandum, Coyote Dam Spillway, No. 72-2 Santa Clara County

This memorandum presents results of our preliminary engineering geologic assessment of ongoing, progressive, fracturing and spalling of the concrete-lined spillway at Coyote Dam No. 72-2, located in the central Diablo Mountain Range, ~3 miles east of San Martin, in Santa Clara County.

The information in this memo supersedes similar Coyote Dam spillway data and information of the Engineering Geologic Data Report Nelson (2017), presented in Section 9.0 analysis and Discussion of Spillway Floor Engineering Geologic Issues of that report. The failure of Oroville Dam spillway
in 2017 caused dam owners to focus on the safety of existing spillways. Consequently, we decided to extract the information from Section 9.0 of the 2017 Nelson report and provide additional critical information in this memo.

Construction of the dam and spillway in 1936-37 involved mass grading and deep excavation of the spillway channel to create steep cut slopes located directly northeast of the lined spillway channel, which resulted in maximum cut slope height of up to 140 feet in underlying Berryessa Formation Bedrock. Figure 1 shows the concrete lined spillway. Figure 3 and Figure 4 show the depth of excavation in the vicinity of the spillway.

Since construction of Coyote Dam in 1937, the concrete lined spillway walls and floor have experienced progressive damage due to ongoing heaving, bowing and apparent landslide slope failure of the foundation bedrock.

Since 1985, when the District replaced a portion of the spillway concrete lining, ongoing distress has occurred as fracturing and spalling failures, along linear zones that coincide structurally with existing spillway floor foundation grade beams, installed during the 1985 liner replacement project.

The long term, historic failures of the concrete spillway floor appear to be due bedrock rebound forces, producing vertical upward earth pressures, initiated in 1936, when mass grading produced the spillway channel excavation and associated cut slope. Figure 4 shows a strong correlation
between depth of excavation of the spillway cut and rates of floor heave. Figure 2 shows the 1937 finished spillway cut slope (yellow shading) and mass excavation for the original spillway lining.