Wanapum Spillway Crack Development: Root Cause Analysis

In February 2014, significant movement of Spillway Monolith No. 4 was observed at Wanapum Dam. The movement was caused by cracking and vertical and horizontal displacement along a submerged lift joint. This paper describes the spillway incident and Forensic Investigation/Root Cause Analysis (FI/RCA) that was performed to identify the triggering events and contributing factors. The FI/RCA determined that the crack initiated due to tensile stresses in the upstream face, exacerbated by concrete fatigue and uplift intrusion. The spillway was stabilized with post-tensioned anchors and lift joint drains.
Lessons learned during the FI/RCA process include:
- Thermal tensile stresses, often neglected during design, can be high enough to initiate cracking.
- Relying on moderate tensile strength may be inappropriate if the affected section is not stable if cracking occurs.
- Monoliths within the body of the dam may be vulnerable to overturning if drainage measures do not extend into the dam.