Introduction

In late February 2014, a crack was discovered at the Wanapum Dam Spillway. The project owner, Grant County Public Utility District (District) immediately consulted with Federal Energy Regulatory Commission (FERC), lowered the reservoir, and retained engineering consultants and construction contractors to address the problem. Through remarkable cooperation between the District, MWH Global, construction contractors, and regulators full operations were restored in March 2015, just over a year after discovery of the crack.

Project Scope

The extent and orientation of the crack were determined by drilling through the spillway. Complex numerical computer models were developed to assess the behavior of the monolith, determine applicable potential failure modes, and to evaluate the effectiveness of potential remedies.

Installation of Post-Tensioned Anchors (PTAs) was selected as the preferred method to repair the cracked monolith and to mitigate the potential for cracking in the rest of the spillway. The solution consisted of thirty-five 61-strand PTA tendons that rank among the higher capacity PTAs in the world and sixty nine PTA bar anchors.

Teamwork

Remarkable cooperation between the District, its engineering consultants, construction contractors, and regulators allowed for design, construction, and regulatory review and approval to advance concurrently, such that full operations were restored in March 2015, just over a year after discovery of the crack.

Project Challenge

The main challenge was to return the reservoir to functioning operating levels as quickly and safely as possible. Immediately after discovery of the damage, the water level was lowered to ensure stability while remedial measures were being designed and implemented.

Installation of Post-Tensioned Anchors (PTAs) to repair the cracked monolith and to mitigate the potential for cracking in the rest of the spillway required precise drilling to avoid critical features within the spillway structure.

Project Solution

The first remedial action taken was to drill vertical drains through the ceiling of the spillway gallery to provide pressure relief for the lift joints above the gallery. More than 100 lift joint drains were installed in the spillway monoliths. This work was followed by the installation of PTAs in the piers and ogee sections of the spillway. The PTA installation was sequenced to allow an intermediate pool raise in November 2014 where the reservoir level was increased from El. 545 to El. 562 (17 feet) in an effort to reduce operational impacts to the mid-Columbia river system, return the fish ladders to normal operations and protect culturally sensitive sites along the reservoir shoreline.

This project has been recognized by Engineering News Record (ENR) as the Northwest’s Best Project of the Year for 2015 in the Energy/Industry category.

Read the ENR Project Award here.