Structural pier repair, primary load path change from cementitious to epoxy grout at the Keck 1 telescope

Each of the Keck telescopes incorporates a 3” thick layer of grout which serves as the primary load path between the steel azimuth journal—the telescope’s running surface—and the main body of the reinforced concrete pier. Keck has observed increased journal deflections of the Keck 1 telescope under load. These increased deflections affect telescope pointing and can cause azimuth drive faults. Investigation into the source of the deflections found deterioration of the cementitious grout, which is believed caused by a combination of exposure to oil and cyclic loading from the telescope. The JPL team responsible for the Deep Space Network antennas solved a similar issue by changing from a cementitious grout to an oil impervious epoxy grout. Following detailed introduction, history, and background of the issue, this paper presents: material qualification testing of the replacement epoxy grout, procedural validation leading up to a trial repair of a 16-degree section of the Keck 1 pier, and lessons learned from the trial repair which occurred in October 2023—including preliminary performance monitoring. Note: the potential for material creep associated with epoxy grout is one specific area of interest for the new material, which will be monitored and measured for approximately 1 year before a full-scale repair begins in March 2025.