Page - 708 - in Book of Full Papers - Symposium Hydro Engineering

breaker to preserve the true free length even when surrounded by grout. In this case, the load is still maintained by the gripping wedges in the head (in the case of strands), or by the lock nut (in the case of bars), and attempts to expose such heads, or indeed to attempt, somehow, a lift-off test, would be inviting trouble. Fourthly, strain gages or load cells of adequate reliability were simply not available to provide long-term monitoring capability, and indeed are still not available today. Compounding this issue is the stark fact that there is still no completely reliable, calibrated NDT Method that can be used to check the residual load, or the integrity, of wire or strand tendons, fully grouted or otherwise. It may be noted, however, that some recent methods are showing promise for bar tendon assessments. Fifthly, contemporary construction records may not always be available for review and, even if they are, they may not contain vital pieces of information, the clues to the likely adequacy of the construction details to preventing long-term corrosion. A case in point here would be the static (bleed) and dynamic (pressure filtration coefficient) stability properties of the grouts: if either property were deficient, then bleed pockets would develop, especially in the tightly arranged “bundles” of wires so common in early legacy anchors. Sixthly, it is not uncommon for Owners to consider somehow exhuming production anchors and then closely examining the tendon for signs of corrosion. Exhumation is feasible if the dam is being deconstructed for decommissioning. This is a relatively rare occurrence and of course is not an option for evaluating legacy anchors in dams required for extended, long-term service. Alternatively, it is technically feasible – if sufficient access exists to allow for the mobilization of a major piece of drilling equipment – to overcore the entire anchor and remove it for inspection. However, this is a very difficult and typically costly option since the drilling method must be precisely colinear with the anchor, and the tendon will not permit the anchor to be removed in convenient “runs,” for example, of 1.5 or 3.0 m. Furthermore, the grout is typically damaged or lost in the drilling process, and the concern will remain if the particular anchor removed is actually representative in its condition of the others, or if any localized corrosion-induced damage will indeed be recognized. As a final point, it is easy to be critical of the contemporary standards used in the design and construction of legacy anchors. However, at the time of their installation, design lives of 25-30 years were considered satisfactory, whereas some of these installations are now over 50 years old. “Modern” anchors (with Type I Corrosion Protection) are children of the late 1990’s, and a great deal of knowledge has been accumulated since then fuelled, in part, by our current attitudes to risk assessment such as inherent in the Potential Failure Modes Analysis process. 4. KEY FACTORS IN RISK EVALUATION DURING A PORTFOLIO RISK ASSESSMENT PROCESS In the case where an Owner is responsible for a number of projects dependent for stability on prestressed rock anchors, it is logical and essential to conduct a preliminary screening process to identify and prioritize the most worrisome projects. This is akin to the process first adopted by the U.S. Army 708