Anchorage

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"Stressed foundation anchor systems are considered one of the most economical methods of increasing rotational and sliding resistance along the base of the dam."

"Prestressed anchors with double corrosion protection may be used to stabilize existing concrete monoliths, but generally should not be used in the design of new concrete gravity dams. They are effective in improving sliding resistance, resultant location, and excessive foundation pressure. Anchors may be used to secure thrust blocks or stilling basins for the sole purpose of improving sliding stability. The anchor force required to stabilize a dam will depend largely on the orientation of the anchors. Anchors should be oriented for maximum efficiency subject to constraints of access, embedded features, galleries, and stress concentrations they induce in the dam. Analyses of tensile stresses under anchor heads should be made, and reinforcing should be provided as required. Tendon size, spacing, and embedment length should be based on the required anchor force, and should be provided the geotechnical engineer for determination of the required embedment length. Design, installation, and testing of anchors and anchorages should be guided by information in “Recommendations for Prestressed Rock and Soil Anchors” (PostTensioning Institute (PTI) 1985). Allowable bond stresses used to determine the length of embedment between grout and rocks are recommended to be one half of the ultimate bond stress determined by tests. The typical values of bond strength given in the above referenced PTI publication may be used in lieu of test values during design, but the design value should be verified by test before or during construction. The first three anchors installed and a minimum of 2 percent of the remaining anchors selected by the engineer should be performance tested. All other anchors must be proof tested upon installation in accordance with the PTI recommendations. Additionally, initial lift-off readings should be taken after the anchor is seated and before the jack is removed. Lift-off tests of random anchors selected by the engineer should be made 7 days after lock-off and prior to secondary grouting. Long-term monitoring of selected anchors using load cells and unbonded tendons should be employed where unusual conditions exist or the effort and expense can be justified by the importance of the structure. In addition to stability along the base of the dam, prestressed anchors may be required for deep-seated stability problems. Non-prestressed anchors shall not be used to improve the stability of dams."

Best Practices Resources

Gravity Dam Design (EM 1110-2-2200), USACE

Trainings

On-Demand Webinar: Intro to Post-Tensioned Anchors for Dams

Citations:

Revision ID: 7182
Revision Date: 07/11/2023