Creeping Suspicion

by Michael P. Bruen, Sr. Engrg. Geol.; Harza Engineering Co., Chicago, IL 60606,
Nicholas Pansic, Senior Associate; Harza Engineering, Co., Chicago, IL,
M. I. Schwartz, Chf., Engrg. Design; Fairfax County Water Authority, Fairfax, VA 22116,


Serial Information: Civil Engineering—ASCE, 1996, Vol. 66, Issue 5, Pg. 60-63


Document Type: Feature article

Abstract: High-capacity, epoxy-coated anchors were installed at Upper Occoquan Dam to increase the stability of the 70-foot-high concrete gravity dam and powerhouse under revised Probable Maximum Flood (PMF) conditions. The post-tensioned anchorage systems consisted of 56 multi-strand rock anchors with design loads of 700 to 1855 kips, averaging 1500 kips per tendon. A double corrosion protection system was specified to provide protection throughout the entire anchor length. During anchor stressing and testing, significant creep movement under constant loads equivalent to 133% of the design load was experienced and exceeded the requisite Post-Tensioning Institute (PTI) criteria. In addition to the creep phenomena, seating losses during transfer of the load to the end anchorage are at least 2 to 3 times greater than that which has been experienced with bare-wire strand tendons. On the basis of anchor test results, modifications were made to the anchor testing protocol, acceptance criteria, and the approach used for assessment of the long-term performance of the anchorage system.

Subject Headings: Anchors | Creep | Dam safety | Post tensioning | Load tests

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