CS10B - Concurrent Session 10B: An Innovative Approach to Spillway Weir Stabilization Utilizing Underreamed Drilling Methods Adapted from the Oil Industry

Failure of traditional friction anchors at low capacities in high plastic clay shales resulted in an innovative solution by drilling an underream in the anchor bond zone using drill tools adapted from the oil industry. The proposed technical paper will detail the associated challenges and successes of an anchor study program and production anchors utilizing an end bearing anchor in lieu of the traditional side friction anchor, a technique not practiced in over 30 years. Project features include a 560-foot-wide uncontrolled concrete, gravity supported, ogee spillway weir underlain by high plastic clay shales (HPCS). Risk evaluations of the spillway denoted sliding instability of the weir monoliths during unusual/extreme loading conditions. Post-tensioned strand anchors were proposed and have been installed to address the sliding potential of the weir.

An anchor study program was developed to improve understanding of the skin friction capacity and potential creep of the HPCS. Initially, 7-strand anchors were installed to determine the anchor bond zone capacity. During testing of the installed anchor, excessive movement was observed of the anchor when applying the alignment load. This led to the complete failure and extraction of the anchor where a slickened surface was observed along the bond zone, grout column. Due to the failure of traditional friction anchors, additional techniques such as post grouting of the bond zone and an underreamed bond zone were tested. An oil field supply company recommended and supplied the underream tool best suited for the drilling conditions. The underreamed bond proved to provide the required load capacity needed for the post-tensioned anchors.

Seventy-three production anchors utilizing underreamed bond zones have been installed to stabilize the ogee weir. The production anchors consist of 12-strand post tensioned anchors utilizing a 21 feet long underreamed bond zone. Items of discussion include drilling techniques, methods used to determine underreamed diameter, and results from anchor load tests, including extended creep tests. Also, two types of instruments to measure the long-term performance of the anchors were installed and the results to date will be presented. The findings from this project resulted in a new method to install high capacity post tensioned anchors in weak, high plasticity clay shales that could not be done with conventional anchors.