Post Grouted Single Bore Multiple Anchors at Hodenpyl Dam, Michigan

Despite the installation of a row of anchors in 1996, the right downstream retaining wall of Hodenpyl Dam, MI, continued to move inward and downstream. The remediation design called for installation of additional anchors. However, there were concerns about creep-induced relaxation of the anchor load over time given the existence of high-plasticity clays at the site. The solution was the installation of post grouted multi-anchors along the base of the retaining wall (Single Bore Multiple Anchors — SBMAs). SBMAs utilize several "unit" anchors within the same borehole, each with its own short efficient bond length positioned at staggered intervals along the bond zone. This staggered arrangement allows each unit load component to be transferred to the soil in a controlled manner over a discrete length of the borehole, thereby producing a very efficient load transfer mechanism. A larger factor of safety against creep is therefore attainable using SBMAs as compared to that provided by conventional tendons. One sacrificial anchor and 13 production anchors were installed. An innovative testing setup and program were developed to allow extended creep testing of the sacrificial anchor followed by load testing to 2.8 times the design load without anchor failure. This paper describes the design and construction of the SBMAs, the load testing setup, and results of extended creep load testing.