Harsens Island Boat House - Harsens Island, MI

Monday, May 12th, 2014 by Josh Kaiser

Challenge

October 2011: The subject boat house located on Harsens Island was built in the 1930s. The house design includes an indoor boat well in the left rear corner where a boat can dock within the home footprint through a garage door opening along the rear wall. The current owners noticed inward deflection of the partially submerged foundation wall that also served as the sidewall of the boat well. The foundation wall was poured concrete construction with concrete block and exterior brick veneer above. Foundation Systems of Michigan was contacted to propose a solution to stabilize and potentially straighten the wall over time. Given the unique situation of a partially submerged wall, this project posed several challenges for providing and installing a solution to remedy the problem. Some of the challenges specifically dealt with access to the wall and working within and above the water. Also, soil information was not available to assist in the system design.

Solution

Foundation Systems of Michigan proposed helical tiebacks to stabilize the existing 30- foot long foundation wall. Seven Model 150 (1.5-inch round corner square bar) helical tiebacks were installed with 10” single-helix lead sections. Four-inch diameter core holes were made through the foundation wall to allow advancement of the tieback shaft and couplers. The lead section with the 10” helix plate was dropped into an excavation on the back side of the wall and coupled to the shaft. This prevented cutting large holes through the foundation wall. Core holes and tieback installation were completed at 15 to 20 degree downward angles from horizontal. Each tieback was installed to a length of 18 feet into the exterior soil to achieve a torque-correlated ultimate capacity of at least 20 kips for a factor of safety ≥ 2. Due to limited access in the boat well, handheld equipment was utilized to advance the tiebacks. Wall plates were attached to the tiebacks on the interior face of the wall and a torque wrench was used for pre-tensioning. Despite encountering adverse weather and having to install the tiebacks on portable platforms over four feet of water, Foundation Systems of Michigan completed the work in just two days.

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