Helical Pile Underpinning of a Raised, Wooden Restaurant

Foundation repairs utilizing helical piles are common within the industry. In many instances, helical piles are selected for remediation due to their ability to be installed in areas with limited construction access and for their ability to carry high axial loads. This paper explores a case history of the underpinning and re-levelling of an existing structure originally founded on timber piles. A raised, one-story wooden restaurant with a large outdoor deck was constructed and initially founded on timber piles. Shortly after construction, the building began to experience excessive settlement. A helical pile system was selected to replace the existing timber pile foundation due to site constraints and cost of repair. The design aspects of the existing structure repair proved challenging. The downdrag of the existing timber piles was a concern, as this added to the overall axial load that the helical piles were required to support. In comparison, due to the relatively small shaft size used for underpinning, helical piles experience minimal downdrag. Through various analyses, the resulting conclusion was to completely disconnect a majority of the timber piles from the structure and abandon them in place. However, the building foundation still needed to provide lateral resistance due to high wind loads in the area. Due to the installation constraints, the helical piles were limited to a small diameter with low lateral capacity. However, the helical piles alone were unable to provide the required lateral capacity. In order to achieve lateral resistance and mitigate the effects of the timber pile downdrag, select timber piles were left in place and secured to the foundation utilizing wood blocking that effectively created a box around the pile heads, allowing for free vertical movement and while resisting the required lateral forces.