Abstract:
Out-dated utility infrastructure is an issue arising increasingly across the United States. In the San Francisco Bay area of California, the compressible soft silty clay, frequently including organic materials, is commonly referred to as ‘Bay Mud’. Steel lattice transmission towers with reinforced concrete foundations, some on timber piles, have been erected on this Bay Mud since the early 20th century. Now, after nearly a century of erosion and subsidence, concrete can be found partially exposed and providing a fraction of the uplift capacity it had been designed to resist. While replacement of these towers and foundations is inevitable, the design process can span several years. Due to potential safety risks, it is imperative that integrity of the tower is maintained until the replacement solution is available.
This presentation examines temporary remediation solutions for lattice tower concrete foundations utilizing helical piles. Helical piles typically can be installed quickly with smaller, more mobile equipment. Helical piles are ideal for difficult access situations, challenging subsurface conditions, and deep bearing strata. Helical piles can be designed to be installed thru the subsiding new Bay Mud soil, anchoring into the older Bay Mud or sand alluvium below providing additional tension capacity. The presentation addresses unique design challenges from each site such as highly degraded concrete surfaces, unknown conditions and lengths of existing timber piles, varying reveal heights, and existing grade beams and other obstructions. The resulting designs consisted of helical underpinning piles with custom brackets and/or beams and epoxy anchors connecting to the existing concrete.