The First Church of Christ, Scientist, The Mother Church Original
Due to lowered groundwater levels in Boston’s Back Bay area, as a result of underground construction, leaking utility lines, and leaking basements, several area buildings have been affected by severe deterioration of the timber piles that support them. SGH was retained by The First Church of Christ, Scientist to assess the condition of the timber pile foundations at The Mother Church Original.
As groundwater levels decrease, tops of timber piles are exposed to air. Exposure to oxygen with high-moisture conditions is conducive to fungal and bacterial growth which can lead to decay, loss of building support, and consequent settlement.
We reviewed historic movement and groundwater level monitoring surveys performed at The Mother Church Original and determined that timber piles supporting the tower were at high risk for deterioration. Our observations at test pits confirmed that the piles were severely deteriorated. To mitigate future settlement, we recommended and developed a foundation underpinning and repair scheme consisting of two phases:
- Phase 1 included cut and post repair of approximately 100 existing timber piles that support interior and exterior walls of The Mother Church Original. The cut and post repair included cutting the top 3 ft of existing piles and replacing the cut sections with steel posts. In areas with section loss of more than half of the pile, we recommended the installation of jacked piles to replace the severely deteriorated timber piles.
- Phase 2 included underpinning the tower using micropiles. The approach involved cutting rectangular penetrations through the basement walls, installing needle beams through the penetrations, and supporting the needle beams with micropiles on each side of the wall. Flat jacks placed between the top of the needle beam and the top of the wall penetration were used to preload the micropiles and transfer about 80% of the tower load from the existing timber piles to the micropiles.