- Innovative and cost-effective approach to remediate loose urban-fill for support of shallow foundations
- Use of geogrid reinforced stone subgrade to float shallow foundations over soft soils
- Design of steel sheet pile bulkhead to allow buildings to be constructed over the anchor system
- Cost-effective design
Brown and Howard Wharf extends from historic Thames Street into Newport Harbor in Newport, Rhode Island. The original solid filled stone seawall wharf was built in stages over 100 years ago and was most recently used as a small marina consisting of watercraft vessel storage during the winter or vehicular parking during the summer, dingy dockage, and fueling station.
Challenge
A new development consisting of three multi-story condominiums, a new boat house, a new pile supported marginal wharf, and a full-service marina were proposed for the site. The site is underlain by miscellaneous loose fill over the old organic harbor bottom. The development was originally designed using pile foundations with structural beams and slabs. The high foundation costs prompted a search for alternatives.
Solution
GZA investigated several ground improvement techniques to solve the problems and to allow for the safe construction of the building foundations. To use shallow foundations, the loose fill had to be densified in place. Excavation and replacement of unsuitable materials would require a large-scale earthwork operation, with deep excavations extending well below high tide. A combination of Deep Dynamic Compaction (DDC) and Vibratory Probe Compaction (VPC) were used to densify the fill soils in place and surcharging was used to consolidate the underlying organic soil. In addition, GZA recommended the use of a base reinforcement design for the subgrade soils located directly below the proposed foundations since the foundations were constructed 4.5 feet below high tide. The base reinforcement served as a working mat during construction and helped to limit differential settlements between columns. The reinforced mat allowed construction equipment to work on the site even when the subgrade was below the tidal level.
The new steel sheet pile bulkhead was designed such that the anchor rods were located below the new building foundations. The base reinforcement mat was located just above the anchor system to afford some level of protection for the anchor rods. The use of alternative ground improvement techniques for shallow foundation and slab on-grade construction instead of pile foundation and structural slabs provides a cost-effective solution to the client.
Benefit
The goals of the development program were met with an improved and cost- effective foundation design.