Phoenix Project #2002
Construction of a new addition to Deerfield Academy’s Denunzio Building required the use of underpinning piers to provide support to its existing foundation. Excavation for the addition’s foundation was in close proximity to the existing foundation and presented the potential to undermine and compromise the existing structure. As a result, Phoenix was contracted to install 12 unreinforced concrete underpinning piers beneath the existing structure’s foundation. These piers prevented ground loss and provided the necessary vertical support to protect the structure.
Each underpinning pier was hand excavated and formed with dimensional lumber. As each lift was dug, rings of timber lagging were cut to fit and carefully installed to prevent the sloughing of soils that could lead to the undermining of the existing structure. The excavation and lagging process continued in vertical lifts until the designed bottom of each pit was reached. Once each pit was excavated and formed, concrete was placed to approximately 3 inches below the bottom of the existing footing and vibrated to ensure no air voids developed within the freshly-placed concrete. On the following shift, the intentional gap left between the pier and existing footing was filled with a low slump drypack mixture of cement, sand, and water to ensure full contact between the footing and underpinning pier.
To ensure that no damage would result to the sensitive structure above, a minimum of 10 feet between active underpinning pits (i.e those in which excavation has begun but dry packing had not yet cured overnight) was maintained at all times. Strategic sequencing of pier installation was critical to ensure that the optimum schedule was realized, while always satisfying the safe distance requirement. Overall, more than 100 CY of soil was removed by hand, with an equal amount of concrete placed within the 12 piers.