Abstract:
Active Soil Nail Wall Underpinning Solution for the Bruce Museum Project
Jonathan L. Ernst, P.E., Geosciences Testing and Research, Inc.
Les R. Chernauskas, P.E., Geosciences Testing and Research, Inc.
Martin B. Hudson, Ph.D., P.E., Turner Construction Company
Rolff B. Knobel, Turner Construction Company
Andrew K. Cho, Ph.D., P.E., P.G., Turner Construction Company
The Bruce Museum is located in Greenwich, Connecticut, which displays art and natural history exhibitions. A large addition was constructed adjacent to the museum where the new addition footings were proposed to be constructed up to 16.5 feet below the existing foundations. The soil conditions at the site consisted of highly variable sand and gravel, weathered rock and sound rock.
The Contract Documents called for traditional foundation underpinning. However, traditional underpinning on this project was considered highly risky for two major reasons: (1) deep underpinning pits would be required involving multiple levels and staging with tiebacks and (2) the highly variable soil and rock composition and elevations presented uncertainty in the design and construction of the pits. In lieu of traditional underpinning, GTR proposed an “active” soil nail underpinning system. A major reason that the soil nail underpinning option was feasible, related to the fact that the soil nails and facing could be installed slightly in front of the existing footings rather than directly underneath the existing footings. The soil nail wall design involved one to four rows of soil nails with nail positions and wall depth able to be adjusted “on the fly” as excavation progressed depending on the field conditions encountered. To create the “active system,” the soil nails were pre-loaded to mitigate wall deformations that would be associated with a standard passive soil nail wall design.
To evaluate the performance of the wall and confirm design assumptions, an extensive automated monitoring, testing and inspection program was proposed and executed by GTR to monitor deformations of the wall and existing structure, evaluate and confirm rock and soil bond stresses and ensure that each anchor was pre-loaded. Careful analysis, planning, and use of the observational method illustrates the importance of carrying engineering judgement throughout construction, including the potential for modifying design based on conditions observed. This paper presents a unique case study of the “active” soil nail underpinning system and its use as a safe alternative to traditional underpinning.