Research on Composite Pile Behavior and Development of a Rock Strength Database

Project summary:

This project combined two related research needs integral to the successful implementation at the Minnesota Department of Transportation (MnDOT) of the LRFD design code, which allows for local calibrations: (1) testing of rock to determine material behavior; and (2) testing of steel/concrete sections to determine system behavior. Ultimately, the work will lead to more efficient and economical bridges and structures founded on composite-driven piling, drilled shafts bored into rock, or spread footings placed on rock.

Rock strength and stress-strain relationships are used in various projects related to the characteristics of rock formations to develop accurate engineering models. Based on this work and on performance monitoring, the American Association of State Highway and Transportation Officials (AASHTO) code allows for improved LRFD resistance factors and potential improvements to project safety or economy without excessive conservatism. As new behavior parameters and resistance factors are available, they will be incorporated into state practice. For example, MnDOT has several ongoing research studies on pile foundations relating to load transfer, downdrag/dragload, and monitoring of pile performance. Several types of strain gages are being evaluated in-situ, and laboratory testing is needed to evaluate the performance of concrete-steel composite sections. This effort also ties into the MAP-21 framework for structural health monitoring and asset management.

The project had two interrelated components: one associated with uniaxial and triaxial rock testing to determine strength and elastic parameters, and the other associated with composite pile stiffness and performance. Two separate project reports were prepared for each component.