Reliability based design of driven pile groups using combination of pile driving equations and high strain dynamic pile monitoring

Reliability based design (e.g., load and resistance factor design – LRFD) aims at meeting a maximum permissible probability of failure (target reliability) for engineered systems or major elements thereof. For deep foundations, such as driven pile groups, statistical parameters of the random load and target reliabilities are naturally defined for entire pile groups, while pile driving criteria for stopping pile advance are required for individual piles. We present an approach using dynamic equations (e.g., Gates) and dynamic monitoring (e.g., PDA/CAPWAP) for estimating axial pile resistances. Dynamic equations are site-specifically calibrated to dynamic monitoring results from test pile programs, for example, and resistance estimates of production piles from equations (available at all piles) and monitoring (only available at monitored piles) are combined by best linear unbiased estimation (BLUE). Resulting resistance estimates and uncertainties of all piles in a group are further combined to obtain LRFD resistance factors Φ for pile groups as well as explicit pile driving criteria for individual piles. An iteration procedure is presented to account for the possible presence of previously driven piles in a group. A practical example and charts of Φ as a function of the degree of monitoring (percentage of piles monitored in a group) are used to demonstrate and discuss results.