Discussion of “Monitoring the Foundations of a Bridge during Construction and While in Service”

The authors presented the results of a very interesting case study on the monitoring of loads supported by piled bridge piers, which are typically designed to rely solely on the resistance provided by the piles in the design of the bridge substructure.One can rationalize that a piled bridge pier, consisting of a group of piles inserted in the ground and capped at their top with a pile cap, may be similar to a piled raft foundation.Piled raft foundations traditionally provide support to superimposed loads through a combination of the resistance provided to the raft in contact with the ground and the resistance provided by the piles embedded in the ground.The reason for not designing a piled bridge pier foundation similarly to that of a raft foundation may have been because (1) the area of a pile cap is often much smaller than that of a raft, (2) the soil below the pile cap may not be considered to be relied upon for support through the life of a bridge, (3) the pile cap may not be regarded as a load-supporting structure for pile-supported bridges, or (4) neglecting the resistance by soil below the pile cap is conservative.As determined by the authors, neglecting this pile cap support may result in a significant underestimation of the resistance that, had it been considered, could have led to a less costly design.The evaluation was undertaken through instrumentation of Pier 7 of the Sagamore Parkway Bridge over the Wabash River in West Lafayette, Indiana, during construction and while in service.The subsurface conditions at and around the vicinity of Pier 7, along with the sequence of construction of the substructure and the superstructure units associated with Pier 7, were provided by the authors.The instrumentation used, their layout, details of the monitoring sequence, and the monitored loads compared to those determined for the substructure design were provided.In addition, details of the settlement of the pier during construction and in service were addressed.The results of the study undertaken demonstrate the importance of pile caps in contributing to the resistance afforded by the bridge substructure that is not subjected to scour and loss of ground.Some aspects of the determination of loads applied to the pile cap by the authors, along with general comments on the depth of pile termination, warrant some discussion.These are addressed as follows.