Foundation Performance of a 100-year-old Bridge

Abstract

The Miramichi bridges in New Brunswick, each of six 200-ft (61-m) spans, were built as part of the Intercolonial Railway by Sir Sandford Fleming in 1871–75. When construction of the Northwest Bridge started, it was found that early borings were incorrect and the layer of gravel and sand on which the piers were to rest was underlain by deposits of silty sand and clay–silt. The caissons of the piers were therefore enlarged in order to spread the load better. On observing settlements during construction, Fleming stopped the work and preloaded each pier in excess of the design load. He resumed construction when he observed that settlements had stopped.The level of the bridge piers has been measured periodically since 1914. Recent borings have been made, giving a basis for analysis of the bridge performance. Calculated primary settlements are compared with those measured during construction. Surveyed pier movements are adjusted to reduce errors, and compared with calculated secondary settlements. It is deduced that the piers have settled at a rate equal to or less than that indicated by the theory of secondary compression.The bridge is continuing to perform in a predictable and safe manner, providing an outstanding example of early Canadian engineering.