Geotechnical Issues Associated With the Design and Construction of the Middle River Bridge

The existing short single span, single lane, concrete bridge constructed in 1930 across the Middle River in the Town of Mc Kellar, Ontario required replacement as a result of age and sub-standard approach roadway geometrics. In 1986, a geotechnical investigation was undertaken to upgrade the existing bridge. This investigation was undertaken primarily within the river since the new bridge structure was contemplated on an alignment with improved geometrics. The proposed structure was designed but was never constructed. In 2009, the need for a new structure was revived with the idea of placing it on the existing alignment with minor alignment modifications. However, as a result of buried timber being encountered during the geotechnical investigation, conventional drilling had to be abandoned and an air track rig used to ascertain the nature of the subsurface conditions. Thirty two (32) probe holes/boreholes were undertaken to define the preferred location of the new abutment piles. During bridge construction, the designed batter piles slipped along the steeply dipping rock surface at the west abutment and had to be retained by a massive deadman anchor. This paper addresses the issues encountered and concerns raised during the geotechnical investigations and how these were addressed and resolved. This site although small exhibited challenging ground conditions resulting from the lack of historic evidence, physical constraints imposed on the geotechnical investigation by the river and surroundings, and overall expenditures