The McLeod River Bridge is a four span bridge with 33.5 metre long prestressed concrete girders supported on reinforced concrete piers and abutments. The pier foundations are spread footings keyed into shale bedrock. Over the past 25 years the river has shifted within its bed and now strikes the piers at an angle of approximately 40 degrees. This angle of attack has resulted in scour at the piers, particularly at the centre pier (Pier 2). At Pier 2 the scour level adjacent to the pier extends to 1.5 meters below the bottom of the footing. In January 2000, Alberta Transportation decided that a permanent structural solution was required to protect Pier 2 against failure due to scour and that the pier required protection prior to spring break-up. The time frame available to carry out the design and construction of the required work was less that 12 weeks. This aggressive schedule could only be achieved through close co-operation between the client (Alberta Transportation), consultant (UMA Engineering), environmental approving agencies and those contractors invited to tender for the work on an invitational tender basis. A structural solution, involving the use of large drilled piles into each end of an extended pier shaft was developed to underpin Pier 2. The underpinning was designed to resist ice loads striking the pier at angles of up to 40 degrees. After the completion of the design, a decision was made to delay the construction of the underpinning until the fall of 2000 due to a mild winter and the anticipation of an early spring break-up. Construction began in September 2000 and was completed in December 2000. The cost of the work, including the placing of rock rip rap, was approximately $800,000. (Author/publisher)
Abstract