Monitoring the effects of construction on nearby sensitive structures

Chris Crowe and Peter McDonald

The West Gate Freeway Upgrade involved widening and construction of new access ramps for the existing elevated freeway in a congested area of South Melbourne, underlain by the complex geology of the Yarra Delta. There were significant constraints on the locations of the bridge piers and it was often necessary to construct foundations near critical sensitive structures and services, including an old, but still heavily used brick-lined sewer. In view of the risk of damage to such services, a conservative approach to the design and construction of foundations in the vicinity is taken to minimize ground movements and vibrations, for example substituting bored piles for driven piles, but at a significant increase in time and cost. In order to speed up construction and reduce costs, Douglas Partners on behalf of the West Gate Freeway Alliance undertook trials to predict the effects of pile driving on the surrounding ground to allow the use of driven piles in areas adjacent to sensitive structures. Subsequently, monitoring was undertaken during construction to verify the predictions and included inclinometers and a recoverable below ground vibration monitoring geophone. The monitoring demonstrated that ground behaviour was within acceptable limits and subsequent surveys of some of the services confirmed no damage.