Settlement Monitoring of Large Box Culvert Supported by Rammed Aggregate Piers — A Case History

Subsurface conditions at the site of a large box culvert constructed beneath a deteriorating bridge consisted of highly compressible alluvial clay. Concerned with settlements estimated up to 50 cm, engineers considered completely removing and rebuilding the bridge structure. With estimated settlements still exceeding 10 cm, rammed aggregate piers were selected for installation beneath the box culvert to control differential settlement and prevent downdrag on the existing bridge pier foundations. Despite construction challenges including high water table, very soft soil conditions, and low clearance for machinery beneath the bridge structure, it was determined that the benefits of the box culvert and embankment (i.e. ease of future roadway expansion and continual service of the highway throughout construction) outweighed the cost of replacing the bridge structure. Approximately 250 rammed aggregate piers were installed in a grid pattern with lengths ranging from 2.3 m to 6.7 m depending on embankment fill heights. Performance was monitored with settlement plates and compared to predictions.