Choice Criteria for Bridge Foundation Types on Variable Soil Strata

Bridge designers are often confronted with non-uniform subsurface conditions where extreme variations in ground are present. These are very common in valleys and dry rivers where abutments are placed on rock and middle piers are supported on alluvial deposits. The challenge to the bridge designer is to decide which type of foundation and what depth to adopt in order to achieve an economic and yet safe design. This study highlights a protocol to assist designers to make the most appropriate choice regarding foundation type and depth. A case study of a bridge from south west of Saudi Arabia is considered for this study. A bridge consisting of two carriageways each 15 m wide and 200 m long supported on piers at 20 m spans is considered. The two abutments are elevated 23 m above the bottom of the valley. Twenty two boreholes of 20 to 25 m depth were conducted at different pier and abutment locations. The subsurface profile indicated different formations varying from silty gravel with sand to fragmented Schist and intact rock. The initial suggestion of the geotechnical advisers was to go for 15 m pile foundations to be placed on rock in all supports. It was found that an alternative foundation design can still be feasible with respect to allowable differential settlement and other criteria. Cost analysis was carried out to indicate the amount of savings using this protocol.