THE NEW SUSQUEHANNA RIVER BRIDGE FOUNDATION DESIGN—A STUDY IN CONTRASTS

The Pennsylvania Turnpike is carried across the Susquehanna River and Calver Island below Harrisburg via a bridge constructed in 1949. The bridge is in good condition; however, it is no longer capable of handling projected increases in traffic volume. A precast segmental bridge, the first to be built in Pennsylvania, is currently under construction ~100 ft upstream of the existing bridge. The existing bridge is on shallow spread footings founded on strata of the Gettysburg Formation which floors the river channel. In contrast, the new bridge uses drilled shaft foundations since their construction minimizes disturbance in the channel and to potential archaeological sites on Calver Island. The steeply dipping strata are composed of carbonate cemented sandstone, siltstone, and shale that filled a narrow rift basin. Strata were intruded by diabase sheets that thermally altered the host rock on the west side. In contrast, strata on the east side exhibit no thermal alteration and are deeply weathered. Unconfined compressive strength values of the bedrock range from 5 MPa at the east abutment to 172 MPa at the west abutment. Significant differences in engineering properties of the strata between the east and west sides of the project area raised the question of whether the weaker strata could generate the necessary load-bearing capacity via sidewall skin friction. Furthermore, available bearing capacity calculated using empirical design methods and the unconfined compressive strength values of rock cores, resulted in excessively deep shaft embedment lengths, rendering drilled shafts prohibitively expensive. To assess the feasibility of using drilled shafts with shorter embedment lengths, the Pennsylvania Turnpike Commission, for the first time, authorized Osterberg CellTM load tests to be performed during the design phase. This allowed a comparison between actual skin friction values and those calculated using empirical methods which are typically used for shaft length design. Since site specific load test data were available, a reduced factor of safety was permitted, which, in conjunction with the load test data, resulted in allowable side friction values up to 300 percent greater than those calculated using empirical methods. Using these data the shaft lengths were shortened, resulting in significant savings.