Abstract:
The Black Hawk Bridge carrying IA-9 over the Mississippi River at Lansing, Iowa, is currently being replaced. An important connection between northeastern Iowa and southwestern Wisconsin, the replacement structure is a 1,352.4-ft 3-span continuous truss including a 778-ft main span. The truss structure is being supported on single rows of large diameter drilled shafts socketed into very weak sandstone bedrock. Challenging subsurface conditions exist with respect to both design and construction of the drilled shaft foundations. Specifically, the interface between medium-dense to dense overburden sand and the underlying sandstone bedrock, which coincidentally also classifies as medium-dense to dense sand per SPT N values, is of particular interest. To address uncertainty with respect to design and construction, a full-scale sacrificial construction-phase demonstration and load test drilled shaft is being constructed and tested using the bi-directional method. This presentation describes the challenging subsurface conditions, the associated uncertainties, demonstration drilled shaft construction, and load test results. This topic is pertinent to sites where bedrock is of such low strength or quality that it is difficult to adequately characterize by traditional exploration methods, yet economical foundation performance bearing in these materials is still essential. Such conditions are known to exist in California.