Engineering approaches to conditions created by a combination of karst and faulting at a hospital in Birmingham, Alabama

Abstract

Foundations for a major expansion and modification of a multistory hospital in Birmingham, AL, were founded on faulted and karst-dissolutioned dolomite. The foundation approach had to accommodate a high degree of uncertainty concerning local conditions due to limited access for exploration and extremely variable rock conditions. The scope of the construction included excavation of a subbasement into rock with associated tiebacks to support adjacent foundations, installation of rock-bearing shear walls and rock anchors under the existing hospital, and installation of rock-bearing caissons and wall foundations outside the existing hospital. Local complications included areas of highly shattered rock, a generally pinnacled rock surface with average relief of 3–6 m (10–20 ft), locally very deep cutters and pits, areas where dolomite was weathered to sand or weak rock up to 3 m (10 ft) thick, and pockets of flowing sand and mud near the rock surface. Because of the complexity of site conditions and limited initial access to the site, on-site geotechnical services required innovative approaches to gather additional information on the highly variable and ambiguous rock conditions and adapt detailed foundation design and foundation approaches to the actual conditions encountered. These approaches included triple-tube coring of shattered rock at selected caisson locations; development of a technique for installation of rock anchors into shattered rock, determination of required undercut depths, and remediation at individual foundations where rock was shattered, disaggregated, or steeply pinnacled; characterization of individual cutters by airtrack probing for remediation information in wall foundations; low-angle coring for cutter characterization in the tieback area; change in foundations from walls to caissons or caissons to mat foundations in select areas; and above all, careful judgment-based design. Limitations of characterization methods are also discussed. A fundamental understanding of karst processes and three-dimensional conceptualization was an essential part of the engineering required for this project.