Urban development in karst and collapse-prone geologic environments

The development of marginal land necessitated by urban sprawl is producing undesirable consequences derived from inadequate design or construction, or both, of storm water systems in karst and collapse-prone geologic environments. The St. Peters, Missouri karst area is presented as an example. Sufficient knowledge of the geohydrologic setting, combined with proper planning of water discharge, is essential for engineers, developers, and property owners when developing storm water drainage systems. The design of community and individual storm water drainage systems, lot sizes, roads, and selection of vegetative cover are controlled more often by economics and aesthetics than by existing natural conditions. Frequently, the geohydrologic character of the bedrock, composition and stability of the surficial material, existing topography, and adjoining land use may not fully be considered. Improper design and location of storm drainage discharge leads to sinkhole development as well as increased erosion. In developed karst settings with thick loess deposits, residential lot sizes are frequently too small to control erosion; the structures and infrastructure tend to be located adjacent to valley walls and on steeply sloping topography. The vegetative cover is typically stripped, promoting slope instability, resulting in increased erosion and mass wasting. Road placement, structure location, and drainage design can direct surface water to infiltrate localized areas of surficial materials. This promotes adhesive dissolution in weakly cemented loess, resulting in slope instability and eventual collapse. It is clear that three factors influencing collapses are frequently overlooked in the development of marginal land areas. First, it is essential to recognize the presence of soluble bedrock that exhibits a high degree of secondary permeability. Second, loess is weakly cemented and it loses cohesiveness with water infiltration, making it more erodible and collapse-prone. Third, the hydrology of the site is closely associated with the bedrock and surficial material and significantly influences the occurrence of subsidence and collapse events.