VALLEY & RIDGE DOLINES - HOW TO REPAIR THEM

A large portion of the eastern United States, formerly mostly rural, is underlain by solution-prone carbonate rocks. Development pressures are creating a new awareness of the hazards associated with building on karst terrane. This paper will focus on the nature of karst in the eastern U.S., how it forms, how to recognize some of the precursors to sinkhole formation, and what types of failure mechanisms are likely to be encountered, particularly in relation to geologic structure.

A variety of karst occurrences in open fields, beneath roadways, beneath structures, utilities, detention/retention basins and septic disposal systems are reviewed and the special types of remedial measures that each particular problem may require are presented. Precursor warnings such as sagging roadways, cracked pavements, minor settlement and ponding water are examined.

The remedial measures considered range from doing nothing (ignore it) to inverted graded filters, transit-mix plugs after excavation and watering the throat, high- and low-mobility grouts and dynamic compaction. Some of the concerns of using certain types of remedial fixes, such as: 1) concrete plugs where shrinkage of the concrete may be problematic; or 2) the use of low-mobility grouts in clayey residual soils over karst where travel distances are often limited to borehole dimensions because of the effects of pore pressures; and, 3) the use of grout additives such as sand, fly ash and bentonite, are discussed.