Karst in Appalachia - A Tangled Zone: Projects with Cave-Sized Voids and Sinkholes

摘要

Although many people in the world live over karstic terrain, in the karst areas of the Appalachian Mountains they live over the remains of the largest mountains (above the ocean), which were thrown up in massive plate movement, and the accompanying volcanoes. The limestone rock, shale, coal, silt and clay layers laid down millennia ago have been thrust up into a tangled and contorted mess. In many places the strike of the rock is contorted, folded, and may go past vertical. When you drill into this mess, you encounter randomly placed hard rock, soft soil, aggregate layers, and complete voids. For the contractor who is attempting to build, he will find areas where it may take 15 or 20 feet of drilling in order to reach competent rock or hard cemented sand on which to found your structure. And that encounter may take place 50 or 100 feet underneath the surface. Occasionally you may never reach hard rock or competent soil. Drill holes may be wet or dry. Perched water tables and cave-sized voids are common. The designer of projects in karst areas needs to remain aware of the possible difficulties. Rembco does about 2/3 of its work in Appalachia. Decisions sometimes need to be made hourly, or even by the minute, about what action needs to be taken to prevent the loss of drill bits, the loss of casing, and the breakage of equipment, and about how to keep costs within reason. In this paper, we describe a number of projects which illustrate ground rules we have developed to overcome the variables. These projects may sound anecdotal, but when anecdotal evidence constitutes over half the company's work, it is an excellent foundation for approaching such problems. The illustrations come from drilled mini-piles, from soil nailed and anchored retaining walls, and from underground grouting carried out for many purposes including stopping and re-routing water.