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~h ESPEY, HUSTON b ASSOCIATES, INC. :. 3.0 SUBSIDENCE MONITORING PLAN Monitoring the mining of the four steeply-dipping coal seams in the Meeker Area Mines will produce unique subsidence data. Because it will provide the first site-specific information of its type, the U.S. Bureau of Mines has generated much interest in the project. They have offered to install and survey subsidence monuments which will be strategically located over the caving areas. Monuments will probably be placed at 100-foot to 300-foot intervals around the surface- perimenter and down the centerline of each panel. Additional monuments may be placed perpendicular or at an angle to the center line at various locations along the panel. Monument spacing will be dictated by the topography and vegetative cover over each of the nonitoring areas, since each monument must be visible from a survey point located on undisturbed ground. • ' To serve as monuments, 1-inch steel pins driven 3 to 5 feet into the ground will probably be used. These can be .easily installed by a gas-powered jackhammer or a sledgehammer so that no damage will be incurred to the surface by the placement of these monuments. Frequency of the monitoring surveys has not been determined, however, they will be limited to a 5-month period each year. The heavy snowfall that is characteristic of the region will make monuments inaccessible during the winter months. A high degree of accuracy is the surveys will not be required by the Bureau, so their surveys will probably detect subsidence to the nearest half-foot. Both vertical .and horizontal angles will be measured as well zs the distances to the monuments, so that both vertical and horizontal displacement can be detected. Details of the subsidence monitoring plan are presently being negotiated between Northern Coal Company and the_Bureau of Mines, and a contract between the two parties is being drawn up. S-4