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• _2_ • Rules 2.05.6(6) and 4.20: Subsidence The application includes the results of the required survey of structures and renewable resource lands within the potentially affected area. The following inadequacies were determined to exist within that survey discussion: (1) The applicant observes that a dwelling exists within the potentially affected area, but that this dwelling is owned by Colorado Yampa Coal Company. The location of that dwelling should be clearly delineated upon Map No. 4. Further, on page 2.05-62, reference is made to an "unoccupied structure". If these references regard one-in-the-same structure, the applicant should clarify that point. If the structure is inhabitable, then the applicant must discuss "worst case" projected effects of subsidence. In the case of inhabitable dwellings, it has been common to require appropriate precautions be taken, such as a special utility connection modification, in order to eliminate potential hazards to any occupants. (2) County Road No. 27 is exposed to potential effects from subsidence and should be monitored to safeguard public health and safety. In connection with the proposed survey monumentation, visual inspection techniques should suffice to safeguard County Road No. 27. (3) The Denver, Rio Grande and Western Railroad Spur is also within the potentially affected area. As in the case of County Road No. 27, periodic visual monitoring of this facility should suffice to protect the public health and safety. (4) The applicant projects localized impacts upon both bedrock and surficial aquifers, based upon a specific concept of overburden response which has been insufficiently justified within the application. Recent literature regarding subsidence effects upon overburden above underground coal workings has documented roof caving to heights in excess of 10 times seam height and bed separation to heights in excess of 30 times seam height. On page 2.05.61, among other instances, the applicant refers to overburden caving limitations of 5 times the seam thickness. The mechanical and hydrological implications of such a variance in projected overburden response could be significant. The applicant will have to more thoroughly justify the assumptions being made within its projection of subsidence impacts. In addition, the applicant will have to specifically discuss how the proposed mechanical and hydrological monitoring programs will serve to verify the assumptions made in completing their projections of impact. (5~ The subsidence monitoring program presented within the application proposes to survey monitoring subsidence grid networks. These installations are acceptable as proposed. The applicant also proposes to survey the one identified dwelling within the potential area of. subsidence affects. In addition, however, the applicant will be required to conduct periodic visual monitoring of features such as County Road No. 27 and the Denver, Rio Grande and Western Railroad Spur.