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control plan is intended to present a plan for the prevention of subsidence material damage or diminution of foreseeatle use or value, or measures to be implemented to mitigate subsidence effects., as well as techniques to be used in dete rnrining the degree of damage or diminution. As of May 1, 1986, sixteen approved permits and three applications under review contained subsidence control plans. Each of these nineteen permit applications contained a proposal to limit extrac:t:ion in order to prevent subsidence occurrence. Selected portions of the mine plan areas were designated <is retained pillars or areas limited to low recover- development or access extrac:t:ion. In these eases, the applicants have utilized analytical methods to demonstrate proje<:Ced pillar stabi~ity. Analytical procedures developed by Ashwin and Wilson (1972), and Obert and Duvall (1967) have been popular among Colorado applicants. It is anticipated that as site-specific mechanical subsidence projections are verified through subsidence monitoring programs, operators will be able to prudently propose revised plans to recover a portion of the reserves presently committed to controlling subsidence. Unfortunately, prior to the adoption of state regulations, subsidenc~a research was extremely limited within Colorado and its neighboring western states. The majority of subsidence research was limited to projects funded or conducted by the U. S. Geological Survey, U. S. Department of Energy and the U. S. Bureau of Mines, Asa result of recent budgetary constraints many of these projects have been curtailed, eliminating a gravely needed informational data base. As longwalls gain popularity in the western Unified States, plans which require limited extraction impose critical constraints upon mining feasibility. Several operators currently employing longwall equipment, or in the early stages of mine plan development, have emphasized to the Division the conflicts inherent between longwall maximized extraction requirements and the necessity to preclude surface subsidence through limited extraction. For example, the repetitive rectangular panel configuration consistent with optimal longwall mining techniques are not compatible with the need to protect a surface stream and its associated alluvial valley floor deposits meandering diagonally across a mine plan area. 'Afrile each approved subsidence control plan has included limited extraction components, other methods of compliance have been approved. Plans which anticipated subsidence of mobile home style residences and office structures have been approved with appropriate engineering precautions to guard against uti lity rupture. Undermining and subsidence of petroleum and water pipelines have been approved, following demonstrations that the pipeline material could withstand the projected strain levels, development of proper spill prevention plans, and preparation of repair and mitigation plans, including material stockpiling. The undermining and subsidence of roadways and railroad facilities have been approved with requirement of appropriate signage to inform the users of possible hazard, frequent visual inspections by mine personnel, and provision for safety barricading and rapid repair in the event of subsidence damage. Subsidence Monitoring Colorado's subsidence regulations emphasize the importance of subsidence mo itoring. Monitoring is required if an applicant~oncludes that the worst-possible subsidence consequences will not cause material damage or diminution of reasonably foreseeable use or value to inventoried structures and renewable resource lands. The state regulations also require a monitoring program be conducted if the applicant, as a result of a sophisticated subsidence prediction,z~oncludes that no material damage or diminutio of use or value will result from predi ted subsidence phenomena. Further, if the~pplicant opts to implement a subsidence control plan, the regulations specify subsidence monitoring as a desirable component of such a plan. The state regulations specify the minimum acceptable components for a required subsidence monitoring program [Rule 2.05.6(c)]. Monuments shall be installed and initially surveyed prior to mining within the area of potential subsidence effect, Monuments shall be appropriately located to monitor the predicted subsidence phenomena, including the area encompassing the projected T: angle of draw. The orientation, elevation and location of each inventoried structure shall be monitored. All subsidence monuments and structures must be monitored at least quarterly and the results reported to the Division at least semi-annually. The monitoring program shall extend, fora time, beyond cessation of the mining being monitored, consistent with the need for verification of the subsidence prediction. The Colorado regulations state that if subsidence monitoring determines that the subsidence prediction was inaccurate and the Division determines that material damage or diminution of use or value may occur, the operator will be required to submit a subsidence control plan. In addition, the regulations [Rule 2.05.6(6 )(d)(ii)] contain the authority for the Division to suspend mining under structures or renewable resource lands, if imminent danger of material damage or diminution of reasonably foreseeable use or value is determined to exist. Each of the twenty-four approved mines, whose subsidence inventories delineated structures or renewable resource lands within their permit and adjacent areas, have approved subsidence monitoring programs, as tabulated in Table 1. Sixteen of these approved mines have programs including grid lines of subsidence monuments intended to verify vertical and horizontal subsidence movement and the angle of draw. Five additional approved mines have isolated monument