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-2 7- <br /> - Stratigraphy, lithology and structure of the overburden <br /> - Topography <br /> Mining activities which affect subsidence are: <br /> - Size, shape, depth, location and areal extent of workings <br /> - Rate of development and extraction of resource <br /> - Mining methods <br /> The possible impacts of subsidence include: <br /> - modification of surface water hydrology - channel and watershed geometry <br /> and sediment erosion rates <br /> - Modification of groundwater hydrology - changes in spring discharge, <br /> aquifer characteristics and recharge <br /> - Loss of water from surface impoundments due to disruption by surface <br /> cra ckin g <br /> - Increase in surficial mass movement activity - landsliding and rock <br /> falling <br /> Natural physical influences upon subsidence can be controlled to the extent <br /> that mine plans can be designed to avoid obvious hazards (e.g. shallow open- <br /> ings beneath perennial streams, landslide -bodies, impoundments, etc.) , or <br /> to limit subsidence in accordance--with the current state-of-the-art of <br /> ground control (engineered control of roof and pillar failures in mines) . <br /> Subsidence prediction and ground control engineering are not definitive sciences. <br /> Design engineering concepts must be verified through continuous monitoring <br /> (surface topography, groundwater hydrology and surface water hydrology) and <br /> observation. <br /> Evaluation of the impacts of subsidence upon hydrologic systems requires the <br /> collection of extensive baseline data consisting of surface water and ground- <br /> water monitoring (quantity and quality) , precipitation gaging, evapotrans- <br /> piration rates, geological mapping, topographic surveying and recording of mine <br /> inflow rates. The baseline data should be updated continuously as mining pro- <br /> gresses to improve engineering control design parameters or to mitigate any <br /> unexpected subsidence occurrences. Such procedures are necessary at each <br /> mine within the district in order to coordinate a district-wide subsidence <br /> control plan. These procedures are now being required of all operators through <br /> the Colorado Permanent Regulatory Program permitting process. <br /> Subsidence in the area has been studied or observed at the Somerset mine, the <br /> Hawk's Nest mine, the Bear mine, and hypotheses forwarded concerning the Oliver <br /> #2 mine. Dunrud, in a study regarding the Somerset mine (U.S.G.S. Prof. Paper <br /> 969) , discussed numerous subsidence causes and effects, and while no specific <br /> and consistent effects were delineated, it is evident that the physical <br /> parameters mentioned 'above are important topics. For instance, the presence <br /> of strong rock strata (such as sandstone) at the ground surface will permit <br /> the opening of tension cracks which could extend through the effected rock <br /> unit. Such a condition could significantly affect the interception of surface <br /> water and the recharge of groundwater. The existence of weak rocks (such as <br /> shale) or soils at the ground surface could result in much less pronounced <br /> ` surface cracking. <br />