GENERAL46036

• Permit No: C1981025
• IBM Index Class Name: General Documents
• Doc Date: 7/14/1983
• Doc Name: Amended Proposed Decision and Findings of Compliance
• Permit Index Doc Type: Findings
• Media Type: D
• Archive: No

-31-
Mining activities which affect subsidence are:
- Size, shape, depth, location, and aerial extent of workings;
- Rate of development and extraction of resource;
- Mining methods; and
- Number of overlying and underlying coal seams mined.
The possible hydrologic impacts of subsidence include:
- Modification of surface water hydrology - channel and
watershed geometry and sediment erosion rates;
- Modification of ground water hydralogy - changes in spring
discharge, aquifer characteristics and recharge;
- Loss of water from surface impoundments due to disruption by
surface cracking; and
- Increase in surficial mass movement activity - landslides and
rock falls (affecting water quality and water movement).
Natural physical influences upon subsidence can be controlled to the
extent that mine plans can be designed to avoid obvious hazards (e.g.,
shallow workings beneath perennial streams, landslide bodies,
impoundments, etc.), or to limit subsidence in accordance with the
current state-of-the-art of ground control (engineered control of roof
and pillar failures in mines). Subsidence prediction and ground control
engineering are not definitive sciences. Design engineering concepts
must be verified through continuous monitoring of surface topography,
ground water hydrology and surface water hydrology.
Evaluation of the impacts of subsidence upon hydrologic systems requires
the collection and analysis of extensive monitoring data consisting of
Surface water and ground water monitoring (quantity and quality),
precipitation gaging, geological mapping, topographic surveying and
monitoring of mine inflow, discharge, and consumption rates. The
monitoring data should be updated continuously as mining progresses to
improve engineering control design parameters or to mitigate any
unexpected hydrologic impacts of subsidence. Such procedures are
necessary at each mine within the Roaring Fork Drainage in order to
coordinate a comprehensive subsidence control plan. This monitoring is
now being required of operators through the Colorado Permanent Regulatory
Program permitting process.
The dimensions of surface cracks from subsidence appear to relate to the
type of rocks in which they occur, the thickness of coal removed, the
depth of the overburden, and the location of crack occurrence. Cracks'in
competent rocks tend to remain open and allow free movement of waters.
Cracks in incompetent rocks and soils are likely to fill and retard or
limit free movement of waters.