REP50071

• Permit No: C1981044
• IBM Index Class Name: Report
• Doc Name: REGULATION OF COAL MINE SUBSIDENCE IN COLO
• Permit Index Doc Type: SUBSIDENCE REPORT
• Media Type: D
• Archive: No

REGULATION OF COAL MINE SUBSIDENCE IN COLORADO
installations intended to verify subsidence
movement at selected locations. Thirteen approved
mines have visual monitoring requirements as a
component of their monitoring program. For
structures such as roadways, railroads, pipelines,
powerlines, and ponds, operators perform periodic
inspections to detect damage, on frequencies
varying from daily to quarterly. Eleven approved
mine's subsidence monitoring programs include
periodic survey monitoring of the elevation,
orientation and location of specific structures.
One mine's monitoring program consists solely of a
ground water aquifer monitoring requirement,
intended to verify an anticipated negative impact
upon several bedrock aquifers.
The Division's intention is to require
monitoring programs which will collect the
required information while subjecting the operator
to minimum cost and effort. Because of the
implications of monitoring verification of
predicted subsidence phenomena for future mining
activities, both the Division and operators have
placed emphasis on the design and conduct of
subsidence monitoring programs. As of May 1,
1986, na Colorado mine has been determined to
require modification of their subsidence control
program. While the five year sampling period is
inconclusive, the evidence suggests that
prediction of the mechanical aspects of subsidence
has been relatively accurate (See Table 1).
Hydrologic Consequences of Subsidence
The social and economic development of
semi-arid western states, such as Colorado, has
been strongly influenced by water-related
concerns. The constitution and statutes of the
state of Colorado define the rights and
responsibilities related to water ownership and
use. SMCRA and the State Act, adopted in response
to SMCRA's passage, emphasize the minimization of
mining's impacts upon the hydrologic systems of
the permit and adjacent areas. Analogously, the
OSM regulations and the state regulations
emphasize minimization of mining's impacts upon
the hydrologic balance.
The current state-of-the-art of subsidence
prediction, as evidenced by the limited sample of
the first five-year's experience of the Colorado
subsidence program, appears to result in
relatively accurate prediction of the mechanical
consequences of subsidence. However, the
state-of-the-art of the prediction of hydrologic
consequences of subsidence lags behind that of the
prediction of mechanical consequences. Much of
the chronicled research regarding subsidence has
concentrated upon the investigation of mechanical
ground response and the mechanical effects of
structures exposed to subsidence deformation.
Only an occasional reported observation relates to
ground water or surface water hydrologic response
to subsidence phenomena.
Nineteen of the thirty-seven permit
applications re•+iewed for compliance with the
Colorado subsidence regulations have delineated
ground water aquifers or surface water bodies
105
within their permit and adjacent areas. All
thirty-seven permit applications contain
projections of the specific hydrologic
consequences anticipated as a result of subsidence.
Faced with a mandate to minimize impacts to the
hydrologic balance, any permit applicant would
prefer to demonstrate that the anticipated
hydrologic consequences of subsidence are
minimal. Virtually every permit application's
probable hydrologic consequence description
observes that ground subsidence will result in
cracking of the ground surface above the proposed
underground mine workings, but that weathering
processes will rapidly seal and eliminate that
cracking. While this observation may seem
intuitive, little documented evidence exists
within the literature to support it. The physical
expression of the cracking will probably be
relatively rapidly eliminated, but evidence
suggests that the hydrologic consequences may be
of very long duration.
In the mountainous terrain which characterizes
much of the Colorado underground mining areas,
valleys are predominantly structurally
controlled. Many valley axes are lineaments
characterized by ancient faulting or fracturing.
Mining beneath these valleys often encounters the
subsurface expression of these lineaments, shear
zones exhibiting closely-spaced jointing. These
zones are commonly characterized by secondary
permeabilities significantly above that of the
adjoining country rock. In the opinion of the
Division, it is illogical and imprudent to project
that a rock mass which continues to exhibit
increased permeability several millions to tens of
millions of years after a tectonic shearing event
will perform significantly differently in response
to a subsidence induced disturbance.
Subsidence of the interburden, overburden, and
ground surface will have an effect upon the
hydrologic character of the affected materials.
Impacts upon the hydrologic system are not
illegal, but such effects are to be minimized.
Unfortunately, very little documentation exists
with which to judge the prudence of projections of
the hydrologic consequences of subsidence. What
will the impact of subsidence be upon the
hydrologic character of surface channels and
impoundments? Will subsidence effect the
conjunctive nature of surface channels and their
associated alluvial deposits? Wi11 subsidence
modify the hydrologic parameters of interburden
and overburden aquitards and aquicludes? Wi11
subsidence cause mingling of bedrock and surficial
aquifer waters? Will subsidence affect the
quality or movement of ground and surface waters?
Will subsidence affect the quality or quantity of
ground or surface water available to users? How
will these various hydrologic consequences of
subsidence change with the passage of time? What
mitigating techniques might be efficiently applied
to preclude hydrologic material damage in subsides
areas?
The Division, in cooperation with permit
applicants, has attempted to develop prudent