2014-10-10_PERMIT FILE - C1996083 ‎(8)‎

• Permit No: C1996083
• IBM Index Class Name: Permit File
• Doc Date: 10/10/2014
• Section_Exhibit Name: Volume IIIA Exhibit 15 Subsidence Prediction
• Media Type: D
• Archive: No

As the scope of this study is to evaluate the subsidence impacts on various
resources within the mine property, it is important to specify the critical subsidence
conditions; such conditions can be defined as causing the maximum possible damage to
existing resources. Rather than dealing with initial values of subsidence that might be
caused by developing of entries and /or initial room - and - pillar development and partial
retreat pillar extraction, this study is aimed at predictions of maximum subsidence that will
occur during longwall production. It is reasonable to assume that the initial subsidence is
negligible compared to the final, maximum subsidence.
While subsidence of small magnitude will occur during mining of entries and rooms,
the majority of the subsidence will occur during and after longwall mining. The rate of
subsidence above a longwall panel advancing at the same rate as the pillar line retreats.
Subsidence rate above longwall panels are described in detail by Kratzch (1983).
Subsidence is generally described as occurring in three phases. At the time the
working face (retreat line) passes under a point (Phase 1), usually only about 10% of the
total subsidence has occurred. Gentry and Abel (1978) reported an average 7 %, ranging
from 3% to 10 %, for the surface subsidence before the longwall face passed under a
surface point at York Canyon. As the face advances beyond this point, subsidence occurs
rapidly, typically by as much as 20 mm per day shortly after passage of the face. By the
time a length of panel reaches approximately 1.4 times the overburden depth, 90% to 95%
of the total subsidence will have occurred at a point above the center of the panel during
this phase 11. The time for this degree of subsidence to be achieved, depends on the rate
of mining and the depth of the panel.
The main subsidence, which includes phase I, initial subsidence, and phase 11,
accelerated subsidence, is referred to as "active" subsidence. Phase III ( "Residual" or
"delayed ") subsidence accounts for the remaining 5% to 10 %. The period of active
residual subsidence was reported by Collins (1978) as ranging from 2 to 42 years after
completion of mining in Wales.
Gradual subsidence may occur over room - and - pillar areas as the pillars that have
been left will gradually yield. Total subsidence will be less than subsidence above total
extraction zones, since the coal remaining within the pillars will eventually be spread
throughout the abandoned workings, preventing complete closure of the roof and floor.
The rate of surface subsidence will depend on the rate at which the pillars yield. Large
barrier pillars may be too wide to ever yield entirely, with the result that there would be
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