Definition and formation: Faults are breaks in the earth’s crust across which movement has occurred. The relative direction or motion of movement defines the type of fault.
Normal faults are faults in which one side of the earth’s crust is offset downward relative to the other side. Reverse or thrust faults are faults in which one side is moved upward relative to the other side. Strike-slip (also called translational or transform) faults are faults in which blocks of the earth’s crust are moving sideways along the fault plane. Offset along faults may be measured in inches to thousands of feet.
Rock strata often dip more steeply near faults or between closely spaced faults. Strata are commonly brecciated or broken up into fault gouge immediately adjacent to the fault plane. Fault gouge formed from movement along the fault. Fault gouge is commonly mineralized.
Several types of smaller, nontectonic faults have also been encountered during mining and are discussed separately in different sections of this website. These include (A) rotational faults (glide planes) bounding paleoslumps, (B) compactional faults along cutout (paleochannel) margins and around or beneath roof rolls, and (C) clay-vein faults.
Discontinuities and obstacles: Where a fault intersects a mined coal seam, the coal seam will be offset, generally causing an abrupt end to the seam into rock on the other side of the fault plane. Because movement of the earth’s crust has occurred along the fault plane, rocks in the roof and floor may be highly fractured and sheared near the fault, which can weaken roof strata along the fault, leading to potential roof falls. Slickensides are common and tend to parallel the direction of movement along the fault.
Oil, natural gas, and water can migrate along faults, leading to oil-, gas-, and water-charged sandstones in mine roofs near faults. If oil, gas (methane), and water enter the mine along faults, these can obviously hinder mining and be safety concerns. Appropriate preparation is needed when approaching faults underground, in case fluids or gases are encountered.
In some cases, coals become mineralized (calcite veins, etc.) near faults. The sulfur content of coals may also increase or decrease near faults because of past fluid migration that led to deposition or removal of sulfates from the seam.
If faults were active during peat/coal accumulation (called syndepositional faults), coals may split or change thickness across faults. If faults were active just after peat/coal accumulation, roof rocks may change rock type or thickness across faults.
Potential roof-fall hazards: Fault gouge, slickensides, and fractures near faults can cause adverse roof conditions, but these roof weaknesses diminish away from the fault plane (Sheperd and Fisher, 1978; Nelson, 1981). Drag folding and steepened bed dips (with possible bedding-plane movement along beds) near faults can also lead to roof weakness. Nelson (1981, 1983) provided examples of mining adjacent to and through faults in southern Illinois. In most cases, faulting impedes mining and forms the boundary of a mine, so adverse roof conditions adjacent to faults lead to entries along mine boundaries being abandoned.
Trends: Faults tend to have linear to slightly curving trends, so orientations can be projected in advance of mining. Many surface-fault traces are mapped on 7.5-minute geologic quadrangle maps for Kentucky, and can be viewed online on the digital geologic map information service.
Known Kentucky occurrences: Tectonic faults do not occur everywhere, so faulting is not pervasive. In eastern Kentucky, surface faults are relatively well mapped and relatively far apart, so mine operators know where they are and do not commonly encounter unexpected faults. In western Kentucky, however, faulting is common. Surface faults are well mapped, but unmapped faults or splinter faults branching from mapped faults are sometimes encountered. Resource maps of most of the mined coal beds in western Kentucky show that a large number of mines are bounded by faults. Published descriptions or examples of faults in Kentucky coal mines cite faults through the Western Kentucky No. 4 coal (Greb and others, 2001) and Springfield (W. Ky. No. 9) coal (Greb and Williams, 2000). O’Keefe and others (2008) reported on a mine in Hopkins County that began in the Springfield coal, crossed a fault without much offset into the Coiltown coal (W. Ky. No. 14), and then after some distance, crossed another fault. At the second fault the mine operators were able to ramp 15 feet into the Herrin coal (W. Ky. No. 11) and continue mining.
Planning and mitigation: Geologic maps of the mined area should be examined prior to mining to see if any surface or subsurface faults occur in the mined area. Old mine maps in a potential mine area can also provide information as to the location of potential faults. Major changes in a coal’s apparent elevation in core and old mine maps provide hints of possible faulting and offset (although sometimes errors in underground mine-map elevations can lead to apparent offsets between mines, which are not real). Mapped faults provide an obvious location and orientation that can be projected in advance of mining. Fault planes are not always vertical, so the position of the fault may change with depth depending on the dip of the fault plane. Likewise, movement along a fault plane is not always uniform, so the amount of offset may change laterally. In some areas, faults may bifurcate and merge laterally. Where this occurs, minor unmapped faults may be encountered.
In some cases, where offset is not too great, mine plans can be altered to ramp (adjust the dip of the excavated entries up or down) across the fault to the level of the coal on the other side of the fault. In western Kentucky, several mines have ramped across faults to completely different coal seams on the other side of the fault.
Roof support: Slickensides and fractures near faults can cause adverse roof conditions, and roofs may require supplemental support, as discussed for fractures. Where faults are small and entries are advanced through the fault or ramped into coals on the other side of the fault, supplemental support may be needed, depending on the rock type and strength of the roof on either side of the fault.