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Index | Diagnostic Characteristics | Geographic Occurrence | Investigation & Mitigation | Key Contacts & Expert Advice | Photo Gallery | Essential References & Further Reading Fault reactivation is significant because it has caused widespread and sometimes catastrophic physical damage (and financial losses) to land, property, infrastructure, utilities, civil-engineered structures, underground and opencast mining operations. Faults are capable of several phases of reactivation during multi-seam mining operations, separated by periods of relative stability.
Fault
reactivation may cause the first-time failure of natural slopes, high-walls in
opencast mines, engineered cuttings, embankments and can influence stream flows,
aquifers and the reactivation of ancient (postglacial) landslides. There have
been some attempts to date fault gouge using K-Ar dating of claystones, but the
dates appear to reflect diagensis rather than fault reactivation. Reactivated
faults have the potential to cause a reduction in bearing capacity of the
ground. Other possible geotechnical problems include the presence of voids,
groundwater, minewater, mine gas or boulders. These may have implications for
site investigations and the subsequent siting and design of foundations,
structures and utilities. Where Coal Measures are concealed by younger Permo-Triassic
rocks, such as the Sherwood Sandstone and Magnesian Limestone formations,
fissures may appear during construction, the development of the ground or
following prolonged heavy rainfall. These are likely to have been generated at
the time of mining but have been obscured from view due to the bridging of
weathered bedrock and soil cover. The width and depth of these may be
exacerbated by the subsequent erosion of their walls, following their exposure. Some geological faults which have undergone mining-induced
reactivation should be considered as sites where potential residual ground
movements, voids, groundwater discharge, acid mine water or mine gas emissions
may occur. Reactivated faults may represent ground consisting of numerous,
complex interlocking rock mass discontinuities, with, or without granular of
cohesive clay gouge. This may reduce bearing capacity, influence foundation
design, planning, construction or landfill. Reactivated fault scarps, fissures and compression humps
do not always appear at their postulated outcrop position as inferred on
geological maps. This may be attributed to the acceptable mapping tolerances
(since geological maps provide an estimate of their likely outcrop position on
the ground surface). This is complicated by the variable nature of the strata
(or made ground) which a fault displaces, resulting in deflections, grabens,
fissures, splays and runners. Fault scarps are normally temporary features of
the ground surface and may be destroyed soon after their generation by, for
example, repairs to roads and structures, or by the ploughing of agricultural
land. Their absence on the ground surface In some instances, reactivated faults have reduced the
amount of subsidence on the unworked side of a fault, by absorbing ground
strains and safeguarding houses, structures and land which may have been
otherwise damaged. Fault reactivation is not likely to be widespread since
the majority of geological faults in the Britain are ‘relatively stable’. Also,
not all faults reactivate during mining subsidence and, given the decline in
deep coal mining in Britain over the past few decades, cases are likely to
become reduced. Although fault reactivation, in certain circumstances, may
continue for periods of time (weeks to several years) after ‘normal’ subsidence
has been completed, movements along most faults does eventually cease. Those
faults more prone to reactivation tend to be the master and main faults which
define structural blocks and coalfield cells. The triggers for renewed activity
on faults are not fully understood but may include, for example, natural
groundwater recharge, rising groundwater levels or minewater rebound (caused by
the cessation of groundwater pumping). These may elevate pore fluid pressures in
faults and other rock mass discontinuities. This will reduce the stability of
the fault and may induce
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