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Landslides & Slope Instability

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Subsidence & Collapse Hazard

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Seismic Hazard

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Flood Hazard

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Tsunami Hazard

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Volcanic Hazard

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Gas Hazard

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Fault Reactivation Hazard

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Fault Reactivation: Diagnostic Characteristics

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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.

The surface expressions of reactivated faults, their geometry, morphology and size vary considerably. They range from subtle topographic deflections and flexures merely recognisable across agricultural land or road side verges, to distinct, high-angled fault scarp walls 3-4m high and at least 4km long across moorland plateaux. More commonly, they are less than a metre high, less than a metre wide and a few hundreds of metres long. They occur in rural and urban & industrialised parts of Britain on both exposed and concealed coalfields. Reactivated faults may not be easy to recognise. These were historically suspected when linear lines of structural damage (known as ‘break lines’) were observed on brittle ground, through densely populated or built up areas, often without any trace on more granular or cohesive grass verges, gardens or agricultural land. They have resulted in severe damage to surface structures (buildings, houses, industrial premises, bridges, dams, pylons and towers), services and utilities (sewers, water conveyances, gas mains, pipelines and communications cables) and transport networks (tracks, roads, motorways, railways, rivers and canals). Frequently, reactivated faults have also disrupted agricultural land (through alteration of drainage and gradient).


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
however, does not eliminate their risks and potentially problematic ground conditions. Greater thicknesses of superficial deposits, or made ground, will reduce the severity of a fault scarp, but influence a much broader area. Where the cover is thin or absent a distinct, high-angled fault scarp may develop but where these are thick (10m+) a less distinct, broad, open flexure will be generated.

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
renewed movements along the fault, affecting ground stability. Residual subsidence, post-mining settlement of goaf or the collapse of near-surface mines (such as room and pillar workings or bell pits) may also potentially influence the stability of faults. Furthermore, the disposal of effluent fluid waste disposal in boreholes, in close proximity to faults, or the generation of leachates from waste sites, may also penetrate faults and potentially reduce their stability.

































































Engineering Group Working Party on Geological Hazards