Chapter 11 Coal mining subsidence in the UK

One of the geohazards associated with coal mining is subsidence. Coal was originally extracted where it outcropped, then mining became progressively deeper via shallow workings including bell pits, which later developed into room-and-pillar workings. By the middle of the 1900s, coal was mined in larger open pits and underground by longwall mining methods. The mining of coal can often result in the subsidence of the ground surface. Generally, there are two main types of subsidence associated with coal mining. The first is the generation of crown holes caused by the collapse of mine entries and mine roadway intersections and the consolidation of shallow voids. The second is where longwall mining encourages the roof to fail to relieve the strains on the working face and this generates a subsidence trough. The ground movement migrates upwards and outwards from the seam being mined and ultimately causes the subsidence and deformation of the ground surface. Methods are available to predict mining subsidence so that existing or proposed structures and land developments may be safeguarded. Ground investigative methods and geotechnical engineering options are also available for sites that have been or may be adversely affected by coal mining subsidence.

Chapter 17 Mining-induced fault reactivation in the UK

Faults are susceptible to reactivation during coal mining subsidence. The effects may be the generation of a scarp along the ground surface that may or may not be accompanied by associated ground deformation including fissuring or compression. Reactivated faults vary considerably in their occurrence, height, length and geometry. Some reactivated faults may not be recognizable along the ground surface, known only to those who have measured the ground movements or who are familiar with the associated subtle ground deformations. In comparison, other reactivated faults generate scarps up to several metres high and many kilometres long, often accompanied by widespread fissuring of the ground surface. Mining subsidence-induced reactivated faults have caused damage to roads, structures and land. The objective of this chapter is to provide a general overview of the occurrence and characteristics of fault reactivation in the UK.

Chapter 15 Dissolution – carbonates

The dissolution of limestone and chalk (soluble carbonates) through geological time can lead to the creation of naturally formed cavities in the rock. The cavities can be air, water, rock or soil infilled and can occur at shallow levels within the carbonate rock surface or at deeper levels below. Depending upon the geological sequence, as the cavities break down and become unstable they can cause overlying rock strata to settle and tilt and also collapse of non-cemented strata and superficial deposits as voids migrate upwards to the surface. Natural cavities can be present in a stable or potentially unstable condition. The latter may be disturbed and triggered to cause ground instability by the action of percolating water, loading or vibration. The outcrops of various limestones and chalk occur widely across the UK, posing a significant subsidence hazard to existing and new land development and people. In addition to subsidence they can also create a variety of other problems such as slope instability, generate pathways for pollutants and soil gas to travel along and impact all manner of engineering works. Knowledge of natural cavities is essential for planning, development control and the construction of safe development.

About this title - Geological Hazards in the UK: Their Occurrence, Monitoring and Mitigation – Engineering Group Working Party Report

The UK is perhaps unique globally in that it presents the full spectrum of geological time, stratigraphy and associated lithologies within its boundaries. With this wide range of geological assemblages comes a wide range of geological hazards, whether they be geophysical (earthquakes, effects of volcanic eruptions, tsunami, landslides), geotechnical (collapsible, compressible, liquefiable, shearing, swelling and shrinking soils), geochemical (dissolution, radon and methane gas hazards) or georesource related (coal, chalk and other mineral extraction). An awareness of these hazards and the risks that they pose is a key requirement of the engineering geologist.The Geological Society considered that a Working Party Report would help to put the study and assessment of geohazards into the wider social context, helping the engineering geologist to better communicate the issues concerning geohazards in the UK to the client and the public. This volume sets out to define and explain these geohazards, to detail their detection, monitoring and management and to provide a basis for further research and understanding.

Chapter 8 Swelling and shrinking soils

Swelling and shrinking soils are soils that can experience large changes in volume due to changes in water content. This may be due to seasonal changes in moisture content, local site changes such as leakage from water supply pipes or drains, changes to surface drainage and landscaping, or following the planting, removal or severe pruning of trees or hedges. These soils represent a significant hazard to structural engineers across the world due to their shrink–swell behaviour, with the cost of mitigation alone running into several billion pounds annually. These soils usually contain some form of clay mineral, such as smectite or vermiculite, and can be found in humid and arid/semi-arid environments where their expansive nature can cause significant damage to properties and infrastructure. This chapter discusses the properties and costs associated with shrink–swell soils, their formation and distribution throughout the UK and the rest of the world, and their geological and geotechnical characterization. It also considers the mechanisms of shrink-swell soils and their behaviour, reviewing strategies for managing them in an engineering context, before finally outlining the problem of trees and shrink–swell soils.

Chapter 4 Landslide and slope stability hazard in the UK

With its rich lithological variation, upland, lowland and coastal settings, and past climatic changes, the UK presents a wide variety of landslide features that can pose significant hazards to people, construction and infrastructure, or simply add to landscape character and conservation value of an area. This chapter describes and defines the nature and extent of this landsliding; the causes, effects and geological controls on failure; and their mitigation and stabilization. A risk-based approach to landslide management is outlined with qualitative and semi-quantitative methodologies described. Numerous case studies are presented exemplifying landslide and slope stability hazards in the UK.

Chapter 9 Peat hazards: compression and failure

Peat is a highly compressible geological material whose time-dependent consolidation and rheological behaviour is determined by peat structure, degree of humification and hydraulic properties. This chapter reviews the engineering background to peat compression, describes the distribution of peat soils in the UK, provides examples of the hazards associated with compressible peat deposits and considers ways these hazards might be mitigated. Although some generalizations can be made about gross differences between broad peat types, no simple relationship exists between the magnitude and rate of compression of peat and loading. Based on examples described here, land failures resulting from peat compression are locally generated, but due to the sensitive nature of peat these can result in runaway failures that pose great risk. Understanding the geological hazards associated with compressible peat soils is challenging because peat is geotechnically highly variable and the mapped extent of peat in the UK is subject to considerable error due to inconsistencies in the definition of peat. Mitigating compression hazards in peat soils is therefore subject to considerable uncertainty; however, a combination of improved understanding of the properties of compressible peat, better mapping and land use zoning, and appropriate construction will help to mitigate risk.

Chapter 16 Geohazards caused by gypsum and anhydrite in the UK: including dissolution, subsidence, sinkholes and heave

Gypsum and anhydrite are both soluble minerals that form rocks that can dissolve at the surface and underground, producing sulphate karst and causing geological hazards, especially subsidence and sinkholes. The dissolution rates of these minerals are rapid and cavities/caves can enlarge and collapse on a human time scale. In addition, the hydration and recrystallization of anhydrite to gypsum can cause considerable expansion and pressures capable of causing uplift and heave. Sulphate-rich water associated with the deposits can react with concrete and be problematic for construction. This paper reviews the occurrence of gypsum and anhydrite in the near surface of the UK and looks at methods for mitigating, avoiding and planning for the problems associated with these rocks.

Chapter 2 Seismic hazard

It is often thought that earthquakes do not occur in the UK; however, the seismicity of the UK is usually classified as low-to-moderate. On average, a magnitude 3.2 Mw moment magnitude or larger earthquake occurs once per year, and 4.2 Mw or larger every 10 years. The latter is capable of causing non-structural damage to property. The damage caused by British earthquakes is generally not life-threatening, and no-one has been killed in a British earthquake (at the time of writing, May 2013) since 1940. Damage is caused by shaking, not by ground rupture, so the discovery of a fault surface trace at a construction site is not something to be worried about as far as seismic hazard is concerned. For most ordinary construction in the UK, earthquake hazard can be safely discounted; this is not the case with high-consequence facilities such as dams, bridges and nuclear power plants.

Chapter 18 Radon gas hazard

Radon (222Rn) is a natural radioactive gas that occurs in rocks and soils and can only be detected with special equipment. Radon is a major cause of lung cancer. Therefore, early detection is essential. The British Geological Survey and Public Health England have produced a series of maps showing radon affected areas based on underlying geology and indoor radon measurements, which help to identify radon-affected buildings. Many factors influence how much radon accumulates in buildings. Remedial work can be undertaken to reduce its passage into homes and workplaces and new buildings can be built with radon preventative measures.