F.4 Anthropogenic (man-made) voids

F.4.1 General

The range of anthropogenic (man-made) voids and the potential hazards are summarized in Table F.3.

Information on hazardous coal mine gases can be found in the Coal Authority et al., 2012 [5] publication.

F.4.2 Construction and building

Previously developed, brownfield sites might have contained below and above ground structures, which following their demolition and the reworking or remediation of the site could represent sources of voids. Significant types of voids include partially subsided and collapsed basements, ducts, undercrofts, access tunnels, demolition arisings, cavities and those formed by subsidence beneath structures. The voids might be undocumented and be developed over for various uses, often including less sensitive uses such as car parking and open space. Geotechnical failure processes arising from construction activities, e.g. tension cracks formed as a result of slope failure, might also represent sources of voids.

Asbestos was often placed within above and below ground structures as insulation and can often be missed by asbestos building surveys or if an underground structure has been sealed off. Extreme caution needs to be shown if investigations encounter such structures.

F.4.3 Infrastructure

When infrastructure containing voids is operational, these voids are generally maintained to ensure that they do not represent hazards to overlying development. Proposed new developments might, however, represent sources of hazard to existing infrastructure, and on that basis their identification and investigation can be critical. Old or redundant infrastructure can represent a source of voids in the ground, particularly where there is little information or knowledge about these features. In terms of voids, the principal sources associated with infrastructure comprise ducts, drains, sewers, pipelines, tunnels, shafts, wells, underground canals, old bridges and similar structures.

Table F.3 Anthropogenic voids: potential hazards
Potential hazard Anthropogenic (man-made) voids
Construc­tion and building Infrastruc- Mining ture Waste disposal Graves Military sites
The generation of a failed mass of material containing a high volume of voids which might be water-filled. X X      
Unexpected and unpredictable collapse and loss of ground forming holes at the ground surface. X X      
Accumulations of explosive X and/or asphyxiant gases. X X      
Minewater rebound, i.e. rising minewater levels.   X      
Water release from voids X and/or draimage channels. X X      
Health and safety hazards associated with site occupiers falling into uncovered inspection chambers. X X      
Proposed site developments and their investigations damage live services, tunnels, etc. X        
Mining subsidence.   X      
Collapse and settlement of mine entries.   X      
Emissions of mine gas.   X      
Tip fires and spontaneous combustion.   X      
Settlement (for example of infill to surface mines).   X X X X
Generation of landfill-type gases.     X X  
Contamination. X X X X X X
Spontaneous combustion and fire.   X X    
Collapse and loss of ground.         X
Explosion.         X

F.4.4 Mining

Different methods of mining, e.g. underground (partial and total extraction), solution (controlled and uncontrolled), stoping, surface mining and quarrying can all generate voids and potentially reactivate geological faults. The voids might be associated with the actual mine working, the mine entrance, i.e. shaft, adit, etc., or be related to the nature of the backfill in the abandoned working, e.g. non-engineered fill and voids within backfill to abandoned surface mineral workings. Abandoned mine workings might be extremely old, frequently with little or no obvious surface evidence of former mining activities being present, with the surface mine infrastructure generally completely obliterated. Depending upon the age of the mining, there might be no formal records of the former mining activities and the workings could be unrecorded.

CIRIA SP32 [170] (at the time of publication this was being revised by CIRIA as report reference RP940) provides a detailed account of mining activities in the UK, methods of mining, associated hazards, appropriate methods of investigation, and mitigation options. CIRIA RP940 is intended to include a comprehensive list of available information relating to historical mining of all types across the British Isles.

Attention is drawn to the need to consult the Coal Authority when investigation works are to enter mine working in the UK.

F.4.5 Waste disposal

Certain waste materials have a high void ratio and in certain situations they might be prone to settlement (particularly differential settlement), loss of ground and collapse. The void ratio of such deposits can vary over time due to compaction and consolidation processes and, where organic materials are present, by biological degradation.

Mine spoil tips are scattered throughout former/current mining areas across the UK, and such materials might contain voids, with the extent of the voids being a function of material type, method of tipping, age and nature of any post deposition activities. Old surface excavations, opencast sites and quarries can also contain less well-controlled backfill and non-engineered fill. Although much of these backfills are composed of reworked natural soils and rock (excavated during the mining process), prior to the formal licensing of waste disposal activities waste materials in accordance with the Control of Pollution Act 1974 [171], e.g. domestic and industrial wastes, might also have been incorporated into the backfill. In rural areas diseased livestock have also been recorded in former excavations and quarries.

F.4.6 Graves

Overtime, buried organic remains undergo biological degradation potentially resulting in the formation of voids, loss of ground or settlement. These can include ancient archaeological graves and burials, either recorded or unrecorded, or more recent graves and cemeteries. It is rare that ground investigations unknowingly encounter recent graves since the cemetery boundaries are generally well recorded; however, situations have occurred where ground investigations have encountered recent graves without cemetery boundaries.

F.4.7 Military sites

Abandoned, and long forgotten, military sites can potentially contain voids. An additional hazard associated with such sites is the presence of explosive or other harming substances within the voids. Potential sites can be associated with underground storage areas and tunnels; sites of underground bunkers associated with munitions manufacture, storage and testing; civil defence structures; basements; shelters; tunnels; and overfilled gun emplacements.

F.5 Ground investigation procedures

F.5.1 Stage 1: Desk study

F.5.1.1 General

The susceptibility of the site to contain various geohazards, including voids, is usually revealed through the completion of the desk study. In some cases the scale of any potential voids and associated hazards can be estimated with some accuracy from the desk study. Only when the nature of the potential voids affecting the site have been identified can an assessment be made of the ground investigation techniques most appropriate for their investigation. The information obtained from the desk study is also used to identify health and safety hazards associated with the proposed investigations, and their mitigation,

to ensure the safety of site staff, members of the public and site occupiers as well as ensuring the protection of property.

F.5.1.2 Sources of information

Annex A to Annex E give sources of information which can be obtained and reviewed (see C.9 in particular).

F.5.1.3 Field reconnaissance

During the inspection, the site is examined to identify any features, natural or anthropogenic, which might be associated with voids. Such features include crown holes, depressions, changes in ground slopes for evidence of brine runs or collapsed workings, demolished mine buildings, mining waste and damage to existing structures.

NOTE Those making the site inspection need to be made fully aware of the risk of cavities and appropriate protective measures deployed and used.

F.5.1.4 Reporting

The susceptibility to geohazards associated with the presence of voids is to be identified and recommendations given for ground investigation necessary to further assess the risks to enable the progression of design and costing of any site remediation works. The early mitigation of relict coal mining risk is required to be demonstrated by a developer at planning application stage. The UK Coal Authority, as one of the statutory consultees to the planning process, require planning applications in areas deemed to be at increased risk from historical coal mining activities to be accompanied by a Coal Mining Risk Assessment (CMRA) prepared in accordance with their guidance documents.

F.5.2 Stage 2: Ground investigation

F.5.2.1 Review of project requirements

To ensure that the ground investigation provides all the required information for design, it is critical that the project requirements are reviewed, particularly the preliminary assessment of the geohazards. Only when this review is completed can the full extent, scope and design of the ground investigation be determined.

F.5.2.2 Consideration of conceptual model

Typical questions to ask:

  • What voids are expected to be present at the site?
  • What is the anticipated size, nature or frequency of likely voids beneath the site?
  • How extensive are the voids likely to be?
  • To what depths might voids be expected?

The answers to these questions, in conjunction with a good understanding of the nature of the likely geohazards, enable the areas of investigation and investigation strategy to be determined. Non-intrusive investigative techniques such as geophysics can be used as preliminary work to help optimize any subsequent intrusive investigation.

The number of investigation points (trenches, boreholes, etc.) required depends on various factors including the likely nature of the void or feature being investigated; the type and extent of the proposed structure(s); the site conditions; the foundation design envisaged; and the conjectured geology and geological structure. The maximum depth to which exploratory boring is carried out is determined by the nature of the structure(s) proposed and by the nature of the proposed hazard, geology, geological structure, etc.

F.5.2.3 Methods of ground investigation

Methods to investigate for the potential presence of voids, both invasive and non-invasive, are covered in detail in Sections 3, 4 and 5.