You are here: Avoiding Subsidence Over Abandoned Mine Workings

Careful engineering works to avoid old mine works causing subsidence in buildings.

1. This paper has been conveniently split into four sections for descriptive purposes namely:
History of mining and mining methods
Geology and zones of influence
Site Investigation and treatment of abandoned mine workings
Case studies of recent developments

2. Problems associated with abandoned mining workings occur throughout many regions of the United Kingdom and pose varying problems to many sites and developments. These factors should never be ignored when developing a site or the assessment of an existing structure for either sale or redevelopment purposes. The paper discusses:
Mining problems
Mining technology
Early mines from Roman times
Theoretical sections
Problems posed by thin superficial deposits

3. Old mining methods often dictate and leave legacy for engineers to have to now resolve and it is knowledge of these methods that is essential. The paper discuses Bell Pits in areas of shallow seam dip and thin superficial deposits, shafts, radial workings and mine backfilling.

4. Areas of abandoned Bell Pits can cover vast areas in areas of shallow dipping seams following high intensity mining. The workings of Bell Pits could have been extremely intensive and may even have worked more than one seam and once backfilled can be lost until later development. Bell Pits are always located away from what is termed down the dip of the coal seam outcrop and unlikely to existing up from the outcrop.

5. Following on from the development of the Bell Pit and as technology increased, pillar and stall workings developed in the 16th Centaury following the advent of pumps and therefore deeper shafts. There are many different types of Pillar and Stall working throughout the region of the United Kingdom. During this period the pillars intended to be left unworked to support the roof. These are often robbed after extraction to get more coal leaving unsupported roofs. These workings could either be entered via a shaft or a shallow drift from the surface. By the 17th Centaury long wall methods of mining had been introduced. Both the robbing of pillars or the use of long walled miming often resulted in instantaneous collapse however this was never predictable and can remain a problem to this day.

6. Affected on the surface can affect an area as much as 1.4 x the height from the workings to the surface.

7. There are literally 1000’s of shafts throughout the United Kingdom that are now abandoned. Many are known however many are not and laws now exist for the registration and location of these whenever possible. Often in early mining the coal would be extracted from directly underneath the shaft.  This would often lead to collapse and subsequent disasters so legislation was eventually introduced that coal could not be extracted to within a 100m radius of the shaft. Further legislation was then introduced that enforced the publishing of abandonment plans of all the coal worked. Not only have shafts been used to extract coal they can many other uses throughout a mine.

8. As technology increased the depths of the shafts subsequently increased also. When abandoned they may often be filled with many differing materials with varyoing degrees of compaction leading to migration to the surface. Materials used to fill the shaft would be various and whatever available at the time. In addition shafts would not only be used for the extraction of coal but many other minerals.

9. Any mine workings less than 50m below the surface can have severe implications and subsidence. Exceptionally greater than 50m can cause subsidence. The effect on the surface depends on the nature and condition of the overlying rock, depths of the workings and natural arching may be disturbed by the construction process. Shafts require special treatment and these will be discussed later.
Pillar and Stall mine workings need special consideration as well due to:
Collapse of pillar and stall workings occurs unpredictably
The weight of the building and construction process can have an effect
Where more than one seam has been mined
Widespread subsidence does not normally occur
There are many mechanisms of failure of pillar and stall works;
Floor heave
Crushing of pillars
Roof collapse
Modes of void migration
Structural Engineer has to carry out extensive investigation to determine the predictability and risk of failure and therefore consider all the coal seams, competence of overlying rock, assessment of the zones of instability. This is in order to predict the possibility of Crown Holes occurring on the surface and what form of foundation to adopt. With modern longwall mining this is an extremely complex method.

10. Before any design of foundations can occur extensive site investigation is required and development of these findings. The region of development needs to be known.  This paper uses Bristol as an example as there are common problems in this area. On any development a mining assessment has to be carried out which includes studying geological maps and mining archive material. Each site investigation has to be tailored to suite the development, the contractor and consultant need to determine their responsibilities and information needs to be provided for costing. This paper goes onto discuss the requirements of the site investigation, methods of site investigation, the work of the site investigation contractor and methods of drilling to locate shafts.

11. Following the recommendations of the site investigation an engineer has to assess whether ground treatment is required or whether foundations designed for special consideration and these are discussed in detail.

12. The special consideration of shafts and the various methods of making safe and investigate to locate will be discussed in detail.

13. As treatment of the ground is often costly and difficult to predict foundations over unconsolidated workings are often suitable. These have to consider the findings of the site investigation, potential failure of the roof, surface instability, and interaction with foundations.
The paper then goes on to describe six working examples and how the above has been used to make the assessment.

Ian Caldwell Bsc(Hons) CEng MICE MI Struct E
Caldwell Consulting Engineers
3, Sawyers Close, Wraxall, Bristol BS48 1LY
Tel/Fax 01275 854119 Mob 07850 371555
This paper was presented at the CPD Conference training course held March 2005.  

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