Subsidence
Parts of Bathgate have often been affected by subsidence caused by coal mining. You can still see signs of it at the Hopetoun Street end of Mid Street where some buildings are a little of the vertical. If I remember correctly, the police station in Mid Street was badly affected. Balbardie House was demolished in 1956 because it had been undermined. My brother-in-law lived in Balbardie House and at a time about 1950 he said that one night his bed rolled from one side of the room to the other. Here are some notes on subsidence caused by stoop and room mining:
Case histories which break the rules
by P.G. Carter, Babtie Geotechnical
INTRODUCTION
Mining subsidence prediction is fraught with golden rules when dealing with old stoop and room workings. Often these well known rules apply in practice and all is well. Sometimes they do not apply because the actual conditions depart from the ideal conditions assumed by the rule. This contribution looks at several ways in which these rules can be broken.
THE RULES
These are
- seam thickness rule
- safe depth rule
- age of workings rule
- stoops do not fail rule
1. The seam thickness rule can be written as:
minimum thickness of rock cover for safety = y times seam thickness
Where y ranges from 5 to 10 depending on conditions.
This rule is based on the observation that roof material tends to collapse with time into the worked-out rooms. As it does so, the broken roof material bulks and gradually fills the upward migrating void. Stability is reached when the collapsing zone is completely choked with fallen material. Given assumed conservative bulking factors for the rock of between 10 and 20%, values of y between 5 and 10 are obtained.
This rule works well enough provided:
- the collapsed roof material remains in the rooms
- the roof is able to collapse
Roof material can be removed from the rooms by sliding down and washing down. Sliding down will only occur in very steeply dipping workings. Washing down can occur at lesser angles providing the workings are above the water table and there is water flow through the workings. If collapsing roof material is continually removed from the rooms then choking due to bulking cannot occur and void migration can continue to much higher levels than this rule suggests. If the roof is unable to collapse the stoops are left unprotected so are at greater risk of collapse.
2. The safe depth rule is an empirical rule which states that collapse in old workings will not cause subsidence at ground level, provided the workings are below a certain safe depth. This rule is based on local observations of the depths at which worked seams cause subsidence.
In past times the oft quoted minimum depth was 60 feet (about 18m). Safe depth is now considered to be between 30 and 50m below rockhead.
The safe rule breaks down if collapsing roof material is removed from the rooms permitting higher void migration or if areas of stoops collapse.
3. The age of workings rule is not so much a rule as a frequently held belief that when workings are several centuries old all consolidation and surface settlement must be complete. This is mistaken if processes such as roof collapse and stoop erosion are still taking place. It may apply if the voids are fully choked with collapsed roof material; the stoops and roof supporting pillars have either completely failed, or punched into underlying strata; or the stoops were completely removed on retreat.
4.The stoops do not fail rule is a widely held belief that in stoop and room workings it is the roofs over the rooms which fail, not the supporting stoops between the rooms. This belief is based on various factors, including the observation that extraction rates are frequently stated on mine plans to be less than 50%, the stoops left were generally much wider that they were tall, and the protection given to stoops by falling roof material. Only a small proportion of subsidence is caused by stoop failure but failure may occur due to:
- Robbing of stoops on retreat from a worked area leaving just enough material to give the miners a measure of protection.
- Strong roof material preventing roof collapse. The rooms are left open and the stoops are unprotected.
- Strong floor material preventing punching in of the stoops.
- Erosion and weathering of the stoops caused by running water, oxidation of pyrite (iron pyrites), etc..
- Undermining by deeper longwall mining, or voids migrating upwards from underlying stoop and room workings.
The importance of stoop failure is that such failure can affect much larger areas of ground surface and arise from seams at significantly greater depth than the more common plump hole failures caused by roof collapse.
Case history – Bathgate
A series of mining subsidences affected the Hopetoun Street area of Bathgate between 1975 and 1977. Details of the subsidences are shown in the table. The Balbardie Coal Seam lies above the Jewel Seam and the Jewel Seam lies above the Bathgate Main Seam. The coal seams dip to the west.
| Subsidence number | 1 | 2 | 3 |
| Commencement | December 1975 | February 1976 | December 1977 |
| Maximum settlement (mm) | 115 | 235 | 82 |
| Area of subsidence (metres2) | 1500 | 1000 | 200 |
| Volume of subsidence (metres3) | 275 | 330 | 50 |
| Thickness of rock cover to workings (m) | 34 | 18 | 12 |
| Thickness of Jewel Coal Seam (m) | 1.5 | ||
| Interval to Bathgate Main Coal Seam from Jewel Seam (m) | 15 | ||
| Thickness of Bathgate Main Coal Seam (m) | 2.0 |
For subsidence 1, measurements commenced after subsidence had started so these values are an underestimate.
Mode of failure
A combination of adverse factors appear to have been responsible for the Hopetoun Street subsidence, these being:
- High rate of extraction – 65 to 75%
- Small stoop sizes – 2.7m wide or less
- Workings more than two centuries old
- Steep dip of 22o
- Strongroof and floor strata – mainly sandstone
- Proximity of workings in the Jewel and Bathgate Main seams
- Workings partly above mine water level
This combination of factors resulted in small stoops having to carry large loads for a long period of time whilst being exposed to weathering, water erosion and general degradation. The most likely explanation of the subsidences is that an area of such stoops collapsed in December 1975 to give the first subsidence, followed by other collapse up-dip as greater stresses were thrown onto adjoining stoops. The sequence of subsidences only ceased when they reached the level at which the Jewel Seam met the overlying glacial boulder clay.
Subsidence Number 1, which eventually resulted in the demolition of some of the affected property and grouting beneath adjacent property broke the following rules:
Seam thickness rule
- Rock cover to the Jewel Coal seam was 23 times seam thickness
- Combined rock cover to the combined Jewel and Bathgate Main seam thickness was 14 times seam thickness
In both cases this is substantially more than the usually quoted limiting figures.
Safe depth rule
- Total depth was 42m to upper workings and nearly 60m to lower workings
- Rock cover to upper workings was 34m
These depths are greater than the 60 foot and 30m depths commonly quoted, but a little less than the more conservative 50m sometimes used
Age rule
- The subsidence area was already shown to be undermined by old workings on a mine plan dated 1777
Subsidence, therefore, took place more than two centuries after the mining occurred.
Stoops do not fail rule
- The subsidence areas were far greater and the subsidence volume far larger than could be accounted for by normal roof failure
It is likely that the subsidences were caused by areas of collapsing stoops.
Bathgate Old Times 