The Surveyor’s Job At A Coal Mine
An instrument for levelling, or determining the difference in height between one point and another, referred to as a ‘dumpy level’, originally called a Gravatt level, after the inventor, was introduced in the railway mania of the 1840s. This instrument had a small spirit bubble attached to the telescope and was used in conjunction with a staff or stave, which was graduated with marks corresponding to feet and decimals of a foot. It was used sparingly in a few mines, usually along a main road. Many years later an easily portable staff with a linen / plastic tape which extended using a spring system, was devised for use underground, commonly called a ‘Gees’ staff, after the inventor. Similar staves are used today, but now graduated in metres and centimetres. Today levelling surveys are carried out in all roadways underground at a mine.
Information from The Craig Telescope
Thomas Fenwick, a colliery Viewer and Surveyor of mines, published a book in 1804, on the variations of the magnetic needle. ‘Loose needle’ surveys were being carried out at this time, this being the method by which the magnetic needle on the instrument was released and allowed to swing freely and upon the needle settling, the angle between the base line being used and the magnetic North direction given by the needle, being noted. This system was fine providing that there was no iron or steel in the vicinity, otherwise the needle would be attracted to it and give a false reading.
In the past special headings in coal were driven and any steel or iron was removed so that the Surveyor could obtain a ‘true’ magnetic reading down the pit. A system was adopted whereby an adjustment could be made for subsequent readings of the magnetic needle as the survey was carried on inbye to arrive at the correct bearing.
Also it was known that the magnetic North was to the West of True North and moved to a maximum position West then crept back at the rate of about 8 minutes of arc per year.During the year 2020 the two coincide so there will be no correction necessary. The magnetic North then swings East of True North. Also there is an erratic diurnal or daily variation of the needle that is at its steadiest at 4 am. It is not known whether any of these adjustments were taken into account on all or any of these early surveys. For this reason many old plans must be treated with a certain amount of caution, as to their accuracy.
It is apparent that in later years it was noticed that many underground surveys were done on the night shift to take account of the erratic behaviour of the magnetic needle during the day. After the 1930s the use of the magnetic needle died out, surveys being done using the fixed needle system.
There was certainly a mining knowledge passed down by older colliers of this system, as even in the 1950s, when if the Surveyors happened to be in the way of a job of work being done by others, say on the day shift, in their ignorance the call would come for them to do the job on nights - ‘like they used to’!
Photo Michael Minchin
Measurements were taken using a Gunter’s chain of 100 links, each 7.92 inches long, with a total length of 66 feet, or 22 yards (or a cricket pitch length) or 20.1m. It was difficult to use under poor and undulating conditions and there was also a knack of undoing it from the fastened up bundle. One held on to one end of the chain and cast it out with a sharp jerk, and the chain would straighten out into a line. Of course it was made of steel and was fairly heavy and would kink or sag easily, thereby giving an incorrect measurement, and if not underpacked when measuring in an undulating roadway gross error could arise. Great care had to be taken to ensure that the correct measurement was booked, particularly if the end of the line being measured was partway down the chain, because a series of tallies every ten links, beginning with one and increasing to 4 and from the centre round tally was repeated but, in reverse until 90 was reached. In other words it was easy to mistake a measurement of 40 odd links for one of 60 odd links, without due care and attention. At least two men are required to measure distances. Of course the opposite of undoing the chain had to be applied when the measuring was finished, as there was a knack of folding up the chain into a neat bundle for transporting. Also continued use of these chains created wear on the links and gradually lengthened giving an error in distance if not checked regularly and the relevant distances adjusted to true length. A long chain or tape will give a short distance.
These chains were still used in the 1950s at some pits where water and sludge on the floor quickly ruined a linen tape, which would tend to stretch and the printed readings would be obliterated also. This was the next step in the measuring field. The linen tapes were later replaced by more modern plastic or nylon type or steel, which tended not to stretch and the later flexible steel tapes and bands being plastic coated which were easily cleaned using a rag. The early tapes were graduated in links also and on the reverse side with feet and inches generally up to 100 feet long. The tapes were contained in leather cases originally. The later tapes were in metal cases and were in lengths of 5m, 10m, 20m and 30m. The steel bands were in lengths of 25m, 30m, 50m and usually wound on a cross reel and had detachable handles at either end. These bands had to be carefully used as they could kink easily or snap. All types of measuring tapes need to be cleaned after use to preserve them for further use as they could rust quite easily, rendering them unusable.
The chains were used as part of ‘pole and chain surveys’ which were carried out on the surface and later this exercise was a part of the Surveyor’s training, no survey instrument being necessary. Measurements of fields were undertaken with poles usually painted red or black and white and graduated in feet set in line around the extremities. All lines would be measured with the chain and offsets taken to the field boundaries. Cross ties for check measurement would be taken to give the polygon geometrical strength. A base line would be set out parallel to a hedgerow or road as an orientation or additionally a magnetic bearing could be taken along the main base to give the plotted field some direction if necessary. Distances across rivers or inaccessible areas can be calculated by geometry using this system.
Mining had increased at a dramatic rate in the 1800s and numerous pit shafts were sunk, to satisfy the insatiable appetite of the industrial revolution.Unfortunately many of these new mines were sunk in areas where coal had been mined before, sometimes many many years before, and all knowledge of them had been lost. Numerous inrushes of water or gas occurred, inevitably with loss of life on almost all occasions. A notable one being Redding colliery in Scotland, where 40 men were drowned by an inrush of water from an old pit in September 1923 only to be followed by another inrush in March 1925 at Montagu Main, Northumberland which claimed 38 lives. A more local colliery inrush was at Clay Cross where teens of men drowned.
These old mines were casually abandoned in the past and generally filled up with water. Even though the new mines were not necessarily working the same seam that the old miners had taken, the new seam may have only been a few feet below and the water would break in under pressure. Sometimes the same seam would be called by a different name, just to confuse things!
On occasion it has been found that a new seam has been called the same name as one that had just been abandoned, so that a different type of coal could be sold under the old name, because existing customers ordered on reliability, and a new name may have caused the customer to go to another mine in order to obtain the reliable coal.
Only in certain places would borings be made in advance of the coalface or heading, where it was thought that old waterlogged workings could be. The Surveyor would set out the boreholes and monitor the distances and plot them on the plan of the workings. This type of work in the past hindered the coal output and would not be done unless imperative, whereas today stringent rules on the boring for old workings takes precedence over production in the face of safety.
Examples of this type of boring can be seen on plans of Sutton colliery in the Meden Valley, dated 1884 notably after the inrush at Molyneux colliery in 1869 where 4 men and a donkey had drowned.
Later, the use of boring holes in advance of a heading with flanking boreholes overlapping to stringent directions would become mandatory when approaching known bodies of water. Also the use of borehole survey instruments (e.g. Pjajri Tropari) with clockwork mechanisms were used to enable the position of a borehole to be plotted along its length, particularly when drilling from one horizon to another, e.g. to be able to drill from a gateroad in one seam to a gateroad in another seam above, so that the water could be drained, through a proper pumping scheme, therefore allowing workings to advance beneath a previous ‘no go’ area with complete safety. A typical machine used for this purpose was a Burnside boring machine used in conjunction with a stuffing box.
Surveys of deep boreholes from the surface were done using several methods and one major technique was by the Schlumberger method. The position of the borehole at different depths would be calculated and the position of the hole at the relevant seam horizon would be plotted on the plan of that seam. Some boreholes veered off direction quite considerably and it has been known for the position at the base of the hole to be over 100 yards different at about 700 yards deep to the position of the hole at the surface, as at Blidworth colliery in the 1950s. Here a panel was approaching a borehole position at the Top Hard horizon. The gate was turned to avoid it but the panel still struck the borehole position that was measured some 100 yards out of position. Fortunately the open hole was dry. It had been thought that water would cascade down the hole from the bunter sandstone measures above. Later boreholes would be completely filled with special cement to make sure in future that the problem with water could not arise. A full seam thickness borehole core was recovered by me at Ollerton in the 1970s on 1s face in the Parkgate seam. The borehole core was within 2m of the position I had informed the workmen about.
Boring would be done over a period of months or years and an advance of around 5 feet a day would not be unusual. It was only during the 1970s that due to the NCB’s ‘Plan for Coal’ many boreholes needed to be drilled for exploration and these were contracted out to a Canadian firm who completely revolutionised the boring process by drilling deep holes in 2 or 3 weeks instead of the 5 or 6 months expected. It was always thought that slow boring in the coal measures would enable a good core recovery and that fast boring would destroy the core. They proved that fast boring was better and excellent core recoveries were made. This system is carried on today.
It was suggested in 1797 by William Thomas that an establishment in the form of a Records Office at Newcastle be started for the collection, registration of plans and sections of the various collieries in the district, one of the principal objects in view being the prevention of future accidents from unknown colliery workings filled with water or gas being unexpectedly encountered.
However it was only in 1845 that the Government appointed a Keeper of Mining Records, Robert Hunt, but these records were only voluntary donations of plans and information regarding old workings.It was hoped that all coal owners would send their old plans and information, if any, in to this office which was located in the Science Museum in London. However, there was much secrecy about where the best coal was or might be, etc, so there was a great reluctance on the part of the owners to do this, as many neighbouring mines were in competition with one another.
The correlation of the underground workings with the surface continued, using the free swinging magnetic needle method, albeit using somewhat advanced methods of simultaneous magnetic readings with two different instruments, one on the surface and one underground right up to the late 1930s.
Where the coal workings are accessed by an adit or drift from the surface, a straight-forward traverse is carried out using a theodolite or dial and measurements taken from the surface baseline at the mine down the drift and into the workings where another baseline would be set up in settled ground and from this further surveys would be made to the workings inbye.
Other methods of correlating the underground workings with the surface where shaft access was the only way had been devised and perfected. Several systems using wires hung down a shaft, weighted at the pit bottom were tried, the Weiss system in 1851, with 4 wires, and the Weisbach system with 2 wires and sighting small angles in 1857.
Both these systems were based on sightings to the wires from a close distance with an accurate instrument and using triangles to work out bearings. A third method used widely was the Co-plane system, where again an instrument was set up close by, but this time the instrument was lined in with the two wires and the same bearing was transposed and a base line was created. This was a popular method as fewer calculations were required. The instrument used was a theodolite. Observations would be taken to the wires at the top and bottom of the shaft simultaneously and sometimes from insets in the shaft at other seams between the shaft top and the pit bottom.
However all these systems relied on the steadiness of the wires, (high tensile piano wire being used), and the erratic oscillation used to be dampened down by immersing the heavy plumb-bob weights in water and later barrels of water and oil. It also necessitated the mine fan being switched off for a time to stop the turbulance of the wires, which used to swing in very erratic and irregular patterns for hours on end, depending upon the depth of the shaft or natural ventilation current. Sometimes the wires would be observed from two points at right angles in the shaft bottom for maybe several hours and then clamped in the most probable position, to make the job of observing the wires from the instrument easier. I assisted on such a job in Sutton Colliery downcast shaft sump where readings on 2 scales at right angles were conducted until a mean was achieved.
These correlations were very time consuming and heavy on manpower (up to a dozen Survey staff and half a dozen other pit personnel such as blacksmiths to assist) and could take a period of two days to complete, usually over a weekend, or pit holiday period when normal coal working was at a minimum. It also meant that that particular shaft could not be used for that period. Sometimes correlations were curtailed due to build up of dangerous gases and the mine fan would have to be restarted. Mine managers were always reluctant to allow correlations to be done but of course it was necessary and usually arrangements were made about a year in front so that jobs could be rearranged for that period.
Of course all these systems used a base between two wires, and it must be remembered that until the mid 1800s most shafts were only up to about 9 feet in diameter. In the latter half of the century the diameter increased to around 13 feet. After the turn of the century they increased to around 18 feet at the bigger and deeper mines, and by the 1920s up to 24 feet diameter shafts were not uncommon.
In a 9 feet diameter shaft, allowing clearance from the sides, a 7 feet base would be the maximum.Allowing the same criteria, in a 24 feet diameter shaft it was possible to have a base of 22 feet. However in our local area many shafts were in the region of 13 to 14 feet diameter. This would only allow base lines of around 11 to 12 feet between wires. Allowing for the best possible conditions for the wires to settle, a difference of only a fraction of an inch would be magnified greatly, and using a rough rule of thumb method, one minute of arc is equivalent to 1 inch in position over 100 yards distance (0.025m over 91.44m). By the same rule 1 degree of arc is equivalent to 60 inches or 5 feet (1.524m). If the workings extended inbye to 1 mile, then an error of 1 minute of arc would give an error of 17.6 inches (0.447m), and assuming an error of 1 degree, the workings could be in error by 88 feet or 29.3 yards (26.82m). This is assuming that all the error was one way and that no other compensating errors were present.
Lead collets were loosely fastened around the wires at the top of the shaft and allowed to fall, and several timings were taken at the pit bottom when the collets arrived to make sure that the wires were not fouling the shaft side or any of the shaft furnishings.
Other methods were experimented with such as sighting down the shafts using theodolites with long telescopes but of course there was still only the short base due to the shafts being of relatively small diameter and the system was not very successful.
Nowadays the correlation of the underground workings with the surface is done using a Gyro theodolite. This instrument developed in Germany during the 1940s is powered by a 12v battery. A high revving gyroscope around 25,000 revs per minute connected to a special theodolite searches out, and by a series of complicated calculations etc, the North Magnetic pole direction is found. Various other adjustments are made after firstly testing the instrument on a known guaranteed surface baseline before the underground or surface lines are checked or established by the instrument, and again after the survey. The local base used for orientation was at Nottingham University. This system used in Britain since about 1970 does not require the shaft to be occupied by the Surveyors, other than for a positional mark to be transferred underground from the surface, again using modern optical methods, as a starting point. This point can be established at a different time, and can be one that was established by other conventional means. The Gyro theodolite can be used underground inbye in any roadway where the methane gas content is less than 1¼ %, with the permission of the Inspector of Mines. Again the system is time consuming but not too heavy on manpower but does not in many ways hamper the normal running of the mine and observations can be made on a normal coal shift. Further calculations are made to obtain bearings relative to Grid North for plan work. I was the first in the North Nottinghamshire Area of the NCB to use this system in 1971.
Mine plans were first required to be kept by law following the Coal Mines Inspection Act, 1850 when mine owners were required to produce an ‘accurate plan’ of the mine. At this time a Mines’ Inspectorate was established, and although the Inspector could request that the up to date workings be marked on the plan, he could not take a copy. The mine owners were still full of mistrust as in the past, and secrecy was paramount.
The Coal Mines Act of 1855 strengthened the Inspectorate’s position, with four more Inspectors being appointed, and introduced penalties for proved offences against the Act.