Only a negligible proportion of coal is mined from under the sea in the great coal producing centres, and neither of the large coal producing countries -- the United States and Russia -- mines submarine coal.
The known submarine areas from which coal is mined at the present time, are Chili, Japan, the East and West Coast of Northern England, under the Firth of Forth in Scotland, on Vancouver Island, in British Columbia on the West Coast and Cape Breton Island, Nova Scotia on the East Coast of Canada.
Submarine coal has been mined in Great Britain for a great many years, but the percentage from this source has been very small, not exceeding 2% to 3%. In contrast to this, nearly 88% of the coal produced in Nova Scotia comes from the undersea workings in the Sydney Coal Field, and as 80 per cent of the entire coal resources of Nova Scotia are submarine, the mining of submarine coal is of tremendous importance to those engaged in the industry in this province.
The pioneer colliery in submarine mining to any appreciable extent in Nova Scotia is Princess Colliery of Old Sydney Collieries, Nova Scotia Steel & Coal Company, where the workings first went under the sea in 1877. This mine has been in continuous operation ever since and at the present time the active workings are 3.2 miles from the shore although the maximum distance penetrated seaward along the main slope is 3.4 miles, and the maximum cover 1970 feet under the sea bottom.
Except for three small independent collieries producing a total of less than 1,000 tons per day, all other operating collieries in the Sydney Coal Field are submarine. The Dominion Coal Company operates four collieries in this field and the entire output from these is submarine.
The workings in these mines extend from a minimum of 0.7 miles to a maximum of 2.2 miles from the shore.
It may be of interest to note that Dominion No. 4 Colliery, recently closed, had penetrated further seaward than any other colliery in the field, the main deeps having reached a distance of 3.6 miles from the shore. It is also of interest to note that the deepest submarine mine in Nova Scotia, or probably anywhere, was, prior to its closure, No. 2 Mine of the Inverness Coal & Railway Company, where nearly 3,000 feet of cover had been reached.
There is a greater concentration of mines operating in the submarine coal in the Sydney Coal Field than exists elsewhere, and the workings extend further seaward than those in the British oversea mines, with the exception of those on the Cumberland Coast, where a series of upthrow faults makes the winning roadway comparatively level, allowing of long extension at comparatively moderate depths.
It is necessary in mining submarine coal to take precautions to prevent flow from the sea, by leaving in pillars of sufficient size to support the sea bottom until a certain depth has been reached, at which it is considered prudent to remove the pillars or extract the whole seam.
Disasters in mining submarine coal have not been unknown in the past, the most notable being in the north of England in the early thirties of the last century and in Japan within comparatively recent years; whilst in Nova Scotia, two mines on the West coast of Cape Breton Island were flooded by the sea breaking in. In Mabou in 1909 and Port Hood in 1911, respectively.
The mining laws of Nova Scotia prohibit the extraction of coal at a depth of less than 180 feet of solid cover under the sea, although Section 104, Subsection (i)A permits roadways to be driven to win the undersea coal at a lesser cover than 180 feet, but not less than 100 feet of solid measures between the roadway and the sea bottom.
In the Sydney Coal Field it has been the practice to restrict the width of rooms up to the 700 foot cover line to 20 feet wide and crosscuts 18 feet wide and not to extract pillars under less than 700 feet of solid measures between the workings and the sea bottom. In some cases a balanced room and crosscut system is developed, each place being 16 feet wide, with a maximum extraction of 59 per cent, 41 per cent being left in pillars to support the sea bottom.
The method of work and the percentage of coal extracted in each lift must be approved by the Chief Inspector of Mines for the Province.
The practice has been to vary the size of pillar from 32 ft. x 48 ft. to 47 ft. x 63 ft., according to the depth of the workings. These pillars give an extraction of 50% and 41% respectively Although in past years pillars have been drawn where local conditions were suitable, the present practice is to revert to longwall when sufficient cover is gained to permit total extraction.
In the case of mines operating under land, it is always possible when the workings reach an excessive distance from the shaft bottom, to sink another shaft near the working faces, if not for coal hoisting, at least to lessen the distance the miner has to travel to work. But in submarine mining, when the shafts or slopes are located on the shore, there can be no such relief and the roadways and workings must progress seaward until a limit is reached at which it is no longer economically possible to mine coal. The distance it is possible to penetrate seaward will depend on the nature and thickness of the seam, the character of the roof and floor, the rate of dip which determines depth, and the amount of water encountered. These are physical conditions over which there is no control, but from the economic standpoint the proximity of competing mines under more favorable conditions must have a material bearing.
Apart from these factors, the life of a submarine mine may be increased or lessened, dependent on the care and forethought of the operating company, the points which require particular attention being:
(1) General layout of workings
In regard to these points, the submarine mines of the Dominion Steel & Coal Corporation will stand favorable comparison with those anywhere in existence.
As regards the layout of workings, the most experienced engineers in submarine mining have been consulted and reported on the operations from time to time.
The policy of constructing main roads of as permanent a nature as feasible has been adopted.
The reconstruction of airways and the adoption of a policy of providing adequate ventilation for future working which started following 1920 has been continued and is being vigorously maintained. Approximately half a million dollars is spent on this work each year and is probably the most necessary of all expenditures -- one on which there is no immediate return -- but which safeguards the future of the industry and the coal resources of the province.
Unless main roads and airways are constructed in a substantial and fairly permanent manner, the upkeep of these roads as the workings extend seawards would preclude the possibility of further mining due to the heavy cumulative maintenance costs, and this is a cost which submarine mines must bear to a much greater extent than those mining under land.
All submarine mines of the Corporation, except No. 18 Colliery, are equipped with special means other than the coal hoisting equipment to transport the men a considerable part of the way to their working places. The Corporation transports its workmen a total distance of 15 miles toward their working places, eight miles of roadway being especially equipped and maintained for this purpose.
The total extraction of the coal seams in the undersea working is dependent on the nature and the thickness of the superincumbent strata over coal, the thickness of the seams and the position of the seams in the order of working.
In this field it is considered prudent to err on the safe side and to permit total extraction only when the cover over the topmost seam and the sea bottom is not less than 700 feet.
In general, where the topmost seam is worked first, 700 feet of cover is considered necessary with the additional thickness of the intervening strata between subsequent seams to be worked added to the initial 700 feet.
For example, if total extraction in the Harbour Seam is commenced at the 700 ft. cover line first, then total extraction of the underlying Phalen Seam would commence at 1100 ft. of cover, depending on the thickness of the intervening strata between the Phalen and Harbour seams. In this instance, the combined thickness of the coal as at Nos. 12 and 16 in the New Waterford district would be 11 ft., or approximately 100 ft. of strata for every foot of coal extracted.
If, however, the Phalen Seam, a lower seam in the stratigraphical column, was worked first and total extraction commenced at the 700 ft. cover line in the Phalen Seam, total extraction of the overlying Harbour Seam would not commence until the 1100 ft. cover line was reached in the Harbour Seam.
If the Hub Seam, which overlies the Harbour Seam, was worked at the same time as the underlying seams, then the minimum cover at which total extraction in the Hub Seam would commence would not be less than 1200 ft. and more likely 1500 ft., adopting the general basis of 100 ft. of solid measures for every foot of coal extraction.
The policy followed by the Corporation in respect to total extraction under the sea has been substantially approved by expert advice obtained by the Corporation in 1927.
W. Forster Brown, in his report dated April 1927 on the subject of total extraction, states on page 5:-
However, in 1930 when the above wording was questioned, he replied under date of April 3rd, 1930:-
It will be noted that the opinions expressed by W. Foster Brown still leave an ambiguity. Therefore, in practice the comparison in respect to its policy for total extraction in the undersea area was on the side of safety.
Further, the policy as applied to the total extraction of contiguous seams simultaneously, as in the Victoria and Lingan Seams in the New Waterford area, the upper seam is extracted in advance of the lower seam with a time lag of approximately 6 years, or one working lift; and where the lower seam is extracted in advance of the upper seam, a minimum time lag of four years should be the aim, with all main roadways for haulage and ventilation superimposed in substantial pillars of solid coal.
Consideration has been given in the past to the replacement of coal pillars left in at lesser depths, by sand or other debris by hydraulic stowing, but the increased cost which this method entails precludes its use.
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Last Modified: 98-01-06
Authored by: Louis Frost
The information contained on this site is not provided for the purpose of factual
representation. Instead, it is provided in an historical context. Every effort has
been made to ensure that this information represents the actual content of the
original document authored by Louis Frost for the Dominion Coal Company
on or around 1962. Nevertheless, no warranties are provided in any respect.
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