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July, 1955

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Low Main seam, Horden Colliery

The investigation was carried out in conjunction with Mr. G. R. Rowell, Divisional Strata Control Engineer, who was responsible for measuring the support loads on the face during the time the ploughability tests were made. We are indebted to him for the prop-load results included in this section.

Many unsuccessful attempts were made to plough the face concerned, between October, 1952, and April, 1954. Experiments with pre-cutting, water infusion and shot firing failed to render the coal ploughable, and the plough was finally removed in April 1954. Since that date the face has been worked by conventional methods.

Seam section. The Low Main seam is 54 in. in thickness and consists chiefly of clarain banded with durain. A strong 3-in. durain band occurs in the middle of the seam and iron pyrites was found immediately below it. Two dirt bands ½ in. and 1 in. thick are found at 6 in. and 8 in. from the floor respectively. The bottom 2 in. of the seam are of durain varying to band. (Fig. 21.)

The cleavage plane development is very poor and in many cases appears to be non-existent.

The tests were carried out on a 160 yard single unit retreating longwall face and in an area of solid coal adjacent to it. The face was at a depth of 1,200 ft. and retreating over the Hutton Goaf. The right-hand gate ran approximately along the line of the Hutton pillar edge. (Fig. 22.)

The face was supported by Dowty props on 6 ft. bars, the waste packs were 12 ft. wide and built at intervals of approximately 55 ft.

The support loads were measured using 40 Dowty props fitted with pressure gauges. A minimum of 16 props was employed at any one time and these were set in two rows of eight immediately behind the test stall.

A visual inspection of the face showed that the coal was standing well, between the tailgate and a point approximately 60 yards down the face. From this point to the main gate the coal was spalling off the face to a depth of up to 2 ft. From the tailgate to a point about 55 yards down the face a roof break was visible at the coal head. It had the appearance of a cutter break and according to the men, was formed each day.

Test results

The first set of test stalls was made by pneumatic pick at a distance of 47 yards from the tailgate, the stall being made each day immediately the face was clear.

The first tests were made at 24 in. from the floor with the 45 deg. planing blade taking a 3-in, cut. (Fig. 23.) This mid-seam position was selected as we thought that the roof load on the coal edge would have a maximum effect at this point.

A cutting force of approximately 8,000 lb. was required on successive days. It was also noted that the coal planed off was only 6 in. in height. The total load on the block of 18 dynamometers set behind the stall was 158 tons and 168 tons on successive days. The length of face covered by the block was 29 ft. on the first occasion and 24.3 ft. on the second occasion. Attempts to take a 6-in, cut were made on both days and both failed.

On the third day tests made at the same horizon encountered pyrites and the cutting force required to take a 3-in, cut jumped to 15,000 lb. Further tests on the same day which did not encounter visible pyrites also required 15,000 lb. (Fig. 24.)

The block of props on this occasion covered a distance of 25 ft. and the total support load on the block was 193 tons.

The test site was moved to a distance of 74 yards from the tailgate into the soft coal area. A test stall was again made and the Dowty dynamometer props installed.

The first 24 in. of coal was removed by hand since it was standing in a broken condition and was useless for testing.

A test at 24 in. above the floor, taking a 6-in, cut with the 45 deg. blade, only required approximately 2,300 lb. A 3-in, cut at floor level with the same blade required only 1,000 lb.

Further tests taking 5-in, cuts at floor level, again required under 4,000 lb. (Fig. 25.)

The load on the dynamometer props, covering a face length of 25 ft. was 269 tons.

The measurement of prop loads continued on this section of the face for 18 days and block loads varying from 269 tons to 321 tons, averaging 309 tons, were recorded.

A considerable number of ploughability tests were made in a panel of solid coal, adjacent to the retreating face, at floor level, and at 15 in. and 24 in. above the floor. On all occasions, cutting forces of between 6,000 and 8,000 lb. were required. These forces are similar to those obtained in the first two days' tests on the face.

The tests made at 15 in. from the floor were made with an undercut at floor level, yet no reduction in cutting force resulted. This was probably a function of the cleavage plane development.

Conclusions

(1) There is strong evidence to suggest that a relationship exists between ploughability and support loads on this face, but the possible influence of the roof break on both should not be overlooked. The presence of the Hutton pillar edge should also be remembered.

(2) Between the tailgate and a point about 60 yards down the face, the ratio of the maximum cutting force required for the Low Main seam to the maximum cutting force required for the Busty seam appears to be 2 :1, with more difficult spots having a ratio of up to 4 :1. If it is also remembered that a cut with a 6 in. high wedge only takes 6 in. of coal and in addition does not substantially weaken the remainder of the section, it will be appreciated that a very high force indeed is required to win the full 4 ft 6 in. section.

(3) Between the main gate and a point 60 yards from the tailgate the coal ploughability compares very favourably with that of the Busty.

(4) Where the coal was more difficult to plough, the prop loads were less than in the section where the coal ploughability was comparable to the Busty.

(5) The ploughability in the solid coal area where static load conditions existed, was comparable to that on the section of the face where ploughing was difficult.

B Seam, Waterhouses Colliery

At the time the tests were made at this colliery a Gusto Multi-Plough was working on the face concerned. Full details of this installation and its operation have already been given in a paper to the N.A.C.M. (Northumberland and Durham Branch) by Messrs. Brighton & Hill.

Seam section

This seam is 19 in. high and consists mainly of well cleated vitrain. The coal is friable and contains very few dirt bands, those that do occur, however, are near the bottom of the seam. (Fig. 26.)

The floor parting is uneven and irregular and on occasions inter-grown. The immediate floor is a seggar containing thin dirt partings. A good parting exists between the coal and the roof. The seam is naturally wet throughout.

The Gusto Multi-Plough face, on which the tests were carried out, was a 120 yard, single unit advancing longwall face, Fig. 27. The face was supported on wooden props and 5-ft. steel bars. The two gate roads were maintained approximately 10 yards in advance of the face, the stone from them being stowed in the goaf using a slusher. (Fig. 28.)

At the time the tests were made, the face was being worked through a fault from which a considerable quantity of water was flowing. When first encountered, this fault displaced the seam 6 in. The throw increased, as the face advanced, to at least seam thickness.

A test stall was made on the face at a distance of 18 ft. from the left-hand side of the tailgate. A block of eight electrical-resistance strain-gauge dynamometers was set on the props immediately behind the test stall.

A series of tests, Fig. 29, was made at floor level taking a 3-in, cut with the 45 deg. blade. The normal resistance is evidently about 3,000 lb. with peaks of up to 5,000 lb. when heavier concentrations of band are encountered.

Similar tests made at 6 in. above floor level required less than 1,300 lb.

The force required to take the coal at floor level after an initial cut at 6 in. above floor level was less than 2,000 lb. except when concentrated band was encountered, in which case no reduction was effected.

A series of tests was made taking only a 2-in, thick slice of coal. Very little, if any, difference existed between the force required for the 2 in. compared with the 3-in, cuts. This is probably due to the fact that the band, bedded in what can be regarded as a coal matrix, is continuous around the cut and is just as likely to be firmly held at 2 in. from the free face as at 3 in.

The tests on the face were made over a 7-day period during which the face only advanced 4 ft. The dynamometers remained in their original positions over the whole of this period, the average total load on the block of eight props being 48 tons. The support loads did not relate to the normal rate of advance but, at least they demonstrated the efficiency of the slusher packing.

Tests made in a 9-yard coal pillar between this and the adjacent face only confirmed the results already given.

Conclusions

(1) The chief source of trouble encountered on this installation was the inability of the ploughs to maintain their cutting horizon. Crop coal was left or the floor was ploughed up.

From the test results it can be seen that the ratio of cutting force required at floor level to that required at 6 in. above floor level is 5,000 lb. 1,300 lb., i.e., approximately 4 :1. This probably explains why crop coal has been left on numerous occasions, the ploughs having evidently selected the coal which has the least resistance. Where soft spots occurred in the floor, they selected the floor.

(2) The ratio of the maximum cutting force required for the "B" seam to the maximum cutting force required for the Busty seam at floor level is 5 4. This cannot be considered excessive and one can only come to the conclusion that this seam should plough and it is on record that at first this installation was very successful. It appeared that many of the difficulties at the time of the tests were associated with the fault.

(3) Horsepower calculations of the type already described showed that 564 b.h.p. should normally be sufficient for the six ploughs cutting the bottom 12 in. of coal (tops falling). As the installation is powered with two 50 b.h.p. motors this would leave 43.6 b.h.p. to overcome the various frictional forces and drive the conveyor.

The force exerted when the motors develop 56.4 b.h.p. for planing is 16.6 tons. It can be shown, however, that if the six ploughs cut the floor to a depth of 3 in. the total force required can rapidly mount to a theoretical maximum of 41.8 tons.

It is interesting to note that rope breakages have been experienced on this installation, and we understand that the original rope employed had a breaking strength of 32 tons. This figure is twice that to be expected with normal cutting, but it would be easily obtained if three or four ploughs were cutting the floor to a depth of 3 in.

General conclusions

(1) The apparatus devised and used for these tests is satisfactorily serving the purpose of these investigations.

(2) The results of repetitive tests at a given horizon in a particular seam show sufficient agreement to be of scientific value.

(3) It has been shown that a pre-cut in advance of the normal blades can in some cases be of great value. The importance or otherwise of such a cut can be determined.

(4) That the angle of the cutting tip is a very important factor, has been clearly shown. The blade angle should be kept as near 30 deg. as possible, probably the smallest practical angle from a strength point of view.

(5) Calculations of theoretical cutting force and power requirements, based on these experiments, show close agreement with actual consumption.

(6) The tests at Horden appear to indicate that a close relationship exists between coal ploughability and the loads carried by the face supports. A thoughtfully conceived research programme will have to be conducted to establish the exact relationship.

(7) We believe that we have shown that a scientific approach can be made to both the problem of selecting a seam for ploughing, and also selecting the best design of plough for a particular seam.

Drawings and Photographs accompanying the article