250 THE COLLIERY GUARDIAN. July 31 1914 and, if improperly constructed, will fail before the arch is taking its full load. In fig. 8 are shown proper shaped skew-backs, and it is advisable to have these shaped in one piece. More particularly does it apply when the arches are made of brickwork in concentric rings; otherwise as each ring may be imperfectly bedded, so that as the load is applied, instead of the whole arch taking the load simultaneously, each ring takes it until it fails, and the load is transferred to the next ring, and so on. ■If the arch is built up on vertical walls, as in fig. 7,- and a squeeze comes on in the direction of the arrow S, it can be seen that neither the w’all nor the arch can offer any substantial resistance, and deformation on the dotted lines is bound to occur sooner or later. More- over, it must not be overlooked that one-half of the load on the arch y in fig. 7 is transferred and concentrated on to the small area represented by the footing of the wall x, and this great pressure may result in the ground in the invert squeezing into the roadway, as it offers the least line of resistance. Much depends on what the footing of the wall is founded; but care should be taken to see that it is not overloaded, and, if necessary, the footing of the wall made wider in order to spread the weight and reduce the pressure per square foot. In fact, the vertical side-walls should be designed something after the style of a retaining wall, with the base thicker than the top,, the proportion to be determined by the nature of the ground. In practical arching in mines there are two systems of construction based on opposing theories. Some hold that the excavation should be taken out very generously, leaving a wide annular space between the back of the arch and the excavated hole. This is especially the case in the roof. After the arch has been turned and com- pleted, the space referred to is filled in with ashes, sand or other soft filling material; the idea being that Fig. 9. R.S.J / \ ' u > \ < K d K H I * I! f"/ V f J / / ,/ 1/ Fig. 9a. any squeeze would gradually compress and consolidate the ashes, thus protecting the arch from shock, and more or less convert a concentrated load into a distri- buted load. The ashes produce a cushioning effect, and although this principle may be right to a certain degree, yet it cannot be denied that by the ashes compressing it allows the abutments or skew-back to move. In this way, on the application of the load, the arch flattens owing to the spreading effect of the abutments yielding. Having this in view, the suggestion has been proposed that the abutments should be made as shown in fig. 8 in black. The spaces behind the arches between these abutments should be filled to back right up to the solid formation, so that there is no fear of spreading. The remainder of the arch could be filled with ashes, if deemed necessary for the protection of the arch. If the arching is a four-centre one as shown in fig. 8, - then each abutment forms a double skew-back, with each face formed and shaped to the proper angle. This is necessary, as each arch would then be capable of taking its load without any fear of the junctions giving, and at the same time it would still fall in with the principle of those who adhere to the ashes packing. On the other hand others consider that the arch should be tightly packed and leave as little space as possible. It would be advisable to use concrete for this, as it would set hard and not be affected by the water. Both of these theories evidently aim at the protection of the arch proper, and this course seems very desirable when the arches are constructed of masonry or brick- work, or any material with adhesive joints. If the principle is accepted that the load or squeeze is the result of movement and more or less proportional thereto, then the ashes, by easily compressing, give more latitude for the surrounding stata to move, and any ordinary filling will also compress a little, as it is impossible to get it back and fill up every little space so tightly as it was in its original state. It appears desirable to arrest any movement as quickly as possible, and it has been suggested this might be done by leaving the space between the back of the arching as small as possible and filling it up with concrete. This would then render all solid and prevent undue distortion of the arch; in fact, the concrete filling would act as a relieving arch. Rolled steel joists embedded in concrete would appear to be a move in the right direction, as the steel joist need have very few joints or connections; but it is rather expensive, and each joist being independent of the adjoining one, does not form any lateral bond. Each joist has to take the load without any assistance from the others, and for this reason is not economical, since more material is required. Concrete is comparatively cheap, and if the arch could be made in situ and turned, there is no doubt it would be stronger than either equal thickness of brickwork or rubble masonry, as all joints would be eliminated (see fig. 6). When subjected to tention, the concrete would stand from between 60 to 1001b. per sq. in. before breaking, which is much more than an adhesive joint between two bricks or blocks of stone. As concrete is capable of standing something between 2,000 to 4,0001b. per sq. in. in compression, it is obvious that only about one-sixth of its compressive strength would be developed when it would fail in tension; and to remedy this rolled steel joists have in one instance been enveloped in the concrete 2 ft. to 3 ft. in thickness. The inside diameter of the barrel was lift., and it has stood very well for a number of years (figs. 9 and 9a). Fig. 10. Fig. 10a. Fig. 11. Fig. 13. Fig. 12. However, it will be seen that the concrete does very little duty beyond spanning between the joists. Any concentrated load would be carried by one joist only, as the concrete would fail in shear before transferring much of the load from one joist to the adjacent joists. If, instead of using rolled steel joists, round steel bars had been used, as shown in figs. 10 and 10a, the arch would have been much stronger and more econo- mical, as less steel and concrete would have been required. As will be seen, by the interlacing of bars in both directions, any load is distributed and the steel is used to better advantage. The reinforcing bars would pro- vide against both tensile and compressive stresses, and the whole arch would be without any joints; in fact, it would resemble a huge casting, and in order to demon- strate the great strength of this kind of construction, i.e., without joints, it may be of interest to refer to a ferro-concrete arch made in Austria some years ago. The arch was made of ferro-concrete, of 32 ft. clear span, with a rise of only one-tenth, i.e., 3-2 ft. The arch was 6 in. thick at the crown and 8 in. thick at the springing, and fig. 11 represents the span and thick- ness to scale. This arch had a moving load over it in the shape of a locomotive weighing 53 tons, and the deflection was very small. With a running locomotive the worst test is made as it approaches on one haunch, then gets over the centre, and then loads the opposing haunch, so that you get a very quick reversal of stresses. After the locomotive test the arch was loaded up on one-half of the span only, until they had piled up some 220 tons, as shown in fig. 11. This represents something like 2,3501b. per sq. ft. The arch then failed, only because the abutments gave way. This example not only shows the great strength of ferro-con- crete, but also illustrates the importance of unyielding abutments. Although the strength of ferro-concrete has been illustrated, it may be thought that it would not stand any pressure from the inside, or sudden shock, and the following example will be of interest. At Lievin, in the Pas-de-Calais, a series of experi- ments were carried out to obtain some data with regard to the explosions of coal dust. x\ gallery some 700 ft. long was constructed, being in area some 81 sq. ft. The first portion for about 100 ft. was made of ferro-con- crete, as per fig. 12, 7 in. thick and strengthened with counterforts every 5 ft. The remainder of the gallery was made of steel tube, 7 ft. diameter and fin. thick, with timbering on the inside, and the outside of the tube was covered with earth. The ferro-concrete work was designed for a pressure of 571b. per sq. in., or 3*6 tons per sq. ft. However, in some of the experi- ments, the pressure must have gone up to 1141b. per sq. in., or 7’2 tons per sq. ft., and it may of interest to quote from M. Taffanel’s report, dated November 4, 1909 “We tested some coal dust a little more favourable than those previously investigated. We were unable to determine the exact pressure, because the gauges were damaged, but the explosion was formidable; the steel gallery gave way for the last 40 ft. Three fragments of steel plate,* several timber linings, and the earthen embankment took part in a magnificent display of fire- works, and fell on all sides at distances ranging up to 500 ft.” The result of this report speaks for itself, and gives Fig. 12a. ample evidence of ferro-concrete to bear big pressures and also instantaneous shocks. Another instance may be quoted of some colliery arching in the Nuneaton district, and in fig. 13 is shown a longitudinal section of some ferro-concrete arching constructed in 1912. It appears that prior to this brick arches had been made, and these failed from time to time, and the arches shown in figs. 14 and 15 were put in. The arch in fig. 14 is 18 in. thick at the crown and at the springing it is 3 ft. 9 in., while the invert is 2 ft. 6 in. thick. In fig. 15 the thicknesses are about the same, but the over-all dimensions are much larger. The platform in the middle of fig. 15 acted as a roof for the chamber, under which is 17 ft. wide and 7 ft. 6 in. high. This chamber is used as engine or pump-house. During construction some little difficulty was experienced at first with water percolating through the clod, but this was eventually overcome, and the arches are now watertight and standing satisfactorily. It is interesting to note that after the concrete in the arch had set, the irregularities and space between the outside of the arch and the roof were packed with concrete; and with respect to the arch it is also interesting to note that no settlement or distortion was noticed prior to the setting of the concrete. In an arch of this kind the concrete will get harder as time goes on, and the strength will increase with age. Any tendency of the arch to spread through the application of a load on the roof would be noticed at the junction of the arch with the invert, but this is prevented, as the whole is monolithic, and the steel reinforcement in the invert, which is continued right up into the arch, prevents this.