THE COLLIERY GUARDIAN AND JOURNAL OF THE COAL AND IRON TRADES. Vol. CXV. FRIDAY, MARCH 8, 1918. No. 2984. The Use of Concrete for Mine Supports.* By Prof. GEO. KNOX, F.G.S., M.I.M.E. Until recently timber has been the principal means of support used for underground workings, but owing to the continued rapid growth of the mining industry, the available supply of suitable timber is difficult to obtain and costly to provide, particularly at the pre- sent time when freight rates are so exorbitant. The increase in cost and temporary nature of timber led to the introduction of brick and stone linings for main roadways, and of steel girders with wood lagging for secondary roadways. Brick and stone linings— particularly the latter—are too brittle to be of much service in ground where the strata have only partially subsided, and unless steel girders are put in very close together, with only light timber lagging, they are apt to be badly twisted, necessitating constant renewal. In ground which has completely subsided, particularly in return airways, brick stone, or steel are preferable to timber, on account of the time they will stand with- out renewal. For general roadway supports, steel is cheaper than timber on account of the saving in renewals and freedom from falls of roof and sides. During the past 10 or 15 years many Continental mining engineers have introduced concrete and rein- forced concrete for shafts, galleries, tunnels, etc., with great success; and during the few years preceding the war, in Belgium and the Pas-de-Calais coal fields particularly, the use of concrete had become extensive, replacing almost entirely the use of brick, stone, or steel for permanent supports. In discussing this matter with several Continental engineers who had adopted this process, they all expressed surprise that it had not been adopted on a large scale in Britain, as it had proved an assured success in their collieries, their claim being that it (а) was cheaper and superior as a means of support; (б) reduced stoppage in haulage to a minimum; (c) produced smooth, airtight roadways, which in dusty mines could easily be kept clean and safe by watering or dust extraction; (d) reduced leakage in ventilation. The first attempt at forming concrete linings on the Continent was by making reinforced blocks and build- ing as in the case of bricks, but this was found to be costly and less stable than the method now used of forming the whole lining of “ concrete • puddle ” behind a framework which was allowed to remain in for about 14 days before being withdrawn. By this time the whole of the concrete has set in one complete block, and if reinforced with old wire ropes, etc., will stand a greater pressure than either stone or brick linings. Underground concrete lining, to be cheap and suc- cessful, must be constructed of materials suitable to the conditions prevailing in each particular case. In wet ground, for example, it would be useless to use ordinary lime, whereas in dry workings cement need only be used in very small quantities (about 1 part of cement to 8 of lime). Again, in ground where a heavy crush was expected, or in large excavations such as pit-eyes, the “aggregate” should consist of hard rock, whereas under ordinary circumstances the shale from the mine rippings will be found strong enough. The amount of reinforcement required will also depend on local circumstances, being greatest in large excavations or in subsiding or faulted ground. In choosing cement, use always the best class pos- sible, which can be determined by its fineness, specific gravity, and tensile strength. In very particular work, where strength is the all-important factor, the cement may be improved by re-grinding immediately before being used. Where rapidity of setting is required (initially and finally), cement should be re-ground very fine. If, however, the cement and aggregate had to be mixed on the surface and sent into the mine in trams, slow-setting cement should be used, as concrete which becomes “ dry ” and has to be re-mixed is as friable as bricks. Where great strength is required, the best concrete is made from 9 parts (36 per cent.) 1| in. granite, 8 parts (32 per cent.) | in. granite, 5 parts (20 per cent.) sand, and 3 parts (12 per cent.) cement. In dry underground roadways the concrete may consist of 1 part (3 per cent.) cement, 8 parts (23 per cent ) freshly burned lime, and 24 parts (74 per cent.) hard shale (from ripping) up to in. In other words, the strength of concrete depends upon the hardness of the aggregate and the purity of the cement, and the cost will be determined likewise. Continental engineers say that cement and lime should never be mixed in concrete, but the mixture has been used successfully in several cases in this country, with only one failure, and in that case the lime had been lying in the store for a considerable time, which in all likelihood accounted for the failure. Where cement only is used, fine material (stone and sand) must be mixed with the coarser aggregate, so that all the interspaces are roughly filled with only a thin layer of cement between the parts to consolidate them. * A memorandum prepared for the Board of the South Wales and Monmouthshire School of Mines. Careful mixing is very important to ensure success- ful results. Where this is done by hand (as will be the case in the experimental stage), it should be care- fully turned over when dry until well mixed, and the process repeated again when wet, care being taken to keep it free from any oil, grease, or slimy muds. For this reason, it will be better to have the dry mixing done on the surface, and conveyed underground in trams to the place where the concrete is required, when the wet mixing could be carried out. FI& 1 2 ? When putting reinforced concrete in position, it is best to shovel it in against either side (from the bottom upwards), allowing it to come to rest at the angle of repose, and during this process the reinforced bars (old ropes usually) should be jarred occasionally by knocking with a hammer, so that the whole space has an opportunity of getting completely filled. Concrete has a low tensile strength, but fortunately for its use in mines compressive strains are what have to be most frequently guarded against, and its strength in compression is much higher than brick or stone. Ordinary concrete (without reinforcement) is three to five times as strong as the best brick, and five to eight and a-half times as strong as ordinary brick: — Laid in best mortar. Ordinary brick. Best brick. 834 .. ' Concrete. . 2,000 to 4,000 after 1 month. Compressive strength in lb. per square inch. 486 ... On the Continent adopted are as follow: the mixtures most Cement. Sand. Rock. frequently Compressive strength *in lb. at end of 1 month. For vertical walls 1 ... 1 ... 2 ... 4,000 Retaining walls., .... 1 ... H ... 3 ... 3,400 Shaft linings .... 1 ... 2 ... 4 ... 2,700 Engine, etc., foundations ... 1 ... 2f ... 5 ... 2,300 Underground dams .... 1 ... 3 ... 6 ... 2,000 As illustrating the effect of reducing the percentage of cement and increasing the percentage of sand, the following results were obtained from experiments recently carried out: — Proportion of sand 0... i... 2... 3... 4... 5... 6... 7... 8 Comparative strength 1... |... |... j... $... |... b... }... I To show the value of re-grinding to produce a quick- setting concrete, the following results may be quoted: Sample. End of 24 hours. Lb. per sq. in. End of 7 days. Lb. per sq. in. End of 28 days. Lb. per sq. in. Ordinary 135-9 . .. 555-3 ... 657-8 Re-ground for 5 hours . 348-9 . .. 568-3 ... 631-3 Re-ground for 14 hours . 365*0 . .. 596*7 ... 598-8 The relative cost and strength of wood, brick, sand- stone, and concrete are given by Habets (“ Cours d’Exploitation des Mines”) as follows: — Wood. Sandstone. Brick. Concrete. Cost per metre in francs ........... 150 ... 120 ... 32 -20 Resistance in kilos, per sq. c. 45 ... 80 ... 12 ... 30 C< st per cubic yard ......... <£4 Us. 6d. . <£3 13s. 3d. . 18s. 10d.,..12s. 2d. Where mine rubbish is used to form the aggregate, the cost of material is about 7s. 2d. per cu. yd. for 6-1 concrete (cement); about 8s. 2d. per cu. yd. for 5-1 concrete (cement); about 4s. per cu. yd. for 4-1 con- crete (lime); and about 3s. 2d. per cu. yd. for 5-1 concrete (lime). Mixing, transit, and fixing would cost 3s. to 5s. per cu. yd., according to the nature of the ground to be filled in. Adding to this the fixing of the “ cradling ” or “ centring ” and the old rope strands for reinforce- ment, the cost would probably be, at the present time, between £4 and £5 per linear yard of roadway 9 ft. by 7 ft. Where cement only is used to consolidate the aggre- gate, care should be taken to prevent dust adhering to the rock forming the aggregate before mixing. If the material used produces a lot of dust in the crush- ing, the aggregate should be sprayed (or, better still, subjected to a jet of compressed air) to get rid of the dust. Where a mixture of lime and cement is used, this is not necessary. One of the best materials to be had in colliery districts from which the aggregate may be constructed is the burned shale from colliery rubbish tips. It is harder than ordinary shale, is rougher on FIG 3 !o c* FH>5 the surface, and therefore gives a better surface for the cementing material to adhere to. When mixed with ground furnace slag, it forms an excellent aggre- gate for concrete. In many cases boiler clinker is used to mix with the mine shale. It is crushed and passed through a f in. mesh. This fills the interspaces between the pieces of coarser shale, but as clinker has only a very low strength in compression, it is not advisable to use it if a plentiful supply of hard mine shale can be obtained. Where side support is required (as well as roof sup- port), the concrete lining should be put in as shown in fig. 1, so that all of it comes into compression when weighted. On the Continent, where the main roads are concreted, this work is done as soon as the maxi- mum of subsidence has been reached, and as the road is then ripped to get the required height, the finished area remains practically constant. It is, therefore, an advantage to made “centres” or “cradles” (figs. 1 and 2), as in use at Bethune Colliery, where the cross staff can be withdrawn, allowing the whole thing to collapse and be carried forward for re-erection. Where the area of roadway is very variable, this would require too much concrete to fill up the inter- vening space. It is, therefore, advisable to have some form of frame which can be expanded (fig. 3) to suit the local conditions. This is essential where rein- forced concrete is being substituted for other supports (in repair work), particularly where large falls of roof have taken place. The cheapest reinforcement for this kind of work is the strands of old steel haulage and winding ropes. Under normal conditions, it may only be necessary to