THE COLLIERY GUARDIAN AND JOURNAL OF THE COAL AND IRON TRADES. Vol. CXV. FRIDAY, APRIL 26, 1918. No. 2991. Modern Methods of Shaft Sinking.—II. By W. H. MAXWELL, A.MJ.C.E. Sinking by the Freezing Process. When sinking in loose, water-bearing strata, either as a surface deposit or occurring at some depth—as is sometimes the case with quicksands—the freezing process is often employed. By this method the water- bearing beds are frozen immediately around the shaft, which is then sunk by hand and tubbed while the ground is still in the frozen state. The process was introduced by F. H. Poetsch in 1883, and has been largely used in Germany, France and Austria, and also to a smaller extent in Belgium, United States and England. It is applied by first drilling a number of small boreholes down to the beds to be frozen, usually around the outside of the shaft, and spaced at about 35 in. apart. In these holes freezing tubes are placed, and are then connected up in such a way as to enable the freezing liquid to circulate GXQUN0 (A J * $ * I L O N 3 I *1 1 0* § I % N I Fig. 1.—Cement Grouting in Shifting Ground, by Means of 6 in. Bores Lined with Perforated Steel Tubes. by the action of suitable machinery through the bore- holes and back again to the refrigerators. The tubes are about 4 in. diameter, sealed at the bottom, and have inner tubes about 1 in. diameter, opening into the larger tubes at the bottom, and connected at the top by means of valves to the inlet and outlet pipes conveying the liquid. The freezing liquid, or brine, is forced down the 1 in. tubes in the bores by a force pump, and returns slowly through the wider tubes into a collecting pipe leading to tbe refrigerators, where it is again cooled. The object is to establish a complete wall of frost around the shaft, and so make it possible for hand sinkers to work inside this ring. The frozen wall must be main- tained until the shaft has reached the required depth, and has been lined with tubbing. Where a shaft is already partially lined, the action of the frost, owing to the contraction and expansion set up, may result incracking some of the existing tubbing, To meet this trouble an improvement has been proposed, whereby the freezing tubes are insulated down to the level of the tubbing, and shafts may now be frozen with a minimum of risk to existing tubbings. The freezing liquid employed in this process is generally a solution of magnesium chloride or calcium chloride, refrigerated by an ammonia plant. After the friable water-logged ground has been frozen to a solid block, sinking proceeds by hand with the aid of light explosives used with care. The spaces behind the linings are filled in with dry concrete. The freezing process is not capable of universal application, and the depth to which it can be employed is limited by physical considerations. The rigidity of ice depends upon the pressure to which it is subjected, so that a limit is imposed by the depth at which a frozen wall will stand with the necessary firmness. A depth of 720 ft. is the greatest the writer has seen suggested as practicable. British shafts to which the freezing system has been applied are—Washington Colliery (1901), and Dawdon, Easing ton, and Wearmouth, all in the county of Durham. At the Dawdon Colliery, Durham, the sinking by the freezing system proved a great success. “ The sand, which was 92 ft. thick at a depth of 367 ft., was frozen solid. So great was the intensity of freezing that the sand resembled hard grey freestone, although pieces readily crumbled when held for a short time in the hand. On being exposed to the atmosphere the sand soon became soft and fell to pieces. In the shaft bottom the frozen sand was so hard that blasting had to be continued through the deposit. The total time occupied in sinking through the “ quicksand ” itself, tubbing it off, and securing the strata below the freezing pipes and the frozen ground, was six weeks.* At Washington Colliery, the work of freezing two shafts to the stonehead was undertaken by Gebhardt and Koenig, of Nordhausen. One of the most troublesome accidents in the appli- cation of the freezing system is the leakage of the freezing liquid through burst pipes into the frozen beds, because such portions of the strata as become impreg- nated with the solution cannot be readily frozen again, and is therefore liable during sinking, to break through into the shaft. The pipes should consequently be of the best and most durable material, and all joints and connections securely made. Use of Cement Grout in Shaft Sinking*. The writer has applied the process of forcing Portland cement grout down small boreholes, placed around the main boring for the purpose of consolidating the sur- rounding strata which had given trouble during the course of sinking (fig. 1). The small borings, 6in. in diameter, were put down to depths where “falls” had occurred, and were lined with thin, perforated mild-steel tubes. Down these cement grout was forced by compressed air at a pressure of 1001b. per square inch, and, by this means, cavities around the main boring were filled and the surrounding ground consolidated. The injection of cement grout has long been prac- tised with the object of making tight joints in leaky tubbings in existing shafts; but in 1904 the Cie. des Mines de Bethune decided to extend the system, and to sink a new shaft through bad ground in the cretaceous beds by the grout injection process. Vertical boreholes were made around the site of the proposed shaft, and Fig. 3.—Sinking Drum Process in Loose Sandy Ground, with Brick Shaft above Ground. Fig. 2.—Sinking Drum Process. grout was forced in as the holes were carried down, thus consolidating the material through which the shaft was to be sunk. The quantity of cement injected varied according to the inclination of the water-bearing beds. When these approached the vertical, the consumption of cement was excessive and the process less satis- factory. In the case of steeply-inclined beds, it has proved advantageous to sink the shafts by short lengths, and inject each new length in advance by means of radial boreholes driven from the last com- pleted length of shaft. To remove fine mud and prevent it from mixing with the grout, it is well to force in, if possible, a good supply of fresh water before the * E. S. Wood, Proc. Inst. C.E., vol. clxxiii. grout is injected. Rapid pumping from tbe boreholes, especially by an “ air-lift,” also has the effect of drawing away fine debris, thus leaving the fissures clear. The cement grout injection process is cheaper than the freezing system, and the plant employed is relatively simple. The process is most likely to be successful in loose, gravelly beds or in fissured, water-bearing rock, but is not suitable in cases of heavy clay or close, compact strata. The Sinking Drum Process. For loose, unstable surface deposits and water-bearing ground, this is one of the oldest methods of sinking employed, but there have been various improvements in detail during recent years. Fig. 5.—Interlocking Channel-Bar Steel Piling. pF5 I 'r I K------------------->1 Fig. 6.—Steel Piling to Shaft (by the British Steel Piling Co.). u fiOCK Fig. 4.—Piling through Quicksand. In this method a “ drop-shaft,” built up of cast iron tubbing rings, bolted together to form a cylinder, with a cutting shoe attached to the bottom of the cylinder, is lowered or forced down from the surface as the sinking proceeds. The ground within the cylinder is excavated in the ordinary way by hand at the bottom of the shaft by means of a grab or sack-borer, according to the nature of the material to be removed. This system was employed, for instance, in sinking the new pits of the Carriden Coal Company, Bo’ness {Colliery Guardian, September 15, 1916, page 497). In some cases it is necessary to start with the masonry or brick shaft and to force down the drop shaft of iron tubbing rings inside this. The cylinders are forced down by hydraulic rams, the back pressure from which is taken by a special ring