Tunnels and Tunneling

lining, material, masonry, excavation, poling, boards, timber and usually

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Excavation.— The statement on the preced ing page shows the general character of excava tion for thetion for the different kinds of tun nels to which allusion has been made. It is taken from (Tunneling' by Charles Prelini, C. E.

The Timbering of As excava tions are made for tunnels not only in rock, but also frequently in soft material, it is neces sary to support the sides and top of the ex cavation to prevent material falling in, and this support is usually given by timber struts and planking, called either lagging or poling boards, suitably disposed concurrently with the excavation. As round sticks are more economical than sawed timber they are usually employed for this purpose. Methods of putting this timber in place vary largely with the methods of excavation employed and the sequence in which various portions of the tun nel are excavated. If the material is reason ably self as clay and much firm earth as well as most rock, the entire excavation may be completed closely in front of the tim bering already in place if it is not desired to follow the excavation immediately with the masonry lining. In rock tunneling the masonry lining may usually be completed, where needed, immediately following the excavation without much or any timbering.. In soft material, how ever, it is usually necessary to use timber sup ports, even for the first excavation, in the head ing or drift. In fact, it is commonly necessary to insert poling boards, as they are called, ahead of the actual excavation, as shown in Figs. 4 and 5. The poling boards are inserted over the crown bar or top horizontal round stick of the bracing and under the block on which one end of the previous poling board rests, thus enabling the soft material to be ex cavated at the front part of the heading without the newly exposed material falling down. After the heading or drift is thus excavated and timbered. other portions of the cross-sec tion are excavated concurrently with placing the supporting timbers, so that when the entire section is excavated there will be a complete sys tem of supporting struts and cross timbers with the poling boards or lagging resting directly against the soft material of the sides and roof. These supporting timbers are so placed as to permit small dump or tram cars to be used for carrying out the excavated material. Obviously, the precise method of using poling boards will depend upon the manner or sequence in which the different portions of the excavation are completed and the kind of material found. It

is e9ually evident that the poling boards and lagging may be used lengthwise of the tunnel or transversely, as shown in Figs. 4 and 5, the former necessitating the excavation of the least amount of material and being preferable in most cases. After thus completing the excava tion and timbering, the masonry lining may be put in place, the timbering being removed as fast as the masonry progresses. The lagging and poling boards usually remain undisturbed back of the masonry lining. The timber which has been removed when the masonry .is laid may be repeatedly used, not more than about 20 per cent of it usually being destroyed in any one use.

The Lining of Reference has already been made to the masonry lining of tunnels. This lining may be constituted of almost any grade of suitable masonry, in ac cordance with its durability, strength and facility of put in place. The kind of lining masonry is determined largely by the convenience and economy of the local supply. If the tunnel is in saturated water-bearing material the static head or pressure on the lining may be very great, and the thickness of masonry must be sufficient to carry that water pressure. Occasionally also there may be some little motion of the rock through which the tunnel is driven. In such cases the masonry lining must be of sufficient thickness to resist that movement. While no general rule can be given, the thickness of masonry lining may be from 1 to 4 or 5 feet, the least thickness being suitable for rock not very liable to fall and the greater thickness for such soft material as wet clay and sand and in cases where the static pressure of the saturat ing water may be very great. In some special locations for temporary purposes timber lin ing has been used, but it is not ordinarily em ployed for permanent work. Iron lining has also been employed under certain special con ditions, but it is not often used for other than those methods of driving tunnels through soft material with the aid of the pneumatic process and the shield, the circular cast-iron lining being put in place immediately behind the shield. This cast-iron lining is made in sec tions with flanges for joints and for stiffening purposes which when put in place form a com plete circle. These sections are sometimes not more than 2 feet by 4 feet, while in other cases they may be as much as 5 feet or 6 feet square. In all cases they are cast so as to form complete circular cylinders when put in place and bolted together.

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