Considering now the side wall masonry, it will be remembered that in excavating the part No. 5, [Fig. 68], of the section, frequent side trenches were excavated, and struts inserted to take the weight of the masonry. These struts are inserted on a batter, with their feet near the center of the tunnel floor, so that the side wall masonry may be carried up behind them to a height as near as possible to the springing lines of the arch. When this is done the struts are removed, and the space remaining between the top of the partly finished side wall and the arch is filled in. This leaves the arch supported by alternate lengths or pillars of unexcavated earth and completed side wall. The next step is to remove the remaining sections of earth between the sections of side wall, and fill in the space with masonry. [Fig. 71] is a cross-section, showing the masonry completed for one-half and the inclined props in position for the other half; and [Fig. 72] is a longitudinal section showing the pillars of unexcavated earth between the consecutive sets of inclined struts and several other details of the lining, strutting, and excavating work.
Fig. 71.—Sketch Showing Method of Underpinning Roof Arch with the Side Wall Masonry.
Fig. 72.—Longitudinal Section Showing Construction by the Belgian Method.
The invert masonry is built after the side walls are completed. This is regarded as a defect of this method of tunneling, since the lateral pressures may squeeze the side walls together and distort the arch before the invert is in place to brace them apart. To prevent as much as possible the distortion of the arch after the centers are removed, it is considered good practice to shore the masonry with horizontal beams having their ends abutting against plank, as shown by [Fig. 71]. These horizontal beams should be placed at close intervals, and be supported at intermediate points by vertical posts, as shown by the illustration. Since the roof arch rests are for some time supported directly by the unexcavated earth below, settlement is liable, particularly in working through soft ground. This fact may not be very important so long as the settlement is uniform, and is not enough to encroach on the space necessary for the safe passage of travel. To prevent the latter possibility the centers are placed from 9 ins. to 15 ins. higher than their true positions, depending upon the nature of the soil, so that considerable settlement is possible without any danger of the necessary cross-section being infringed upon. In conclusion it may be noted that the lining may be constructed in a series of consecutive rings, or as a single cylindrical mass.
Hauling.
—Since in this method of tunneling the upper part of the section is excavated and lined before the excavation of the lower part is begun, the upper portion is always more advanced than the lower. To carry away the earth excavated at the front, therefore, an elevation has to be surmounted; and this is usually done by constructing an inclined plane rising from the floor of the tunnel to the floor of the heading, as shown by [Fig. 72]. This inclined plane has, of course, to be moved ahead as the work advances, and to permit of this movement with as little interruption of the other work as possible, two planes are employed. One is erected at the right-hand side of the section, and serves to carry the traffic while the left-hand side of the lower section is being removed some distance ahead and the other plane is being erected. The inclination given to these planes depends upon the size of the loads to be hauled, but they should always have as slight a grade as practicable. Narrow-gauge tracks are laid on these planes and along the floor of the upper part of the section passing through the center opening mentioned before as being left in the centers and strutting.
In excavating the top center heading there is, of course, another rise to its floor from the floor of the upper part of the section. Where, as is usually the case in soft soils, this top heading is not driven very far in advance, the earth from the front is usually conveyed to the rear in wheelbarrows, and dumped into the cars standing on the tracks below. In firm soils, where the heading is driven too far in advance to make this method of conveyance adequate, tracks are also laid on the floor of the heading, and an inclined plane is built connecting it with the tracks on the next level below. In place of these inclined planes, and also in place of those between the floor of the tunnel and the level above, some form of hoisting device is sometimes employed to lift the cars from one level to the other. There are some advantages to this method in point of economy, but the hoisting-machines are not easily worked in the darkness, and accidents are likely to occur.
