AMERICAN METHOD.
In this country tunnels through loose soils are excavated according to the “Crown Bar” or American Method. This consists in opening the whole section of the tunnel before the construction of the lining as in the English Method. It differs from the English method, however, in that many timber structures are erected for the support of the roof, and that the excavation and construction of the lining are far apart, so allowing the miners and the masons to work continuously and without interfering with each other.
Section A-B.
Fig. 88.—Sequence of Excavation in the American Method.
Fig. 89.—Strutting the Heading in the American Method.
Section C-D.
Fig. 90.—Temporary Timbering of the Roof in the American Method.
Section E-F.
Fig. 91.—Showing Crown Bars Supported by Segmental Arches.
Excavation.
—The diagram in [Fig. 88] shows the sequence of excavation. The work begins by driving a central heading usually 7 × 8 ft., strutted by means of vertical or batter posts and cap-piece. [Fig. 89],[11] the props resting on foot blocks. Between the cap-pieces of the consecutive frames are placed planks driven upward at a slightly inclined angle. After the heading has been excavated and strutted, the floor is lowered by removing the part marked 2 in the figure. The two batter posts supporting the cap-piece are now substituted by two longer ones resting on the floor of part 2 and abutting against longitudinal beams which are inserted underneath the cap-pieces. These longitudinal beams are called crown bars. The new batter posts are resting either on foot blocks or sills according to the quality of soil and they are strongly wedged to the crown bars. On each side of these crown bars are inserted poling-boards or planks close to each other, which are driven downward. The part marked 3 in the figure is removed by enlarging the cut 1 × 2 on both sides. The plank, inserted above the crown bar, is driven in either preceding or following the excavation and another crown bar is inserted at the end of this plank. This second crown bar is supported by a prop whose other end abuts against the foot of the rafter strutting the heading. Between this crown bar and the roof of the excavation, other planks are placed transversally to the axis of the tunnel and are driven in until they are supported by a new crown bar, etc. The various props supporting the crown bars are placed radially or in a fan-like manner, similar to the characteristic arrangement of the timbering in the Belgian method. Bracers to strengthen the timbering and the roof of the excavation are inserted longitudinally between the various posts and transversally between the crown bars, [Fig. 90]. As a rule, only three or four of these radial structures are temporarily erected. A trench is excavated at the side of the part marked 3 in the figure to receive the wall plate which is a heavy timber laid on the floor parallel to the longitudinal axis of the tunnel. On the wall plates are erected the arched timber sets composed of five or seven segments of hewn timbers so as to form a polygonal frame which is wedged to the crown bars and which will support the arch of the roof. After one of these segmental timber sets is erected the temporary radial structure is removed and the upper section of the tunnel is cleared of any obstruction as the pressures are transferred to the wall plates, [Fig. 91]. The bench marked 4 in the figure is taken away and the vertical props inserted under the wall plates, [Fig. 92].
[11] [Figs. 89] to [91] are taken from a paper by S. W. Hopkins in Harvard Engineering Journal, April, ’03, on the Fort George tunnel.
Section G-H.
Longitudinal Section.
Fig. 92.—Transversal and Longitudinal Section of a Tunnel Excavated and Strutted According to the American Method.
Strutting.
—The longitudinal strutting is used in connection with the American method of tunneling. In fact, the strutting consists of a series of longitudinal bars supporting planks laid transversally to the axis of the tunnel and abutting against the roof of the excavation. These crown bars during the excavations and immediately after are temporarily supported by radial timbers forming almost a fan-like structure, but this is soon substituted by a permanent one composed of a polygonal timber frame of five or seven segments which are cut to dimensions. The batter posts of the heading, the radial posts of the temporary timber structure and the crown bars are all round timbers from 10 to 12 ins. in diameter. All the other timbers are square edged, the usual dimensions being 10 × 10 ins. or 12 × 12 ins. with the exception of the wall plates which are 14 × 14 ins. The dimensions of the various members of the strutting and the distance apart of the different frames vary with the quality of the soil. For instance, in ordinary loose soils the frames are placed between 4 to 6 ft., but in very soft soils they are erected only 3 or 31⁄2 ft. apart.
Chiefly in the southwest, in tunnels excavated according to the American method, the timbering has been left as regular lining and it was only after many years when this temporary structure had decayed or was burned down, that the tunnels were lined with masonry. But in many instances the whole timber structure was left in place even when the tunnel was lined with masonry immediately after the excavation had been made. This was usually done when the tunnel was lined with concrete masonry. In such a case the timbering was left to support the pressures of the roof while the concrete was plastic and before it hardened.
Centers.
—In the American method the whole section of the tunnel is open before the construction of the lining, thus the masonry can be built from the foundations up. The centers are designed so as to support only the weight of the masonry during its construction and not the pressures of the tunnel as in the other methods and consequently they are of light construction. The centers described in the Murray Hill tunnel, page 123, may be advantageously used in building the concrete lining in tunnels through loose soils excavated by the American method.
Hauling.
—The excavation of the heading and the upper section of the tunnel is usually far ahead of the bench, consequently the hauling of both the débris and the building materials is made at two different levels, viz., on the bench and on the floor of the tunnel. When the face of the heading and the excavation of the bench are not more than 50 ft. apart, the hauling can be conveniently done on the tunnel floor, while the materials and débris on the upper section of the tunnel are hauled by wheelbarrows or light cars propelled by handpower. For a greater distance, however, it is more convenient to use light cars running on narrow-gauge tracks all through the tunnel. In this case the tracks on the tunnel floor and on top of the bench are connected by means of an inclined platform where the cars may ascend and descend without interfering with the excavation of the bench. Here, as a rule, tunnels have been excavated in soils considered good, generally through rock, while loose soils have been encountered only in small sections. The same method of excavation for whatever material is encountered is certainly very convenient, as it affords a great regularity in the work; hence its extensive use. A great disadvantage of this method is the double strutting, viz., the polygonal and the longitudinal strutting succeeding each other, whereas one of them could be easily spared. Another defect is that it requires a larger amount of excavation, in case the strutting is left in place.