GENERAL DISCUSSION.
It may be set down as a general truth that the excavation of tunnels through soft ground is the most difficult task which confronts the tunnel engineer. Under the general term of soft ground, however, a great variety of materials is included, beginning with stratified soft rock and the most stable sands and clays, and ending with laminated clay of the worst character. From this it is evident that certain kinds of soft-ground tunneling may be less difficult than the tunneling of rock, and that other kinds may present almost insurmountable difficulties. Classing both the easy and the difficult materials together, however, the accuracy of the statement first made holds good in a general way. Whatever the opinion may be in regard to this point, however, there is no chance for dispute in the statement that the difficulty of tunneling the softer and more treacherous clays, peats, and sands is greater than that of tunneling firm soils and rock; and if we describe the methods which are used successfully in tunneling very unstable materials, no difficulty need be experienced in modifying them to handle stable materials.
Characteristics of Soft-Ground Tunneling.
—The principal characteristics which distinguish soft-ground tunneling are, first, that the material is excavated without the use of explosives, and second, that the excavation has to be strutted practically as fast as it is completed. In treacherous soils the excavation also presents other characteristic phenomena: The material forming the walls of the excavation tends to cave and slide. This tendency may develop immediately upon excavation, or it may be of slower growth, due to weathering and other natural causes. In either case the roof of the excavations tends to fall, the sides tend to cave inward and squeeze together, and the bottom tends to bulge or swell upward. In materials of very unstable character these movements exert enormous pressures upon the timbering or strutting, and in especially bad cases may destroy and crush the strutting completely. Outside the tunnel the surface of the ground above sinks for a considerable distance on each side of the line of the tunnel.
Methods of Soft-Ground Tunneling.
—There are a variety of methods of tunneling through soft ground. Some of these, like the quicksand method and the shield method, differ in character entirely, while in others, like the Belgian, German, English, Austrian, and Italian methods, the difference consists simply in the different order in which the drifts and headings are driven, in the difference in the number and size of these advance galleries, and in the different forms of strutting framework employed. In this book the shield method is considered individually; but the description of the Belgian, German, English, Austrian, Italian, and quicksand methods are grouped together in this and the three succeeding chapters to permit of easy comparison.