=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. THE BELGIAN METHOD OF TUNNELING THROUGH SOFT GROUND. [Illustration: FIGS. 68 and 68A.--Diagrams Showing Sequence of Excavations in the Belgian Method.] The Belgian method of tunneling through soft ground was first employed in 1828 in excavating the Charleroy tunnel of the Brussels-Charleroy Canal in Belgium, and it takes its name from the country in which it originated. The distinctive characteristic of the method is the construction of the roof arch before the side walls and invert are built. The excavation, therefore, begins with the driving of a top center heading which is enlarged until the whole of the section above the springing lines of the arch is opened. Various modifications of the method have been developed, and some of the more important of these will be described farther on, but we shall begin its consideration here by describing first the original and usual mode of procedure.