COMPARISON OF METHODS.
The differences between the drift and heading methods of excavating tunnels through rock, consist chiefly in the excavations, strutting, and hauling. When the drift method is employed an advanced gallery is opened along the floor of the tunnel before the upper part of the section is removed, and when the heading method is employed the upper part of the section is completely excavated before any part of the section below is excavated. When the drift method of driving is employed polygonal strutting is usually used, and longitudinal strutting is employed with the heading method of driving. In the drift method the hauling is done by one system of tracks at the same level, while in the heading method two systems of tracks are employed at different levels.
It is, perhaps, impossible to state without qualification which method is the better. European engineers who have been connected with both the Mont Cenis and St. Gothard tunnels, driven by the drift and heading methods respectively, had the opportunity to practically observe the advantages and disadvantages of these two methods. Their conclusion was that the drift method was more convenient for tunnels driven through hard and compact rock, and that the heading method was better for tunnels of fissured and disintegrated rocks. To prove this opinion, experiments were made in one of the tunnels approaching the great St. Gothard tunnel. On a short tunnel the excavation was made by the drift method from one portal, while at the other, the heading method was followed. Although the general rule was fully confirmed still the conditions at the portals were not identical. More conclusive experiments were made by Mr. Ira A. Shaler, the contractor for Section IV., of New York Rapid Transit Railway. He had the opportunity of driving two parallel tunnels under Murray Hill only 17 ft. apart. The eastern tunnel was driven by the drift method, the western one by the heading method. After the work had proceeded for a few months, Mr. Shaler stated that in his case the drift method was more convenient. He could spare drilling several holes at each advance, thus obtaining economy in time, labor and material without considering the advantage of a simpler transportation of the débris. He promised to publish his results for the benefit of the profession, but, unfortunately, lost his life in an accident in the tunnel before the completion of the work.
An advantage that the drift method affords in long tunnels is, that the water, which is usually found in large quantities under high mountains, is easily collected in the drift and conveyed to the culvert, while in the heading method the water from the advance gallery, before being collected into the culvert built on the floor of the tunnel, must pass through all the workings. This may be a serious inconvenience when water is found in large quantities, as, for instance, was the case in the St. Gothard tunnel, where the stream amounted to 57 gallons per second.