134 TUNNELING 



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 nat- 

 ural 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 pres- 

 sures upon the timbering or strutting, and in especially bad 

 cases may destroy and crush the strutting completely. Out- 

 side the tunnel the surface of the ground above sinks for a con- 

 siderable 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 char- 

 acter entirely, while in others, like the Belgian, German, Eng- 

 lish, 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 frame- 

 work employed. In this book the shield method is considered 

 individually ; but the description of the Belgian, German, Eng- 

 lish, 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. 



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 char- 

 acteristic of the method is the construction of the roof arch 



