{_In hard rock._ {By drifts. { {By a heading. { {_By upper half:_ } { { the arch is built }Belgian method. { { before the side walls. } { { { {_By the perimeter:_ } { { excavated and lined } { { before the central }German method. { { nucleus is removed. } { { {_In loose soil._{_By whole section:_ {English method. { { the lining begins after{Austrian method. { { the whole section is {American method. { { excavated. { { { { {_By halves:_ } { { the lower half is } { { excavated and lined, }Italian method. { { followed by the work } { { of the upper half. } METHODS OF{ EXCAVATING{_In quicksand._ TUNNELS. { { {In resistant soils. {By two lateral { { {narrow trenches. {_Open-cut_ { {_tunnels._ {In loose soils. {By one very large { { {trench. { { { {Built up. By slices. { { {At great depths under }By any method. { {the river bed. } { { { { {By shield. { {At small depths {By compressed { {under the river {air. {_Submarine_ {bed. {By shield and {_tunnels._ { {compressed air. { { { { {By coffer dams. { { {By pneumatic { {On the river bed. {caissons. { { {By built-up { { {sections. The above diagram gives in compact form the classification of tunnels according to materials penetrated and methods of excavation adopted, which have been described more fully in the succeeding paragraphs. It may be noted here again that this is a purely arbitrary classification, and serves mostly as a convenience in discussing the different classes of tunnels without confusion. CHAPTER V. METHODS OF TIMBERING OR STRUTTING TUNNELS.