68 and 68A. Diagrams Showing Sequence of Excavation in the Belgian Method 145 69. Sketch Showing Radial Roof Strutting, Belgian Method 147 70. Sketch Showing Roof Arch Center, Belgian Method 147 71. Sketch Showing Method of Underpinning Roof Arch with the Side Wall Masonry 149 72. Longitudinal Section Showing Construction by the Belgian Method 149 73. Diagram Showing Sequence of Excavation in Modified Belgian Method 152 74. Sketch Showing Failure of Roof Arch by Opening at Crown 153 75. Sketch Showing Methods of Repairing Roof Arch Failures 154 76. Diagrams Showing Sequence of Excavation in German Method of Tunneling 155 77. Diagram Showing Sequence of Excavation in Water Bearing Material, German Method 156 78. Sketch Showing Work of Excavating and Timbering Drifts and Headings 157 79. Sketch Showing Method of Roof Strutting 157 80. Sketch Showing Roof Arch Centers and Arch Construction 158 81. Sketch Showing Method of Excavating and Strutting Baltimore Belt Line Tunnel 162 82. Roof Arch Construction with Timber Centers, Baltimore Belt Line Tunnel 163 83. Roof Arch Construction with Iron Centers, Baltimore Belt Line Tunnel 164 84. Diagram Showing Sequence of Excavation in English Method of Tunneling 167 85. Sketches Showing Construction of Strutting, English Method 168 86 and 87. Sketches of Typical Timber Roof-Arch Centers, English Method 169 88. Sequence of Excavation in the American Method 172 89. Strutting the Heading in the American Method 172 90. Temporary Timbering of the Roof in the American Method 173 91. Showing Crown Bars Supported by Segmental Arches 173 92. Transversal and Longitudinal Section of a Tunnel Excavated and Strutted According to the American Method 174 93 and 94. Diagrams Showing Sequence of Excavation in Austrian Method of Tunneling 177 95, 96 and 97. Sketches Showing Construction of Strutting, Austrian Method 178