(iKKMAN METHOD 151 



forated pipes into the sides of the excavation, and forcing 

 cement grout through them into the soil to solidify it These 

 pipes penetrated the ground about 10 ft, and the method 

 proved very efficient in preventing the inflow of water. 



Excavation. - The excavation was carried out according to 

 the German method of tunneling. Bottom side drifts were 

 first driven, and then heightened to the springing line of the 

 roof arch. Next a center top heading was driven, and the 

 haunch sections taken out. The object of beginning the exca- 

 vations by bottom side drifts, was to drain the soil of the upper 

 part of the section. The center core was removed after the 

 side walls and roof arch were completed, its removal being 

 kept from 50 ft to 75 ft. to the rear of the advanced heading. 

 The dimensions of the side drifts proper were about 8x8 ft., 

 but they were often carried down nuu h below the floor level 

 to secure a solid foundation bed for the side walls. 



Strutting. The side drifts were strutted by means of 

 frames composed of two batter posts resting on boards, and 

 having a cap-piece extending transversely across the roof 

 of the drift These frames were spaced about 4 ft apart 

 The excavation was advanced in the usual way by driving 

 pol ing-boards at the top and sides, with a slight outward and 

 upward inclination, so that the next frame could be easily 

 inserted with additional space enough between it and the 

 sheeting to jM-rmit the next set of poling-boards to be inserted. 

 These poling-hnards were driven as close together as practicable 

 so as to prevent as much as possible the inflow of water and 

 earth. 



The center lop heading was strutted in the same manner as 

 were the side drifts. The arrangement of the strutting em- 

 ployed in enlarging the center top heading is shown clearly by 

 Fig. 78, which also shows the manner of strutting the side 

 drifts and face of the excavation, and of building the masonry. 



Centers. -- Both wood and iron centers were employed in 

 building the roof arch. The timber centering was constitnt..! 



