THK FILL SKCTION METHOD 163 



springing lines of the roof arch. When this drift has been 

 driven ahead a distance varying from 12 ft to 20 ft. or some- 

 times more, the excavation of the center top heading No. 2 is 

 driven for the same distance. The next operation is to remove 

 part No. 3, Urns forming a central passage the full depth of the 

 rumel section at the center. This trench is enlarged by 

 r.-moving parts Nos. 4, 5, 6, 7, and 8 in the order named until 

 the full section is opened. A modification of this plan of 

 i-xcavation is shown by Fig. 87 which is used in firm soils. 



Strutting. Each part of the section is strutted as fast as 

 it is excavated. The center bottom drift first excavated is 

 strutted by laving a transverse sill across the floor, raising 

 two side jM>sts from it, and capping them with a transverse 

 tiinlHT having its ends projecting l>eyond the side posts and 

 hahed as shown by Fig. 88. The top center heading No. 2, 

 which is next excavated, is strutted by means of two side posts 

 resting on blocking and carrying a transverse cap as also shown 

 bv Fig. 88. Sometimes the side posts in the heading strutting- 

 frames are also carried on a transverse sill as are those of the 

 bottom drift. This construction is usually adopted in loose 

 soils. When the sill is employed, the middle part, No. 3, is 

 strutted by inserting side posts between the bottom of the top 

 sill and the cap of the frame in the drift l>elow. When, how- 

 ever, the posts of the top heading frame are carried on blocking, 

 it is the practiee to replace them with long posts rising from 

 the cap of the bottom drift frame to the cap of the top heading 

 frame. Further, when the intermediate sill is employed at the 

 Imttom level of the top heading it projects lioyond the side 

 and has its ends halved. 



After the eompletion of the center trench strutting the next 

 i i>k is to strut parts Nos. 4 and 5. This is done by continuing 

 the upper sill bv means of a timber having one end halved to 

 join with the projecting C0d of the sill in position. This ex- 

 t.-nsiun timlier is shown at a. Fig. 80. The next operation is 

 to place the timU-r /, 1 aving orte end resting on the cap-piece 



