216 TUNNELING 



tween the rock and the iron lining, to confine the Portland 

 cement grout, which was now introduced back of the plates. 

 To effect this grouting 1^-in. holes had been drilled and tapped 

 through the back of several plates in each ring. Through 

 these holes the grout was pumped by means of a Cameron 

 pump; and after the space between the brickwork and the 

 lining had been thoroughly grouted, the work was found, on 

 taking off the air pressure from the heading, to be perfectly 

 water-tight. It was not until towards the end of July that the 

 work of lining the brickwork was completed and driving ahead 

 in the rock was resumed. Then, when an advance of only 10 

 ft. had been made, a second soft seam was encountered about 

 80 ft. beyond the first one, and a test pipe was driven to a 

 horizontal depth of 70 ft., without encountering anything 

 solid. To avoid further delay, the driving of the test-pipe 

 was discontinued at this depth, and preparations made for 

 advancing the heading. For this test-pipe li-in. common 

 wrought-iron pipe was used, which was driven in by a small 

 machine-drill, and washed out at each lengthening of the 

 pipe with a l|-in. wash-pipe. From these washings the differ- 

 ent materials penetrated were sampled, with the following 

 tabulated results : 



3 ft. gray decomposed feldspar and chlorite. 



11 ft. soft black mud, containing lumps of carbonized wood like charcoal. 

 19 ft. hard black mud and sand, with nodules of pyrites. 

 22 ft. gray decomposed feldspar. 



4 ft. decomposed feldspar and chlorite. 

 11 ft. gray decomposed feldspar. 



Water was again found next to the rock, but was consider- 

 ably held in check by the compressed air. As from the results 

 of the test-pipe there were no special difficulties to apprehend 

 from the indicated material, it was decided to drive ahead, 

 under the open heading method, as this involved no delays in 

 waiting for special machinery. The light steel cylindrical roof 

 was again used in advancing the excavation, but for the perma- 



