594 ANNUAL REPOET SMITHSONIAN INSTITUTION, 19 3 



girders in the side of the caisson, through which the shield was to be 

 advanced, were burned out, after which the plates were removed from 

 the invert to the springing line. The lower pockets of the shield 

 were then bulkheaded and the space between the pockets and and the 

 exposed face was filled with clay. After this, the remaining plates 

 were removed, proceeding upward from the springing line. A semi- 

 circular annular ring was cleared for the hood and packed with clay 

 into which the hood was forced when the shield was advanced. 



The material at the face consisted of timber and riprap down to 

 the springing line, similar to the material encountered in shaft 

 sinking, making excavation very difficult. The stones in the crib 

 varied from 1-man stones to those three-quarters of a yard in size. 

 The voids between the stones were filled with soft black mud, which 

 did not offer sufficient resistance to prevent the escape of air, neces- 

 sitating the mudding up of the entire face with clay. As the excava- 

 tion was carried forward, the escape of air through the heading of 

 the north tunnel at times taxed the full capacity of the power house, 

 40,000 cubic feet of free air per minute. 



On June 10, 1923, a small blow occurred at the face of the shield 

 and it became necessary to drop the air pressure sufficiently to allow 

 the water to flow into the tunnel before the blow could be stopped. 

 The progress through the riprap was very slow, as extreme measures 

 had to be taken to avoid blow-outs. After the shield had passed 

 through the old timber and riprap crib, the river bulkhead was en- 

 countered which did not offer any unusual difficulties. 



Before tunneling through similar material in the south tunnel east 

 5,500 bags of 1 : 1 Portland cement grout were ejected through the 

 east shield bulkhead of the south caisson and six pipes were sunk 

 from the surface east of the caisson through which 140 bags of 

 1 : 1 Portland cement grout were placed. This grout displaced much 

 of the soft mud and filled the voids in the riprap and greatly facili- 

 tated the driving of the shield so that very little air escaped through 

 this material after it had been consolidated hj grouting. 



After about 60 rings were erected in each tunnel, the shields were 

 stopped to build tunnel bulkheads and to install cages at the shafts 

 and then tunneling was resumed. Immediately east of the river 

 bulkhead soft mud, considerably lighter than Hudson River silt, was 

 encountered in the upper part of the excavation. In this material 

 the tunnel began to rise directly behind the shield and also to move 

 northward. 



To hold the shield and the tunnel to the proper grade, it was 

 necessary to take in a certain amount of material through the shield. 

 Accordingly, the shield was advanced with the top pockets bulk- 

 headed and a large percentage of the excavation was permitted to 

 enter the tunnel through openings in the lower part of the shield. 



