222 TUNNELING 



progressed more rapidly, and the shield soon reached undis- 

 turbed material, which was found quite dry and hard. It was 

 still the same black mud, with occasional lumps like charcoal, 

 and numerous nodules like pyrites, which glistened like silver 

 in the black, peat-like mud. Mattocks were used by the men 

 in the working chambers, who would clean out these four com- 

 partments to within a foot of the cutting edge. As soon as 

 this was done hydraulic pressure was put upon the jacks, some- 

 times to the amount of 5,000 Ibs. per sq. in., and the shield 

 forced ahead 16 or 18 ins., enough for another ring of plates, 

 the working chambers again being filled with the displaced 

 material. On Dec. 24 the last of the black mud was passed 

 through, and lying next to it, at an angle of 40 towards 

 Long Island, white decomposed feldspar was found, containing 

 fragments of decomposed quartz charged with sulphureted 

 hydrogen. 



An important departure was now made in the method of 

 erecting the cast-iron lining rings by breaking joints with the 

 segments. In all the iron-lined tunnels it has been the estab- 

 lished custom to erect the rings with continuous horizontal 

 joints. For some reason it was thought inadvisable to attempt 

 breaking joints with the segments. The writer's experience 

 in the Hudson tunnel had shown him the importance of obtain- 

 ing, in soft, squeezing ground, a perfectly rigid tunnel-ring. 

 In a material exerting hydrostatic pressure the tunnel lining is 

 subjected to a resultant strain, tending to flatten the ring, or 

 decrease its vertical diameter. Any yielding to this strain 

 results both in increasing the deforming pressure and in de- 

 creasing the power of the ring to resist the strain. In a lining 

 erected with continuous joints the rigidity of the ring is 

 dependent upon the bolting in the horizontal joints. At the 

 Hudson River tunnel a ring of plates was bolted together 

 lying flat on the ground, the plates all brought to a true circle, 

 and the two 1^-in. bolts in each joint well tightened. Upon 

 raising this ring with a derrick, so that it stood erect, the ring 



