I04 NEW YORK STATE MUSEUM 



Peekskill valley section. The aqueduct after tunneling through 

 Cat hill reaches, at the western edge of Peekskill hollow, a much 

 broader depression than Sprout brook, where again some pressure 

 structure had to be selected. Explorations were made here also on 

 a comparatively large scale to test the amount of drift cover, the 

 rock profile of the valley and the quality of bedrock. These data, 

 together with a drawing illustrating the suggested rock structure 

 may be found in N, Y. State Mus. Bui. 146. 



This valley is occupied by a down-faulted block of the Hudson 

 River- Wappinger-Poughquag series crowded together so closely 

 that the folds are essentially isoclinal and the beds stand on edge 

 across the whole valley. Such a condition introduces no special 

 difficulty in itself, but exploratory borings proved that in the lime- 

 stone beds particularly the circulation of ground water has materi- 

 ally weakened the rock. Comparatively friable material was 

 recovered in many places at considerable depth. 



These explorations were made at a time before the interpretation 

 of such conditions had become standardized by observation and com- 

 parison of the behavior of such ground in the tunnel construction. 

 In the beginning there was much greater fear of the efifect of certain 

 natural conditions and less question of others than they have 

 proved to deserve, and it took several years to put some of them in 

 their true relation. In some places, therefore, where questionable 

 conditions were encountered, an alternative design, wholly avoiding 

 the question at issue, was adopted as the safest procedure. 



This was doubtless the dominant factor in the decision to use 

 steel pipe for the pressure section crossing Peekskill valley instead 

 of the pressure tunnel in the rock, which could also have been con- 

 structed. It was also true that the steel pipe could be laid more 

 cheaply in this section, but this was hardly the governing factor in 

 this case. Those who have had much experience with the behavior 

 in various tunnels since that time, and also had opportunity to 

 check up on the original borings, know that borings are readily 

 misinterpreted. Underground conditions are easily overempha- 

 sized or underestimated. It should be said, however, that as these 

 qualities and behaviors have come to be better understood, the con- 

 viction has grown that a rock pressure tunnel would be entirely 

 ])racticable in such ground as this. Perhaps tunnel construction will 

 be resorted to at some future time if repairs or replacement of the 

 pressure pipe should be required. 



The Peekskill valley exploration, therefore, is of interest to the 



