GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK IO3 



The fact that such material is occasionally encountered introduces 

 an engineering factor, which, previous to these experiences, was 

 generally overlooked. Tliis is the possibility of encountering decayed 

 rock of bad workjing quality, even after passing through the drift 

 cover on high ground and in positions not usually considered ques- 

 tionable. As a matter of fact, good behavior was largely taken 

 for granted in this case without much exploration, and it was only 

 after encountering the difficult ground in actual construction that 

 its quality became known. It is a striking thing in this case that 

 the ground has a northwest exposure and it would appear that 

 such a place high on the valley side should be completely denuded 

 of such superficial material. That such is not the case, however, is 

 a simple fact and it is not the only one discovered in a similar 

 position. 



Sprout brook valley section. At Sprout brook the aqueduct 

 reaches a very narrow valley much too low for crossing without 

 some special structure and too narrow to allow a very economical 

 pressure tunnel plan in bedrock. Before the rock conditions were 

 fully determined, however, explorations were made with the idea 

 of determining whether the pressure tunnel method would be feasible. 

 The rock floor was found at considerably greater depth than the 

 present surface, the whole bottom of the valley being occupied by 

 limestone standing practically on edge. Gneisses and granites 

 occupy the sides of the valley and no unusual structural features 

 were discovered. 



As far as engineering requirements and working conditions are 

 concerned, the rock tunnel is an entirely practical method. That 

 plan was abandoned, however, in favor of steel pipe because of 

 the more economical construction by that method. The tunnel would 

 require a great depth of shaft at either side and that item' thus 

 becomes dominant in the total cost, being as great for this short 

 structure as for a longer tunnel. This makes the cost of such a short 

 section quite out of proportion to the other adjacent parts or equal 

 lengths of other pressure tunnels. On the other hand, since the 

 drift fill in the valley brings the floor up to 150 feet elevation, steel 

 pipe could be used to practical advantage. 



This is an especially good illustration, therefore, of the working 

 of quite a dififerent principle from that of geological interpretation 

 or even of direct engineering questions in the matter of selection of 

 design for a piece of work. The controlling factor in this case is 

 relative cost. 



