CHAPTER XVI 



A STUDY OF SHAFT 13 AND VICINITY ON THE NEW CROTON 



AQUEDUCT 



[See outline location map, pi. 30] 



There has been reference made occasionally in connection with 

 the Bryn Mawr explorations, as well as others, to the remarkable 

 piece of bad ground encountered in 1885 ^^ the New Croton aque- 

 duct near Woodlawn in the Saw Mill valley. This experience has 

 been the source of much misgiving. Because of its evident im- 

 portance and close relationship to conditions that may exist in the 

 same formation at points on the Catskill line, an examination of 

 this ground was made for the purpose of comparison. The mean- 

 ing of that case and its bearing on the Bryn I\Iawr questions are 

 given below: 



Engineer's records 



This ground and its remarkable behavior is described by Mr J. P. 

 Carson in the Transactions of the American Institute of Mining 

 Engineers, September 1890, pages 705-16 and 732-52. 



A description is also given in Wegman's Water Supply of the 

 City of New York, 1658 to 1895, on page 152. 



From Mr Wegman's report is taken the following : 



The south heading was started from this shaft on June i, 1885. 

 It advanced at the rate of about 80 feet per month for 392 feet 

 through good limestone rock (dolomite), which then became softer. 

 On December 9, 1885, when the heading had reached a point 407 

 feet from the shaft a fissure was encountered from which about 

 100 cubic yards of decomposed limestone clay, sand and dirty water 

 poured into the tunnel, partly filling it for a distance of 125 feet. 

 After three days delay, when only clear water was flowing into the 

 tunnel, the fissure was plugged with straw. The heading was ad- 

 vanced 20 feet further until on December 22, 1885, an outoour three 

 times greater than the first occurred, covering everything in the 

 heading out of sight ^^ ^ ^ borings were made on the surface 

 with a diamond drill to determine the extent of the soft ground in 

 front of the tunnel. It was found to lie in a pocket in the rock, 



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