380 



HELL-GATE, IMPROVEMENT OF. 



depth of drill-hole per cubic yard of rock, 

 0.7; average number of pounds of nitro-gly- 

 cerine required per yard, 5.54 ; number of 

 feet drilled per machine per shift of 8 hours, 

 6.5 ; average cost per foot of hole drilled, in- 

 cluding placing and removing of scow, lower- 

 ing and raising cupola, the expenses of drill- 

 ing, of sharpening drills, and loss of steel, 

 $2.05 ; cost of dredging and dumping debris 

 per cubic yard, $4.29. 



For the removal of Hallett's Point Reef a dif- 

 ferent method was employed that of tunnel- 

 ing. This plan for the destruction of dangerous 

 reefs under water was suggested in!868 by G. 

 0. Reitheimer, former superintendent of the 

 Holyhead Breakwater Works, and in the same 

 year it was advanced by General Alexan- 

 der, of the U. S. Engineer Corps, who pro- 

 posed it for the removal of Blossom Rock 

 in San Francisco harbor, and by A. W. von 

 Schmidt for the same object, who projected 

 making an excavation deep enough to receive 

 the exploded fragments. The plan of each 

 was to sink a large shaft of iron or other ma- 

 terial to the surface of the rock, and, after so 



VERTICAL SECTION OF COFFER-DAM AND EXCAVATION AT 

 BLOSSOM BOCK. 



adjusting it at the bottom as to prevent the 

 entrance of water, to bore downward into the 

 interior. Mr. von Schmidt exploded Blossom 

 Rock in 1869 according to this plan, but with- 

 out following out his project of a deep excava- 

 tion to receive the debris. 



Hallett's Point Reef has been a serious ob- 

 struction in the East Channel, dangerous to 

 large and small craft alike ; it did not leave 

 enough sea-way for vessels floating down with 

 the ebb and steering clear of Flood Rock, and 

 vessels have often been cast against it ; it cre- 

 ated, besides, dangerous eddies at either tide, 

 particularly a strong drift toward the Frying- 

 Pan, when the tide set in from the sound. 

 The reef was semicircular in form, 720 feet 

 across, and running out 300 feet into the 

 channel. 



The work was commenced in July, 1869, by 

 the construction of a coffer-dam, of irregular 

 pentagonal form, between high and low water 

 marks, reaching 95 feet out upon the rock, and 

 having a breadth of 145 feet at the shore. The 

 shaft, 105 feet by 95 feet, was sunk to the 

 depth of 33 feet below mean low water. Tun- 

 nels, 10 in number, were then driven into the 

 rock and connected by cross-cut galleries at 

 regular distances. The tunnels were carried 

 out to distances varying from 51 to 126 feet 



before any transverse sections were made. 

 The tunnels radiated from the centre, and the 

 galleries followed concentric lines around the 

 shaft. As the distance between the tunnels 

 became greater, additional branching tunnels 

 were made until, at the last, they numbered in 

 all 35. There were 10 galleries, whose mean 

 distance apart was 25 feet from centre to cen- 

 tre. The area covered by the tunnels and 

 shaft was 2f acres. Tunnels varied at the 

 shaft from 13 to 22 feet high, with an average 

 height of 19J feet, and from 9 to 12f feet wide, 

 with an average width of 10 J feet; but they de- 

 creased rapidly as the reef declined, until their 

 dimensions were little greater than the head- 

 ing. The first gallery had a height of 12 feet 

 and a width of 9 feet, but the outer ones were 

 much smaller. The number of piers left stand- 

 ing was 172. The total length of tunnels was 

 4,857 feet, and of galleries 2,568 feet, making 

 in all 7,425 feet. 



The process of driving ahead the tunnel was 

 by drilling a number of holes converging tow- 

 ard each other, and springing out a mass of 

 rock, thus creating a cavity, into which the 

 surrounding portions could be exploded from 

 holes drilled perpendicularly to the face. When 

 large charges can be safely used in tunneling, 

 the convergent lines are drilled deep around a 

 pyramidal mass with a rectangular base, and 

 perpendicular holes (one or more) are made 

 within the base, and all are charged and fired 

 together, thus producing a deep excavation. 

 This rapid process could not be much used, 

 however, in the Hallett's Point excavations, 

 owing to the danger of heavy charges and si- 

 multaneous firing. Charges of not more than 

 eight ounces of nitro-glycerine were generally 

 employed, and the Bickford fuse used to set 

 them off. The cartridges were made with 

 cases of pasteboard, coated with an impervious 

 composition, and the percussion was produced 

 by means of a cap containing fulminate of 



GROUND-PLAN OF TUNNELS AND GALLERIES AT HAL- 

 LETT'S POINT. 



mercury. The explosive material was nitro- 

 glycerine and the compounds of nitro-glycer- 

 ine, except in spots where the rock was suf- 

 ficiently laminated to require only gunpowder. 

 Owing to the impracticability of heavy blasts 



