Removal of Hell Gate rocks

Up to 1876

The great obstruction impeding the ship-travel between the Atlantic Ocean and New-York City, via Long Island Sound, was located at a promontory of Long Island called Hallett's Point. It extended out into the East River, approaching Ward's Island, which occupies three-fifths of the width of the river at that point; and some dangerous rocks were found in the immediate vicinity. The narrow channel thus formed has been a danger and a difficulty to navigators ever since this part of the country was first explored; and the rush of water taking place through the pass gave it the name of Whorl-gate, afterward Hurl-gate, whence the name by which it is now known was easily derived.

The first mention of preparations for commencing the work of removing the obstructions is found in the Report by Lieuts. Davis and Porter, of the United-States navy, made in the year 1848. This document gives a very accurate description of the course of tidal currents, the dangers to navigation caused by the rocks, obstructions, etc.; and it recommends that Pot Rock, the Frying Pan, and Way's Reef be blasted and scattered. The two former are single rocks of a pointed shape: the latter is long, and has the character of a ledge. The report also recommends that the middle channel be improved by blasting, so as to make a clear channel of sufficient depth for common vessels and steamboats ; and it also speaks of the increased facilities for naval defence which this improvement would afford. The difficulty of blockading the port of New York, with two outlets instead of one, would be, at least, doubled. Lieut. Porter did not exactly agree with Lieut. Dayis as to the best plan for improving the channel. They both recommended the removal of the small rocks — Frying Pan and Pot Rock — from the middle of the channel; and Porter included a part of the reef at Hallett's Point, the shell of which was blown in atoms, its interior having been removed and deposited far away on dry land. The art of blasting under water was almost unknown at the inception of this work; and engineers agree, that even the little improvement recommended by them could not have been effected without the inventions and discoveries which have since been made. The process adopted at that time for submarine blasting was to take down cans of powder, place them against the side or top of the rock, and explode them by means of a galvanic battery. This did well enough for rough and jagged rocks and bowlders; but so soon as the surface had been levelled off, it was of little or no use to attempt to continue the operation.

In 1832, Congress having made an appropriation of twenty thousand dollars for the removal of the rocks at Hell Gate, Major Fraser, of the engineers, began operations according to the Maillefert process. The sum of eighteen thousand dollars was expended on Pot Rock, and the depth of water was increased from 18.3 to 20.6 feet. This was all that had been accomplished up to 1868, when the duty of an examination of Hell Gate was committed to Gen. Newton of the United-States engineers, who made a report in January, 1867. For operating on the rocks in the middle of the channel, a steam-drilling scow was constructed. It had a well-hole in it thirty-two feet in diameter, through which twenty-one drills were worked; while the scow lay on the surface of the water directly over the rock to be operated upon. This formidable machine was first used in the spring of 1869, on Diamond Reef. A large number of holes were drilled into this rock, varying from seven to thirteen feet in depth, four and a half inches in diameter at the top, and three and a half inches at the bottom; and the rock was broken up by charges of nitroglycerine of from thirty to thirty-five pounds. Coenties Reef was operated upon in 1871. Ninety-three holes were drilled, and charged with nitroglycerine ; and seventeen surface-blasts were made. In 1873 three hundred and seven holes more were drilled, and thirty-nine surface-blasts were made. The amount of nitroglycerine consumed was 17,127 pounds, and the reef was thoroughly broken up. The debris had been partly removed, when, in 1875, Congress, owing to a mere clerical blunder, failed to include Diamond Reef in the appropriation; and work at that place had to be suspended. In 1872 the drilling-scow was towed to Frying-pan Rock. Seventeen holes were drilled, and eleven surface-blasts made.

Operations for removing the reef at Hallett's Point were begun in August, 1869. A coffer-dam was built of timber, securely fastened to the rocks by bolts passing through the framework. The coffer-dam was pumped out about the middle of October; and operations on the interior for sinking the shaft were begun early in November, and continued till the middle of June, 1870, when work was suspended on account of the funds appropriated for this part of the work being exhausted. At that time four hundred and eighty-four cubic yards of rock had been taken out, at a cost. of $5.75 per yard. In the latter part of July operations were resumed ; and, during that fiscal year, the shaft was sunk to the required depth of thirty-three feet below mean low water, and the heads of the ten tunnels opened to distances varying from fifty-one to one hundred and twenty-six feet. Two of the cross-galleries had also been opened. The amount of rock excavated from this place that year was 8,306 cubic yards, and the drilling was all done by hand. During the next year the use of steam-drills partially succeeded hand-drilling, and the work was pushed more rapidly. The number of feet of tunnel driven during the year was 1,653, and of transverse galleries 653.75. The quantity of rock removed was 8,293 cubic yards. A ground-plan of the work herewith gives an excellent idea of the excavation as completed. An exceedingly well-executed model of the works was exhibited at the Centennial Exhibition at Philadelphia. It is made exactly to scale, and well represents the nature and extent of the vast operations that have now been successfully completed.

The rock-bed of the river is, in the model, raised from the pillars that support it, so that a close inspection of the interior may be made. There are one hundred and seventy-two of these pillars, pierced with about four thousand drill-holes; and the shell, or roof, or bed of the river, varies from six to sixteen feet in thickness. No less than thirty thousand cubic yards of broken stone was left under water, all of which was removed by dredging.

A detailed survey of the upper surface of the reef was made in 1871, by Mr. William Preass, assisted by Mr. F. Sylvester. They took more than sixteen thousand soundings, each separately located, by means of instruments, from the shore. Great pains were taken to delineate exactly the surface of the rocks. The appropriation of 1871 was two hundred and twenty-five thousand dollars, — just one-half the amount asked for by Gen. Newton, who regretted that the beginning of operations on the Gridiron was thus prevented, as he considered this rock more dangerous to the navigation of large vessels than Hallett's-Point Reef. For the next year he asked six hundred thousand dollars, but got less than half that sum. About the middle of November, 1873, work was suspended for want of funds; at at the end of the fiscal year, June 30, 1874, it was found, that, for the four months and a half during which operations had been carried on, 896 linear feet of tunnels had been opened, and 4,648 cubic yards of rock removed. The total length of tunnels and galleries then amounted to 6,780.67 feet. The excavation now being nearly finished, the manner of finally blowing up the whole mine began to exercise the minds of the engineers. Gen. Newton finally suggested his own plan for blowing up the reef at Hallett's Point, which was to perforate each pier with drill-holes entirely or partly through its mass, a sufficient number of these being provided to complete the destruction of the pier when fully charged. The charges in the different holes of the same pier were to be connected together; and a fuse, composed of a quick explosive, would connect the system of charges in each pier with those of the neighboring piers. By this mode, the communication of heat or the electric spark to a few centres of explosion would suffice to propagate it through the whole system, because the explosion of the connecting fuse would advance more rapidly than the demolition of the rock. Gen. Newton's plan was adopted, with few slight changes, principally suggested by himself. Instead of depending on explosives to convey fire from pier to pier throughout the mine, an electric spark was sent directly to every centre, insuring the simultaneous explosion of the whole mine. It was decided that the minimum amount of explosives could be determined by placing one charge in each square pier and two in each oblong pier ; but this mode would make the lines of least resistance the maximum, and thus increase the shock, which would be propagated through the reef to the dwellings upon the land. It was therefore determined to decrease the lines of least resistance, which would multiply the number of blasts, and increase the quantity of explosives, but would, at the same time, reduce to a minimum the vibrating influence through the reef. It was, therefore, calculated that the exterior effect, except an agitation of water, would be small.

The proximity of the reef to habitations at Astoria, Ward's Island, and Blackwell's Island, made it necessary to devise a system of explosion, which, effecting the work of demolition would, at the same time, do no damage to life and property.

The atmosphere and the rock being the mediums through which the shock would be transmitted, it was essential that the waves propagated through these should be made as small as possible.

It was evident, in the first place, that, if to each charge its full capacity of useful work in the breaking-up of the rock were assigned, regard being likewise had to the superincumbent weight of water, no external effect of moment would be perceived in the atmosphere.

In the second place, it was evident that the magnitude of the rock-wave would depend greatly upon the amount contained in individual charges; that is, if eighty pounds were required for the individual charge, the vibration of the rock would be much greater than if these charges did not exceed twenty pounds. It was known that eighty-pound charges of nitroglycerine, fired in numbers of twelve to twenty, did not cause a destructive wave.

Again: the reef, after excavation, being connected with the rest of the rock formation only through the piers and the outer edge of the roof, it was inferred that the shock propagated in the rock should be estimated as due mainly to the charges necessary to disrupt the piers and roof from their connection with the bed-rock; and, also, that the additional number of charges required to break up the roof and piers would not enter largely into the amount of shock transmitted under ground.

These were the fundamental ideas upon which the system of mines was established. As the tunnels in radial lines concentrated upon the land, an accumulative explosive effect in that direction was prevented by so proportioning the charges that the roof should be broken through by the first impulse in many places, and thus give vent upward.

To prevent a concentric explosion, by which the debris might be heaped up in large masses near the centre of the area, the charges in the outer zone of the semi-elliptical reef were increased beyond those of the other portions; and by this means the masses of debris fell generally back in the places to which they originally belonged. Some portions were thrown beyond the limits of the reef toward the channel, and constituted what may be termed a dispersive explosion.

Hallett's-point Reef is in the shape of an irregular semi-ellipse; the major axis, which lies next to the shore, being seven hundred and seventy feet in length, and the minor axis projecting straight into the channel about three hundred feet. The cubic contents above the depth of twenty-six feet at mean low water amount to fifty-one thousand yards. Besides the risk of striking the reef, it produces eddies on both sides of it according to the direction of the tidal currents, and is much in the way of vessels coming down in the ebb in the effort to hug the shore, and thus avoid being drawn upon Middle Reef. The explosives used in tunnelling at Hallett's Point have been nitroglycerine and its compounds, and gunpowder; the' latter being used only when the rock was weak and seamy. Nitroglycerine was always used for driving the headings of the tunnels. To drive a heading, the drill-holes are made at an angle with the fuse, so that the charge lifts out the rock by its explosion. A cavity being made in the middle of the heading, holes are drilled around it, and the surrounding rock blown into it. Only one blast is exploded at a time, as great care has to be taken not to shake the structure overhead by too heavy vibrations. There is consequently no volley firing, and the galvanic battery is not used for discharging the blasts. The average twelve months' work, with six Burleigh drills, was the excavation of two hundred and thirty-five lineal feet of heading per month. Up to June, 1872, the work had been prosecuted by hand-drilling, with the exception of 20,160 lineal feet of drilling by the Burleigh drill, and 7,000 feet by the diamond drill. That by the Burleigh drills was done by contract, at so much a foot; and the diamond drill, purchased for the purpose of exploring the rock ahead, was put in competition with it.

The cost of drilling, after a long trial with the Burleigh, is found to be between thirty-six and thirty-seven cents per foot, including repairs, etc. The cost of hammer-drilling was found to be about ninety-five cents per foot. The number of feet of holes drilled by each machine per shift of eight hours was thirty feet.

The diamond drill, owing to the encounter of frequent veins of pure quartz in the rock, often gives out, and has to be repaired. Owing to the restricted area of the tunnels and galleries, the work of excavation was almost exclusively that denominated heading, without the advantage of enlargement. The rock, after being blasted, was lifted by hand into a box resting on a truck-car, which was run down to a point upon a rail-track, and thence drawn by a mule to the shaft, where the box was hoisted by a derrick, and its contents emptied into dump-cars, to be rolled away, and deposited in the pile. Calling the cost of blasting and removing one cubic yard one dollar, the following gives the proportion of each item of expenditure:

The work of excavation having been finished, the drills were set to work perforating the roof and piers with holes to receive the final charges which are to explode the mine.

The mode of calculating and arranging the charges was to consider the roof-holes as the receptacles of explosives enough to form common mines.

The line of least resistance was assumed as the distance from mid-length of the charge to the surface of the rock. Since the charges were perfectly tamped by the confined water within the excavation, this rule of measuring the line of least resistance was assumed to be practically correct. With less perfect tamping, the lines of least resistance for such mines designed to break through the roof would have required estimates quite different.

The average amount of explosives required to break up and dislodge one cubic yard in enlargement had already been found to be .97 pound ; and from this resulted

C = charge in pounds = 0.038 L 3 ;

L being line of least resistance in feet.

All roof-holes, excepting those over piers, were treated by this formula. The piers, being very irregular in shape and size, would have exacted much care and time to have located the holes and proportioned the charges to the exact mathematical requirements in each case. One pound and a half of explosives were assigned, as a rule, to each cubic yard of the piers; it being considered of the first importance to demolish completely these supports of the roof.

The roof-holes above piers were charged from the formula:

C = n L s ;

being successively .038, .05, and .06, increasing from the shaft outward.

The bodies of piers within the outer zone were charged with two pounds per cubic yard. Within the inner zone, where the depth was comparatively little, it was considered proper to reduce the charges to the smallest limit capable of affording a good result, both to avoid disturbance of the atmosphere and to prevent a concentrated action, due to the direction of the tunnel upon the land. The increased proportion given to the charges within the outer zone favored this intention, by giving quick vent to the gases in that direction.

The cubic contents of the roof and piers were 63,135 yards and the amount of explosives as follows: rend-rock 9,127 ½ lbs., vulcan powder 11,852 13/16 lbs., dynamite 28,935 ¼ lbs. — total 49,915 9/16 lbs. Being at the rate of 0.79 pound to each cubic yard.

The explosives were packed at the respective places of manufacture, in tin cartridge-cases, the last being furnished by the Government.

The number of holes charged was 4,427, and the number of tins used was 13,596; eighty-seven percent being twenty-two inches, and the remainder eleven inches, in length.

The holes being tapering, the cases varied in diameter from one and three-eighths to two and a half inches; the intermediate sizes differing by one-eighth of an inch.

One end of the tin case was fitted with a screw cap, with rubber washers to exclude water ; the other end being arranged with four short lengths of brass wire, soldered on the perimeter of the bottom, and spread out. When the cartridge was pushed home, the wires, by their elasticity, pressing against the side of the hole, prevented a falling-out.

On the 11th September the charging of holes was commenced, and finished at nine p.m. on the 20th, — consuming nine days. Had the cartridges been delivered in good condition, this operation would have consumed only about four days.

These holes were made from two to three inches in diameter, and from six to ten feet apart, and their average depth about nine feet. The size of the holes, and their direction and distances apart, were made to vary according to the character of the rock to be broken. The drilling of these holes up into the roof of the mine soon increased the leakage of water into the works from three hundred gallons per minute to five hundred, it being impossible to avoid tapping a seam occasionally. Many of the holes that were found to be leaking were plugged up temporarily, and the leakage thus reduced. The outside gallery and the No. 4 heading were deepened so as to concentrate all the leakage, and cause it to flow to the shaft-end of that heading, where pumps were placed. The following shows the amount of the appropriations made by Congress each year for Hell-gate and East-river improvement, and the whole amount expended up to the date of the last report of Gen. Newton to the chief engineer:

After this report was made, Congress appropriated $250,000.

Care had been taken to test the various kinds of explosives. Up to the middle of 1874, nitroglycerine had been principally used for blasting purposes. Several hundred pounds of mica-powder were then tried, some giant-powder, several thousand pounds of rend-rock, and, later, considerable vulcan-powder was used. All of these are nitroglycerine compounds. Neither of them was found to be as powerful as the glycerine itself; but it was repeatedly demonstrated, that, with ten ounces of rend-rock or vulcan-powder, they could break as much rock as they formerly did with eight ounces of nitroglycerine, while the cost per pound was less than one-half that of glycerine.

The Mode of Firing (Final Explosion 1876)

After the holes had been charged with tin canisters, the next operation was to insert the priming-charges, which were contained in brass tubes. Brass was preferred to tin on account of greater durability in salt water and better protection against leakage, — conditions insuring the detonations at least against moisture, should the exposure be of long duration.

The amount of these charges — three-fourths of a pound to each primer — has been included in the grand total already given. The primers contained also, as detonators, fuses holding each twenty grains of fulminate of mercury. The terminals of two connecting-wires were inserted in each fuse, and bridged with .001-inch silver platinum wires a quarter inch in length.

The fuses, in groups of twenty, were connected in continuous series with connecting-wires. A lead and a return wire were attached to each group.

Twenty primers, with fuses and wires properly arranged in a box, with lead and return wires on reels, were carried to each party engaged in this work. The time consumed in placing 3,680 primers, unreeling the lead and return wires, and leading these out of the shaft, was two days and a fraction.

The connecting-wires, in length varying in the different groups from twenty to thirty-five feet, were copper wires of No. 18 American gauge (.04303 inch), insulated by a coat of guttapercha; the size after coating being No. 9 American gauge (.11443 inch). The total amount used was 118,525 feet.

The lead and return wires were copper wires of No. 12 American gauge (.080808 inch), insulated with two coats of guttapercha; size of coating. No. 4 American gauge (.2043 inch). The total amount used was 147,703 feet, in lengths from 250 to 625 feet.

The batteries used consisted respectively of forty, forty-three, and forty-four cells of zinc and carbon, or nine hundred and sixty cells in all, divided into twenty-three distinct batteries, each battery to fire a hundred and sixty fuses, arranged in divided circuit, in eight groups of twenty each. The fluid was made in the proportion of six pounds of bichromate of potassa, one gallon concentrated pure English sulphuric acid, and three gallons of water.

The separate batteries were so arranged in two frames, that all the cells could be immersed by the same operation. The system then consisted of 3,680 mines and twenty-three batteries ; each battery assigned to a hundred and sixty mines, which were divided into eight groups of twenty each.

The mines of each group were connected in continuous series, and a lead and return wire to the battery closed the circuit. To insure the simultaneous discharge of the whole system, a "circuit-closer" was introduced.

The method, which will be explained, for one division of a hundred and sixty charges, will suffice for the others.

One lead-wire from each group of the division — i.e., eight in all — was connected with one pole of the battery. The other pole was then connected with a brass pin, penetrating through a wooden horizontal disc, which, being let go by the run, would cause the brass pin to enter a cup filled with mercury, planted in a second wooden horizontal disc fixed in position. If the eight return-wires of the same group were then connected with the brass cup containing mercury, it is evident, that, when the brass pin entered the mercury-cup, the circuit would be closed, and explosion would result. Obviously, if, instead of one pin and one mercury-cup, twenty-three pins and twenty-three cups were attached respectively to the two discs, and the same connections as just described made for each and every division of a hundred and sixty mines, a simultaneous explosion would result at the moment when the upper disc should fall upon the lower.

The upper disc was held over the lower one, and apart from it, by a cord passing and looped over the tin case of a torpedo, securely attached to a frame. This torpedo, or cartridge of dynamite, was provided with a detonator, from which two wires passed to a small battery situated twenty-one hundred feet distant. The torpedo was fired by closing this circuit with a Morse's key : the cord being severed, allowed the upper disc to descend upon the lower, and thus close the circuit of the great batteries.

The siphon was started at 12.07 a.m. on the 23rd of September, and at 7.30 p.m. the excavations were filled to the level of the tide.

The mines were fired at three seconds past 2.50 p.m., on Sept. 24, 1876.

The explosion was distinguished by the absence of hurtful shock in the atmosphere, in the water, or under ground.

The elevation of spray, vapor, and gases, projected upward, reached to the height of a hundred and twenty-three feet, measured at the centre and highest point. The quantity of water actually raised was trifling, as evidenced by the almost total absence of a propagated wave. The explosive effort in the air was not perceptible ; the glass in buildings close to the dam, and of one in particular along the shore-line of the shaft itself, not having in a single instance been broken.

The underground shock was trifling, but perceptibly felt in the cities of New York and Brooklyn. Along the line of the reef, a little plastering was dislodged from a ceiling in a house a hundred and fifty yards, and in two houses six thousand yards, from the work.

The new facts obtained by this experience were:

1st, that an unlimited amount of explosives, distributed in blast-holes in moderate charges, proportioned to the work to be done, thoroughly confined in the rock, and tamped with water, may be fired without damage to surrounding objects. 2nd, that an unlimited number of mines may be simultaneously fired by passing electric currents through the platinum-wire bridges of detonators.

The total cubic contents demolished by the explosion were 63,135 cubic yards solid. On the different suppositions for the broken debris of once and a half, and of twice, the original volume, there would result respectively 94,702.5 and 126,370 cubic yards.

The contractor was set at work removing the broken rock with a steam-grapple. The cost to the government is $2.40 per ton of 2,240 pounds.

The quantity of broken stone to be grappled in order to obtain a depth of twenty-eight feet was 45,488 cubic yards.