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July, [O12 hang still closed in and: the sands boiled. The work was again aband- oned and the contract cancelled. An unusual step was now taken by the navy department. Five construc- tion companies of the requisite finan- cial strength were invited to bid for the completion of the dock. The Hol- brook, Cabot & Rollins Corporation, of Boston,was the low bidder, at $1,- 389,000, and contract was made with them on Nov. 13, 1909: Almost. co- incident with this, Civil Engineer Frederic -R, Harris, United States navy, was detailed by the department to take charge of the work, and as soon.as he had made his preliminary examination of the site, reported that it was. not: feasible to build the dock by the use of:-ordinary methods. A new type of construction was advo- cated and finally adopted by which the foundation work would be prose- cuted by the use of pneumatic cais- sons. Preliminary Work. An exhaustive set of core borings were driven at intervals over the site of the work, and caisson sinking be- gan. Bed rock was found at a prac- ticable level near the head of the dock, but at the entrance and for about half the length; it was founda that re- liance would have to be placed on the typical boulder and sand _hard- pan encountered. fe The size of the dock was again in- creased to these final dimensions: 720.4. tong. over yall; lad at. .6° in: wide .at copie: 110 ft wide. ata point' 35. ft. below M. -a W.:. and 30 ft. over sill at mean high tide. The dimensions as given above will take any ship capable of passing the locks of the Panama canal, not over 690 ft. long. Finding' that the orig- inal plan of surrounding the site with a pneumatic caisson cut-off wall, with- in which a dock of the original type was to be built in the impounded material, would not be possible, ow- ing to the impracticability of reach- ing rock with compressed air work at the entrance end, this design was of necessity abandoned. It had been proposed to build a heavy or gravity type structure resting on piles driven at regular intervals and close together over the entire area. The inverted arch floor of concrete was to be 17 ft. thick, to resist the hydrostatic thrust when the dock was empty, and was so designed as to transmit this thgust to the side walls, of massive construction, whose addi- tional weight was to make the whole structure stable, while the weight of the structure itself and of the suov- THE MARINE REVIEW ported ship was:to be taken by the piles. The construction, as outlined above, was found to be impossible without penetrating some distance into the un- stable quicksand which underlaid the entire: area. . This. was.a fine sand, saturated with water, and when so saturated, flowing almost as freely as water itself. The removal of all top soil. over the area would release this unstable material and its removal would become practically an impossi- bility. The attempt to do just this thing was, in a large measure, responsible for the early failures. Final Design. In this dilemma, new methods were devised and it was fially decided to sink a continuous wall 5 ft. in thick- ness around the entire dock, either to rock or hardpan, and to finally in- corporate this wall with the super- structure in such a way that it should become practically monolithic con- struction. Caissons 36 ft. long and 5 ft. wide were sunk end to end, in such a position as to be in line with the center of gravity of the dock wall, above. Each end of these caissons was provided with a semi-circular re- cess, and these, abutting, were finally sealed' with a solid concrete key, placed under pneumatic pressure. The structure as now designed contem- plated a reinforced concrete floor slab, approximately 8 ft. thick. This was supported along the center and on two lines parallel to the center line by reinforced ,concrete piers, 7 ft. square. These piers were sunk in caisson to a pvint determined by the material encountered, till they reached a reliable bearing below the quick- sand, when they were flared by pol- ing boards "to "ay minimum of 11 {i square. The piers were located in ranks 20 ft. center to center longitud- inally, and the side lines were each 24 ft. from the center line. This pro- vided for the weight of a ship in dock being supported as a direct thrust on these piers,-each of which pro- vided 121 sq. ft. of bearing area on the firm stratum in which they were anchored. They further served the purpose of providing resistance against uplift from the hydrostatic pressure, being in effect immense' mushroom anchors, the center line providing a resistance of 500 tons each and the side piers, which were not sunk to as great a depth, took care of 400 tons per piér. < Reinforcement. The floor reinforcement was placed in such a way as to make the slab capable of beam action in both di- 2285 rections. This was necessitated by the fact that the three governing con- ditions of greatest stress, viz. with the dock empty and water pressure acting upward; with the dock flooded, and the weight of the structure and contained water acting downward; . and with the dock pumped out with a ship in it, producing a heavy con- centration on the supporting piers and floor; demand that a complete reversal of stress shall be provided for. It was necessary to provide for a maximum possible thrust upward on the bottom at mean high tide of 2,900 lb. per square foot, in addition to the ship loads given above acting down- ward and the weight of the structure itself; and 3.8 sq. in. of twisted steel rods were provided for each running foot of the dock floor, both top and bottom. The surrounding caisson wall was reinforced by 60-lb. steel rails, placed vertically, on 2-ft. centers outside and 8-ft. centers inside of the wall. These rails projected so as to furnish me- chanical bond with the superstructure at the pdint when the floor joins the sidewalls, l-in. square twisted rods were placed at an angle of 45 deg., projecting toward the center line, 1 ft. center to center. Alternate rods were bent to a horizontal posi- tion so as to lie in the plane of the floor; the balance projected into the buik of the side wall. Above the floor level the face of these walls was provided with short rods, 5 ft. by 4 ft. center to center, to provide bond with the bulk of the superstructure wall. The piers supporting the floor were reinforced vertically by 60-lb. rails and 3-in. diameter bolts which -- were allowed to project to interlock with the steel in the floor. Arch Head. The head of the dock is a semi- circle of approximately 70 ft. radius. The caissons which enclose this space are made segmental in shape, and are 7 ft. thick, so that a horizontal arch -- of approximately 4 ft. thickness lies entirely within their mass. These features will be easily identified by reference to, Fig. 1. The cut-off fea- ture of the foundation wall was com- pleted by two lines of caissons sunk across the mouth of the dock. These were 40 ft. apart, and so constructed that their vertical continuation formed a cofferdam, within which the gate sills, 25 ft. apart, and the connective entrance floor and sidewalls were laid in the open cut. This open working was braced inbothdirections by tim- ber framing at right angles, successive tiers being placed as the excavation proceeded, the removal of excavated