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ie place puddle back-fill. around the out- side of the sheeting up to the low- tide level, thus greatly reducing the head against the sheeting, and the possibility of underscouring in the ir- regular formations. This method had previously been contemplated, but it was desired to avoid it at this time, in order to make the fill while the concrete work was in progress, since the contract had been badly delayed, due to the inability to secure materials. A borrow pit was opened in a hill above the shop building of the navy yard to obtain material for the back- fill. This hill is an irregular forma- tion of shale and clay, exactly similar to the material around the head of the dock. The material, when deposited around the sides of the sheeting, pushed back the silt, which had accum- ulated there to a depth of several feet, and settled into the stratum of black clay under this silt, forming a quite satisfactory puddle. In all about 52,000 cu. yds. of the puddle was required to make the back- fill up to low-tide level. A steam shovel was installed in the borrow pit and the material was delivered over the tracks in the navy yard in trains of 5-yd. dump cars. A track was laid along each side and the inshore end of the cofferdam to permit the material to be deposited directly in place. Var- ious conditions in the navy yard hin- dered this work, but on an average 1,000 cu. yd. of material were deliv- ered every 24 hours over a single track %4-mile long leading through the yard, with 17 switches in the track. | After the fill had been carried up to low-water level, the two 15-in. pumps were started and the water level low- ered to the tops of the piles in three days, exposing the mud which stood over the heads of the piles. This might appear to be too rapid work, but the cofferdam had been pumped out previously, so the sheeting had all taken bearing; as the fill had also been deposited through the water it was- therefore thoroughly packed, so, on the whole, no danger was anticipated in removing the pressure from the in- side of the cofferdam. It was not con- sidered necessary to install additional pumps, since the small openings in the sheet piling had all been wedged when the cofferdam was pumped out before, so the latter was practical'y tight, the backfill making it even more se- cure. During the 21 months which elapsed after the cofferdam was built, until it was finally unwatered, 18,000 cu. yd. of silt had been deposited, from the water, around and over the heads of THe Marine REVIEW the piles driven to provide a founda- tion for the dock. - At the time this article was prepared the silt was be- ing sluiced to the pumps at the land end of the dock. The bottom of the cofferdam was divided into three longitudinal sections of four panels of the timber crib each, with the inten- tion of cleaning the mud from one side section and placing the concrete in the latter to a depth of 2 ft. Then the other side was to be cleaned and concreted, after which the mud would be cleaned from the four center panels and the concrete placed in them, thus forming a 2-ft. concrete seal over the entire bottom. The mud was loos- ened with jets from nozzles supplied by a reciprocating pump on top of the crib. Sufficient water to carry the loosened material to the pumps was admitted through gates in the sides of the cofferdam. By working laborers in three 8-hr. shifts, it was possible to remove, in this manner, 2,000 cu. yd. of material a day, with an 8-in. and two 15-in. pumps, the latter carrying 10 per cent solid matter in solution. Two 15-in. centrifugal pumps, on which dependence will be placed to keep the water level in the cofferdam 'lowered, are each arranged on an ele- vator cage so they can be raised and lowered with the variations in the water level, or lifted above the top course of the timbers of the crib. Each pump is placed on a 6% x 10-ft. platform, built of 6 x 8-in. timbers, and:<is -belt-driyen by a 120-H. P. electric motor on a platform 14ft. above it, the two platforms being con- nected by eight 6 x 6-in. uprights, which are cross-braced thoroughly and form the cage. The latter operates in a timber guide frame, built up on the top timbers of the crib, and: is 'sus- pended from this frame by a steel ca- ble on triple purchase sheave wheels. The two guide frames are set at the inshore end of the cofferdam, where the discharge pipes can be dropped over the sides. Each cage can be readily raised and. lowered by travel- ing derricks, which operate on top of the crib. The suction head is thus greatly reduced by being able to alter the position of the pumps, and danger of the pumps being stitbmerged in case the cofferdam is filled suddenly is avoided. Since danger of flooding the cof- ferdam will exist at all times under the most favorable circumstances that can be obtained, it is essentially neces- sary that arrangements be made to place the volume of over 80,000 cu. yd. of concrete required in the dock with all possible speed. To this end, a very complete material and concrete. handling plant has been installed. The controlling features of the arrangement of this plant were to be able to pass . all materials through it in as nearly a straight line as possible, and to in: sure a continuous output during three 8-hr. shifts' under practically all con- ditions. As no. suitable sand is ob- tainable in the immediate vicinity, one of the scows that had been rigged previously as a pile-driver outfit, was fitted up as a dredge to pump sand from the bed of the Sacramento river, 42 miles above the site. In that lo- cality the bed of the stream is com- posed of a remarkably clean, sharp sand. The dredge has an 8-in. centti- fugal sand pump, steam engine, and can deliver 50 cu. yd. of sand an hour to scows under the operating conditions. The scows were towed to the dock by a gasoline tug until a larger tugboat could be placed in commission for the purpose. The crushed stone is furnished by a quarry 24 miles distant on the shores of San Pablo Bay. It is loaded 'into 600-cu. yd. scows, which are also towed to the site. An elevated storage pocket divided into two compartments, one for sand and one for broken stone, has been erected on a pile foundation, with its adjacent end about 50 ft. downstream from the dock, and with its longitud- inal center line coincident with the longitudinal axis of the dock. The concrete mixing plant is a three-story tower against the off-shore end of the cofferdam. This tower is carried by piles, and has' its transverse center line coincident with the longitudinal center lines of the cofferdam and stor- age pocket. The elevated pocket is 30 x 300 ft. in plan, and rises to a total height of 32 ft. above mean low tide. It stands on 89 transverse pile bents, each of which contains four piles ranging from 80 to 100 ft. in length. These piles were driven to a penetration of as much as 75 ft. in places without fetch- ing up, as the stratum of blue clay un- derlying the site of the dock drops away at the off-shore end of the latter to an undetermined depth. Batter piles were driven on both ends of every fourth bent to prevent lateral motion, and as all of the piles under the struc- ture assumed a comparatively large carrying capacity soon after they were driven, no apparent settlement or weakness has been detected when both pockets were fully loaded, bringing ap- proximately 30,000 pounds on each pile. A tight floor is erected over the caps of the pile bents, the bottom of belt-driven by a. SED POR Le RBS Meme ee Bisa pt od Sie aS