The second method of depositing concrete under water, namely, by placing the freshly mixed concrete in coarse sacks and immediately lowering them to place, is very convenient under certain conditions. This method is of especial value in leveling a foundation to receive concrete blocks, or to form a base for concrete deposited in situ. Small bags of concrete have been successfully used in filling the spaces between pile heads which were to support an open caisson. In such a case the bags should be lowered to a diver who places and rams them. If the bags be properly leveled and the earth firm, a part of the load is thus transmitted to the material between the pile heads, while if the earth be very unstable, the bag construction compels the piles to act together, giving lateral stiffness to the foundation and tending to prevent over turning.
1 "Concrete Work under water," Proc. Inst. C. E., Vol. lxxxvii. See also "Notes on Concrete," by John Newman, pp. 116 and 117.
548. The bag method was successfully used in replacing with concrete the timber superstructure of the breakwater at Marquette, Mich.1 The main portion of the breakwater was built of monolithic blocks on the rock-filled timber substructure. After removing a portion of the rubble filling, a bed was made for the monolithic blocks by laying concrete in place two feet thick, extending from one foot below to one foot above low water datum. This method was afterward replaced by the use of concrete in bags, which made it safe to remove a lesser amount of the rock filling of the crib at the center, and thus decreased the expense of the work. The bags were of eight ounce burlap made 6 feet long and 6 feet 8 inches in circumference, and held about one ton of concrete. They were filled while lying on a skip specially constructed, so that when the skip was in place it could be tripped and the bag placed in its exact position in the work.
549. In connection with this work a practical indication of the character of the concrete deposited in this manner was given by some small bags of concrete that were laid to protect, during the winter storms, a portion of the crib filling. Mr. Coleman says of this,2 "Only one layer of these sacks, laid slightly overlapping from the lake side of the crib, was used. The sacks were so lightly filled that when laid as described, the average thickness of the concrete covering was not more than six inches. The crib was storm swept many times without-displacing a single sack, and when they were removed in the following spring to facilitate the work, they came away, when pulled up with the floating derrick, a dozen or more at a time, so firmly were they cemented together, and in many cases large rubble stones were lifted up along with them, because of the adhesion of the cement to their surfaces".
550. The Cost of the concrete in bags was as follows: —
Materials, cement, sand, stone, burlaps, etc......$5,281
mixing concrete and filling bags...........912
Total Cost per cubic yard..........$6,758
Or, Cost in bags, exclusive of materials . . . . , 1.477
Major Clinton B. Sears, Corps of Engineers, in charge; Mr. Clarence Coleman, Asst. Engineer.
2 Report Chief of Engineers, U. S. A., 1897, p. 2620.
The Cost of the first plan, placing a two foot layer of concrete in situ, where different methods of handling were employed, was, for labor: —
Loading scow with materials............$0.411
Cost in situ, exclusive of materials......$1,781
551. When concrete bags are used in forming a foundation, the lower layers should usually cover a considerably greater area than that required for the top. Especially is this true if building upon insecure earth. This increased area at the bottom may be obtained by building the sides on a batter, or by the use of footing courses. If the latter are used, they should be so designed that in any case the projection beyond the course next above is not greater than the thickness of the layer.
Before filling the concrete into the bags it should be thoroughly mixed, as for deposition in the ordinary manner. The practice of using dry concrete for this purpose is reprehensible for the same reason as has been given in § 545. It has also been found that if the concrete is mixed and filled into the bags in a dry state, a layer of concrete on the outside may cake before the water has had time to reach the interior portion. The bags should be filled about three-fourths full, leaving the mass free to adjust itself to inequalities in the rock, or to the irregular surface of the previously deposited layer. When strength and compactness are desired, the bags should be placed by a diver and gently rammed. In this way the mass may be well bonded by "breaking joints".
Very large bags of concrete are sometimes employed, as in the construction of a breakwater at New Haven, England.1 "The top of the breakwater has a width of thirty feet, is ten feet above high water, and is surmounted by a covered way and parapet running along the outer side, both sides battering one in eight. The breakwater is unsheltered from the force of the Atlantic, is founded on the rough, natural sea bottom, and the foundation course has a width of fifty feet; the lower portion of the structure, from the bottom up to a level of two feet above low water, consists of one-hundred-ton sacks of concrete deposited while plastic. The canvas with which the concrete was enveloped was of jute, weighing about twenty-seven ounces per square yard. The sacks were dropped into place by a specially designed steam hopper barge. The 'sack-blocks' in the finished work became flattened to a thickness of about two feet six inches. With the exception of this sack work the breakwater is built of plastic concrete laid in situ" Similar sack-blocks of one hundred sixty tons have been employed in breakwater construction.