vided by lines handled manually. The smallest of these slabs were 14 feet by 18 feet 6 inches, of which five were constructed. The largest were 100 by 40 feet, of which two were formed. One slab of intermediate dimension, 48 feet 6 inches by 19 feet, was also built. The slabs referred to were launched in depths ranging from 10 to 40 feet. The outer ends of the largest rested in about the last-named depth. A diver was employed to determine their condition, with the result of finding them intact, subject merely to anticipated cracking, and fully effective for revetment purposes. The handling of these slabs appeared to justify the general conclusion that with proper equipment large sheets of reinforced concrete constructed upon a properly designed barge could be launched therefrom and controlled to the bed or bank to be revetted without insurmountable difficulty, objectionable disruption, or excessive cost. As a result the Mississippi River Commission recommended, under date of April 18, 1915, a further allotment for "Experimental revetment" of $25,000, which was approved by the Acting Secretary of War April 26. Under this allotment two barges were built upon which to form concrete sheets 125 by 50 feet, and upon a third barge was installed the machinery and equipment for handling the same. The general proposition contemplates the construction of a concrete slab or mat, 2 to 4 inches thick, upon what may be termed a mat barge. After sufficient hardening, this barge is withdrawn, the mat being withheld from relative movement and provided during the operation with substitute support and suspension from a series of overhead trusses. The latter have hinged bearings upon a second barge and roller bearings upon the ways of that withdrawn. This support and suspension is arranged for by a suitable number and disposition of lines from attachments upon the overhead trusses to rings or loops formed in the mat and connected to its reinforcement. the mat barge is withdrawn the lines referred to successively take weight, with the result that upon sufficient withdrawal for complete clearance the mat is wholly suspended between the two barges concerned, and thereafter may be tripped to position under the continued control and direction of upstream and shore lines of proper length and position. As Plant for the experiments in discussion was provided at a cost of $17,379.41. Creosoted lumber mat barges were built by day labor, and the required equipment secured from the Bucyrus Co., of South Milwaukee, Wis., who made several valuable suggestions as to mechanical and structural details. During the present spring six concrete mats have thus far been constructed and launched, at a cost of $3,019.97. The first was 50 by 50 feet, 3 inches thick, and weighed about 62,500 pounds. The second was 122 by 47 feet, weighing about 215,000 pounds. The last four have been 122 by 42 feet, and have weighed,, each, about 192,000 pounds. All of these several mats were handled without especial difficulty, and were controlled to position within reasonable limits, as determined by the fact of buoys attached at the corners of the structures. Incidental troubles were encountered, all of which, however, were satisfactorily corrected. The slabs just described were launched upon a sloping bank in depths of from 8 to 60 feet of water. Three of the sheets, launched during a high river, have been partly inspectable at low stages. All are but slightly disrupted, and fully effective for their intended purposes. Since in any actual revetment operation of the kind in question the time within which concrete slabs will harden sufficiently to withstand launching is an important, or, in fact, a controlling element of plant capacity, request was made of the Bureau of Standards that it seek an inexpensive material which, added to ordinary concrete in the mixing, would accelerate its hardening. After exhaustive investigation, the said bureau reported that the addition to the mixing water of perhaps 3 to 4 per cent, by volume, of a saturated solution of calcium chloride would produce marked acceleration. No evidence was developed of any probable ultimate deterioration or other objection. This ingredient was employed in the construction of four slabs, with results consistent with the bureau's conclusions. In what may be termed reasonable concrete weather, sheets will harden sufficiently to withstand launching within about 48 hours of their formation. The experimental operations outlined are thought to establish the fact that with proper equipment a concrete revetment of the character described can be constructed with advantage and economy. As to the latter about 60 cents per square yard would appear to be a reasonable estimate of field cost under conditions of going work. This figure is comparable with 70 to 80 cents for standard willow fascine revetment. As a further step in the matter, it is proposed to recommend the construction of one unit of working plant in steel, eccentric loading, etc., rendering the use of wooden plant hazardous. Four photographs accompany from which a fair general understanding may be had of the character and operation of the experimental plant described. III. WOLF RIVER. Location-Two hundred and thirty miles below Cairo, left bank. Original condition.-Wolf River is a hill stream cutting through the town of Memphis and entering the Mississippi at the north end of the Memphis Harbor. On its banks are located a number of hardwood manufacturing activities, part of whose log supplies are transported on the Mississippi, thence by way of Wolf River to the several plants. The natural depths of the stream are sufficient for the purpose indicated except that shoaling has been perennially experienced, incident to backwater from the Mississippi and the consequent deposit of material near the junction of the two. Previous projects.--None. Existing project.-The existing project was adopted in 1894, when, by the river and harbor act of August 7, $50,000 was appropriated for Memphis Harbor, $10,000 thereof to be used in dredging out the mouth of Wolf River, in the discretion of the Secretary of War. Operations and results prior to the fiscal year.-Such dredging as was required was done by contract until in 1898, when the dredge Wolf was purchased from the then contractor and operated under the Memphis office. The necessity for dredging was intermittent, and during the year 1914 none was required, Wolf River being apparently capable of maintaining sufficient depths since the junction of one of its branches with the Mississippi, incident to caving of the latter in 1912. The total cost of the improvement of this stream prior to the fiscal year amounts to $70,786. Operations and results during the fiscal year.-None. Condition at the end of fiscal year.-Conditions during the past two years are such as to suggest that no further dredging will be required, natural depths being sufficient for all needs. Local cooperation.-None. Effect of improvement.-The mill interests located on Wolf River have been conveniently accommodated as to their supply of logs. The transfer landing of the Rock Island Railroad at the mouth of the stream was provided with sufficient depths for its boats until 1912, when transfer operations were discontinued. Proposed operations.-None. Commercial statistics of Wolf River. Number of lumber manufacturing plants.... Capital invested.. Value of finished product from May 1, 1915, to Apr. 30, 1916. Amount paid in wages during same period. Value of logs at mill during same period.. Timber sawed, feet b. m.. Towing plant employed: IV. LEVEES. (a) Upper St. Francis district. Location. On the right bank of the river from Cape Girardeau, Mo., 54 miles above Cairo to New Madrid, Mo., 70 miles below. This district comprises an area of about 700 square miles. Original condition. The United States made its first levee expenditure in this district in 1899. At that time there existed fragments of levees constructed by local boards and individual land owners, but nothing to be dignified as a levee system. The most important of these several detached lengths of levee were found between Commerce and the Iron Mountain Railroad, and from Birds Point for about 3 miles south. They merely joined the relatively higher ridges found variously along the upper St. Francis front and were capable of restraining only the mildest overflows. |