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1 MARINE REVIEW AND MARINE RECORD. IMPROVEMENT OF THE DETROIT RIVER. One of the Assistant United States Engineers gives an Interesting Account of Methods Pursued in Deepening Lake Channels, of Dredging Plant used, Costs, etc. Mr. Charles Y. Dixon, assistant United States engineer at Detroit, has for a number of years been in charge "on the work," as the term goes, of extensive dredging operations in the lower part of the Detroit river-operations that are costing the govern- _ ment millions of dollars and in which the best practice of the lakes is developed. At a recent meeting of the Detroit Engineering Society Mr. Dixon presented a great deal of valuable information regarding this work. His paper on the subject follows: ' The improvement of the channels connecting the waters of the great lakes has been in progress under the United States engineer department for more than a quarter of a century. Dur- ing this time the commerce has grown to proportions which would not have been predicted at the time of the beginning of the work. In the early stage of this improvement, vessels were limited to about 13 ft. draught and the improvement required consisted only in the removal of isolated obstructions to navigation. With the gradually-deepened channels came larger and deeper draught vessels, resulting in demands for still further improvement. To meet these increasing demands, the United States has expended and is now expending more money for the improvement of the harbors and channels of the great lakes than has any other coun- try for a like purpose. In preparing this paper it was hoped to interest you by a statement of what has been and is now being done to improve the channel of the Detroit river, and also by a description of the methods of carrying on this work. For a number of years after the beginning of the improvement of the channel of the Detroit river this work was carried on intermittently under a number of small appropriations and it was confined to critical points where the results would be immediately available. Prior to any improve- ment there was a greater depth of water near the Canadian shore at the mouth of the Detroit river than elsewhere and it was but natural for traffic to seek this deeper channel. Thus it was that nature is mainly responsible for the selection of that channel for improvement which has since come to be known as the Ca- , nadian channel. Other routes have since been surveyed for the purpose of preparing estimates in order to make a comparison of costs, but the Canadian channel was fourid to be the cheaper. It must be remembered that the main purpose for which appropria- tions for the improvement of channels are made is to cheapen transportation, and it affects this purpose not at all even though trafic does tcllow the Canadian shore for a few miles. It remains for the engiieer to select that route through which the desired re- sults may be obtained for the least cost. Thus has continued the improvement of the channel near the Canadian shore at the mouth of the Detroit river by the United States. At the beginning of the improvement of this channel (in 1874) vessels were limited in draught to 13 ft., the main obstruc- tion at this depth being then (as now at a greater draught) a ledge of limestone bedrock extending over about %4 mile length of channel near the mouth of the Detroit river. The first work done towards the excavation of a deep channel through this ledge of liniestone was performed by the Canadian government in 1875, and but very little was accomplished when their funds were ex- hausted. They have since preferred to expend their money for channel improvements at other places, leaving this work to be 'done by the United States. _ The first excavation at the Lime-Kiln crossing under the » United States war department was made in 1876 by Messrs. Case & Jennings of Dunkirk, N. Y., at the then exceedingly low price of $7.50 per cubic yard, scow measurement. The first project un- der which this expenditure was made contemplated a curved chan- nel, 300 ft. wide and 20 ft. deep, following the natural direction of the current, but this project was afterwards so modified that the improvement under later appropriations provided a straight chan- nel 440 ft. wide and 20 ft. deep (or 19 ft. deep at the last average midsummer or high-water stage). This work was completed in 1890, when it was found that in order to make this increased depth available much. additional work was required both north and _ south of Lime-Kiln crossing. This work has since been carried on with the view to obtaining a channel of 600 ft. width and 21 ft. depth (at the last midsummer or high-water stage) with an additional widening to a maximum width of 800 ft. at critical points in order to straighten the channel. The channel is now improved to this depth for a width of 600 ft. along the Ballard's reef range (north of Lime-Kiln crossing); for a width of 180 ft. along the west side of a 600-ft. channel at Lime-Kiln cross- ing; for a width of 500 ft. along Amhertsburg reach (work on the additional 100 ft. along the east side being retarded by the claims of the adjacent shore property owners); and for a width of 600 ft. throughout the greater part of the length of Hackett range. Through Bar point shoals the channel depth secured was 20 ft. for a width of 800 ft. The material removed in order to secure this improvement was mainly limestone bedrock, except from Lake Erie to a point about half a mile south of Bois Blanc island, where the material was sand and clay in places mixed with small boulders, these boulders being mainly loose pieces of limestone. At Ballard's reef and Amherstburg reach the limestone was generally underlying from 1 to 3 ft. of loose material, consisting of sand, clay, and small'stones. The money expended to date on this improvement by the United States is approximately $2,750,000; and the unit prices for the removal of rock have ranged from $7.50 to $2.27 [Dec. 17, r cubic yard, and for earth, including loose boulders, from $5.60 to $0.18 per cubic yard. ee : METHODS EMPLOYED IN MAKING SURVEYS. A brief description of the methods employed in making the necessary surveys prior to the actual work of excavation may be of interest. A' system of triangulation is first established with a sufficient number of stations at convenient places to locate all points within the area to be improved, these stations being used for the location of points both during the progress of the sur- vey and during the actual work of excavation. The triangulation stations are, for convenience, referred to rectangular co-ordinate axes with the origin at a centrally located station for the whole system, the axes being north and south and east and west lines. The equations of the channel lines are then computed referred to these axes, and a system of cross-sections (at right angles to the channel lines) adopted. The co-ordinates of the ends of cross- _ sections at 500 ft: intervals are then computed, and from these co-ordinates and the co-ordinates of two triangulation stations _ are computed the azimuths of lines to the ends of the cross-sec- tions from the stations, which stations are so selected that the lines give a fairly good intersection angle (not less than 45°). Having made this preparation (which consists largely of office work), the actual work is started of making the survey, which survey consists in the taking of soundings at regular intervals (usually 10 ft. apart) and in sweeping with suspended bars to locate obstructions. The work of taking soundings is usually done through the ice when opportunity offers, or, if there is ur- gent need during the open water season the soundings are taken by the use of stakes driven or floats anchored at the ends of cross- sections; these stakes or floats heing connected by lines with tags equally spaced, and the soundings taken at the tags from boats. If the work is located conveniently near to shore, open water soundings are preferably taken from a raft (say 100 ft. long and zo ft. wide) anchored in the desired positions. By what- ever method the work is done, whether through the ice or in open water, the location of all points is determined by the com- puted azimuths from properly selected triangulation stations. | On the ice soundings are taken through holes made by boring machines with a lead suspended by a fine wire wound on a wheel - to ft. in circumference, both boring machine and sounding wheel being mounted on sleighs. This method of making surveys is much cheaper and more satisfactory than when made in open water, but the formation of ice on the Detroit river of sufficient thickness for safety cannot be depended upon every year at the places where needed. The cost of making surveys through the ice varies from 1% to 3 cents per sounding, and in open water the cost varies from 3 to 7 cents per sounding; in either case this variation is due to weather and other causes. Whatever the method of making the survey, the time when the sounding is taken and the depth of water are both recorded, and also at a convenient place near the work an observer records the elevation of the water surface at equal intervals of time (say _ every ten minutes). This is necessary in order that the proper correction may later Be applied to each sounding to reduce it to the assumed plane of reference. The necessity for water-gauge readings being taken during the progress of a survey will be ap- preciated by all when it is known that the daily fluctuation may be as much as a foot, even under moderate weather conditions. The field work of making the survey being completed, the ' soundings are reduced to the reference plane, and the reduced soundings plotted. The estimates of the amounts of material above the required depths are then computed, the specifications to control the work of excavation prepared, and the contracts let to the lowest bidder after due advertisement, provided that the necessary funds for the work are available. MEASUREMENTS IN PAYING FOR THE WORK. In making payments for excavation, one of three methods is usually adopted, viz., on the basis of bank measurement, on the basis of scow measurement, or on the basis*of time work. It: has been found more desirable to make the payment on the basis of bank measurement, or material measured in place, where accurate surveys may be made covering the area to be improved. In con- tracts of this nature, payment to the required depth is made at the full contract rate; and, for rock work, payment is also made at half the contract rate for all material removed for the first 2 (or in some cases 3) ft. below this depth, but in earth this half rate usually applies only to the first 1 ft. below the required depth. By this means payment is made for all material necessarily re- moved by the contractor in order to secure the depth required. During the progress of the work monthly payments are made on the basis. of scow measurements, these payments being corrected from time to time by computations from surveys covering the areas improved. = -. Payment is usually made on the basis of scow measurement where accurate surveys covering the area to be improved have not been made and the outlines of the work cannot' be clearly de- fined. This' method is not satisfactory, as there is always the temptation for the contractors' employes to dig deep pools in soft material, to underload the scows, and in other ways to "beat the government." It is only fair to state, however, that dredging contractors generally do not encourage this, their employes being actuated mainly by a desire to make. a good showing for their work; which desire is truly commendable when applied in a le- gitimate way. Payment is made on the basis of time work, or at a stated price per hour for the actual time worked by the dredg- ing plant, only when the work to be done cannot be definitely