extent, to the action of currents, the qualities of hardness and strength are of secondary importance. Mass being here the desideratum, there is no reason to doubt that a natural cement concrete will answer every purpose.

For that portion of the walls above 105 feet one of the two arrangements shown in the accompanying cuts is proposed. If experiments now in progress show that a section along the general line of union of the two kinds of concrete possesses a strength equal to or greater than the weaker concrete, then the arrangement in Cut (1) is recommended. If, on the other hand, it results that such a joint is weaker than a similar section of natural cement concrete, the arrangement shown in Cut (2) is proposed. Very careful experiments with concrete blocks are now in progress, and will be continued until it is settled whether or not a good bond can be secured between contiguous masses of Portland and natural concretes when deposited at the same time.

The saving in cost by the use of natural cement will be considerable. If the lock were built all of Portland, it would cost $20,000 more than if all of natural cement. If built on the plan of Cut (2), it will cost $9,000 more than if all of natural cement. If built on the plan of cut (1), it will cost less than $3,000 more than if all of natural cement.

It is therefore recommended that the arrangement shown in Cut (1) be approved, subject to the results of pending investigations, and that, if these results are unfavorable, then the arrangement shown in Cut (2) be adopted.

THE DAM.

Necessity for a movable structure.-The reason which led to the adoption of a movable dam for the site of Lock No. 1, Osage River, was that the high-water cross section at this point may be contracted as little as possible. In order to make the pool of the dam extend to the most favorable point for the construction of Lock No. 2, should such a lock ever be built, the crest of the dam should stand not lower than 116 feet. The contraction of waterway resulting from a fixed structure of this height would be such as to cause a rise in the flood plane above the lock of 3.08 feet above its level in the natural condition. (Pl. XV.) With a movable dam, as proposed, this rise in the flood plane is reduced to less than 1 foot. The topography of the valley above the dam is such that a difference of 2 feet in the flood plane of the river will represent a considerable acreage of overflow, and the damages which such an overflow would represent would probably be much greater than the extra cost of a movable dam over that of a fixed structure.

Detailed description of dam.-The dam rests on a pile foundation (Pl. VIII), surmounted by a timber grillage on which rests a watertight floor of 4-inch plank. The second row of piles on the upstream side consists of triple-thickness sheet piling reaching entirely across the river on a line with the sheet piling under the lock. It extends downward to reference 75 feet and will cut off the underflow of the river for 25 feet beneath the river bed. The foundation of the dam proper is 25.5 feet broad, and the structure forming the apron of the dam, also resting on piles and joined to the main foundation, is 16.5 feet wide, giving a total breadth of base of 42 feet.

The greatest possible relief of the dam, or difference in elevation between the upper and lower pools, is 16 feet. This, however, as explained in the discussion on the strains of the gate, can very rarely occur. In fact, when the present work of opening a direct connection between the Osage River and the Missouri near Cote Sans Dessein is finished, the lower pool can not fall below 105 feet during the naviga

tion season, giving a relief of only 11 feet. In the preparation of these plans, however, and in determining the strains upon the gate, a possible maximum relief of 16 feet has been assumed.

Upon the foundation just described rests an iron framework consisting of two parts, MLNOPD and DEFG, which, with the concrete mass HIJK, forms the fixed weir. The arrangement of these frames is shown in detail in the drawings. The frames DEFG occur every 5 feet. The frames LMNOPD occur every 2 feet.

The frame LMNOPD forms the lower wall of the chamber AZQ and sustains the pressure on the concave surface D'Q. It also supports the upper end of the apron RR'; and, when the gate is closed for repairs, it supports also a part of the weight of the gate and the pressure on the bulkhead VWX.

The frame DEFG supports the movable part of the dam aud forms the upper wall of the chamber. The wooden partitions D'Q and E'Z are water-tight.

The concrete mass HIJK forms the impervious barrier of the fixed weir and supplies the weight necessary to the stability of the whole

structure.

The apron of the dam, like the main structure, rests upon piles, and is not liable to undermining from the agitation of the water below the dam. Its construction is fully illustrated by the drawings. The space beneath it is left vacant, except as filled with backwater from below, or with the sediment that may collect there. The escape of water from the chamber at Q and from the interior of gate at B passes into this space. The superstructure, or movable portion of the dam, is a sector of a circle in cross section. The arc subtended is 67° 30'. It consists of an interior iron framework A' B' C', with a wooden exterior ABC. The upper face AB is air-tight, the lower face is water-tight, and the cylin drical face is air-tight about two-fifths of the distance from C to B. The ends of each section of the gate are closed and air-tight from C down to a line shown in fig. 2, Pl. XIV. The gate is held by the hinge A, of which a full-sized drawing is given on Pl. IX. When the gate is in operation it is supported by water pressure and by the pin A. When not in operation it falls into the chamber AZQ and rests against the stop Z. The other details of the structure are readily understood from the drawings.

The triangular space DEH is a longitudinal culvert by which water is conveyed to or from the chamber AZQ. In order that the pressure of the water may be applied to or withdrawn from the face AB of the gate uniformly throughout its entire length, the connection between the chamber AZQ and the culvert DEH consists of a narrow opening, ZQ', extending the entire length of the gate. Its entire area slightly exceeds that of DEH.

The piers (Pl. XI) separating the sections contain the culverts and valves by which the supply of water to the chamber AZQ is controlled. A rectangular culvert 3 by 4 feet enters centrally from the upper end of each pier and passes out at the lower end. It is intersected at the center of the pier by a cross culvert of trapezoidal cross section, but of the same area. A heavy iron girder, embedded in concrete at the ends, cuts both culverts in two diagonally at their junction, so as to restrict communication through the upstream culvert to the chamber to the right of the pier, and that through the downstream culvert to the chamber to the left of the pier. The culverts are closed by sliding valves operated by oil cylinders actuated from shore.

The operation of the gate is as follows: The outlet valve being closed,

the inlet valve is opened. The head of the upper pool is brought to bear on the lower surface AB of the gate. As shown in the discussion on the strains in the gate, there is always a sufficient head to raise the gate except in a certain contingency to be considered further on. As the gate rises and approaches its normal position when up, it is not brought to rest by a stop, but by closing the inlet valves, or, automatically, by the escape of water at Q'. RQ is a gate 23 feet long. (Pl. X.) There are seven of these to each section. Their combined free space for the flow of water is about 10 square feet. The area of the inlet culvert is 12 square feet. When Q' passes above Q, water begins to escape, and the outflow increases the farther the gate rises. By the time Q'reaches R the outflow through the gate, with the leakage at other points, will fully equal the inflow, and the gate will cease to rise. By gradually closing the inlet valve the gate will settle back to its normal position when Q' is just below Q. The valve is then left in this position, and the friction of the gate will preserve a balance of forces. Ordinarily the gate would be stopped by the operator when it has reached its normal height, but in case of inadvertence or carelessness no harm can result, for the gate will come automatically to rest without shock or sudden stop, as just explained.

The contingency already referred to, when there will not be sufficient initial head to raise the gate, is fully considered in the discussion on the strains in the gate. The air necessary for the expulsion of sufficient water from the interior of the gate to give it the requisite buoyancy to rise in still water is supplied through a 2-inch pipe leading from each section to an air pump on shore. These pipes, with those conveying oil to the valve cylinders, are buried in the concrete mass, as shown in the drawings. The operating room, or house to cover the air and oil pumps, will be located close to the head walls of the lock on the shore side.

For the purpose of making repairs to any section, the bulkhead VWX (Pl. VIII) may be erected, supported by the frame DEFG and by the gate through the struts CX and SW. The gate is supported at A and by braces resting on the apron and against the circumference of the gate. By closing the upper valve and opening the lower, the structure is uncovered down to the level of the lower pool. By closing the lower culvert also and applying a pump, the entire structure is rendered accessible.

The gate will never be kept up after the upper pool reaches stage 120 feet. The piers will not form obstructions to drift, as drift does not begin to run in the river until they are entirely submerged. At such times their location will be marked by buoys for the information of pilots.

The cost of the dam, with a liberal allowance for every item, is a trifle under $120,000, or $140 per linear foot.

For a comparison of the probable cost of this form of gate with that of other forms, the following examples are cited:

Dam at Lock No. 7, Great Kanawha River.-(Authority, Col. Peter C. Hains, Corps of Engineers.) Elevation of bed of river, 535 feet; low water, 539.6 feet; crest of dam, 550.5 feet; type, Chanoine wicket; cost, $270 per linear foot. The relief of this dam, both above low water and above the bed of the river, is very nearly the same as at Lock No. 1, Osage River.

Lumbermen's dam in Minnesota.-(Authority, A. O. Powell, United States assistant engineer, who prepared the estimates.) Common bear

trap type; head of dam, 10 feet. Estimated cost, $280 per linear foot. Big Sandy River, Kentucky.-(Authority, Board of Engineer Officers-Craighill, Lockwood, Turtle.) Type, Poiree needles; length of dam about 300 feet; clear height of crest above sill of navigable pass, 13 feet; estimated cost, $93,029.25. (Annual Report Chief of Engin eers, 1892, p. 2108); cost per linear foot, $310.

HIRAM M. CHITTENDEN,

Captain of Engineers.

REPORT OF THE BOARD OF ENGINEERS.

THE BOARD OF ENGINEERS,
New York City, June 23, 1897.

GENERAL: In reply to your indorsement of May 24, 1897, The Board of Engineers has the honor to state that it has examined the project submitted by Capt. H. M. Chittenden, Corps of Engineers, for the construction of Lock and Dam No. 1, Osage River. This project was forwarded with request for your approval by Lieut. Col. Amos Stickney, Corps of Engineers, president of the Missouri River Commission, with the statement that the Commission had approved it in all of its general features.

In examining the project and making its recommendations The Board of Engineers has acted with the understanding that its opinion has been requested solely upon the novel features of the dam proposed and not upon the question whether the improvement as a whole is a desirable one, nor upon points of foundations, levels, material, etc., upon all of which no opinion can be formed without a personal knowledge of the locality. From the action of the Missouri River Commission it may safely be assumed that the constructive details are suited to local requirements.

As stated in the memoir descriptive of this project (see page 3949), a dam 16 feet in height above extreme low water in the lower pool is necessary in order to pool the water to the required distance upstream. A fixed dam is not regarded as desirable since it would unduly raise the flood level above. On this account Captain Chittenden recommends a dam consisting of a fixed concrete portion 9 feet high supporting a movable portion which can be raised by the hydrostatic head to the full height of 16 feet above extreme low water below. The movable portion is a novel modification of the drum weir. A detailed description is given in the memoir and the accompanying illustrative plate (see page 3947). The whole dam relies for its stability mainly upon the concrete mass of the fixed portion, which rests upon a flooring supported by piles and grillage. With proper care in construction to insure no leakage between the concrete and floor, this part of the dam possesses ample stability against overturning. Proper precautions against sliding can readily be taken.

The movable dam is built in lengths of 75 feet, separated by piers in which the valves of the operating culverts are placed. Captain Chittenden states that it can be raised by a head which is normally obtainable at times of low water, and estimates that it will respond to a pressure of less than 1 foot. In the unusual case of its being drowned by backwater from the Missouri at times when the discharge of the Osage is small, provision has been made to raise it mechanically by pumping air into the drum. The pipe through which the air is pumped

has a diameter of 2 inches, and must serve not only to convey the air under pressure to raise the drum, but to relieve the internal pressures by letting air in and out when the gate is worked by the hydrostatic head. It is believed that there is but one such pipe for each section of the dam 75 feet long. It is possible that freer communication for the air should be provided in order to allow sufficiently quick and easy operation. This fact will be developed in the experiments which are proposed.

Captain Chittenden states that a model on a scale of one-third natural size has already been built, and has worked satisfactorily. He states further that should his plans meet with approval a short section of full size will be built at the sight of the dam to develop any latent defects in the design. The Board of Engineers considers the dam sound in principle and especially well adapted to a locality where it may be placed on the crest of a fixed weir to increase the height of the latter. For such service it preserves to a great degree the advantages of the drum weir while avoiding its one notable disadvantage, that of the deep and expensive foundation usually necessary. Accepting Captain Chittenden's estimate of say $140 per linear foot as accurate, the cost of the dam as proposed will not be greatly in excess of a reasonable estimate for a fixed dam of similar relief and foundation, while much below the ordinary cost of a movable dam.

The Board recommends the design of the dam for approval, and considers it advisable that experiments should be made with a full-sized model, as proposed by Captain Chittenden, before undertaking the final construction.

Respectfully submitted.

Brig. Gen. JOHN M. WILSON,
Chief of Engineers, U. S. A.

HENRY M. ROBERT,
Colonel, Corps of Engineers.
G. L. GILLESPIE,

Colonel, Corps of Engineers.
H. M. ADAMS,

Major, Corps of Engineers.
JOHN G. D. KNIGHT,
Major, Corps of Engineers.
H. F. HODGES,

Captain, Corps of Engineers.

NOTE.-The project as recommended by the Missouri River Commission and by the Board of Engineers was approved by letter of the Chief of Engineers, dated June 29, 1897, with the understanding that experiments will be made with a full-sized model before final construction is undertaken.

ESTIMATES.

REVIEW OF PREVIOUS ESTIMATES AND TABULATED STATEMENT UPON WHICH PRESENT ESTIMATE IS BASED.

Inasmuch as the final estimate of the cost of Lock and Dam No. 1, Osage River, as submitted with the present revised project, is apparently largely in excess of previous estimates, it will be of interest to present a condensed summary of the earlier estimates.

ENG 97-248