Hollow Frame Dam

Where economy in timber is necessary, a hollow frame dam offers an equal degree of security with the log dams previously described, at the same time requiring far less material in its construction.

A good, reliable dam, in fact, can hardly be built with a more economical use of timber than is shown by the plan here given. Our artist has presented very clearly the peculiarities of construction, leaving it necessary to supply little more than the figures, dimensions and etc., to give the reader a clear idea of the design. The dam here represented is built upon a solid rock bottom, but with slight modifications is adapted to streams with a soft or sandy bed, as hereafter explained.

Foundation Construction

The first step in the construction of this dam is to lay the foundation blocks A A, each of which is a stick of timber
ten inches square and about four feet in length. Three rows of these blocks are to be laid across the stream, one at the face of the dam, one at the up-stream extremity, and another midway between them. The distance between the centers both ways across the stream and from one row to another is eight feet, giving three blocks or bearings to each bent of the frame-work. Three additional blocks are placed in the front row and twelve in the rear row to receive the bolts by which the dam is fastened to the rock.

Upon these blocks are now laid the mud-sills B B, which form the immediate foundation of the dam, consisting of logs about sixteen inches in diameter, hewn on the upper and under sides so as to give a thickness in that direction of thirteen inches. These are laid across the bed of the stream in three tiers, one for each row of blocks. Where joints occur, a two-feet splice should be made, and the two ends firmly pinned together.

The end of the front sill at each bank should project into the abutment about fifteen inches ; while that of the second or middle sill projects an equal distance just behind the up stream wall of the abutment, the center of which is near the front sill, bringing part of the abutment against and the other part below the dam. The front sill has three bolts passing through it, one at each splice, an extra block being placed underneath as already stated. The up-stream sill has twelve bolts, under each of which is a block, in addition to the blocks on which the bents of the frame-work are to rest.

The bolts should be 1 3/8 inches in diameter. Each one of them passes through the sill and block down into the rock, which it penetrates about three and a half feet, making the total length of the bolt five and a half feet. The bolt is made after the hole has been drilled, the necessary length being ascertained by careful measurement. A " stoved head " as it is called, is given to the bolt, and a washer placed underneath the head, which is drawn tightly down by the tapered shape ofthe head. In order to prevent any possibility of the bolt working loose, the lower end is split five or six inches up, and an iron wedge inserted. When the bolt is driven down, the wedge, coming in contact with the rock, is driven up, and spreading the point, holds the bolt firmly in its place. Fine wet sand afterward put in will make it perfectly tight and solid, being as effective for this purpose as lead or cement.

In drilling the hole in the rock, an ordinary stone drill slightly smaller than the hole to be made, is employed, and is put down through the sill and block, which are previously bored and placed in position.

Frame Bent Construction

The bents of which the framework of the dam is constructed, and which come next in order, are built throughout of timbers ten inches square the same size of material being used in the lower horizontal pieces C C, the uprights D D, and the upper pieces E E, which form the slope of the dam.

The length of the lower timbers is sixteen feet, and that of the upper timbers the same, the effect of which is to give the face of the dam a slight inclination up stream. The lower timbers are framed into each sill, a gain being cut two inches deep, and the timbers secured with a dove-tail key driven to the side of each bent.

The upright posts connecting the upper and lower timbers of the bent have a length of two feet three inches in the clear at the face of the dam, and half that length at the middle sill ; and they are to be mortised into the upper and lower timbers in the same manner as in the framing of a house. The bents are the same distance apart between centers as the blocks under the sills eight feet, and the distance from the front to the middle upright is the same. The upper and lower timbers of each bent are hewn obliquely or beveled at the up-stream end so as to fit snugly together and give a combined thickness at the extremity equal to one piece.

Frame Ties and Planks

The last step in the building of the frame is placing the ties F upon the top of the structure, extending transversely across the stream in the same direction as the sills. There are five series of these ties, one over each sill and one between. They should consist of timbers 4 by 7 inches and lie on the narrower side.

Each tie is let into the upper or inclined timber of the frame wherever it crosses, the depth of the gain being one and a half inches, giving the tie five and a half inches thickness above the frame. The gain is cut into the frame at right angles with the upper timber, the ties being thus slanted slightly upstream and presenting a level surface for the planking. The forward tie is let into the frame piece about four inches from the end, in order to give sufficient strength to the gain to prevent it from breaking out.

The whole upper surface of the frame is now planked over. The planking, which is strongly spiked to the ties, should be one and a half inches thick, and the wider the better, as the fewer the number of joints, the more secure from leakage will be the covering of the dam. A greater thickness of plank than that given will increase the liability to rot, as the wood is wet on one side and dry on the other.

Dam Abutment

The abutments, as already stated, extend but half way from the face to the up-stream end of the dam. To protect the exposed portion of the sides, the dam is enclosed with stout upright planking from the middle sill to the upstream end, the ends of the planks resting on the rock bottom. In like manner, the rear of the dam is closed with sheet piling extending from bank to bank, closely matched and of sufficient hight to meet the planks which cover the top of the dam, the lower ends of which are footed by the piling, which extends to their upper side and is flush with the surface of the dam.

The abutment is built of timbers fifteen to eighteen inches in diameter and is eleven feet square. The logs are hewed on one side to give a face to the abutment. The first or foundation timbers are laid in the same direction as the sills of the dam, transversely to the stream, the lower one about three feet below the face of the dam, and the upper one just below the middle sill, which it touches and helps to hold in position.

The first cross piece on the side toward the dam is laid over and across the end of the front mud-sill, which extends beneath it, as already stated, about fifteen inches into the interior of the crib. The up-stream end of the cross-piece reaches to the middle sill of the dam. The timbers of the crib are notched and saddled where they rest upon each other, and the structure is thus firmly held together. The ends of the first two ties on the surface of the dam extend to the crib, and the third tie passes directly behind it in the same manner as the center sill below.

The crib is filled up with rough stone or coarse gravel, and covered with upright planking on the upper side and on the side against the dam. A joist, I, two by ten inches, is spiked against the crib along the top of the dam from its crest to the up-stream corner of the crib.

Application

The dam here shown is ninety-three feet long and its total hight from the rock bottom to the surface of the planking is six and one-half feet. There are eleven bents in the complete dam, only half of which is shown in our engraving. The dam here represented was built by Messrs. Bookwaiter & Claypool of Attica, Ind., to furnish power for a large and very complete flouring-mill just erected by them, in which three Lettel Double Turbines were placed, with all other necessary machinery, furnished by the same establishment.

The design of the dam-, which was drawn in the office of James Leirel & Co., can be adapted with some alterations to a stream having a soft instead of a rock bottom. For that purpose, it would be necessary to lay a foundation of two and one-half inch plank, instead of the blocks, for the sills to rest upon. These planks should be laid lengthwise of the stream, and project ten or twelve feet below and an equal distance above the dam, making a total distance of about forty feet. As it is difficult to obtain planks of this length, the foundation may be laid in two sections, the planks in each having a length of twenty feet.

About midway between the breast and the- up stream end of the dam, where, if the planks are twenty feet long, a joint will occur, a wide sill should be placed beneath them, upon which the ends of the planks can be firmly spiked. At the down-stream end of the planks, constituting the edge of the apron, a light sill or binder should be placed underneath not to support, but rather to hold together the planks. At the up-stream end, the planks will be simply imbedded in the soil, and the planking at this point, and the whole back of the dam, covered with gravel, sand and dirt. A layer of brush at the bottom of this covering will make it hold all the more firmly to the bed of the stream.