Soil incompressible, but liable to scour. Sands, gravel, &c., found directly on the soil, and give the floors a thickness of from 2 to 6 feet, depending upon the lift, the width of the lock, and the tenacity of the masonry; oppose subterranean filtrations by cross-walls of beton or masonry descending lower at the head and foot of the lock and under the miter-sills than the general foundations, or by carefully-driven rows of matched sheeting-piles under the whole width of the lock; make the floor thicker under the miter-sills and under the lower gate-chambers. Make an apron below the lock whose thickness decreases as it recedes from the lock, and whose total length depends on the lift and the resistance of the soil. Sheeting-piles are very efficacious for intercepting subterranean communications. I have seen locks a hundred years old on the Picardy Canal which still worked passably, although the lock-chamber no longer had a floor, because the rows of sheeting-piles under the miter-sills were in good condition. To have the rows of sheeting-piles well joined it is necessary to use the system which was formerly followed and which is yet in use among the Dutch. The piles are so arranged as to be capped by two parallel stringers, leaving between them an interval equal to the thickness of the sheeting-piles; the latter can then be driven by continuous panels, and by slight successive penetrations along the whole length of the row, so that they reach their ultimate penetration without losing contact, and mutually sustaining each other; which, as is well known, is the advantage of driving by panels. On the other hand, when they are driven by the ordinary method of first driving piles held between two rows of stringers, and then sheeting-piles in the interval between the clamps, the piles obtain isolated holds, independent in direction one of the other, and it is difficult to form a connection between them and the intermediate sheetingpiles. Ties that are parallel to the length of a lock are the cause of dangerous filtrations, because when the earth settles which was placed under them it leaves a void which cannot be filled, and which establishes a continuous communication from the water above the lock to that below it, whilst similar voids under the caps are interrupted at each pile. If, as often happens in these kinds of soil, the springs are very abundant, after having excavated until the pumping becomes too costly, the trench for the foundations should be finished by dredging. The bottom should be graded to suit the drainage; the sides of the excavation should be slightly raised; then drainage-wells should be dug in the lowest parts; after which the whole should be covered by a bed of from 1 to 2 feet of beton, so as to have a kind of large, flat, impermeable canal, in which pumping can be done after the mortar has set. Beton, placed on the soil, chokes or diminishes the bottom springs, and makes pumping much less expensive. I found in a similar case that ten Archimedean screws were sufficient to lay bare an excavation covered with 16 inches of beton, while seventeen screws had not succeeded in getting water lower than 24 feet above the bottom of this excavation. If the foundations are much below the level of the springs, it will be necessary, after dredging, to drive an inclosure of piles and sheeting-piles at the feet of the main slopes of the excavation, which must be somewhat widened; then a layer of beton, of from 2 to 3 feet in thickness, must be poured into the inclosed space; next, by means of scaffolds resting on the heads of the piles of the inclosure, whose top must be above the level of the springs, vertical or inclined posts must be planted in the beton, which will serve to support panels, so as to make a second interior inclosure, forming with the first one a perimetrical coffer-work, which should be filled with beton up to the level of the springs, supporting it on the exterior by earth filling. We will thus have a coffer-dam, inside of which we can pump out after the mortar has set. The posts and panels will then be removed, and the masonry will be built. The masses of beton in the coffer dam, cut in steps if the posts were inclined, will form part of the side-walls and of the lift-wall. At the lower end of the lock they must be removed to below the surface, in order to open communication with the lock, unless from motives of economy this part of the coffer-dam was made of clay, which can more readily be removed. If there is danger of cracking the beton by driving in the posts, their feet can be buttressed by long timbers extending from one side of the coffer-dam to the other. The interior posts ought to be somewhat inclined; if they are much inclined, considerably less beton is required. But that part which fills the acute angle of the cofferwork can only get there by flowing down a slope, and at this part all the milk of the béton (laitance) will be accumulated. This has but a very moderate consistence, and may give rise to accidents, which can be avoided by using vertical or slightly-inclined panels. I have given the above translation on account of its intrinsic value, and because it is contained in a very valuable treatise, (Minard's Navi gation des Rivières et des Canaux,) which is now out of print. This book was recommended to me by a distinguished French engineer, (M. Malézieux,) as the best authority on such work, and by good fortune I succeeded in securing a copy. I ought to add that "beton" and "concrete" are synonymous terms. I think that I am perfectly safe in saying that every lock on the Ohio will be founded on rock, gravel, or sand. Having estimated for a lock on rock foundation, it remains to deter mine what modification will be required in the estimates for sand and gravel foundations. The great difficulty occurs in the lock-chamber. Although by using sheet-piling we may greatly reduce the percolation of water through the soil under the lock, it is impossible to stop it entirely. The effect of this under-current of water is to cause an upward pressure on the floor of the lock whenever the chamber is empty. This upward pressure must be met by dead-weight, or by weight aided by tenacity. If we use nothing but concrete, it will resist partly by its weight, (due allowance being made for reduction of weight by immersion,) and partly by its construction as a monolith with its ends firmly held under the side-walls. If we fill the area with piles and a less amount of concrete in the spaces between the piles, we will then have a resistance due to the weight of the concrete in water, increased by the resistance of the piles to extraction. Lastly, we may use masonry built in what is known as plate-bands. or reversed arches with an infinite radius for the intrados. The keystone is wedge-shaped, with its widest face lowest; the other voussoirs have their sides inclining toward the key, and their under-widths are slightly greater than their widths at the intrados. The plate-band may therefore be considered as the extreme case of a flat arch. It may be built on a foundation of concrete, or on a wooden platform, thus making two additional methods. All the plans described above require the same expenditure for cofferdam, and for the rows of sheet-piling designed to prevent subterranean filtration. The cost of these works will therefore be estimated before going into the details of the floor. COFFER DAM. This will be built of two rows of piles and sheeting-piles, 8 feet apart. and the space between the rows will be filled with gravel. The outside sheeting-piles will be 3 inches thick and 12 feet long; the inner ones being 2 inches by 10 feet long. The latter will the driven by hand. Sheeting-piles. The sheet-piling, to prevent filtration, should extend along the whole length of the river-wall, across the head, across the foot, under the lower miter-sill, and on the prolongation of the line of the dam. Its total length will be 1,136 feet. The price of pumping will be taken at the price previously deter mined, viz, $3,020 for the two seasons that will probably be required for constructing the lock. FLOORS OF LOCK-CHAMBERS. Concrete only. To determine the necessary thickness of the concrete, De Lagrené, (Navigation Intérieure, vol. iii, p. 77,) gives the following formula: = safe tensile-strain on concrete = 5 tons per square meter. Substituting these values in the formula, we get e= -144+ 12 √ 144 + 2 x 2 x 5 1.9 meters = 61 feet. This result is a large one, and, as experience has shown (Minard, Navigation des Rivières et des Canaux, p. 184) that the under-pressure is always less than the theoretical head, I have estimated on a uniform thickness of 6 feet. 17,333 cubic yards concrete, at $5 22,000 cubic yards gravel-excavation, at 30 cents.. $86, 665 6, 600 93, 265 Piles and platform with concrete. The usual practice in France is to put the concrete on top of the platform, while the contrary is the practice in this country. It seems S. Ex. 19, pt. 8—2 to me that where concrete is used under the platform voids may occur under the bottom of the lock by settlement or otherwise, and that under these circumstances the concrete would probably become de tached from the piles and the under-surface of the platform, with which its bond is necessarily weak, and would fall into these voids. If this should happen, the platform would have to withstand the under-pressure without any help from the concrete. This would not occur, however, where the concrete was placed above the platform, and for that reason I prefer the French practice. In the following estimate the supporting-piles are placed 7 feet apart over the whole area occupied by the chamber, and 33 feet apart under the walls, and 3 feet of concrete is placed on the platform. The maximum upward pull on each pile under the chamber, allowing for the maximum under-pressure due to the head, is calculated at 5 tons, but experience has shown that this is much greater than will be found in practice. The friction on the sides of the piles will be ample to withstand this upward pressure even at its maximum. Plate-bands of masonry resting on concrete and on piles and platform. The thickness of the plate-bands will be taken at 24 feet, resting on? feet of concrete in the first case, and on piles and platform in the second. In the first case, therefore, there will be a substitution of 24 feet of plateband masonry for 4 feet of concrete. The volumes of the two will therefore be in the proportion of 5 to 8. Equality in cost would require that the price of a cubic yard of masonry should be one and three-fifths greater than that of a cubic yard of concrete. But as this masonry must be of cut-stone, it is evident that its cost would more than exceed this limit. This method of construction, therefore, need not be exam ined in detail. The same remarks apply still more strongly to the case of plate-bands or piles and platform, as in this case the 2 feet of ma sonry only replaces 3 feet of concrete. Where concrete is used, with or without piles and platform, the bed of concrete must extend under the side-walls, replacing a portion of the masonry. This will make a reduction in cost of about $5,000 in lock masonry. Summing up the results thus far obtained, we get the following: Foundation and floor. Lock, as per estimate for rock-foundation... Total Deduct from estimate on rock-foundation, coffer-dam, and pump Saving on lock-walls.. Add 10 per cent. for contingencies... Total cost of lock................... $93, 265 179, 610 293,962 $6,000 20,000 5,000 31,000 262, 962 26, 296 289, 258 Lock, as per first estimate, with deductions as indicated above................ 89, 547 The foundation of concrete on piles and platform being the cheaper of the two, will be the one that will be used in the estimates. The costs of the navigable pass, the weirs, and the piers will also be different on gravel from what they were on rock. The following are the estimates on this part of the work: The coffer-dams are allowed to remain and become a part of the work, care being taken to cut them down to a foot or two below the level of the sills. The high weir has practically no coffer-dam, as what might be considered such is filled with concrete, and thus made the foundation for the wickets. |