767. The use of concrete in the construction of canal locks is comparatively recent, but it has met with much favor, and its use is extending. The requirements for a lock wall are that it shall be reasonably water-tight, that its strength shall be sufficient to withstand the thrust of the gates and support the earth filling behind it (or in a river wall, the difference in water pressure on the two sides), and that it shall withstand the impact and abrading action of boats using the canal. In all of these respects concrete is believed to be the equal of a good class of stone masonry. At St. Marys Falls Canal, portions of the lock walls which have been injured by boats and repaired with concrete have given entire satisfaction, although in such cases the concrete had to be patched on, and sometimes in places difficult of access for work of this character.
The present accepted method of concrete lock construction is to build the walls in alternate sections, filling in the intermediate sections after the others have set. It is sometimes thought necessary to make the work on a section continuous from time of starting the concreting to its completion. That the exterior appearance of the work may be somewhat better if such a course is followed, is true, but it is very questionable whether the attainment of this desirable result is worth the additional expense and the additional liability of having poor work done under the cover of darkness when work at night is necessitated by such a rule. With proper precautions, such as making steps in the top surface of work left for the night, as already detailed elsewhere, and being careful that the limit of work on exposed faces is bounded by true horizontal and vertical lines, the plane of weakness occasioned by a horizontal joint extending only partially through the work cannot be a serious defect in a concrete wall.
769. The molds, so far as the walls alone are concerned, are comparatively simple and have already been described under the head of forms (Art. 62). Cable passages, gate recesses, hollow quoins, culverts, etc., call for special carpentry work, sometimes of quite intricate character. While the efficiency of the machinery and the lock as a whole should not be sacrificed to obtain easy construction, yet sharp corners should always be avoided, and simplicity of outline should be the constant aim. Linings of hollow quoins (when steel quoins are considered necessary), gate anchorages, cable sheaves and other parts built into the masonry, are in general placed with greater difficulty in concrete forms than in stone masonry. Aside from such special constructions, the walls may be built up much more rapidly of concrete than of stonework.
As to the proportions to be used in concrete for locks there is no rule of thumb. As a guide, the stresses in each part of the structure should be determined as well as the knowledge of the forces will permit, but the proportions will depend on the question of water-tightness and freedom from deterioration quite as much as upon required strength. It may be said, however, that in a considerable portion of the cross-section of the walls, weight is the main consideration and the concrete need not be very rich. The concrete surrounding the culvert, however, should be of good quality, as the stresses which may be developed here do not admit of close analysis.
770. The walls should be faced with mortar made of one and one-half or two parts sand, or, better, two parts of granite screenings one-half inch and smaller, to one part of the same kind of cement used in the body of the concrete. This facing need not be more than three inches thick, and if made of sand and cement, it will probably be better if not more than one inch thick, though this may depend on the materials and local conditions. In any case this facing should be laid with the concrete by means of a removable steel plate similar to that described in § 528. The top of the wall should be finished with mortar or granolithic similar to a concrete walk or driveway. While the walls should in general have a vertical face, a slight batter is allowable at the top, starting at about upper pool level, to protect the concrete from being chipped by the impact of boats, and for a similar purpose the outer corner of the wall should be rounded with six to twelve inch radius.
Special care must be taken in lining the culverts, particularly in silt-bearing streams, and in such places as a change is made in the direction of the flowing water. For high heads it may be necessary to line the culverts with cast iron for a portion of their length. Granite and hard burned bricks have also been used for this purpose, but in locks of moderate lift, granolithic lining will usually be found sufficiently resistant.
All necessary irons and bolts should be built into the masonry as the work progresses, as they will be much more secure than if set later in recesses left for them.
The large lock in the canal at the Cascades of the Columbia was one of the first in the United States to be designed of concrete in this country. In this lock the walls, wells, copings and portions of culverts were faced with stone. The foundation rock was covered with eight inches of rich concrete, one part Portland cement, two parts sand to four parts gravel. Fourteen feet of the chamber walls and ten feet of gate abutments or wide walls were of concrete, one to three to six, while balance of masonry was of one to four to eight concrete.
The molds were of four by six posts four feet apart, and lagging of two-inch lumber, dressed to size for exposed faces. The work was carried up in horizontal layers, not more than two feet being placed in one day. The set concrete was picked and washed when fresh concrete was to be laid upon it so as to get as good a bond as possible. The inlet pipes to the turbines to operate the machinery were built in the lock walls, and as it was not desirable to place an iron pipe in this location, the pipe was molded of concrete and afterwards laid in the wall. The pipe was thirty-nine inches diameter, walls six inches thick and contained about 0.22 cubic yard of concrete per foot. It was made in three foot lengths in vertical molds, and the Cost of about six hundred feet of it was at the rate of $3.56 per foot, or $16.19 per cubic yard.
In the locks for the Illinois and Mississippi Canal the walls are entirely of concrete, and were built in alternate sections about thirty feet long. Work on a given section once commenced was continued to completion without intermission. The top was finished without any plaster or wet coat, the excess concrete being simply cut off with a straight edge and rubbed smooth and hard with a float. Vertical wells one foot square were left in the walls at intervals, and these were kept filled with water for about three weeks after the completion of the section, and then filled with concrete. To avoid weak places due to single batches made from cement of poor quality which might have passed inspection, the cement was mixed in lots of five to ten barrels before being used in the concrete.
The quoins of these locks were of cast iron. The foundations and the spaces in rear of lock walls are cut off from upper pool by cross-walls, and are underdrained to the lower pool to prevent the action of water pressure due to the upper pool level tending to force up the foundation. Ten inch and twelve inch tile drains were used for this purpose.
The proportions used in general were one part Portland cement, three to three and one-third parts gravel, and four parts broken stone, the concrete containing about one and four-tenths barrels of cement per yard. The average cost of concrete in quantities of two thousand to four thousand yards was from $8.50 to $9.15 per cubic yard, distributed approximately as follows: —
Materials.............$5.00 to $6.00
Molds...............82 to 1.42
mixing and placing........ 1.64 to 1.82
Miscellaneous............12 to .47
In the Herr Island Locks, Alleghany River, the failure of the cofferdam to exclude water from the lock pit on account of porosity of the river bed, led to the adoption of a concrete foundation, laid under water, of sufficient weight to balance the hydrostatic pressure. After this foundation was in place, the cofferdam was pumped out and the concrete side walls built in the dry.
The concrete was placed in one foot courses covering the entire area of the wall, the forms being made of one course of two by twelve inch plank set on edge and halved at the ends to form two inch lap splices. Iron rods one-quarter inch diameter were placed six feet eight inches apart to tie face and back plank together. A two by twelve inch cross-plank was placed on edge beside each tie rod, dividing the work into short sections. After completing the concreting to the top of the forms throughout, the cross-planks were removed and the space filled with concrete, thus making a vertical joint. The forms for the next course were then put in place in a similar manner. The size of stone used as aggregate was first two inches in one dimension, but this size was afterward reduced to one and one-half inches, and finally to one inch, the smaller size stone being preferred.
The lock in the Mississippi River between Minneapolis and St. Paul was founded on a soft sandstone rock having many water-bearing seams. The lock was surrounded on three sides by a cut-off wall. A trench two inches wide and ten feet deep was cut in the soft rock by jetting a series of holes in close juxtaposition and then breaking out the intervening wall with a drill and saw of special construction. In this trench was first laid a double thickness of three-quarter inch boards and the remaining space was grouted full. Sections of this wall afterward uncovered, showed the method to have been very effective. Similar methods of sealing open seams in rock by the use of grout under pressures have been used elsewhere.
The forms for the construction of this lock were of excellent design1 and have been described under the head of "forms" (§ 514). The walls were built in alternate blocks, twelve feet long. At the ends of the blocks are left vertical spaces five by seven inches, to be filled with mortar and other water-tight composition. The forms are lined with sheet iron, and to obtain a smooth face the concrete is thrown against the lining, the stones rebound, leaving only mortar on the face. The face is rammed with tampers of special form, wedge shaped, and measuring 3/4 inch by 5 inches on the lower edge. This is followed by a flat rammer. The finish is said to be excellent.
Proportions in the concrete were varied somewhat from time to time, though in general it was mixed one part silica cement, two and one-third parts sand and six and two-thirds parts of crushed stone without screening. Tests showed that about ten per cent, of this crusher product was fine enough to be considered sand, and account of this fact was taken in fixing the proportions as above. The Cost of the concrete, over 11,000 yards, was as follows: —
1 Mr. A. O. Powell, Asst. Engr., Report Chief of Engrs., 1900, p. 2778.
Total stone........... $2.49
Total materials.......... $5.77
mixing and placing concrete....... 1.44
Total Cost per cubic yard concrete $8.42