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to lack of stone sufficient to sink, the ballasted mat has lain for four or more days on the surface; it began to sag along the middle, this sag in one case amounting to as much as 15 feet. In such a case there is some danger of rupture and distortion.
When ready for ballasting, barges loaded with stone are placed along the outer edge of the mat; planks are run out from them to and across the mat; then stone is carried out by wheelbarrows, distributing the weight on the mat uniformly until it is barely atloat. The ballasting is begun at the head and continued downstream, the barges being dropped down along the mat as each section is ballasted. A considerable quantity of stone is also placed on the mooring barges along their downstream edge for use when the mat is being lowered at the commencement of the sinking. The amount of ballast used is ordinarily about a third of a cubie yard per square of mat.
After the ballasting is completed the mat is pulled close in to the shore and held there by wire strands to fastenings on the bank. Loaded stone barges, lashed end to end, having a combined length sufficient to cover the width of the mat, are brought to and are hung from the mooring barges along the upper outside edge of the mat. Stone from the mooring barge is cast onto the mat near its head, and by slacking the slip lines the head of the mat is allowed to sink a few feet. The stone barges are then pulled in over the mat alongside the mooring barges, and long lines are passed from the mooring barges to the stone barges to control the movements of the latter. When all preparations have been completed a large force of men is placed along both sides of the stone barges, considerable stone is cast on the mat, the slip lines are slackened, and the mat is gradually lowered to the bottom. The drop lines to the stone barge are then payed out, and the stone barge is thus permitted to drift downstream over the mattress, the laborers throwing off stone from all sides as rapidly as possible, this being continued until the mat is completely sunk. Under favorable conditions the time of sinking is less than an hour for a thousand-foot mat, counting from the time of sinking the mat-head. To facilitate the sinking, it is usual to have a steamer attached to the outer end of the stone barge and line to men on shore from the inner end to control the barge from swinging too far in either direction. After dropping the stone barge over the entire length of the mat and
dis That th
cking the latter it is usual to bring it back to the mooring barges this
at repeat the operation, this time permitting it to drift slowly over mat and casting off the stone, so as to secure as uniform a disbution as practicable and insuring the mat being well down.
It sometimes happens that a mat is sunk in a locality where the arrept is reversed. In such a case provision against the mat
rawling" upstream must be made by anchoring the lower edge o the bank below with one or more cables, according to the strength the eddy. It is very seldom, however, that this reverse current extends outstream more than half the width of the mat; therefore, ay such part of the mat as will be subjected to the eddy need be mo-hored; but this precaution should never be neglected. When bp mat is supposed to be safely sunk the mooring lines are slackbed
, the shackle pins pulled up, and all cliains and cables are reavered and the mooring barges removed,
The total quantity of stone needed to ballast and sink a mat is urdinarily three-fourths of a cubic yard per square of mat.
RAPIDITY OF WORK
With a full and well-trained force and under average conditions about 120 feet of mat can be constructed each day, and under 'avorable conditions a mat 1,000 feet long should be built and sunk -rery two weeks.
When the bank is straight the mat is made so as to lap on the shore a few feet above the water, and if the subaqueous slope near the water surface be gentle it may be desirable to use mat barges itted with a short overhang, so that the mat built on it may lap on the shore.
If the shore line be gently curved the mat can be made to fit it hr sliding off one end of the mat from the barge more than the other and making the fascines conical instead of cylindrical and thus nake the mat curved. (See Photo. No. 13.)
But it frequently happens that some of the indentations of the shore are so deep in comparison to the width that the main mats can not be well fitted to them. In such cases recourse must be had to connecting mats. For their construction a mat barge is brought up along the bank to be covered so that the ends of the ways lap over the dry bank. In most cases they will do this, but along the middle, owing to the concavity of the pocket, they may be 10 feet or more away from the dry bank. In this case stout poles are laid from the deck of the mat barge to the shore, and the mat resting upon these can then be launched, or, if necessary, hauled out and louhi where its edge should rest. These mats are built similar to ti per l'ust main one, except that the fascines lie parallel to the shore instea 1 it was of normal to it and that no top cables are used. It is built out b successive launches until it overlaps the main mat by 20 feet when as 8 it is ballasted and sunk. Before this is done the mat is anchored t, wheret the shore about every 50 feet by wire strands fastened to stumps o. trees. After the bank has been graded these anchorages can, igla un necessary, be transferred to dead men sunk beneath the grade, ishul slope, but their principal duty is to prevent sliding down of the ani 1 mat while it is being sunk.
After the sinking of all the subaqueous mats work is begun or the upper bank, and before the paving can be laid on this it must be first cut to a proper slope.
GRADING UPPER BANK
The slope to which the upper bank is graded is usually one vertical to three horizontal, though in bad soils and on exposed salients vedru - this slope is made more gentle.
If, as sometimes happens, the bank --knis is already at or near that slope and merely needs a little smoothcall w ing off this can be done with shovels, but in general the hydraulic he tot method costs less. Two streams are generally used. The nozzle for tiku each hose pipe is fastened by a universal joint to a piece of iron pipe firm driven into the ground and is operated by a long wooden lever. This holds the stream steady and permits it to be thrown in any neces- sust sary direction. The nozzles are held quite close to the face of the also cut and for protection from the spray the men are furnished with waterproof clothing. One stream is used to undercut the bank and de the other to wash the fallen material into the river. The usual practice is to work downstream, cutting the top of the bank well in ad- is the vance, so that the wash from this will flow as closely as practicable inalong the face of the cut. In this way a uniform grade can be most a tak easily obtained, but with some banks composed of more or less sand it is practically impossible, however much care be taken, to prevent the washing out of gullies, which must, of course, be subsequently
edia filled up. Generally, it is necessary after grading to do some little trimming with shovels, but the amount of this should be made as small as possible, as a bank is better able to stand the attack of the current the less it has been disturbed. The principal obstacles to rapid work are stumps and buried logs. All stumps not washed out by the grading must be afterwards removed by chopping down
ut to o the
with the slope, otherwise, when they become submerged, small
will be created behind one, which may damage the paving. It was formerly the custom to grade the bank before building the peous mat, as it was thought that the material washed down vuld make a more regular subaqueous slope. This was subseatly abandoned, as a rule, but it can, if desired, be used in
sure at a place where there is no danger of caving going on durDit 2 mat construction and thus destroying the slope already graded.
waiting to grade until after the mats are sunk the subaqueous pe is held, the inshore edge of the mat rests on firm and undisshed material, and the material washed down on the mattress ps to silt it up, hold it in place, and fills up any cavities be
After the bank has been graded paving is begun. There is first
read over the bank a layer about 3 or 4 inches thick of quarry uts salls or crushed rock. Upon this a carefully pitched layer of ripank
with a thickness of from 6 to 8 inches, is placed. Care is taken oth.
leave as small voids as possible, and the top surface is left rough. clic
This makes the total thickness of the paving from 8 to 12 inches,
spering in thickness from the bottom to the top of the bank. Along ipe ne edge of the mat work, and in order to cover any narrow belt his
vít bare by the settlement of the mattress, there is laid quite a
s-ary ridge of stone 18 inches or more in thickness. he
Other details of the revetment work are seen in photographs Nos. th
10.11 (pages 139, 141, and 143), 12, 13, and 14 (pages 255, 257, d
ind 259), and drawings in accompanying atlas,
The bank revetment completed, as above described, covers the .
hank from the top of the paving to the foot of the mattress with a novering, non-erodable itself and containing no holes through which xour can take place, and erosion can, as long as the work remains intact, take place only above the paving or outside the mattress, and by a proper extension of paving and mattress damaging seour at these places can be prevented.
The weak point is at the junction of the two kinds of work. The mat can always be expected to settle a little after sinking, but the amount of this settlement is ordinarily quite small and soon ceases, and in all probability will have stopped entirely before the paving is laid, and should any slight settlement take place afterwards the
stone ridge will probably fall down into the gap and prevent an damage.
TIME OF WORK
Bank revetment should always be built at as low a stage as pos sible, and preferably when the water is falling slowly, not only be cause the absence of drift and slack current at such a time make the work easy and reduce its cost, but for other reasons. ing can only be laid on the visible parts of the bank, and it is neces sary, therefore, that the mats should lap far enough above the water surface to permit of a proper connection being made, and every foot of upper bank covered with a mattress means that much taken from its outstream reach. Then, again, all that part of the mat above low water is subject to rapid decay and must eventually be replaced by stone. In practice, it is, of course, impossible to put all the work in at extreme low water, and it is therefore necessary to go over the work during some succeeding low stage and carefully pave over the part of the mat exposed, so as to protect the bank even after the brush decays.
The stone of the upper bank work is, of course, not subject to deterioration, while as much of the brush as is always wet is fairly durable, but it can not be expected that the work can be left to it. self absolutely and will need no care or repairs. Bank revetment, like most constructions, may be damaged in many ways, and it is only by constant care and by repairing all faults as soon as they occur that it can be maintained in good condition. The paving just above the low-water line must be watched, especially after the first high water. The paving higher up may be injured by boats as they land, pressing holes into the soft banks, by floating logs, ice, and in other ways, and all such holes must be repaired before the action of the current enlarges them. One special cause of damage is concentrated surface drainage, flowing under the paving and cutting gullies into the bank. When not practicable to divert this drainage it must be carried over the slope in specially prepared surface drains. Places where the subaqueous bank, when revetted, is very steep, where the bank is known to be shattered, or where seepage is expected, must be especially carefully watched, and all faults that may develop must be repaired at once, as it is only by such quick repairs that such a bank can be held.