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purpose. The sides were so constructed as to admit of their easy removal from the bottom of the caisson when it had been sunk to its bed. Rankine mentions a caisson described by Becker which measured 63 feet long, 21 feet broad, and 15 feet deep over all. The bottom beams used in constructing this large caisson were 10 inches square and 2 feet 10 inches apart from centre to centre, and the uprights forming the sides were 10 inches square and 5 feet 8 inches apart from centre to centre.
But to return to the subject of this article. The dams used in soft bottoms, where piles can be driven, are con-structed of timber, and vary in strength according to the head of water they have to sustain. But the general style of construction is in all cases the same. The dams are formed of parallel rows of piles driven into the bottom, the space between the rows being filled by a mass of clay puddle of sufficient thickness to exclude the water which percolates between the joints of the piles. In cases where the head of water is not great, the coffer-dam is generally constructed as shown in fig 1., where the gauge piles a
Pro. 1.Coffer-dam for Soft Bottom.
are driven at distances varying from 4 to 8 feet apart, to which walings b are fixed, and between the walings sheet piles c are driven. The sheet piles are shod with iron, having a sloping edge to cause the piles to cling while being driven, and in the centre of each bay there is a key pile e, having a slight taper which on being driven down compresses the sheet piles on either side of it closely together. In cases where the water pressure is great the sheeting piles are dispensed with, and the dam is formed of two or sometimes three rows of whole timbers having the clay puddle between them. Fig. 2 is a dam on this principle, used in the construction of the Thames embankment, and described in the Transactions of the Institution of Civil Engineers by Mr Thomas Ridley, and after the explanations that have been given, its construction will be easily understood as an outer and inner dam formed of two rows of close-driven whole logs with intervening spaces of 6 feet filled with clay puddle. In all cases the dams must be supported by sufficient stays or struts, abutting on firm ground, or, when this cannot be got, on dwarf piles driven deep enough to afford sufficient resistance. It is also important to remove the soft matter between the rows of piles to as great a depth as possible, and to fill in the excavated space with clay puddle, for
The coffer-dams described illustrate the general construction of such works, but various arrangements of the timber work have been adopted to suit particular situations, such as Mr James Walker's coffer-dam for constructing the founda-tions of the river terrace of the Houses of Parliament at Westminster (vide Min. of Proc. of Inst, of C. E., vol. i.), and Sir John Hawkshaw's dam for the middle level drainage of Lincoln (Min. of Proc. of List, of Civil Engineers, vol. xxii.)
All the examples that have been given are applicable to situations where the bottom is sufficiently soft to admit of piles being driven. But cases occur where this is impossible. Such, for example, as the removal of obstructions from the beds of rivers where it may be necessary to lay dry a large area of solid
although the timber-work of the dam must be constructed so as to resist pressures, it will generally be found that the greatest risk of failure arises from the filtration of water under the bottom of the sheeting piles and puddle.
rock, and in that case it is necessary to adopt a totally different construction of dam. The accompanying cut (fig. 3) shows a coffer-dam designed by Mr D. Stevenson, which is specially adapted to a hard bottom where piles cannot be driven. It is formed of two rows of iron piles placed 3 feet apart and jumped into the rock, which supports two linings of planking, the inter-mediate space being filled with puddle and the whole structure properly stayed. This dam has been successfully employed on many works, and on the Ribble navigation, where it was first introduced, it was used to excavate a bed of rock 300 yards in length and of a maximum depth of 13 feet 6 inches. The maximum depth at high water against the dam was 16 feet, but in high river floods the whole dam was completely submerged, and on the water subsiding it was found that the iron rods, although jumped only 15 inches into the rock, were not drawn from their fixtures.
Dams must be designed with a special regard to their sufficiency to resist the water pressures they have to bear, and Professor Rankine gives the following formulae, in his Manual of Civil Engineering, p. 612, for calculating the pressure which the struts may have to bear.
Let J=breadth in feet of the division of the dam sustained by one strut,
x = the depth of water in feet,
tt> = the weight of a cubic foot of water in lbs.
P=the pressure of water against that division of dam;
*? = wbx*-r1
and the moment of that pressure relative to a horizontal axis at the level of the ground is
M = wta3-M>.
Let h be the height above the ground at which the strut abuts against the dam, and i its inclination to the horizon; the thrust along the strut is
T = M sec.
from which the scantling required, depending on the sort of timber employed, can be calculated.
In conclusion it may be noticed that the introduction of iron cylinders and compressed air for founding the piers of bridges has not only superseded the use of timber coffer-
dams for piers in soft bottoms, but has enabled bridges to be securely placed in situations where no timber dams could have
answered the purpose. On the other hand, there are many engineering works connected with river, harbour, and dock improvements, to which the cylinder system is quite
inapplicable, and for which extensive and costly coffer-dams
of the kind we have described must continue to be
employed. The method of founding by iron cylinders has
been described in the article BRIDGE, to which the reader
is referred. (D. S.)
Transactions of Institution of Civil Engineers, vol. iii. p. 337.