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BREAKWATER DESIGN.
141
rubble mound, surmounted by a vertical superstructure, is peculiarly suitable
for tidal seas and for seas with a slight tidal rise and fall, provided the water
in that case be very deep. In the case of tidal seas, there is no objection to
stopping the superstructure at low water level.
“ The type of construction in which the superstructure is made entirely of
blocks laid in regular courses, can be adopted for seas which have but a slight
tidal rise and fall, provided the site be sheltered. This type is not sufficiently
reliable for heavy seas, and in some cases the system of large monolithic
caissons can be adopted instead with advantage, on condition that the width
of the blocks be suitably proportioned to their length, that the loose rubble
of the hearting is perfectly compact, and that the very dangerous effects of
the backwash at the seaward base of the blocks, which are produced by the
impact of the waves, be counteracted by using exceptionally good material
for the upper part of the apron or outside road on the sea face, or by loading
and preserving this apron by means of protective blocks deposited at the base
of the caissons.”
The recommendations of this report formed the subject of some discussion
and not a little adverse criticism on the part of the Congress in regard to
several of the opinions therein expressed. It was evident that unanimity
could not be attained, and finally, the Congress limited the expression of its
views to the following resolution :—
“ The Congress refers to the information furnished by the written reports
and oral observations; it considers that engineers will find there information
of great value for the construction of breakwaters, especially in regard to the
force of waves, but, by reason of the great diversity of cases, it does not think
that it should formulate any absolute conclusions.”
With this summation of the special ad vantages and disadvantages attaching
to thè varions types of breakwater exemplified at the present day, we bring
our remarks on breakwater design to a close, simply adding some detailed
reference to a few selected cases, chosen in illustration of the principles laid
down in the preceding pages.
Breakwaters at Marseiiles.—The main undertaking, begun in 1845,
has a length, at the present time, of 4530 yards, including an extension of 600
yards completed in 1904. The same principle of construction has been main-
tained throughout a period of sixty years with unvarying success.
A section of the breakwater is exhibited in fig. 116. The core is a bed of
small rubble, having a depth or thickness of 10 feet, and lying upon the sea
bottom at a depth of 55 feet below low water level. It is overlaid by layers
of natural stone of increasing dimensions, ranging from 2 cwts. to nearly 4
tons a-piece. The quay shelter wall is a masonry structure founded upon the
topmost layer of blocks.
The exterior slope is 4 to 3 for its lower portion, extending from the
foundation to low water level. At this point it flattens abruptly to nearly
3 to 1. The effect of this sudden transition is to create a sharp ridge at the
water-line, with the result that the waves are cut at the point where thei