MASONRY PIERS.
281
Since upright piers from the sea bottom are inevitably expensive in
construction, where the depth is at all considérable, and further, since the
rubble mound offers a suitable means of bringing the foundation level
tolerably near the water level without incurring too much danger of
disturbance, a combination of the two types is a very comrnon feature
of modern practice.
The level at which loose rubble of different sizes may be trusted to
remain stationary in stormy weather is a matter of considérable importance
in piers of this type. Sir John Coode states that he found the shingle of
Ohesil Beach in motion during winter storms, at a depth of 8 fathoms.
The line of permanent mud, which marks at any rate the extreme limit of
wave action, whatever other agencies may assist in its determination, lies at
a depth of 12 to 16 fathoms below low water off the coast of Holland, and at
Pig. 214. —Jetty at Algiers.
a depth of 80 to 90 fathoms in the vicinity of the Shetlands. But even
assuming the motion of waves to be perceptible throughout so great a
range, it is manifest that the force diminishes with the distance below the
surface, and that, at a certain depth, the effects become of trilling import-
ance. In fact, it appears that the really injurions effects of wave action are
confined to a zone extending from the surface level to a distance of about
25 or 30 feet below. Beyond this point, small rubble and quarry rubbish
may be deposited, with comparative impunity, in mounds which will stand
at slopes of 1 or 1| to 1.* Upwards of this, stones of larger bulk and
greater weight must be employed, culminating in blocks of not less than
There are, of course, abnormal cases in which these statements do not accord with
experience. Por instance, at Peterhead Harbour in October, 1898, blocks weighing
upwards of 41 tons each were displaced by the waves at a depth of 36J feet below low
water of ordinary spring tides, but this and one or two other examples at Wick and
elsewhere are exceptional.