212
DOCK ENGINEERING.
two lowermost tiers weigh about 35 tons each, the upper tiers average
5 tons less. An interval of from three to four months is allowed to elapse
between making and using,
when the mortar is com-
posed of hydraulic lime,
but only three weeks,
when of cement. The
Fig. 151. —Section of Quay Wall at Bougie.
Mud
blocks are set by a float-
ing crane with the assist-
ance of a diver. When the
four submerged courses
have been constructed, the
wall is weighted with a
temporary surcharge of
two tiers of blocks, which
causes the structure to
settle bodily to the extent
of about 3^ feet in a
period of two months, at
the end of which time the
rate of settlement is found
to be insignificant, the sur-
charge is removed and a
coping course substituted.
The backing behind the
wall is of rubble with a
covering layer, 3 feet thick,
of quarry spalls, above
which is discharged the
mud dredged from the
foundations.
The cost of this type of
wall works out to rather
more than £14 per foot
run, made up, approxi-
mately, as follows :—
Dredging site, . £1 1 0
Rubble filling, . 4 15 0
Artificial blocks, 6 3 0
Surcharge, . 0 9 0
Coping, . . 0 11 0
General, . . 1 5 0
Experience has shown inadequate stability in a portion of the wall, as
constructed above, and several important modifications are being introduced
into another section of the same undertaking. The dredged mud is no