202
DOCK ENGINEERING.
set upon a concrete bed and covered by ashlar masonry (moellons d'appareil)^
The concrete, 6 feet 6 inches thick in the floor of the look, 10 feet thick in
the gate platforms, and 12 feet thick in the aprons, was composed of equal
parts of hydraulic lime mortar (Tournai lime, trass, and sand), pebbles
(galets), and broken material (briques roches concassées). The sectional pro-
file of the floor (fig. 187) exhibits a flat centre, 42 feet 6 inches in extent,.
flanked by curves which are tangential to the side walls.
Fig. 187.—North Lock, Dunkirk—Section.
The side walls were executed generally in local brick and limestone, set
in Portland cement mortar, with a facing of ashlar masonry. Normandy or
Brittany granite was used for the sills, hollow quoins, caisson quoins,
copings, square quoins, culvert apertures, and for the rounds of the
pierheads above low water. The mortar was composed of 1 to 1J parts of
Portland cement to 1 of sand.
The North Loek at Buenos Ayres.*
It was at first proposed to lay out the northern entrance to the Madero
Docks in a north-easterly direction from the north basin to the outer roads,
where there is a long stretch of water having an average depth of 20 feet
3 inches below low-water level, and thence in a S.E. direction to the
bar anchorage. This line, however, was abandoned as likely to involve an
increase in silting, owing to its directly transverse situation in regard to the
stream, and it was eventually decided to turn the channel as quickly as
possible into the run of the river (see fig. 8, p. 37).
The north loek is 82 feet wide at the coping, has a length of 508 feet
6 inches between sills, and a draught of 22 feet over sills at low water. It
is traversed by a swing bridge. A main service subway, 9 feet 10 inches by
7 feet 6 inches, in rubble masonry, lined with brickwork, passes under the
floor. The general disposition of the lock will be readily grasped from an
* Dobson on “Buenos Ayres Harbour Works,” Min. Proc. Inst. C.E., vol. cxxxviii.