The air-chamber, which is similar to that of the Plymouth gates, is placed so that the top is at the level of high-water neap tides, about half-way up the gate.
There are two sluices at low-water level, each having an area of 8 square feet.
The entrance invert being subject to the influx of sand from the outer basin, and to the deposit of coal rubbish dropped into the dock, Mr. Brunel decided in this gate not to use wheels, but to make the hinge and heel-post strong enough to carry the whole weight of the gate, even if it were unsupported by the buoyancy of the air-chamber.
The heel-post is a massive piece of cast iron; the bottom part is bored out, and into it is fitted a cast-iron cylindrical pin, 1 foot 6 inches diameter and 7 feet 6 inches long. On this are ground discs of steel, lubricated with oil, whereon the gate rests and turns. Thus the surfaces exposed to friction are above the sand or grit at the bottom of the dock. The lower end of the pin fits into a cast-iron socket fastened to the masonry, and is prevented from turning round by being made hexagonal. The sides of the hexagon have sufficient play to enable the gate to adjust itself, when shut, to the masonry sill, so as to be water-tight. The top of the heel-post works in a brass bush, 18 inches diameter and 15 inches broad, enclosed in a massive wrought-iron collar, which is strongly fastened by anchor chains to the masonry. After the gate was completed the strength of the hinge was tested by moving the gate before the water was admitted into the dock. The only resistance to the motion of the gate is the slight friction at the hinge.
No coffer-dam was used in the construction of this work, but advantage was taken of a large bank of slag and earth enclosing a portion of the site of the dock. This was extended and raised, and a sea dam formed. The dam was cut through when the works were completed, and a channel dredged to the depth of 6 feet below low water.
Brentford Dock.
In 1855 an Act was obtained for making a dock on the Thames at Brentford, and a railway to join the Great Western Railway at Southall. The dock has an area of about 3½ acres.
The works were begun in July 1856, and were completed, and the dock opened, three years later.
The walls are founded in the London clay, which here underlies a bed of gravel of some thickness; from this there was a considerable influx of water.
The chief peculiarity of the dock is the form of construction adopted for the sides. Piers of brickwork, 10 feet long and 2 feet 3 inches thick, are placed at right angles to the sides of the dock at intervals of 26 feet. The backs of these piers are connected by horizontal arches, carried up with a curved batter. The piers are about 20 feet high, and arches are turned upon them, which support the front part of the quay, and meet the horizontal arches at the backs of the piers. Thus the sides of the dock consist of a series of vaults, arched over at the top, and also at the back towards the pressure of the earth.