Part 46 (1/2)

The town of Monkwearmouth is situated at the mouth of the river Wear, on the north side, and opposite to the towns of Sunderland and Bishopwearmouth, which extend for about a mile along the south side. In order to improve the entrance of the river, and to diminish the sand-banks which lay near its mouth, piers were proposed as early as the middle of the last century, and were partly built on both sides of the river before the year 1800. From that date until 1831, although the question of making docks had been considered, and designs proposed by different engineers, no steps had been taken for their construction, and the only works executed for the improvement of the port were the extension and alteration of the piers already existing. In 1831 designs for docks to accommodate the increasing traffic were made simultaneously by Mr. Brunel and Mr. Giles. Mr. Brunel's docks were to have been on the north side of the river, and to have had an area of 25 acres, with quays, warehouses, &c. Mr. Giles's were to have been on the south side.

Neither of these schemes was approved of by Parliament; but shortly afterwards a private company was formed for the construction of a dock on a plan designed by Mr. Brunel, though on a much smaller scale than his scheme of 1831, the dock being only about 6 acres in area, with a tidal basin of about an acre and a half. The company encountered considerable opposition from the authorities of the town of Sunderland, but succeeded in obtaining a royal charter for the construction of the dock. They subsequently obtained an Act of Parliament empowering them to make the entrance from the dock to the river. The dock was constructed, and eventually became the property of the North Eastern Railway Company, to whom it now belongs; they have erected coal drops along the quay, and have made it a s.h.i.+pping place for collieries connected with their railway.

The work was begun in 1834, and the dock and tidal basin occupy part of the site chosen by Mr. Brunel for his larger scheme of 1831.

The quay wall was built with a curved batter, the chord line joining the top and bottom having an inclination of 1 in 5. The masonry was carried up in courses, and made solid by filling every part thoroughly with mortar. A course at the face and at the back of the wall was built up; an abundance of mortar was then spread in the heart of the wall, and the stones built in the mortar. Thus no crevices could be left in any part of the work, and the back of the wall was soundly built throughout.

The entrance to the dock is 45 feet wide, with side walls of the same profile as the quay wall. Except at the gate floors, there is a segmental invert of dressed stone of such curvature that it is 6 feet 6 inches lower in the middle than at the side walls. The gate floors are formed with inverts, curved to correspond with the under sides of the gates.

The masonry of the entrance was executed within a four-sided coffer-dam, the sides of which were slightly convex outwards. This coffer-dam was constructed in the usual way, there being two rows of close piling with puddle between them; and it was strengthened by internal horizontal sh.o.r.es which connected the opposite sides, and by diagonal bracing. The piles were driven until they met with so much resistance as to render it unsafe to drive them farther. When the ground inside the coffer-dam was excavated, it was found that the piles had been driven into sand and gravel, and that, to enable the masonry to be built on a good foundation, it would be necessary to excavate about 7 or 8 feet below the piles. They were therefore driven down gradually, as the ground was removed from the inside, until the requisite depth was obtained. The whole coffer-dam was thus an immense caisson, the sides of which were lowered by gradual driving, instead of being simultaneously forced down by weights.

The masonry of the walls of the tidal basin is similar to that of the walls of the dock; some parts of the foundation were laid by means of a diving-bell.

In the entrance between the dock and the tidal basin there is a pair of gates pointing inwards, which serves to retain the water in the dock during the fall of the tide, and there is also a pair of storm gates pointing outwards, which protects the inner pair from the force of the waves.

The construction of both pairs of gates is similar.

The two leaves of each pair meet at an angle of 125. Each leaf is about 30 feet long; the bottom beams are curved to the form of the segment of a circle; the height at the meeting-post is 27 feet, and at the heel post 22 feet (see woodcut, fig. 17). This arrangement is, to a greater or less extent, followed in the dock gates Mr. Brunel afterwards constructed. By raising the pivot, the gate floor can be made of ample strength, and the cills and heel-posts are free from mud and deposit.

The gates are constructed of horizontal beams of yellow pine timber, 21 inches thick, placed close together for a height of 12 feet above the bottom. Above this there are beams of timber and of cast iron at intervals. The whole is planked over on the inner side with 4-inch planking. The heel-post and meeting-post are socketed into cast-iron uprights, which also receive the ends of the horizontal beams. To preserve the gate from any change of form, a diagonal iron tie-bar extends from the top of the heel-post to the timber beams forming the lower part of the gate.

[Ill.u.s.tration: Fig. 17. Monkwearmouth Dock Gates.

_Elevation. Section.

Plan.

Scale of feet._]

Nearly under each meeting-post is placed a bevelled cast-iron wheel, 18 inches in diameter, which supports part of the weight of the gate.

There are four sluices in each leaf, placed in pairs, with a small interval between them. Each pair of sluices counterbalances the weight of the other pair by being attached to opposite ends of a lever at the top of the gate. A screw works in the segment of a large worm-wheel formed on the end of the lever, and, being turned round, opens and shuts the sluices. After the timber work of the gate had been fitted together, it was taken to pieces, and subjected to the preserving process called Kyanising, which consists in immersing the wood in a solution of corrosive sublimate. This process has been so successful, that when the gates were recently taken out for examination the timber was found to be nearly perfect, only slight surface repairs being required in one or two places.

The great bulk of light wood at the bottom of these gates gives them a certain amount of flotation at all times of tide. After the gates had been in use many years it was found that one of the wheels had been detached for some time, but the buoyancy of the gate had prevented any mischief resulting. The buoyancy of the lower part of the gate is somewhat a.n.a.logous to that of the air-chamber which Mr. Brunel introduced afterwards in his wrought-iron dock gates.

_Bristol Docks._

About the year 1804 that portion of the river Avon which flows in a serpentine course through the city of Bristol was enclosed, and the water in it retained at a constant level, a new cut or shorter channel being made for the river. The portion separated, called the Floating Harbour, or Float, is about two miles long and 100 yards broad. At its lower end it is connected with the river by the c.u.mberland Basin, a half-tide basin, with two locks, and at its upper end by a feeder, which brings the water of the Avon into it, the river in dry weather being stopped from pa.s.sing into the new cut by the Neetham Dam, About half-way up, the Float is entered on the north side by the river Frome; and, a little above this junction, it is crossed by the Prince's Street drawbridge, which divides it into two parts. About 170 feet above the bridge the Float is connected with the new cut by another basin with a lock, called Bathurst Basin.

Mud and other deposits had acc.u.mulated to such an extent in the Floating Harbour, that at the end of the year 1832 the directors of the Dock Company employed Mr. Brunel to suggest remedial measures.

In order to effect his object at the least possible cost, he proposed certain works, together with an improved system of managing the water of the river, so as to allow more of it to pa.s.s through the Floating Harbour, by means of which great benefit might fairly be antic.i.p.ated.[178] He remarked that,--

By systematically following this course, the object of which is simply to keep in continual action all the means, however small, which can at the moment be brought to bear, and thus day by day to remove or neutralise, or merely diminish (as the case may be), the continual deposit which is going on--in fact, by applying a constantly acting remedy to oppose a constantly acting evil--I have little doubt that the formation of shoals similar to the existing ones may be entirely prevented, or at all events that they will be of such a nature as to be easily removed by two or three yearly scourings, and without that time and labour which are now expended with so little effect.

It should be observed that the yearly scourings, which became so objectionable to the trade, were not introduced by Mr. Brunel, but were part of the original arrangements of the docks.