Railway Construction

by William Hemingway Mills

diverted to one side, and a small artificial island made to receive the curb above water-level. When a curb is fairly fixed in position, the work of building the brick well can be commenced. With the wrought-iron curb the triangular cavity between the vertical plate and sloping plate must be filled with concrete to form a level base for the first course of brickwork. The wooden curb being composed of horizontal layers of timber, is ready to receive the brickwork without further preparation. To strengthen and keep the brickwork firmly tied together, strong wrought-iron vertical tie-rods, 1¼ or 1½ inch in diameter, are generally built into the work—as shown in the sketches—at distances about four feet apart. The lower end of the bottom tier of tie-rods is secured to the curb, and the upper end passed through a strong wrought-iron plate-ring, which is continuous all round the brickwork. A long deep nut is screwed down over the top or screwed end of tie-rod until the plate-ring is down tight on the brickwork. The tightening nuts are made sufficiently deep to receive the lower ends of a second series of vertical tie-rods, which in like manner pass through another wrought-iron plate-ring on the next section of brick well, and the same arrangement is continued for the full height of the well. The lengths of the tie-rods will depend upon the lengths of the section of brickwork to be built at a time, and may vary from 10 to 15 feet.

As the work of building proceeds the curb and brick well will sink gradually into the ground, and down to a certain depth, varying according to the material of the river bed, the weight of the brick well itself will effect the penetration and lowering. Beyond this depth the lowering must be done by scooping or dredging the material from the inside of the well, and placing heavy weights of old railway iron or other convenient masses on the top. When one section or length of well has been sunk down, then another set of tie-rods are inserted into the deep nuts, and another section of brickwork commenced. The operation of lowering is rather tedious, as all the weights have to be hoisted up on to the top of the length in hand, and piled so as to leave space for lifting out the material dredged from the interior; and then, when the length has been lowered, all the weights must be removed before the brickwork can be resumed on another length. Where the river bed consists of soft material, the excavation inside the well can generally be

effected by suitable dredges or scoops worked from the surface or top of brickwork. Should trees or other obstructive masses be met with embedded in the strata, it will be necessary to employ divers to remove them piecemeal out of the way of the curb.

When the brick well has been lowered down to the full depth, and is thoroughly bedded in a stratum of strong material, the test weights should be left on for some time to ascertain if there is any further sinking. After all the weights have been removed the bottom of the well can be dredged out clean, and the interior filled in with concrete to such height as may be considered necessary.

Brick wells must be watched carefully to ensure that they sink down in a perfectly vertical position. Any inclination away from the perpendicular must be corrected at once by means of guys and struts, the same as in sinking iron cylinders. The principal difficulty will be with the first 20 or 25 feet.

The diameter of the well will depend upon the weight it has to carry, and its height from river bed to under side of girders. The wells may be either circular or polygonal in section, and built singly or in pairs, as shown in sketches ([Fig. 121]).

Many piers and abutments of bridges in shallow or moderately deep water are built by means of coffer-dams of timber and clay puddle. The coffer-dam forms a water-tight wall round the site of the foundation, from which the water is pumped out, and the excavation carried down to the depth required. In very shallow water it is sometimes sufficient to drive only a single row of piles, and form a bank of good clay puddle on the outside, as shown in [Fig. 122]. In deep water it is necessary to drive a double row of piles, 3 or 4 or more feet apart, and fill in the space between with clay puddle, as shown in [Fig. 123]. The piles for coffer-dam work should be carefully selected, of good timber straight, and correctly sawn on the contact faces. Guide-piles are first driven in proper line and position round the intended foundation. To these strong horizontal double waling pieces are securely bolted, one on each side of the guide-pile, one pair near the top, and the other pair as low down as can be placed. The sheeting piles, which are lowered down between the horizontal waling or guiding pieces, are driven as close to one another as possible, being assisted in doing so by the sheet-pile shoe, shown on [Fig. 124], which is made not with a point like

an ordinary pile shoe ([Fig. 125]), but with a cutting edge slightly inclined, so that in driving the tendency of the pile is to drift towards the pile previously driven. Sometimes the outer row of piles consists of whole balks, and the inner row of half balks; the size of the piles must, however, be regulated by the depth and current of the water. When both rows of piles have been completed, the space between should be dredged out, and then filled with carefully prepared clay puddle. To enable the puddle to adapt itself thoroughly to the wooden sides, it is desirable to remove the inside walings after all the piles are driven, as any internal projections interfere with the proper punning and settling of the puddle. The swelling of the puddled clay has a tendency to force apart the two rows of piles, and to counteract this as much as possible, iron tie-rods should be passed through from side to side every few feet, and screwed up against large washers placed on the outside of the outer walings. Strong struts or cross-bracing of timber must be placed from side to side inside the coffer-dam to resist the pressure of the water in the river. This cross-bracing can be removed gradually as the work of building progresses upwards, and be replaced with short struts wedged in against the sides of the finished courses.

In cases where the ground is soft, and when it is not considered prudent to excavate the foundations deeper for fear of disturbing the stability of the coffer-dam piles, rows of large, square bearing-piles may be driven in the floor of the foundation, as shown in [Fig. 111]. The tops of these bearing-piles must all be sawn off to the same level, and a platform of strong double planking securely fixed to the piles to receive the foundation course of concrete, masonry, or brickwork. The spaces around the tops of the piles and the under side of the timber platform should be filled in with good cement concrete.

The interior of the coffer-dam is kept dry by constant pumping, either by hand pumps or steam pumps, according to the volume of water finding its way into the foundations. When the finished pier or abutment has been carried up above the river water-level, the coffer-dam is no longer required, and may be removed. Sometimes, to save the timber, the piles are drawn by means of strong tackle fitted up for the purpose; but in doing this there is considerable risk of disturbance to the foundations, and it is better to leave the piles in the ground