401. Setting the lay out of Bridges
1. General
a) It is necessary to accurately lay out the centre line of a bridge and the locations of its piers and abutments and to establish a system by which they can be checked with ease during the progress of the work. b) Position of the principal reference lines and level pegs should be so selected and laid that they are easily accessible for check during the execution of the work .
c) The Principal reference lines to be established are the longitudinal centre line and the transverse centre lines of abutments and piers. If the bridge is on a curve, the tangent points of the curve and the directions of the tangents at either ends should also be established by pegs.
2. Setting out bridges without a base line :
Where deep excavations are not involved and where there is no water flow in the river during the working season, setting out primarily involves fixing the alignment correctly using a theodolite. The distance between the abutment at either end and the nearest pier and the pier - to-pier distance can be set out by directly measuring and marking the centres using a good steel tape (See Annexure 4/1).
The centre points of each structure (pier or abutment) should be punch marked on a flat or angle iron piece fixed flush with the top of a concrete block at the correct location .
3. Setting out bridges with the help of a base line :
a) Where deep excavation, pile driving or well sinking is involved, and where there is standing water, base lines are set out at right angle to the centre line of the bridge, one on either end on the high banks, or on one side of the bridge or anywhere between the abutments where level ground is available.
b) The actual position of the piers/abutments is determined by the intersection of three sight lines, one along the alignment sighted from stations located on either end, a second from a station on the base line on the down stream side and a third from a point on the base line on the upstream side. Theoretically all these three lines should intersect at one point. Normally a triangle of error gets formed and the correct centre is fixed by judgment within this triangle. (Refer Annexure 4/2) .
4. Important points to be observed while setting out base lines: a) Linear measurement should be carried out with invar tape or Electronic distance measuring equipment.
b) Concrete pillars with steel plates fixed over them should be located at tape lengths for accurate measurements.
c) Spring balances should be used for giving specified tension to the tape.
d) Tape readings should be corrected for tension, temperature and slope.
402. Pipe and box Culverts with open foundation
Pipe and box culverts can be constructed after removing the top soil in bed to the required depth and replacing it with a layer of lean concrete after the bed is levelled and well consolidated by ramming or rolling. Reference may be made to Annexure 4/3 for a typical arrangement. Pipe and box culverts should not normally be provided where the bed is likely to be scoured .
403. Other Bridges with open foundation
1. Open foundations must rest on a stratum with adequate bearing capacity. In order to reduce the bearing pressure the base can be sufficiently widened by providing footings. The footings will rest on a lean concrete bed of adequate thickness.
2. The foundation should be taken to a depth not less that 1.75 metres below the lowest anticipated scoured bed level in ordinary soil. In rocky soil, it will be adequate if it is properly keyed into the rock for a minimum of 0.3 metre in case of hard rock and 1.5 metres in case of soft rock. Sloping rock may be suitably benched. Fissures and weathered rocks should be avoided. A typical arrangement is shown in Annexure 4/4.
3. In soft soils, rafts may be provided as foundation. Such rafts should be protected by means of suitable aprons and cut off walls or launching aprons, both on the upstream and downstream sides to prevent undermining of the foundations .
4. Excavation for open foundations with shoring :
Excavation should be done in such a way that the surrounding soil can stand by itself by suitable sloping the sides. When excavations have to be deep or when the side slopes are not stable, suitable shoring may be provided from top, using timber planks, walling pieces and struts. Typical arrangements of shoring are shown in Annexure 4/ 5. In deep foundations and large size excavations, where the seepage is heavy, suitable pumps may be used for dewatering. A small sump on the side or corners should be provided for collection of the water to be pumped.
404. Excavation using coffer dam
1. Shallow foundations :
Where excavation is required to be done under flowing or standing water, coffer dams of steel sheet pile, RCC or timber may be constructed. Driving is done from a floating platform. Annexure 4/6 shows a typical arrangement with steel sheet piling.
2. Deep foundations :
When the depth of water is more than 10 metres, coffer dams with single wall sheet pile will not be adequate. In such cases double wall sheet pile coffer dams as shown in Annexure 4/7 may be provided. The two rows of sheet piles are connected by tie rods and the space in between is filled with rock or soil. Suitable beams are provided on the inside.