Bridge form an important part of the transportation rules. They are constructed across the rivers to carry highways and railways.
Nature of Rocks
The rocks over which the piers and abutments are to be founded must be strong and durable. They should be free closely spaced joints, fissures, shear zones, solution channels and other zones of weakness. Poorly cemented, thin bedded and soft sedimentary rock should be avoided. The rocks having joints and fracture may be consolidated by grouting.
For placing the abutments of a bridge, a valley walls are thoroughly examined. The valley walls where the strata dip into the river channel from unstable slope because they have a tendency to slide into the river channel. Such unstable valley slopes should be avoided.
Foundation of Bridges
The weight of the bridge, the load of the traffic and pressure of the wind and flowing water are ultimately transmitted to the foundation of the piers and abutments. Therefore the design and construction of bridges is governed largely by these points, these geological features should be studied during the selecting a site for a bridge.
Structure of Rocks
If a bridge is aligned across the strike of the country rock, different types of rock beds having varying strength and composition are met with along the foundation. In such cases a close examination of the foundation rock under each pier and abutment should be done by putting bore holes. If thinly bedded soft rocks such as shakes are exposed in the river bed, the water currents would easily cut deep grooves parallel to the bed and hence would undermine the foundation of piers.
A fault, if it is running across the bridge alignment, is a source of many troubles. The highly crushed and weathered rock which exist in the faults zones, make the foundation treatment extremely expansive. It is therefore advised that the possibility of avoiding the faults by shifting the bridge alignment upstream or downstream may seriously be considered.
Type of River Channel
In alluvial channels the thickness of loose sands and gravels may be so great that it is economical to reach the bed rock for placing the piers. In such cases pile foundation is used. The piles are generally driven through the alluvial material to the bed rock. Friction piles are used where the bed rock is not available upto a great depth.