Important Geological Information related to the Bridges:
- Complete and detailed geological investigation is required if the piers are to be founded directly on the bedrock.
- Bore-holes are needed to be bored up to the depth of bedrock along the central axis of the proposed piers. Geological information and maps of that area are obtained and studied. If a bedrock is uniform under whole the bridge, then bore-holes are made under the alternate proposed piers. First of all, the nature and uniformity of the bedrock (under the bridge) is checked
- In most of the cases, nature of the bedrock varies from one abutment to the other
- Deep boring and testing at pier points are necessary for long, high or massive bridge or all of these characteristics in one bridge, in order to find out the nature of the subsoil and rock.
Previously Well-Foundations for the foundations of above-said bridges were made but presently ‘Socketed Piles extending into the Bedrock’ are commonly made
Case studies of the foundations of some bridges:
Case 1: George Washington Bridge, connecting New York and New Jersey, USA; built in 1934:
- Formation of the river bed is highly variable in nature of rocks. Left end i.e. NJ End consists of stratified shale above and highly folded limestone underneath. Right end i.e. NY End consists of the same continuing limestone underlain by highly folded schist. There is a fault in the schist on NY side. Sufficient fault-seam and face were filled with concrete and anchorage was provided on huge concrete block.
Case 2: Formation with unfavourable and unstable slanting stratification of abutment (slanting towards the river)
Case 3: Formation of the river bed comprising slanting igneous rock and two different types of slanting sedimentary rocks with a big fault between the igneous and sedimentary rocks situated just under the centre of the river’s width.