Investigation for Bridge Site
• A bridge may be defined as a structure built over a river, a dry valley or any depressed part of land to provide a link between the two opposite sides.
• The foundation of bridge piers and abutments require a serious geological investigation.
• An ideal site for the construction of a bridge in is the one across the valley cut in a sound rock and where the river flow is free from scouring due to bends, tributaries and other causes.
• The chief factors which govern the stability of bridges are lateral forces, earthquake forces and scouring action of river.
• Pressure from bridge and running water are the main lateral forces.
• Piers should be founded at depth safe from scouring erosion.
• In many cases, the location of a bridge is decided more by socio-economic factors than geological considerations.
• In big cities divided by streams and rivers, a bridge has to be places where necessary irrespective of subsurface geology.
• But in highways there is often some flexibility available in the choice of placement of a bridge.
• In any major bridge construction project, the bridge abutment and piers should be kept on sound, strong and stable rock foundation below as possible.
• River bed are covered by varying thickness of unconsolidated natural deposits of sand, gravels and boulders not safe for foundation of bridge pier.
• The piers placed directly on them would be unstable.
• The depth of sound rock may vary from 5-20 m in some to more than 100 m in others.
• Drill holes are made all along the centre line of the proposed bridge, even on the banks sound rock sequence are reached.
• Height of individual piers may vary according to the depth of the sound bedrock below the surface as each them should be founded on the stable rock.
• The nature of the bed rock is commonly determined by the study of petrological characters and engineering properties, especially the strength values, using the core samples obtained during drilling of test bore holes.
• A decision to place the pier on a particular rock at a particular depth is matter of judgement and design requirement.
• Most igneous and massive sedimentary and metamorphic rocks like gneiss, quartzite are considered sound for bridge foundation.
• Weak rocks which might behave badly in presence of water include cavernous limestone, fractured sandstone especially with clayey cements, shales, clays, slate, schist and the layer of peat and compressible organic material.
• Presence of harder rock over weaker rocks, rock heterogeneity, zones of weathering etc. are not favourable sites for bridge foundation which should be treated.
• Horizontal attitude and uniformly massive structure with depth are desirable characters on the foundation rocks as these offer resistance against failure.
• Even inclined rock in a confined situation are considered safe if they possess normal strength values.
• Fracturing and highly jointing is undesirable for the foundation as they might cause settlement beyond allowable limits.
• When bridge is aligned across the strike, various types of rocks with varying strength may be encountered along the foundation, which necessitates a close examination of foundation rock under each pier and abutment.
• Fault zones are to be avoided as the foundation, as any further displacement along these planes will adversely affect the bridge.
• A bridge may be defined as a structure built over a river, a dry valley or any depressed part of land to provide a link between the two opposite sides.
• The foundation of bridge piers and abutments require a serious geological investigation.
• An ideal site for the construction of a bridge in is the one across the valley cut in a sound rock and where the river flow is free from scouring due to bends, tributaries and other causes.
• The chief factors which govern the stability of bridges are lateral forces, earthquake forces and scouring action of river.
• Pressure from bridge and running water are the main lateral forces.
• Piers should be founded at depth safe from scouring erosion.
• In many cases, the location of a bridge is decided more by socio-economic factors than geological considerations.
• In big cities divided by streams and rivers, a bridge has to be places where necessary irrespective of subsurface geology.
• But in highways there is often some flexibility available in the choice of placement of a bridge.
• In any major bridge construction project, the bridge abutment and piers should be kept on sound, strong and stable rock foundation below as possible.
• River bed are covered by varying thickness of unconsolidated natural deposits of sand, gravels and boulders not safe for foundation of bridge pier.
• The piers placed directly on them would be unstable.
• The depth of sound rock may vary from 5-20 m in some to more than 100 m in others.
• Drill holes are made all along the centre line of the proposed bridge, even on the banks sound rock sequence are reached.
• Height of individual piers may vary according to the depth of the sound bedrock below the surface as each them should be founded on the stable rock.
• The nature of the bed rock is commonly determined by the study of petrological characters and engineering properties, especially the strength values, using the core samples obtained during drilling of test bore holes.
• A decision to place the pier on a particular rock at a particular depth is matter of judgement and design requirement.
• Most igneous and massive sedimentary and metamorphic rocks like gneiss, quartzite are considered sound for bridge foundation.
• Weak rocks which might behave badly in presence of water include cavernous limestone, fractured sandstone especially with clayey cements, shales, clays, slate, schist and the layer of peat and compressible organic material.
• Presence of harder rock over weaker rocks, rock heterogeneity, zones of weathering etc. are not favourable sites for bridge foundation which should be treated.
• Horizontal attitude and uniformly massive structure with depth are desirable characters on the foundation rocks as these offer resistance against failure.
• Even inclined rock in a confined situation are considered safe if they possess normal strength values.
• Fracturing and highly jointing is undesirable for the foundation as they might cause settlement beyond allowable limits.
• When bridge is aligned across the strike, various types of rocks with varying strength may be encountered along the foundation, which necessitates a close examination of foundation rock under each pier and abutment.
• Fault zones are to be avoided as the foundation, as any further displacement along these planes will adversely affect the bridge.