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A bridge may be defined as a structure built over a river, a dry valley, low land or an estuary or any depressed part of the land to provide a link between the two opposite sides. It is essentially a communication link on a road or railway track or a highway. Bridges] especially over major rivers and in hilly and mountainous areas are very important civil engineering structures. Their role in socio-economic development and defence strategies can hardly be overemphasized.
In most cases the location of a bridge is decided more by socio-economic factors than by geological considerations. Thus, there are seven bridges over the River Jhelum connecting the two parts of Srinagar city (in Kashmir) within a total distance of 5 km. On the contrary, there is only one bridge over the River Chenab (at Ramban, Jammu Province) connecting the valley of Kashmir with rest of the country.
That may be true for most of the other big cities, states and countries of the world. In other words, within big cities divided by rivers or streams, a bridge has to be placed where it is needed, irrespective of the subsurface geology. However, on highways, there is often some flexibility available in the choice of placement of a bridge.
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This is unlike tunnels, where alignment is primarily and essentially controlled by geological considerations. But, in the case of bridges also, the design, stability and durability depend, to a great extent, on the subsurface geological conditions that must be properly investigated and cautiously interpreted.
In any major bridge construction project, the designer is keen to place the bridge abutments and piers on as sound, strong and stable rock foundation below as possible.
This being so, the geological characters that need to investigate and thoroughly established are:
(a) The depth to the bed rock;
(b) The nature of the bed rock;
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(c) The structural disposition of rocks.
(a) Depth to Bed Rock:
In most cases, the river bed below the water is covered by varying thickness of unconsolidated natural deposits of sand, gravels and boulders.
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Such loose materials are not safe as foundations for bridge piers for at least two reasons:
Firstly, piers placed directly on them would be unstable;
Secondly, the cover material is liable to be removed due to scouring by river water.
As such, the pier must be placed on stable foundation, preferably of rock, under a suitable thickness of cover material so that it is safe from scour by river water.
The height of pier from under the span to the foundation level, therefore, depends on the ‘depth of the bed rock’ below the river water.
Such sound bed rocks might be available within a depth varying from 5 to 20 meters below a river bed or they might not at all be available even up to 100 meter or more. All that depends on the local geology which has to be investigated and understood.
To achieve this, drill holes are made all along the centre line of the proposed bridge, even on the right or left of it, till they reach the sound rock sequence or up to a reasonable depth. Utmost care is needed not to mistake isolated big boulders buried underneath the river bed as the bed rock. Boulders are rocks but they are not bed rocks and cannot be trusted as foundations for bridge piers.
(b) Nature of Bed Rock:
The very first rock encountered below the bed cover material may not be suitable as a foundation.
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It should be kept in mind that three types of loads are to be borne by a bridge pier foundation:
i. The compressive, vertical loads due to the weight of the bridge span and that of pier material;
ii. The horizontal loads due to the thrust of the water flowing above as transmitted directly and through the pier;
iii. The dynamic, complex load, often inclined and shearing in character, due to heavy traffic on the bridge.
Consequently, the bed rock selected as foundation for the pier must be strong enough to bear the sum total of all these loads, not temporarily, but throughout the proposed life of the bridge.
The nature of the bed rock is commonly determined through study of petrological characters and engineering properties, especially the strength values, using the core samples obtained during drilling of test bore holes. In fact complete and very useful geological profiles could be prepared all along the centre line of the proposed bridge from the study of such core logs.
These (profiles) would depict complete sequence (and even structural disposition) of the rock formations existing below the surface material up to a desired depth. A decision to place the pier on a particular rock at a particular depth is then matter of judgement and design requirements.
Most igneous and massive type of sedimentary and metamorphic rocks is quite strong, stable and durable as foundations for bridge piers and abutments. The group of weak rocks which might behave badly in the presence of water includes such types as cavernous limestones, chalk, friable sandstones especially with clayey cements, shales, clays, slates, schists and the layers of peat and compressible organic material. Many of them are amenable to treatment by artificial methods.
(c) Structural Disposition:
Ideally, the horizontal attitude and uniformly massive structure with depth are desirable characters in the foundation rocks as these offer inherent resistance against failure. However, even inclined rocks in a confined situation under the bridge piers are considered quite safe if these possess normal strength values.
Folding and faulting might cause some uncertainty in establishing a perfect geological profile but are not otherwise negative factors. Acute fracturing and profuse jointing is, however, undesirable at the foundation levels as these might cause settlement beyond the allowable limits.
When the bridge sites are located in the zones of seismic activity, the foundations are required to be designed for additional seismic loads as specified in the codes of respective areas.
In the glaciated areas, special care must be taken to establish the existence of drowned or buried valleys that might be filled by secondary material of most heterogeneous characters. In such cases a bed rock may be encountered only at great depth and it may be desirable to reach it through piles. In fact, occurrence of drowned valleys is considered one of the major complications in bridge foundations that limit the options of a design engineer.
Similarly, the factor of scour must never the overlooked. Riverbed materials and rocks under them at shallow depths are liable to removal by scouring. The scour itself is a function of river velocity and direction of the currents on the one hand and nature and degree of consolidation of the rocks on the other hand.