I will answer all of it for you.
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. 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. In most cases, if there is river bed below the water is covered by varying thickness of unconsolidated natural deposits of sand, gravels and boulders. 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.
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.
The very first rock encountered below the bed cover material may not be suitable as a foundation.
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.
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.
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. Therefore the low bridge is better because;
It will cost less and be more structurally sound. The only thing is you would have to make a road down both sides of the valley but that would be more structurally efficient than trying to take the whole span of the valley on one bridge. The low bridge would also be better if there is water beneath the bridge in which is not stated.
The high bridge would cost a lot more and not be as structurally sound accordingly.
