Geological Characters For Investigation

GEOLOGICAL CHARACTERS FOR INVESTIGATION

Geology of the Area

Preliminary geological surveys of the entire catchments area followed by detailed geological mapping of the reservoir area have to be conducted. These should reveal

main topographic features, natural drainage patterns,

general characters and structures of rock formations such as their stratification, folding and faulting and igneous intrusions, and

the trend and rate of weathering and erosion in the area.

Geology of the site Lithology.

Ø    The single most important feature that must be known thoroughly at the site and all around and below the valley up to a reasonable depth is the Lithology, i.e. types of the rocks that make the area.

Ø     Surface and subsurface studies using the conventional and latest techniques of geological and geophysical investigations are carried out.

Ø    Such studies would reveal the type, the composition and textures of the rocks exposed along the valley floor, in the walls and up to the required depth at the base.

Ø     Rocks are inherently anisotropic materials, showing variation in properties in different directions.

Ø     Complex litho logy definitely poses challenging design problems.

Structures

Ø This involves detailed mapping of planes of weakness like bedding planes, schistosity, foliation, cleavage, joints, shear zones, faults and fault zones, folding and the associated features.

Ø     While mapping these features, special attention is given to recording their attitude, spacing and nature.

Ø     Shear zones have to be searched, mapped and treated with great caution.

Ø In some cases, these may be developed to such an extent that the rock may necessitate extensive and intensive rock treatment (e.g. excavation, backfilling and grouting etc.).

Following is a brief account of the influence of more important structural features of rocks on dam foundations

Dip and Strike

Ø     The strength of sound, un fractured stratified rock is always greater when the stresses are acting normal to the bedding planes than if applied in other directions.

Ø     This being so, horizontal beds should offer best support for the weight of the dam.

Ø     But as is shown in a latter section, the resultant force is always inclined downstream.

Ø     the most UNFAVOURABLE strike direction is the one in which the beds strike parallel to the axis of the dam and the dip is downstream

Ø     It must be avoided as far as possible.

Ø     Therefore, other conditions being same, beds with upstream dips are quite favorable sites for dam foundations.

Faults

These structures can be source of danger to the dam in a number of ways. Thus,

v       The faulted rocks are generally shattered along the rupture surfaces;

v       Different types of rocks may be present on either side of a fault plane. Hence, sites with fault planes require great caution in calculating the design strength in

various sections of the dam.

v Dams founded on beds traversed by fault zones and on major fault planes are more liable to shocks during an earthquake compared to dams on non-faulted rocks.