Joints in Rock
The term joint indicates a crack or separation or fracture in a rock without displacement, Or Along which no noticeable displacement has taken place.
A joint can be open or closed and closed joints may be invisible.
We can have three sets of joints in rocks, one along the bed and the other two at right angles to it.
In Igneous rocks like basalt that have been formed by rapid cooling, the joints are closely spaced, like the columnar basalt which has regular close vertical joints. Rhyolite has closely spaced irregular joints.
The coarse-grained rocks like granites (cooled slowly) can have all three sets of joints and they are generally spaced at varying distances. Sedimentary rocks such as sandstone usually contain all three sets of joints.
Joints are important in relation to engineering works such as the following:
1. Formation of rock slides along rock slopes after heavy rainfall
2. Availability of groundwater for water supply
3. As water can easily flow through these joints, they are important also in all stages of dam construction and tunnelling.
Faults in Rock
The fault is commonly defined as a fracture in the rock along which displacement of one side with respect to the other.
In the process of faulting, the earth’s crust is broken up into individual strips.
If one of the strips goes down with respect to its original position, it is called a normal fault.
If the block rises up from its original level, it is called a reverse fault.
Faults that are approximately at the right angle the folds are called dip or cross faults.
Some faults are called active or live faults. There are many active faults near the Himalayan mountain areas.
The faults are considerably important in dam construction, bridge construction, etc.
The locality where civil engineering structures are built should be a ‘dead fault’ a fault which is not at all active at present but in historic times.
Otherwise, further movements can occur and it can considerably affect the dam or bridge structure.
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