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Structural Geology: Joints And Jointing

Structural Geology: Joints And Jointing
Joints are defined as divisional planes or fractures along which there has been no relative displacement.

JOINTS AND JOINTING

 

Terminology

 

    Joints are defined as divisional planes or fractures along which there has been no relative displacement.

 

    These fractures divide the rocks into parts or blocks and unlike the faults, the parts have not suffered any movement along the fracture plane.

 

     There may be or may not be an opening up of blocks perpendicular to the joint planes.

 

    Nature. Joints may be open or closed in nature.

 

Open joints are those in which the blocks have been separated or opened up  for small distances in a direction at right angles to the fracture surface.

 

     These may be gradually enlarged by weathering processes and develop into fissure in the rocks.

 

     In closed joints, there is no such separation.

 

     Even then, these joints may be capable of allowing fluids (gases and water) to pass through the rock

 

Similarly, the joints may be smooth or rough on the surface and the surface may be straight or curved in outline the joints may be small in their extension


Attitude

 

    Joints are fracture planes or surfaces and their occurrence often takes place in such a way that their position in space or attitude (dip and strike) may be described conveniently either independently or with respect to the attitude of the rocks in which they occur.

 

CLASSIFICATION

 

     Joints have been classified on the basis of spatial relationships, geometry and genesis.

 

A. Spatial Relationship

 

All joints are divided into two main groups on the basis of presence or otherwise of some regularity in their occurrence:

 

1. Systematic joints (regular joints).

 

     These show a distinct regularity in their occurrence which can be measured and mapped easily.

 

    Such joints occur in parallel or sub-parallel joint sets that are repeated in the rocks at regular intervals.

 

     The columnar joints and the mural joints described below are examples of regular or systematic jointing.

 

2. Nonsystematic (or irregular) joints.

 

     As the name implies, these joints do not possess any regularity in their occurrence and distribution.

 

They appear at random in the rocks and may have incompletely defined surfaces. In many cases these are related to the systematic joints in that these occur between them.

 

     At other times, the non- systematic joints may show no relationship with the systematic joints and their curved and rough surfaces may even cut across the former.

 

B. Geometry

 

In stratified rocks, joints are generally classified on the basis of relationship of their attitude with that of the rocks in which they occur.

 

Three types recognized on this basis are :

Strike joints in which the joint sets strike parallel to the strike of the rocks.

 

Dip joints in which the joint sets strike parallel to the dip direction of the rocks; Oblique joints are those joints where the strike of the joints is at any angle between the dip and the strike of the layers. These are also called diagonal joints when they occur midway between the dip and strike of the layers.

 

C. Genesis (Origin)

In such cases, joints are classified into one of the following genetic types:

 

Tension joints are those, which have developed due to the tensile forces acting on the rocks. The most common location of such joints in folded sequence is on the outer margins of crests and troughs. They are also produced in igneous rocks during their cooling. Joints produced in many rocks during the weathering of overlying strata and subsequent release of stresses by expansion are also thought to be due to the tensile forces (Fig. 7.31).

 

Shear joints. These are commonly observed in the vicinity of fault planes and shear zones where the relationship with shearing forces is clearly established (Fig. 7.32). In folded rocks, these are located in axial regions.

 

Compression joints. Rocks may be compressed to crushing and numerous joints may result due to the compressive forces in this case. In the core regions of folds where compressive forces are dominant, joints may be related to the compressive forces.


 


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