Joints and unconformity

By

Rahul sinha

• What are joints?

• Terminology related to joints

• Classification

• Engineering considerations

What are joints? • Joints may be defined

as the fractures that divide the rocks into parts or blocks which have not been suffered any relative motion along that fracture.

• Joint set: it may be defined as group of or more joint surfaces trending in the same direction with almost the same dip.

• Joint system: A joint system is a group of two or more joint sets. (May have two or three intersecting sets)

Terminology related to joints • Nature: joints may be open or closed.

• Open joints: open joints are the joints in

which the blocks have been separated for

the small distance to right angles to the

fracture surface.

• Closed joints: closed joints are the joints in

which the blocks are not separated from

each other but allow the fluids(water and

gases) to pass through the rocks.

• Veins: generally open joints gets filled up

by secondary materials which crystallize or

precipitate there forming thin or thick

steaks of material. These steaks are called

veins when thin and fissure veins when

there thickness is greater than 20 cm.

Classification • Joints are classified on the basis of

I. Spatial relationship

II. Geometry

III. Genesis(origin)

On the basis of spatial relation

• Systematic joints: joints that shows

distinct regularity in the occurrences

which can be easily measured or

mapped. Such joints occur in parallel

joint set that are repeated in the rocks at

the regular intervals.

• Non-systematic joints: the joints that

don’t posses any regularity in their

occurrence and distribution.

On the basis of geometry There are three types of joints on the basis

of geometry:

1. Strike joints

2. Dip joints

3. Oblique joints

• Strike joints: the joints

which are parallel to the

strike of the rock.

• Dip joint: the joints which

are parallel to the dip of

the rock.

• Oblique joint: joints which

are neither parallel to the

strike nor to the dip of the

layer in which they occur.

• In stratified rocks some

joints may develop

essentially parallel to the

bedding planes. These are

called bedding joints.

On the basis of origin of joints

1. Tension joints: joints which are developed due to the tensile force acting on the rocks. The ,most common location of such joints are the outer margins of crests and troughs in the folding.

2. Shear joints: these joints are commonly observed in the vicinity fault planes and shear zones where shearing stress prevails. In folds they occur in axial regions.

3. Compression joints: rocks may be compressed too crushing and numerous joints may result due to compressive forces. These occur in the compressive regions of the folds like on in innermost margin of the axis of folds.

Engineering considerations DISAVANTAGES

a. joints are important because they split the

rocks into a number of pieces which, in turn,

reduce the competence of rock mass,

increase the porosity and permeability and

make them (rocks) susceptible to quick

decay and Weathering.

b. Joints become avenues for the leakage of water in case of reservoirs. If they are closely spaced in the upstream side, silting problems also arise in reservoirs.

c. The incompetence, leakage and other effects introduced by joints in rocks may create foundation problems at dam sites.

d. Joints may pose ground water problems in tunnelling.

e. Depending on the relation of the dip of joints in rocks with reference to the surface slope, they may pose problems in laying roads and railways along hill slopes.

ADVANTAGES 1. Increase the ground water potentiality in any

place.

2. Suitable spaced joints (i.e., with neither very

close nor very wide joint intervals) not only

facilitate the quarrying process or tunnelling

process but also reduce the cost by

decreasing the use of explosives.

• What are unconformity?

• Origin of unconformity

• Classification

• Engineering considerations

What is unconformity? • An unconformity may be defined as surface

of the erosion on non-deposition occurring

within the sequence of rocks. An

unconformity is developed due to the change

in the process of deposition of sedimentary

rocks.

Origin of unconformity

•If the process of deposition remain

uninterrupted for considerable time then

layer will be deposited in order. Older beds

occupy the position at the base of the

sequence and younger beds are on the top.

The sequence of deposition is called

conformable.

•But if in any case the deposition is

interrupted at a certain stage and exposed

to erosion, then top layers will be eroded.

This is followed by the phase of deposition of

new beds over eroded surface. The

sequence so formed is not conformable.

Types of unconformities? • Angular unconformity: it is characterized by

different inclinations and structural features

above and below the surface unconformity. The

sequence below the unconformity may be steep,

faulted or folded and sequence above may be

horizontal or inclined.

• Disconformity: it is the type of unconformity in

which the beds below and above the surface of

erosion are almost parallel.

Angular unconformity and disconformity

Nonconformity

Non-conformity: it is the term used for

unconformity in the sequence of the rocks

composed of plutonic igneous or

metamorphic rocks as older and sedimentary

rocks as younger or newer.

Engineering consideration

• Unconformity indicates the

discontinuity in the sequence of the

rocks. The behavior of the rocks

above and below the unconformity

shows the variation in their

mechanical properties and hence

affect the stability of the project.

• Unconformity marks the a weak

contact which can allow percolation

of water and can also act as fault

plane towards forces imposed from

above.

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