Joints are fractures in rocks that divide the rock mass into blocks. There are three main types of joints based on their orientation: strike joints parallel to bedding, dip joints parallel to slope, and oblique joints with other orientations. Joints are classified based on their pattern, geometry, and origin. Unconformities represent gaps or erosional surfaces in the geological record where rock layers are missing. The main types are angular, disconformity, and nonconformity. Both joints and unconformities are important in engineering projects as they can impact stability, water flow, and excavation difficulties.
2. • What are joints?
• Terminology related to joints
• Classification
• Engineering considerations
3. 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)
4. 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.
• 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.
• 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.
6. 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
7. • 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.
8. 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
9. Engineering considerations
DISADVANTAGES
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
10. ADVANTAGES
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.
b.
The incompetence, leakage and other effects introduced by joints in rocks may create
foundation problems at dam sites.
c.
Joints may pose ground water problems in tunnelling.
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.
d.
e.
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.
11. What are unconformity?
Origin of unconformity
Classification
Engineering considerations
12. 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.
13. 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 .
14. 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
15. 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.
Nonconformity
16. 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 .