Joints in Rocks: Origin and Classification | Geology

In this article we will discuss about:- 1. Origin of Joints 2. Classification of Joints 3. Occurrence of Joints 4. Engineering Considerations for Joints in Rocks.

Origin of Joints:

Joints are caused in different rocks due to different reasons. No single theory can, explain origin of all types of joints. At present it is agreed that there are at least three principal processes due to which joints may be caused in different rocks.

These are outlined as follows:

(a) Contraction during Formation:

Sedimentary rocks especially those of plastic nature and rich in moisture in the initial stages (clays, shales, limestones and dolomites etc.) undergo some contraction on drying up which might have resulted into irregular jointing.

Similarly, igneous rocks which form by cooling and crystallisation from an originally hot and molten material (magma or lava) also necessarily undergo considerable contraction during the cooling process giving rise to tensile forces strong enough to break the congealing masses into jointed blocks. Such contraction or shrinkage is generally accepted to be the cause of the vertical type of joints in granites and the so well-known columnar joints of basalts and other effusive rocks.

(b) Expansion and Contraction:

Rocks, like many other solids, expand with rise in temperature and contract on cooling. Such repeated expansion and contraction is characteristic of regions with dry hot (arid) climates where day and night temperatures on the one hand and summer and winter temperatures on the other hand vary within a very wide range – 50° to 60° C. The outer parts of rocks exposed to direct sun heat may develop cracks and fissures due to such repeated expansion and contraction.

Removal of overburden due to weathering or other processes of rock wasting may also cause expansion of the underlying rocks due to unloading. The previously loaded rocks get relaxed with the release of the forces. These stresses are compressional – tensional in nature. Joints may appear parallel to the surface of erosion in such exposed rocks. The sheet joints of sedimentary and other rocks are attributed by many to the process of erosional unloading through geological ages.

(c) Crustal Disturbances:

Many joint types, especially those associated with folded and faulted rocks are clearly related to the processes of crustal disturbances that are responsible for building of mountains and continents. These processes are easily capable of exerting sustained and strong forces on rocks that virtually cut them into slices along certain directions irrespective of the composition and strength of the rock components.

Sudden seismic shocks have also been suggested by some as a possible cause for the development of joints in many rocks.

Classification of Joints:

Joints have been classified on the basis of:

A. Spatial relationships,

B. Geometry and

C. 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. Non-Systematic (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 (Fig. 7.28):

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

2. Dip joints in which the joint sets strike parallel to the dip direction of the rocks;

3. 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.

In stratified rocks, some joints may develop essentially parallel to the bedding planes. These are simply referred as bedding joints.

In the folded regions, joint orientation is conveniently described with reference to the hinge of the fold. A line running parallel to the hinge-line is assumed as b-axis; the a-axis is normal to it and the c-axis is normal to the plane containing the a and b axis.

The joints running parallel to b-axis are called radial joints. These cut the layers almost perpendicularly. Similarly, joints running parallel to the layers are designated as C-joints. In these areas, joints can also be distinguished into dip joints, strike joints and diagonal joints. (Fig. 7.29).

In igneous and metamorphic rocks, the joints may be classified on the basis of their geometric relations with planar structures of those rocks such as lineation or cleavage etc.

Two terms are commonly used in such cases:

Cross or Q joints, which are joints traversing the linear structures at right angles.

Longitudinal or S joints, are joints traversing parallel to the linear structure. In these rocks all the joint systems traversing at any other angular inclination with the linear structures are described as diagonal joints (Fig. 7.30).

C. Genesis (Origin):

Joints are very common and at the same time very complex structures in rocks. As regards their origin it is often very difficult to attribute a particular type or group or system of joints to an exact cause of origin. Only in a few cases, the predominant force (compression or tension or shear) that has been responsible for the development of joints in the rocks can be established easily.

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

1. Tension Joints:

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 is also thought to be due to the tensile forces (Fig. 7.31).

2. 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.

3. 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.

Occurrence of Joints:

Joints are perhaps the most common structural features of all types of rocks occurring everywhere in the world. It is seldom that we find any big rock mass on the surface free from joints.

Rocks of all the three main classes:

(a) Igneous,

(b) Sedimentary and

(c) Metamorphic, show joints of various types.

(a) Igneous Rocks:

The igneous rocks are formed by cooling and crystallization of hot molten material called magma or lava. As such, in most cases they show joint systems related to the tensile stresses developing during the process of cooling and crystallization.

The three regular or systematic types of joints observed in igneous rocks are:

1. Sheet joints,

2. Mural joints, and

3. Columnar joints.

1. Sheet Jointing:

In granites and other related igneous rocks, a horizontal set of joints often divides the rock mass in such a way as to give it an appearance of a layered sedimentary structure, called in this case as a Sheeting Structure.

Sheet joints are sometimes caused due to weathering and removal of overlying rock masses, which cause expansion of the underlying igneous and other rocks as a consequence of unloading.

2. Mural Jointing:

In granitic and other rock masses, there may occur three sets of joints in such a way that one set is horizontal and the other two sets are vertical, all the three sets being mutually at right angles to each other. This sort of geometrical distribution of joints dividing the rock mass into cubical blocks or murals is called mural jointing.

3. Columnar Jointing:

These types of joints are typical of volcanic igneous rocks although they may also be observed in other rocks.

These are also called prismatic joints. The joints divide the rock mass into polygonal blocks, each block being bounded by three to eight sides. Five and six sided blocks are common. Normally, the main joints are vertical or perpendicular to the cooling surface and may extend to varying depths ranging from a few centimeters to many meters. In surface area also, the blocks may vary from a few centimeters to a couple of meters.

The polygonal cracks are thought to be directly related to the tensile forces developed during cooling (accompanied by contraction) of the hot molten material (lava of magma).

Their formation is explained somewhat as follows:

(a) In a homogeneous mass undergoing uniform cooling throughout the surface, contraction is equally developed in all directions.

(b) Centres of contraction are developed at equally spaced intervals and the lines joining these centres are the directions of tensile stresses.

(c) When the strength of the rock is overcome, fractures appear at right angles to these lines of tensile stresses.

The rock mass is thereupon divided into polygonal cracks. When the cooling is not exactly uniform throughout, the blocks formed may be irregular in outline. At certain depths, the continuity of the fractures is broken by disc shaped joints called cup and ball joints.

(b) Sedimentary Rocks:

Most sedimentary rocks are generally profusely jointed. Joints may be of systematic and non-systematic classes. These joints may be closely and regularly spaced sets, parallel or sub-parallel to each other and bearing varying relationships with the attitude of the rocks. Since sedimentary rocks are often folded and faulted, the joints in them are genetically related to those forces that have caused the major structural deformations.

In deeply stratified rocks, removal of overlying strata due to weathering gives rise to compression-tension forces that may cause regular and irregular jointing of various types. In some sedimentary rocks systems of joints may develop parallel to the bedding planes-the bedding joints. In many cases these may be confused with stratification.

(c) Metamorphic Rocks:

These rock types are heavily jointed in many cases, the joints being of irregular or non-systematic types. These joints are often the result of local and regional stresses acting on rocks as a source of metamorphism. In many cases, the metamorphic rocks may show those joints which were pre­existing at the time of metamorphism of the rock with little or no modification.

Engineering Considerations for Joints in Rocks:

Joints affect the properties of rocks both in a negative as well as positive manner with respect to the activities of a professional civil and mining engineer. Negatively speaking, joints influence many engineering operations. The selection of sites for dams and reservoirs and alignments for highways and tunnels through rocks will require very thorough investigations of joints for arriving at safe and economic designs.

Joints are always to be considered as a source of weakness of the rocks and as pathways for the leakage of water through the rock. Both these properties of joints destroy the inherent soundness of the rock to a great extent.

If a rock forming the foundation of a dam or reservoir happens to be heavily jointed and the region is one of low water table, the risk of leakage of water from under the dam or from the reservoir may be of substantial magnitude demanding very heavy cost for treatment of the rocks.

Similarly, if the roof or side rocks in the case of the tunnel are much fractured, slippage of rocks along these fractures and leakage of water may cause many troubles, often insurmountable by ordinary methods of treatment. Lining of tunnels may remain only solution in such cases and this will involve huge extra cost.

Joints are a major cause of instability of the rock masses in the hilly regions. Jointed rocks get easily lubricated in the presence of moisture and start sliding or falling from the original places of occurrence. Many landslides and slope failures are directly related to the jointed nature of the rocks.

Treatment of Joints:

Treating the negative qualities of rocks due to joints will differ in different projects. The first requirement in all cases is, however, detailed investigations to establish full characteristics of the joints in terms of their type, frequency, intensity, pattern of distribution and the extent to which they have influenced the rock.

This may form the single most important work of site investigation in some cases. Great care has to be exercised in locating the presence, distribution patterns and magnitude of micro joints that are typical of many rocks. Such joints, if left unnoticed and untreated, may widen after the construction of the project and endanger its stability.

Treatment of joints may involve grouting with a suitable grout material for increasing the strength of the rocks or for reducing their permeability or for achieving both these objectives.

As regards the positive effects of joints in rocks, these are greatly sought after in the exploration for groundwater and oil reserves in a given area. Only a well-jointed and porous rock can form a good aquifer or a good oil and gas reservoir. Similarly, mineralisation with economically valuable minerals from hydrothermal solutions takes place in jointed rocks and fissures, which are formed due to widening of joints.

Hence joints are a desirable character from the point of a mining engineer, water resource engineer and oil-prospecting engineer. In quarrying of stones for whatsoever purpose, widely spaced joints in the rocks make a desirable quality for the workforce.