CAUSES OF FOLDING
The Tectonic Folding may be due to any one or more of the following mechanisms:
Folding Due to Tangential Compression
Lateral Compression is believed to be the main cause for throwing the rocks of the crust into different types of folds depending upon the types of rocks involved in the process and also the direction and magnitude of the compression effecting those rocks.
In general, this primary force is believed to act at right angles to the trend of folds. under the influence of the tangential stresses, folding may develop in either of the three ways: flexural folding, flowage folding and shear folding.
It is that process of folding in which the competent or stronger rocks are thrown into folds due to their sliding against each other under the influence of lateral compression.
This is also distinguished as flexural-slip-folding in which the slip o r movement of the strata involved takes place parallel to the bedding planes of the layers.
It has been establis hed that in flexural folding, the amount of slip (and hence the ultimate type of fold) depends on a number of factors such as:
thickness o f the layers and nature of the contact; thick er the layers, greater is the slip; further, cohesionless contacts favour easy and greater slips;
distance fr om the hinge point; greater the distance from the hinge points, larger is the
displacement, so much so that it may be negligible at the hin ge point;
type of the rocks involved; siltstones, sandstones and li mestones are more prone t o flexure slip folding compared to soft clays and shales.
Ø It is t he principal process of folding in incompetent or weaker, plasti c type of rocks such as clays, shales, gypsum an d rock salt etc.
Ø Duri ng the compression, the material of the involved layers behaves almost as a viscous or plastic mass and gets buckled up and d eformed at varying rates suffering unequal disto rtion.
Ø In such cases the thickness of the resulting fold does not remain unifor m.
Ø In many cases, folding is attributed to shearing stresses rather than simple compression.
Ø It is assumed that in such a process, numerous closely spaced fractures develop in the rock at the first stage of the process.
Ø This is followed by displacement of the blocks so developed by different amounts so that ultimately the rocks take up folded or bent configuration.
Ø The folded outline becomes more conspicuous when the minor fractures get sealed up due to subsequent recrystallisation.
Folding Due to lnsrusions
Ø Intrusion of magma or even rock salt bodies from beneath has been found to be the cause of uparching of the overlying strata.
Ø In magmatic intrusions, highly viscous magma may be forced up very gradually and with considerable force so that the overlying sedimentary host rocks are bodily lifted up to provide space for the rising magma.
Ø In extreme cases, the magma may even rupture the overlying strata to flow out as lava
Folding Due to Differential Compression
Ø Strata that are being compacted under load in a basin of sedimentation develop, with passage of time, downward bending especially in the zones of maximum loading.
Ø If the strata in question is not homogeneous, the bending may not be uniform in character and results in warping or folding of different types.
Ø Such folds are, however, totally dependent on the load from above and are attributed to superficial causes.
Ø These are, therefore, non- tectonic folds.
Ø Folds developed in the areas of work are important for a civil engineer in that these make his work more complicated.
Ø If these structures are not thoroughly investigated and properly interpreted, any civil engineering project standing on or driven through the folded rocks may prove not only uneconomical in the ultimate analysis but also, unsafe as well.
Ø Due consideration is, therefore, always to be given to the presence of folds in deciding about the designing and construction of such structures as driving of traffic and hydropower tunnels, selection of sites for dams and reservoirs and in fixing the alignments of roads, bridges and highways.
Change in Attitude
1 Folding of any type would cause a change in the attitude (dip and strike) of the same strata in the aerial extent and also in depth.
2 Hence same layers may be repeated along an alignment or one or more different layers may be unexpectedly encountered.
3 If it happens so and the unexpectedly repeated or encountered layers are of undesirable nature, the project costs may be effected as also the time schedule and safety of the project.
Shattering of Rocks.
1 The stresses are often strong enough to break or shatter the rocks, especially in the axial zones, which are the places of maximum concentration of these forces.
2 hence, in folded rocks, axial regions are likely to be the areas containing fractured zones.
3 This effect is of utmost importance because shattered rocks become:
• weak in strength parameters of all types;
• porous and pervious in character;
Axial regions in the folded rocks should be thoroughly studied and if possible, should be avoided for other better alignments or sites as the case may be.
If it is not possible to avoid them, these areas must be subjected to suitable processes of rock treatment for developing in them desired qualities of strength and imperviousness.
1 All the stresses that have acted on the rocks during their folding are generally absorbed by these rocks by undergoing strain.
2 In essence, the folded rocks are considerably strained, the magnitude of strain varying from point to point in the folded sequence.
3 Now, as and when there is an effort by nature or by the engineer to disturb this adjustment of the rocks to the stresses, the rock may respond by release of some strain energy.
4 Enough stored strain energy is released as soon as (or soon after) the excavations are made and huge blocks of rocks start caving in or falling with great force called the rock bursts.
5 This often involves fatal accidents besides causing considerable delay in the progress of the work.
6 A proper planning of the work in folded areas is, therefore, of utmost importance to avoid these possible hazards in construction work.