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Rocks may be broadly divided into two categories in relation to tunneling- consolidated and unconsolidated or soft ground. Geological characters that have a direct bearing on a tunnel project will differ almost in all details in these categories.
Only a brief account is given below:
Consolidated Rocks:
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This group includes the massive igneous, sedimentary and metamorphic rocks that very often form major mountain ranges and sub-mountainous regions. Most tunnels in the mountains pass through these rocks.
Tunnel design, method of its excavation and stability are greatly influenced by following geological conditions:
1. Lithology,
2. Geological structures and
3. Groundwater conditions.
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1. Lithology:
The information regarding mineralogical composition, textures and structures of the rocks through which the proposed tunnel is to pass is of great importance in deciding:
(i) The method of tunnelling,
(ii) The strength and extent of lining and, thus
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(iii) The cost of the project.
Hard and Crystalline Rocks are the favourites with the tunnel engineers. These are excavated by using conventional rock blasting methods (RBM) and also by tunnel boring machines (TBM) of suitable strength. In the blasting method, full face or a convenient section of the face is selected for blasting up to a pre-selected depth during one shooting sequence.
Holes of proper size and angle and at proper spacing are drilled by machines. These are loaded with predetermined quantities of carefully selected explosives of known strength. The loaded or charged holes are ignited or triggered and the pre-estimated rocks get loosened as a result of the blast.
The blasting round is followed by a mucking period during which the broken rock is hauled out of the excavation so created. The excavations in hard and crystalline rocks are very often self-supporting so that these could be left unlined and next round of blasting in the new face created is undertaken, ensuring better advance rate.
Rocks falling in this group include granites, diorites, syenites, gabbros, basalts and all the related igneous rocks, sandstones, limestones, dolomites, quartzites, arkose, greywackes and the like from sedimentary group and marbles, gneisses, quartzites, phyllites and slates from the metamorphic groups.
When any one of these rocks is stressed, such as during folding or fractured as during faulting, tunnelling in these rocks proves greatly hazardous. Rock bursts which occur due to falling of big rock blocks from roofs or sides due to release of stresses or falling of rock block along fractures already existing in these rocks often cause many accidents.
Soft Rocks:
This group includes shales, friable and poorly compacted sandstones, chalk and porous varieties of limestones and dolomities, slates and phyllites with high degree of cleavage and also decomposed varieties of igneous rocks. Their excavation cost, volume for volume, might be lower than those in hard rocks.
However, in most cases, these rocks are not self-supporting. Hence, temporary and permanent lining becomes necessary that would involve extra cost and additional time. Rocks like clays, shales, argillaceous and ferruginous sandstones, gypsum bands and cavernous limestones have to be viewed specially with great caution during tunnelling.
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Many clays (and hence rocks in which clays are present), crumble due to swelling in the presence of water especially when there is a release of confining pressure from around them. Such a situation may develop when they are exposed on the sides of a tunnel, at the inverts and on the arches. In such cases sides might bulge out, inverts pop up and roofs might cave in – all very unfavourable situations. Even slides and subsidence might be caused.
Fissured Rocks form a category in themselves and include any type of hard and soft rock that has been deformed extensively due to secondary fracturing as a result of folding, faulting and metamorphic changes of shearing type. Tunnelling in such rocks is always hazardous and very challenging job for an engineer.
The extent of deformation has first to be thoroughly investigated and comprehended. A very safe (and economical) method of attack has to be planned, which would need, in almost all cases, very careful execution to avoid otherwise sure accidents. Excavation through such zones in tunnel alignments would require large-scale timbering for providing temporary support which is to be followed by permanent lining.
The fissured rocks are very often the zones where groundwater would also be encountered. Sometimes the water is under considerable hydrostatic head, sufficient enough to wash out men and machinery if not forewarned. Adequate arrangements for providing drainage, even at a short notice, would have to be made.
2. Geological Structures:
The design, stability and cost of tunnel depend not only on the type of rock but also on the structures developed in these rocks.
Following main structural features of rocks have to be fully determined along the proposed tunnel route:
i. Dip and strike,
ii. Folding,
iii. Faulting and shear zones and
iv. Joint systems.
i. Dip and Strike:
These two quantitative properties of rocks determine the attitude (disposition in space) of the rocks and hence influence the design of excavation (tunnel) to a great extent.
Three general cases may be considered:
a. Horizontal Strata:
Such a situation is rare in occurrence for longer tunnels. When encountered for small tunnels or for short lengths of long tunnels, horizontally layered rocks might be considered quite favourable. In massive rocks, that is, when individual layers are very thick, and the tunnel diameter not very large, the situation is especially favourable because the layers would then over-bridge flat excavations by acting as natural beams.
But when the layers are thin or fractured, they cannot be depended upon as beams; in such cases, either the roof has to be modified to an arch type or has to be protected by giving a lining. Sides of tunnels, however, could be left unsupported except when the rocks are precariously sheared and jointed.
b. Moderately Inclined Strata:
Such layers that are dipping at angles upto 45° may be said as moderately inclined. The tunnel axis may be running parallel to the dip direction, at right angles to the dip direction or inclined to both dip and strike directions. Each condition would offer a different set of problems.
In the first situation, that is, when the tunnel axis is parallel to the dip direction (which means it is at right angles to the strike direction), the layers offer a uniformly distributed load on the excavation. The arch action where the rocks at the roof act as natural arch transferring the load on to sides comes into maximum play.
Even relatively weaker rocks might act as self- supporting in such cases. It is a favourable condition from this aspect. However, it also implies that the axis of the tunnel has to pass through a number of rocks of the inclined sequence while going through parallel to dip (or across the strike) of strata.
In the second case, that is, when the tunnel is driven parallel to strike of the beds (which amounts to same thing as at right angles to the dip), the pressure distributed to the exposed layers is unsymmetrical along the periphery of the tunnel opening – one half would have bedding planes opening into the tunnel and hence offer potential planes and conditions for sliding into the opening. The bridge action, though present in part, is weakened due to discontinuities at the bedding planes running along the arch.
Such a situation obviously requires assessment of forces liable to act on both the sides and along the roof and might necessitate remedial measures.
In the third case, when the tunnel axis is inclined to both the dip direction and the strike direction, weak points of both the above situations would be encountered and have to be taken care of.
c. Steeply Inclined Strata:
In rock formations dipping at angles above 45°, quite complicated situations would arise when the tunnel axis is parallel to dip or parallel to strike or inclined to both dip and strike directions. In almost vertical rocks for example, when the tunnel axis is parallel to dip direction, the formations stand along the sides and on the roof of the tunnel as massive girders. An apparently favourable condition, of course, provided all the formations are inherently sound and strong when considered individually also.
Conversely, in tunnels running parallel to strike of vertical beds, it is more than likely that a number of bedding planes (which are planes of weakness) are intersected at the roof and along the arch so that natural beam action or arch action gets considerably weakened.
ii. Folding:
Folds signify bends and curvatures and a lot of strain energy stored in the rocks.
Their influence on design and construction of tunnels is important from at least three angles:
Firstly, folding of rocks introduces considerable variation and uncertainty in a sequence of rocks so that entirely unexpected rocks might be encountered along any given direction. This situation becomes especially serious when folding is not recognized properly in preliminary or detailed surveys due either to its being localized or to misinterpretation.
Secondly, folding of rocks introduces peculiar rock pressures. In anticlinal fold, loads of rocks at the crest are transferred by arch action to a great extent on to the limbs which may be highly strained. These conditions are reversed when the folds are of synclinal types. In such cases, rocks of core regions are greatly strained. Again, the axial regions of folds, anticlinal or synclinal, having suffered the maximum bending are more often heavily fractured.
The alignment of a tunnel passing through a folded region has to take these aspects in full consideration. When excavations are made in folded rocks, the strain energy is likely to be released immediately, soon after or quite late to tunnelling operations, very often causing the dreaded rock bursts. Very slow release of small amounts of strain energy might cause bulging of walls or caving in of roofs or popping up of floors.
Thirdly, folded rocks are often best storehouses for artesian water and also ideal as aquifers. When encountered during tunnelling unexpectedly, these could create uncontrollable situations. The shattered axial regions being full of secondary joint systems are highly permeable. As such very effective drainage measures are often required to be in readiness when excavations are to pass through folded zones.
iii. Faulting:
Faults, are surfaces along which rock movement has occurred in the past; these are also potential surfaces for future movements of the rocks. This definition clearly brings about significance of intersection of fault planes, fault zones and shear zones with the tunnel axis. Not only that, faults may bring rocks of entirely different nature to come to lie in contact with each other.
Similarly, fault zones and shear zones are highly permeable zones, likely to form easy avenues for ground water passage. Inclined fault planes and shear zones over the roof and along the sides introduce additional complications in computation of rock pressure on the one hand and of rock strengths on the other.
This discussion leads to a general conclusion – wherever tunnel is intersected by fault planes or shear zones, it is to be considered as passing through most unsafe situations and hence designed accordingly by providing maximum support and drainage facilities.
iv. Joint Systems:
Joints are cracks or fractures developed in rocks due to a variety of causes. Although all types of joints tend to close with depth (due to load of overburden), their presence and orientation has to be investigated. Joints are planes of weakness and must always be suspected when the rocks are folded and faulted. Even originally closed joints may become reactive and open up in the immediate vicinity of tunnel excavation. Jointed rocks cannot be considered as self-supporting although these might belong to massive category.
In many cases problems created by jointing in such rocks can be rectified by grouting. In other acute cases, lining of the tunnel in the fractured zones might have to be applied.
3. Ground Water Conditions:
Determination of groundwater conditions in the region of tunnel project is not to be underestimated at any cost. In fact groundwater level vis-ά-vis tunnel axis is a major factor governing computations of overhead loads on tunnels and also in the choice of method of tunnelling.
Groundwater conditions effect the tunnel rocks in two ways:
Firstly, through its physico-chemical action, it erodes and corrodes (dissolves) the susceptible constituents from among the rocks and thereby alters their original properties constantly with the passage of time. It might have already done much of this type of job when the tunnel is excavated through such water-rich rocks.
Secondly, it effects the rock strength parameters by its static and dynamic water heads. Such an action may become highly pronounced when an artesian acquifer is actually intercepted by tunnel excavation. A sudden release of pressure in the direction of excavation could create worst disaster for the tunnelling menfolk.
There are three general possibilities of relationship between tunnel axis and groundwater level:
(a) The tunnel axis may be passing entirely through impervious formations in which there is no possibility of water seepage or leakage or movement. It is an ideal condition for tunnelling of course, but is very rare in nature. Good lengths in short tunnels or small length of long tunnels might show such a relationship.
(b) The tunnel axis might be located mostly above the water table, intercepting the aquifer only in some sections. This is one of the most common situations and would involve provision for special drainage facilities to be located in water-bearing zones of the section. The head of the water in the zone of interception has also to be given due consideration and might necessitate lining for stopping leakage or inrush of water.
(c) The tunnel axis might be located below the water table. Such a situation should be avoided as far as possible. In some specific cases, however, this might be the only possibility, such as in soft ground tunnelling, under water tunnelling e.g. below rivers and lakes or in the karst regions.
Wherever tunnels enter the saturated zones, effective drainage systems and also support systems have to be planned much in advance and executed with great precision and perfection. Water is likely to enter the excavation with a force proportional to full hydrostatic head of the water body. Waterproof lining is to be provided for the full length.
It may, therefore, be summed up that hydrogeological investigations have to be made with fullest concern and caution all along the proposed tunnel alignment.