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Roads and highways are always very important projects for any country and an index of its development. Their planning, designing, construction and maintenance are among the major duties of civil engineers the world over. As with any other civil engineering project, geological investigations play important role in the design, stability and economical construction and maintenance of the roads.
Such investigations are aimed at providing full details regarding topography of the area, lithological characters of the rocks or soil and the groundwater conditions.
Topography as a Factor for Selection Alignment of a Road Project:
Topography or the landform of a region is single most important factor that controls the selection of alignment of a road project. Topographic maps would reveal the existence of various land features like valleys and the inflowing streams, the hills and their undulations, the plateaus and the plains with all their varying configuration from place to place. Obviously, knowledge of all such features is not only important but very essential for a right alignment.
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Moreover, such a knowledge would also be necessary to decide where cuttings would be required and in which areas it would be filling that would be necessary, or where the slopes could be left at their natural inclination and where these would have to be flattened, protected by giving breastwalls or supported by giving retaining walls and so on. Preliminary surveys, including aerial surveys followed by detailed surveys are often necessary to obtain desired topographical and other details.
Lithological Character:
Geological surveys should invariably provide all possible details regarding the composition, texture, structure and origin of rocks and sediments making the ground through which the proposed alignment of the highways has to pass.
Broadly speaking, ground may be divided into two types – consolidated, massive hard rock type and soft, unconsolidated type.
The Massive groups of rocks include all varieties of igneous, sedimentary and metamorphic rocks which can stand even with vertical slopes. For making roads through them, however, these rocks require extensive blasting operations. They cannot be simply cut out or dug out. Once cut, especially if they are free from joints and fractures and unfavourably inclined bedding planes, these rocks stand erect for year without much maintenance.
The Unconsolidated group presents the engineer many complicated problems. Thorough soil investigations regarding their mode of origin, texture, structures, porosity, permeability, degree of compaction, consolidation characteristics or compressibility, etc. all are required to be known within broad limits to design safe and stable roads over them. Residual soils are generally homogeneous and properties evaluated from selective bore whole samples might prove sufficient.
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In transported type of soils, however, variation in properties both laterally and vertically might be a rule than exception. Hence, the more complicated nature of these soils. Presence of clay seams or layers at critical places should be investigated as some types of these rocks often swell on coming in contact with moisture, and create adverse situations for road stability and safety.
Geological Structures:
The structural features of rocks, especially in those of sedimentary and metamorphic origin, have very important bearing upon the design of cuts as well as on the stability of the road as a whole. A given rock might be quite hard and otherwise sound for a cut as road foundation.
But, if in the same rock some planes of weakness (such as bedding planes, joints, foliation, cleavage) are present in such a way that these are inclined towards the free side of the valley, the rock could likely fail along these planes.
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Such structural features include:
(a) Dip and strike,
(b) Joints,
(c) Fault planes.
(a) Dip and Strike:
There may be three possibilities for making a cut in the inclined beds – it can be made parallel, at right angles or inclined to the dip direction.
The relative merits of the cut vis- a-vis its stability would be as follows, assuming other things are favourable:
(i) Cut is Parallel to the Dip Direction:
In such a case (Fig. 24.11), the layers offer a uniform behaviour on either side of the cut and as such the risk of failure is minimal on this account.
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(ii) Cut is made Parallel to the Strike:
Cut is made parallel to the strike, that is, at right angles to the dip direction. In such a case, strata plunge across the cut, offering different inclinations of the layers on either side of the cut. On the dipping inside of the cut, there is always likelihood of slips, especially when the planes are inclined steeply and get lubricated very often due to rainwater, or groundwater movement. In some cases where the layers dip into the hill rather than in the road, the cut is considered quite stable.
(iii) Cutting Inclined to Dip and Strike:
In such cases also, the strata will dip across the cutting and the slope of cutting will be unequal on both sides. Hence such a condition would give rise to similar difficulties as encountered in cuts parallel to strike.
When there is no alternative to cuts either parallel to or inclined to strike (other than at right angles), special measure might become necessary to ensure stability of slopes.
Such measures would include:
(i) Enlarging of the section of the cutting, particularly on the hillside face, to stable limits;
(ii) Provision of strong, adequately high retaining walls;
(iii) Very efficient drainage system to effectively remove water from the affected slopes.
(b) Joints:
These influence the stability of the cuts in the same way as the bedding planes. When present in great abundance, joints reduce even the hardest rock to a mass of loosely held up blocks on the side of a cut which could tumble down on slight vibrations.
Further, even if the joints are few, but are continuous and inclined towards the free side of the cut, these offer potential surfaces for slips during the presence of moisture. In major road construction programmes, therefore, jointed rocks have to be provided artificial support by breastwalls and retaining walls for ensuring stability.
(c) Faults:
Faulting generally leads to the crushing of the rock along the fault planes and shear zones. Such a condition is, of course, very unfavourable for a cut when it happens to form upper or lower slope or even base of the cut. It worst type of planes of potential failure.
Weathering:
In some cases, when the strata along or under a cut is composed of layers of rocks of different hardness, the softer layers get weathered at a faster rate than the overlying or underlying harder rocks. This generally results in undermining which might cause slips or falls of the whole face. Sometimes, when the top layers are weathered too heavily, the slope might experience a persistent rock fall or debris-fall type of situation from above.
In either case, the state of weathering of rocks is of considerable significance and cuts might need better designing when these happen to pass through weathered zones of the rocks. Cleaning of slopes from loose debris and flattering for avoiding debris fall shall be needed. Sometimes constructing concrete walls against the fracture zones may also be adopted beneficially.
Groundwater Conditions for Roads and Highways:
It is always necessary to investigate thoroughly the position of water table of the area. Not only that, water bearing qualities should also be known along the proposed route. It is quite likely that a water bearing zone (aquifer) might be intersecting the base or slopes of an alignment. Specific care and design would be required for these natural water conduits. These are always to be taken as weak and hazardous zones in the road.
It is also known that water exerts important influence on the bearing capacity of the rocks and soil. Hence when the ground is rich with moisture it would not bear the design loads, unless these properties of ground have also been determined in moist conditions. Sometimes there is a condition of free flow of ground-water through the soil. This is quite dangerous for the stability of the road surface laid above such soil.
Complicated Regions for Roads:
a. Roads in Hilly Regions:
Meandering:
Construction of roads in hilly regions is always a job full of many complications. Thus, the most important principle for road alignment based on connecting two visible points by the shortest route is the most difficult to be followed in hilly regions. In fact, the topographic and the permissible factors necessitate, more than often a meandering, zig-zag course.
Aerial Survey:
Similarly, another very important complication is the area that has to be surveyed in the specified time. This will require, obviously, use of some quicker methods of surveying. Hence aerial surveying may become necessary for successful completion of the project in specified time.
Rock Consideration:
If some solid and stratified rocks are encountered along the alignment, special investigations should be carried out to determine:
(i) Dip and strike of the bed;
(ii) Lithological composition of the rocks;
(iii) Presence and nature of faulting, jointing and permeability due to these secondary planes of weakness.
b. Roads in Marshy Regions:
Subsidence:
In some regions, the apparently strong soil might be underlain by weak, saturated or semi dry or dry and loose material like muds, peat, muck and compressible clay in good thickness. In such regions the road thickness is of considerable importance and if it transfers all or greater part of the load to the underlying weaker zones, failure due to subsidence might be the major threat to the stability of the road. It (subsidence) has to be predicted and controlled before it takes place.
Geological investigations are especially important and should reveal the presence of unstable material at critical depths. Such unstable areas include swamps, marshes, peat bogs, saturated clays and the like.
In areas where such layers or deposits are encountered their geological character and geometric dimensions should be thoroughly investigated and the cuts and road foundations designed accordingly. Of great importance to know are the nature of the soft deposits and their thickness. These could be obtained by test drill holes.
Treatment:
In all such regions, the most satisfactory method for ensuring stability is to excavate the weak soil or peat or other such material and replace it with sound material. This is, obviously, possible when the area involved is quite small and the deposit is shallow in character. In other cases, methods aimed at inducing strength by artificial means may be adopted. Drainage may also contribute effectively in stabilizing such regions.
c. Roads in Water Logged Areas:
It has been observed that roads situated in water-logged areas get deformed and damaged due to reasons connected with water-logging. A simple explanation is that due to rise of water table in such regions, the capillary moisture wets the sub grade soil to varying degrees depending upon its textures. This leads to unequal reduction in bearing capacity of the soils which manifests itself in the road deformation. The deformation becomes more serious when such roads continue to be under heavy traffic.
There are two possible ways to treat this trouble:
(i) Lower down the water-table to the safe limits;
(ii) Increase the thickness of the road crust.
d. Roads in Permafrost Regions:
In permafrost regions the ground remains permanently frozen below some depth. During the summer, thawing may set up in the top layers. When roads are constructed in such regions ignoring such facts regarding the ground, these might fail within short span of time. The reason is simple – when a road is constructed over a frozen ground, the warm fill material acts as a blanket over the intergranular ice below and thus causes its melting, which causes subsidence because the soil below becomes temporarily saturated.
Necessary precautions aimed at stopping development of such a situation are required to be taken. Vegetation, if any, should not be disturbed and a well-planned ‘insulating layer’ should be given in between the warm fill of the road surface and the ice below. Ice crystals should be allowed to remain in their original state.
Geological Problems after Road Construction:
The geological factors are to be considered prior to road construction as these greatly influence the final alignment of the road.
Two geological problems that affect roads after their construction are:
(a) Frost action and
(b) Erosion of slopes along roads.
(a) Frost Action:
In cold humid regions, road surface fails due to freezing of water within the voids of the subsoil below. The freezing starts, in cold weather, from surface downwards and extends from the larger voids to the capillary zone below. The freezing of capillary water in sub grade soil exerts tremendous pressure (140 kg/cm) on the road above and also on the particles all around and becomes the major cause of (frost) heaving up of the solid particles which destroys the surface layer.
In summer, thawing of the ice crystals results in surplus free water that escapes through the cracks in the road producing what is commonly known as the frost boils of the road.
The treatment of frost action lies in:
(i) Removing the porous soil susceptible to capillary freezing and replacing it with non-porous, uniformly mixed soil.
(ii) Lowering down of water-table by providing adequate drainage.
(b) Erosion Problems:
Side slopes of the cuts and fills and drainage ditches are most seriously affected due to erosion by flowing water.
Erosion is further enhanced if:
(i) The soil of the slopes is soft and incoherent and without a vegetable cover;
(ii) The velocity of water that rushes in the side ditch during different periods becomes exceedingly high.
The erosion of side slopes can be prevented to a considerable extent by providing interception ditches that should intercept the water in definite channels and drain it out properly without flowing over and into the body of the soil on slopes. In case of side ditches, erosion can be prevented by reducing the velocity of the water flowing through them by flattening their grades.