2. GENJSRAL

2.1 In areas which have already been developed, advantage should be taken of existing local knowledge, records of trial pits, bore holes, etc, in the vicinity, and the behaviour of existing structures, particularly those of a nature similar to that of the proposed structure. In such cases, exploration may be limited to checking that the expected soil conditions are those as in the neighbourhood.
2.2 If the existing information is not sufficient or is inconclusive the site should be explored in detail so as to obtain a knowledge of the type, uniformity, consistence, thickness, sequence and dip of the strata and of the ground water conditions.
2.2.1 Site Reconnaissance - Site reconnaissance would help in deciding future programme of field investigatiofis? that is, to assess the need for preliminary or detailed investigations. This would also help in determining scope of work, methods of exploration to be adopted, field tests to be carried out and administrative arrangements required for the investigation. Where detailed published information on the geotechnical conditions is not available, an inspection of site and study of topographical features are helpful in getting information about soil, rock and ground-water conditions. Site reconnaissance includes a study of local topography, excavations, ravines, quarries, escarpments; evidence of erosion or landslides, behaviour of existing structures at or near the site; water level in streams, water courses and wells; flood marks; nature of vegetation; drainage pattern, location of seeps, springs and swamps. Information on some of these may be obtained from topographical maps, geological maps, pedological and soil survey maps, and aerial photographs.
2.2.1.1 Data regarding removal of overburden by excavation, erosion or land slides should be obtained. This gives an idea of the amount of pm-consolidation the soil strata has undergone. Similarly, data regarding recent f?lls is also important to study the consolidation characteristics of the fill as well as the original strata.
2.213 The type of flora affords at times some indication of the nature of the soil. The extent of swamp and superficial deposits and peats will usually be obvious. In general, such indications, while worth noting, require to be conflrmed by actual exploration. d will depend upon the permeability ‘of the strata and the head causing the water to flow. The water level in streams and water courses, if any, in the neighbourhood, should be noted, but it may be misleading to take this as an indication of the depth of the water table in the ground. Wells at the site or in the vicinity give useful indications of the ground-water conditions. Flood marks of rivers may indicate former highest water levels. Tidal Auctuations may be of importance. There is also a possibility of there being several water tables at ditferent levels, separated by impermeable strata, and some of this water may be subject to artesian head.
2.2.2 Enquiries Regarding Eatlfer Use of the Site - In certain cases the earlier uses of the site may have a very important bearing on proposed new works. This is particularly so in areas.where there have been underground workings, such as worked-out ballast pits, quarries, old brick fields, coal mines and mineral workings. Enquiries should be made regarding the location of shafts and workings, particularly shallow ones, where there may be danger of collapse, if heavy new structures are superimposed.
2.2.2.1 The possibility of damage to sewers, conduits and drainage systems by subsidence should also be investigated.
2.2.3 Geophysical investigations of the site may be conducted at the reconnaissance stage since it provides a simple and quick means of getting useful information about stratifications. Depending on these information, detailed subsoil exploration should -be planned. Important geophysical methods available for subsoil exploration are:
a) electrical resistivity method, and
b) seismic method.
2.2.3.1 Electrical resistivity methdd - The electrical resistivity method, in which the resistance to the flow of an electric current through the subsurface materiaIs is measured at intervals of the ground surface, may be useful for the study of foundation problems and particularly for finding rock strata under deep soil cover.
2.2.3.2 Seismic method - The seismic method makes use pf the variation of elastic properties of the strata which affect the .velocity of shock waves travelhng through them, thus providing a usable tool for dynamic elastic moduli determinations in addition to the mapping of the subsurf’ horizons. The required shock waves can be generated by hammer blows on the’ ground or by detonating a small charge of explosives. This method is quite useful in delineating the bedrock configuration and the geological structures in the subsurface.

2.3 Outline of Procedure

2.3.1 Number and Disposition of Trial Pits and Borings - The disposttion and spacing of the trial pits and borings should be such. as to reveal any major changes in thickness, depth or properties of the strata over the base area of the structure and its immediate surroundings. The number and spacing of bore holes or trial pits will depend upon the extent of the site and the nature of structures coming on it. For a compact building site covering an area of about 0.4 hectare, one bore hole or trial pit in each corner and one in the centre should be adequate. For smaller and less important buildings even one bore hole or trial pit in the centre will suffice. For very large areas covering industrial and residential colonies, the geological nature of the terrain will help in deciding the number of bore holes or trial pits. Cone penetration tests may be performed at every 50 m by dividing the area in a grid pattern and number of bore holes or trial pits decided by examining the variation in the penetration curves. The cone penetration tests may not be possible at sites having gravelly or boulderous strata. In such cases geophysical methods may be useful.
2.3.2 Depth of Exploration - The depth of exploration required depends on the type of proposed structure, its total weight, the size, shape and disposition of the loaded areas, soil profile, and the physical properties of the soil that constitutes each individual stratum. Normally, it should be one and a half times the width of the footing below foundation level. In certain cases, it may be necessary to take at least one bore hole or cone test or both to twice the width of the foundation. If a number of loaded areas are in close proximity the effect of each is additive. In such cases, the whole of the area may be considered as loaded and exploration should be carried out up to one and a half times the lower dimension. In weak soils, the exploration should be continued to a depth at which the loads can be carried by .the stratum in question without undesirable qettlement and shear failure. In any case, the depth to which seasonal variations affect the soil should be regarded as the minimum depth for the exploration of sites. But where industrial processes affect the soil characteristics this depth may be more. The presence of fast growing and water seeking trees also contributes to the weathering processes. NOTE - Examples of fast growing and water seeking trees are Banyan ( Fkus ben&nsis ), Pipal ( Ficus religiosa ) and Necm ( Azadirachta indica ).
2.3.2.1 An estimate of the variation with depth of the vertical normal stress in the soil arising from foundation loads may be made on the basis of elastic theory. The net loading intensity at any level below a foundation may be obtained approximately by assuming a spread of load of two vertical to one horizontal from all sides of the foundations, due allowance being made for the overlapping effects of load from closely spaced footings. The depth of exploration at the start of the work may be decided as given in Table 1, which may be modified as exploration-proceeds, if required.

2.4 Importance of Ground-Water Tables

2.4.1 For most types of construction, water-logged ground is undesirable because of its low bearing capacity. On sites liable to be water-logged in wet weather, it is desirable to determine the fluctuation of the water table in order to ascertain the directions of the natural drainage, and to obtai.n a clue to the design of intercepting drains to prevent the influx of ground water on to the site from higher ground. The seasonal variation in the . level of water table should also be noted.
2.4.2 If in the earlier stages of investigations, dewatering problems are anticipated a detailed study should be carried out to ascertain the rate of flow and seepage.
2.4.3 For deep excavation, the location of water-bearing strata should be determined and the water pressure observed in each, so that necessary precautions may be taken during excavation, for example, artesian water in deep strata may give rise to considerable difficulties unless precautions are taken. An idea of the steady level of water should be obtained. Bore holes, which have been driven, may be used for this purpose, but since water levels in bore holes may not reach equilibrium for some time after boring, these should be measured 12 to 24 h after boring and compared with water levels in wells that may be available in the area. It is seldom necessary to make detailed ground-water observations in each one of a group of closely spaced bore holes but sufficient observations should be made to establish the general shape of the ground-water table; however, observations should always be made in the first boring of the group. The minimum and maximum ground-water levels should be obtained from local sources and wells in the area would also give useful information in this regard.