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Geotech Investigation
Field Tests
  • The methods of subsurface investigation enable vertical sections of the strata to be drawn and samples to be tested, on the site or in & laboratory for determining shear strength parameters, bearing capacity of the soil, permeability, water table, type classification and other geophysical information in the field. This information, together with the normal topographical survey, provides the engineer with complete details of the site and enables him to prepare economical designs for the foundations.

  • This code deals mainly with subsurface investigation for foundation of multi-storeyed buildings to determine :
    a) Sequence and extent of each soil and rock stratum in the region likely to be affected by the proposed work;
    b) Nature of each stratum and engineering properties of soil and rock which may affect design, and mode of construction of proposed structures and their foundations;
    c) Location of ground water, and possible corrosive effects of soil and water on foundation materials.

  • Ground-water conditions :
  • The ground-water level fluctuates, and will depend upon the permeability of the strata and the head causing the water to flow.
    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.
  • may be applied to materials other than the above, that is, natural beds or large hard fragments or original igneous, sedimentary or metamorphic formations.

  • Importance of Ground-Water Tables :
  • For most type 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 direction of the natural drainage, and to obtain 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.
  • The geophysical methods of reconnaissance are sometimes supplemented by penetrometer tests by cones. The cone penetrometer, apart from its use for delineation of rock strata, may also be utilized for correlation with more detailed borings of soil characteristics like density, bearing capacity, etc.
  • Sounding methods consist of measuring the variation in the resistance of the soil with depth by means of a penetrometer and may be conducted either by the static or the dynamic methods (IS 4968 (Part 1) : 1976, IS 4968 (Part 2) : 1976 and IS 4968 (Part 3) : 1976). The soundings by dynamic method may also be carried out in bore holes using a standard sampler. The sampler used and the procedure adopted shall be as specified in IS 2131 : 1963. The static cone penetration methods are not suitable for exploration of boulderous or gravelly strata and in very stiff cohesive soils. However, dynamic cone penetration methods may be conducted in such area to give an idea about the compactness of strata.
  • Subsurface explorations should generally be carried out in two stages, that is, preliminary and detailed.
  • Preliminary Eploration :-
  • The scope of preliminary exploration is restricted to the determination of depths, thikness, extent and composition of each soil stratum, location of rock and ground water, and also to obtain approximate information regarding strength in compressibility of the various strata. When reconnaissance is not possible, it is essential to carry out preliminary investigation to decide the method of approach of investigation. During preliminary investigation, geophysical methods and tests with cone penetrometres and sounding rods are useful guides.
  • Detailed Exploration :-
  • Detailed investigation follow preliminary investigation, and should be planned on the basis of data obtained during reconnaissance and preliminary investigations progress. The scope of detailed exporation is ordinarily restricted to the determination of engineering properties of strata which are shown by preliminary exploration to be critical. The object of detailed exploration is to determine shear strength and compressibility of all types of soils, density index, natural moisture content, and permeability. It may also be necessary to determine the preconsolidation pressure of the strata from oedometer tests and to determine the consolidation characteristics beyond preconsolidation pressure. Appropriate shear tests should also be conducted on samples subjected to ambient pressure beyond the preconsolidation range also. The detailed investigation includes boring programme and detailed sampling to determine these properties. Field tests which may be performed are in-situ vane shear tests and plate load tests. The field permeability test and the test for the determination of dynamic properties of soils may also be conducted, where necessary. More advanced methods of logging of bore holes by radioactive methods fall under the category of detailed investigation. All in-situ tests are to be supplemented by laboratory investigations. The various phases of currently used methods of exploration and their mode of application are indicated in Appendix A.
A. Reconnaissance Methods : -
  • Electrical resistivity method ac or dc
  • Seismic refraction method
  • Standard penetration test (IS : 2131) - 1963
  • Static cone penetrometer test (IS : 4968 Part-3) - 1976
  • Dynamic cone penetrometer test (IS : 4968 Part-2) - 1976
B. Exploratory Methods : -
  • Shell and auger
  • Hand auger
C. Detailed Investigation : -
  • Thin walled tubes 50 to 125mm
  • Piston type sampler
  • Sampler with special core retainers
  • Sand sampler
  • Solidification methods
  • Open cuts and trenches
  • Plate load test
  • The standard penetration test (SPT) is an in-situ dynamic penetration test designed to provide information on the geotechnical engineering properties of soil.
PLATE LOAD TEST(IS: 1888-1982)
  • Plate Load Test is a field test for determining the ultimate bearing capacity of soil and the likely settlement under a given load. The Plate Load Test basically consists of loading a steel plate placed at the foundation level and recording the settlements corresponding to each load increment. The test load is gradually increased till the plate starts to sink at a rapid rate. The total value of load on the plate in such a stage divided by the area of the steel plate gives the value of the ultimate bearing capacity of soil. The ultimate bearing capacity of soil is divided by suitable factor of safety (which varies from 2 to 3) to arrive at the value of safe bearing capacity of soil.
  • The modulus of sub grade reaction (k) is used as a primary input for rigid pavement design. It estimates the support of the layers below a rigid pavement surface course (the PCC slab). The k-value can be determined by field tests or by correlation with other tests. There is no direct laboratory procedure for determining k-value.
  • Electrical resistivity is a measure of how much the soil resists the flow of electricity. It is a critical factor in design of systems that rely on passing current through the Earth's surface. An understanding of the soil resistivity and how it varies with depth in the soil is necessary to design the grounding system in an electrical substation, or for lightning conductors. It is needed for design of grounding (earthing) electrodes for substations and High-voltage direct current transmission systems. In most substations the earth is used to conduct fault current when there are ground faults on the system.
IN-SITU DENSITY (IS: 2720 (Part 28 & Part 29) - 1975)
  • Whenever soil is placed as an engineering fill, it is usually compacted to a dense state, to obtain satisfactory engineering properties. Compaction on site is is usually effected by mechanical means such as rolling, ramming or vibrating. Control of compaction is necessary to achieve a satisfactory result at a reasonable cost. Laboratory compaction tests provide the basis for control procedures used on site. There are two methods for obtaining insitu density as follows –
  • Sand Replacement method as per IS: 2720 (Part 28) – 1975
  • Core Cutter method as per IS: 2720 (Part 29) – 1975
  • This method covers the determination of the California Bearing Ratio (CBR) of a soil tested in situ, with a selected overburden pressure,by causing a cylindrical plunger to penetrate the soil at a given rate and comparing the relationship between force and penetration into the soil to that for a standard material. At certain values of penetration the California Bearing Ratio (CBR) is defined in the form of a percentage, as the ratio of the force exerted on the soil to a standard force that would be exerted on a specified crushed rock compacted and confined in a given manner.
  • The Dynamic Cone Penetration Test provides a measure of a material’s in-situ resistance to penetration. The test is performed by driving a metal cone into the ground by repeated striking it with a 8 Kg weight dropped from a distance of 575 mm. The penetration of the cone is measured after each blow and is recorded to provide a continuous measure of shearing resistance up to 1200 mm below the ground surface. Test results can be correlated to California Bearing Ratios.
  • Core recovery parameters describe the quality of core recovered from a borehole. Total core recovery (TCR) is the borehole core recovery percentage. TCR is defined as the quotient: Sum of length of total core recovered /Total core run in percentage
  • Rock-quality designation (RQD) is a rough measure of the degree of jointing or fracture in a rock mass, measured as a percentage of the drill core in lengths of 10 cm or more. High-quality rock has an RQD of more than 75%, low quality of less than 50%. Rock quality designation (RQD) has several definitions. The most widely used definition was developed in 1964 by D. U. Deere. It is the borehole core recovery percentage incorporating only pieces of solid core that are longer than 100 mm in length measured along the centerline of the core. In this respect pieces of core that are not hard and sound should not be counted though they are 100 mm in length. RQD was originally introduced for use with core diameters of 54.7 mm (NX-size core). RQD has considerable value in estimating support of rock tunnels. RQD forms a basic element in some of the most used rock mass classification systems: Rock Mass Rating system (RMR) and Q-system. RQD is defined as the quotient: Sum of length of core pieces that are > 100 mm measured along the centerline / Total length of core run in percentage
Lab Tests
  • Soil sample as received from the field shall be dried in the air or in sun. In wet weather a drying apparatus may be used in which case the temperature of the sample should not exceed 60°C. The clods may be broken with a wooden-mallet to hasten drying. The organic matter, like tree roots and pieces of bark should be removed from the matter other than soil, like shells should also be sample. Similarly, separated from the main soil mass. A noting shall be made of such removals and their percentage of the total soil sample noted.
WATER CONTENT (IS: 2720 (Part 2) - 1973)
  • In almost all soil tests natural moisture content of the soil is to be determined. The knowledge of the natural moisture content is essential in all studies of soil mechanics. To sight a few, natural moisture content is used in determining the bearing capacity and settlement. The natural moisture content will give an idea of the state of soil in the field.
SPECIFIC GRAVITY (IS: 2720 (Part 3) - 1973)
  • Specific gravity is the ratio of the weight in air of a given volume of a material at a standard temperature to the weight in air of an equal volume of distilled water at the same stated temperature.
GRAIN SIZE ANALYSIS (IS: 2720 (Part 4) - 1985)
  • The grain size analysis is widely used in classification of soils. The data obtained from grain size distribution curves is used in the design of filters for earth dams and to determine suitability of soil for road construction, air field etc. Information obtained from grain size analysis can be used to predict soil water movement although permeability tests are more generally used.
ATTERBERG LIMIT (IS: 2720 (Part 5) - 1985)
  • The Atterberg limits are a basic measure of the critical water contents of a fine-grained soil, such as its shrinkage limit, plastic limit, and liquid limit. As a dry, clayey soil takes on increasing amounts of water, it undergoes dramatic and distinct changes in behavior and consistency. Depending on the water content of the soil, it may appear in four states: solid, semi-solid, plastic and liquid. In each state, the consistency and behavior of a soil is different and consequently so are its engineering properties.
SHRINKAGE FACTORS (IS: 2720 (Part 6) - 1972)
  • The soil shrinkage is defined as the specific volume change of soil relative to its water content and is mainly due to clay swelling properties.
COMPACTION OF SOIL (Part 7 - 1980 & Part 8 - 1983)
  • Soil compaction is defined as the method of mechanically increasing the density of soil. In construction, this is a significant part of the building process. If performed improperly, settlement of the soil could occur and result in unnecessary maintenance costs or structure failure. Almost all types of building sites and construction projects utilize mechanical compaction techniques.
  • The unconfined compressive strength (UCS) is the maximum axial compressive stress that a right-cylindrical sample of material can withstand under unconfined conditions—the confining stress is zero. It is also known as the uniaxial compressive strength of a material because the application of compressive stress is only along one axis—the longitudinal axis—of the sample.
TRIAXIAL SHEAR TEST (IS: 2720 (Part 11 & Part 12) - 1986)
  • In a triaxial shear test, stress is applied to a sample of the material being tested in a way which results in stresses along one axis being different from the stresses in perpendicular directions. This is typically achieved by placing the sample between two parallel platens which apply stress in one ) direction, and applying fluid pressure to the specimen to apply stress in the perpendicular directions. The triaxial test is carried out in a cell on a cylindrical soil sample having a length to diameter ratio of 2. The usual sizes are 76 mm x 38 mm and 100 mm x 50 mm. Three principal stresses are applied to the soil sample, out of which two are applied water pressure inside the confining cell and are equal. The third principal stress is applied by a loading ram through the top of the cell and is different to the other two principal stresses. The soil sample is placed inside a rubber sheath which is sealed to a top cap and bottom pedestal by rubber O-rings. For tests with pore pressure measurement, porous discs are placed at the bottom, and sometimes at the top of the specimen. Filter paper drains may be provided around the outside of the specimen in order to speed up the consolidation process. Pore pressure generated inside the specimen during testing can be measured by means of pressure transducers.
DIRECT SHEAR TEST (IS: 2720 (Part 13) - 1986)
  • The concept of direct shear is simple and mostly recommended for granular soils, sometimes on soils containing some cohesive soil content. The cohesive soils have issues regarding controlling the strain rates to drained or undrained loading. In granular soils, loading can always assumed to be drained. A schematic diagram of shear box shows that soil sample is placed in a square box which is split into upper and lower halves. Lower section is fixed and upper section is pushed or pulled horizontally relative to other section; thus forcing the soil sample to shear/fail along the horizontal plane separating two halves. Under a specific Normal force, the Shear force is increased from zero until the sample is fully sheared. The relationship of Normal stress and Shear stress at failure gives the failure envelope of the soil and provide the shear strength parameters (cohesion and internal friction angle).
CONSOLIDATION TEST (IS: 2720 (Part 15) - 1986)
  • This test is performed to determine the magnitude and rate of volume decrease that a laterally confined soil specimen undergoes when subjectedto different vertical pressures. From the measured data, the consolidation curve (pressure-void ratio relationship) can be plotted. This data is useful in determining the compression index, the recompression index and the preconsolidation pressure (or maximum past pressure) of the soi. In addition, the data obtained can also be used to determine the coefficient of consolidation and the coefficient of secondary compression of the soil.
  • The california bearing ratio test is penetration test meant for the evaluation of subgrade strength of roads and pavements. The results obtained by these tests are used with the empirical curves to determine the thickness of pavement and its component layers. This is the most widely used method for the design of flexible pavement.
  • The soil permeability is a measure indicating the capacity of the soil or rock to allow fluids to pass through it. The permeability coefficient can be determined in the laboratory using falling head permeability test, and constant head permeability test. The knowledge of this property is much useful in solving problems involving yield of water bearing strata, seepage through earthen dams, stability of earthen dams, and embankments of canal bank affected by seepage, settlement etc.
FREE SWELL INDEX (IS: 2720 (Part 40) - 1977)
  • The clay and specially the black cotton soils have a tendency to swell in small or more proportion when submerged in water. Free swell or differential free swell also termed as free swell index, is the increase in volume of soil with out any external constraint when subjected to submergence in water.
SWELLING PRESSURE OF SOIL (IS: 2720 (Part 41) - 1977)
  • The one-dimensional consolidometer is widely used for the measurement of swelling characteristics of expensive soils. The swelling pressure in the consolidometer test has been defined as the pressure which prevents the specimen form swelling or the pressure which is required to return a swelled specimen back to its original state (void ratio, height) prior to swelling.
  • UCS is one of the most basic parameters of rock strength, and the most common determination performed for boreability predictions. It is measured with the length to diameter ratio of 2 by using NX-size core samples. 3 to 5 UCS determinations are recommended to achieve statistical significance of the results. If the sample length to diameter ratio was greater or less than 2, a correction factor that is applied to the UCS value determined from testing. The unconfined compressive strength of the specimen is calculated by dividing the maximum load at failure by the sample cross-sectional area:

  • Where:
  • sc = Unconfined Compressive
  • F = Maximum Failure Load
  • A = Cross-sectional area of the core sample