Note: This highly informative and elaborate paper was written by the Author many years back and holds valuable information. Readers should update themselves with the latest developments that might have occurred after the presentation of this paper.
Infrastructure development in a country aims at creating favorable conditions for the overall economic and social progress of that country. An increase in it is directly linked to a rise in the Gross Domestic Production ( GDP ) of that country. Expansion of trade, rise in the standard of living of people, diminishing of poverty-line and meeting the challenge posed by increase in population, all depend upon the pace
of infrastructure development. A country can not think of becoming an ‘Economic Power’ unless it has built a strong infrastructure for itself. That’s why India has been concentrating upon this important sector for the last many years now.
However, real increase in infrastructure index is achieved only if the structures, may these be flyovers or expressways, high rise structures or multi-purpose buildings, ports or terminals or any other ones are built with a sound foundation and to meet the challenges that the soil strata pose during the service of these structures. Failure of a structure may drag a country’s the image to a new low and the development plans suffer a big blow. Unfortunately, geotechnical investigations are one area that have often been assigned a low priority. Elaborate reports are developed but hardly based on facts and actual tests. Many where, these have become a mere paperwork and an eyewash.
Importance of geotechnical investigations: All mega infrastructure projects need elaborate geotechnical investigations at the site of construction before evolving the design of various structures involved. A number of tests are required to be carried out for which a variety of equipment is required. These Geo-technical investigations at a site involve considerable field work and laboratory experiments. The aim is to find the nature of sub-strata at sufficient number of points so that the soil parameters and sub-surface conditions for each individual structure are available after the finalization of the layout of the project. The design of foundations for various structures is not to be left wanting for any additional data. A thorough geo-technical investigation is therefore required.
Scope of Geotechnical investigations: The scope of geotechnical investigations to be carried out at a project site depends on the quantum of work involved, area involved, type of structures, their height and likely depth of their foundations below the ground level.For example, a thermal project will be essentially having a tall chimney of 220 metres or 275 metres height. The foundations for such a structure shall most likely be pile foundations, extending to 20 metre or so below the ground level. A hydro project may have its power house base 20 metres or more below the ground. A bridge may be having pile or well foundations for itself. The investigations are to be done in such a manner that a site gets suitably covered for erecting all structures. The following tests shall generally be involved:
1. Borehole investigations.
2. Trial pits.
3. Plate Load tests.
4. Cone penetration tests.
5. Permeability test.
6. CBR tests.
7. Electrical Resistivity tests.
8. Water sample tests.
9. Laboratory tests.
Information provided by the tests: The tests noted above provide valuable information for evolving the design of structures and to carry out field activities like dewatering and compaction. The information provided the tests is as under:
1. Borehole investigations: These provide the following data and information—
2. Undisturbed samples of soil at founding level and at every change of strata for preservation and further tests.
3. Disturbed samples of soil at founding level and at every change of strata for further tests.
4. Depth of ground water table in each borehole.
5. Piezometer record for each borehole showing changes the daily changes in ground water table.
6. Types of soil in each stratum as per IS classification, standard penetration curve and all properties of soil.
2. Trial pits: Trial pits provide the following information—
2. Disturbed samples of soil.
3. Position of water table, if encountered.
4. N-values at change of strata and base of pit.
5. Densities of soil.
3. Plate load tests: Nothing is more reliable than a field test for bearing capacity of foundations, preferably at or near founding level. The plate load tests are therefore very important and give an idea of actual bearing capacity of soil at a level where most of the foundations are most likely to be rested.
4. Cone penetration tests: Static Cone Penetration Test (SCPT) and Dynamic Cone Penetration Test (DCPT) exactly determine the relative shear strength of soil and densities of various strata. SCPT continues till a specified capacity is reached while DCPT is continued till the refusal to penetration is met or desired depth is achieved. Results of Standard Proctor Compaction test help in plotting a curve between moisture content in percent and dry density of soil in gm/cc as coordinates to determine the optimum moisture content of soil (OMC) and corresponding maximum dry density of soil.
5. Permeability test: This test when conducted under constant head and falling head methods, gives the coefficient of permeability of various soil strata.
6. Field vane shear test: This test when conducted in various locations and depths tells the shear strength of cohesion-less soils.
7. CBR tests: California Bearing Ratio (CBR) test result in plotting load Vs penetration curves to arrive at CBR values which are used in design of pavements to be laid in the infrastructure project area.
8. Electrical resistivity tests: These tests determine the electrical resistivity of soil.
9. Water sample tests: These tests include chemical analysis of water samples to know its chloride content, sulphate content, pH value, organic matter content and other harmful salts content. These tests are done to check if the water is fit for drinking purposes and for concrete production.
10. Laboratory tests: These tests determine the characteristics of soil and water. The grain size distribution of soil, its shear strength, modulus of elasticity, liquid limit and plastic limit get known. Grain size analysis shows percentages of gravel, sand, silt and clay and soil classification for each sample.
Geotechnical Investigation Equipment: Following equipment is required to carry out above investigations at site:
Plate Load Test equipment: Conducting field tests for bearing capacity are most essential to know the exact values as soil behavior is sometimes most unpredictable. Plate Load Test (PLT) results are considered so reliable that these sometimes act as a datum for other test results for calibration of values. PLT gives the ultimate bearing capacity of soil. It is conducted under ‘maintained load conditions’ and ‘cyclic load conditions’. Under
Cyclic load conditions, the load increments are removed to know the rebound values. As the load is increased gradually, a situation arrives when the test plate on which the load is being applied, begins to settle at a rapid rate. This situation indicates the ultimate bearing capacity of soil. The PLT equipment consists of hydraulic jacks, hydraulic pumps fitted with calibrated dial gauges, plain and grooved MS plates of required area, high pressure
bearing metallic flexible pipes, extension rods for dial gauges, columns, datum bars, spikes and dial gauges. The plates are kept 25 mm thick. The plates are square in shape and are any of the 30 cm, 45 cm, 60 cm, 75 cm side. Plates are supplied by the equipment suppliers in customized sizes and shapes also. Load to be transferred to the test plate is normally created at site by raising a platform and loading it with sand filled bags. The load is transferred from the permanent supports to hydraulic jacks which further transfer it to the test plate. The test is normally conducted as per IS 1888.
Drilling rigs: Availability of drilling rigs with the geotechnical investigator eases his job to a great extent. Drilling rigs can be hydraulically or pneumatically operated. These are truck mounted or trailer mounted. Diesel engine with gear box for forward and reverse operation is used. Hydraulic system is energized by heavy duty geared pumps connected to the power unit. Hydraulic oil tank of large capacity, radiator cooling system and wire braided hoses help in smooth running of hydraulic system. A clutch system is added to the rigs to shut off the system in emergency and hold drilling operation.
Depending upon their capacity, drilling rigs can drill up to medium and deep depths, expedite soil sampling, make conducting of Dynamic Cone Penetration Test and Static Cone Penetration Test faster, facilitate Vane shear test in field and help in many other applications. These can be installed on most of medium and heavy duty vehicles. Reputed
firms supply many sort of accessories for these rigs. Drill rods, casings, bits, SPT samplers, sampling tubes, monkey weights, cone test attachments are to name a few. Hydraulically operated drilling rigs are more efficient and must figure on the fleet list of geotechnical investigator.
Free Fall hammers: Automatic Free fall hammers prove very useful for conducting the standard penetration tests and cone penetration tests in an accurate manner. An important factor for accuracy of tests is that the impact energy for all blows is equal. Automatic free fall hammers ensure that. These hammers are provided with proper guide system to keep the guide rod and drill rod in vertical position. The height of fall of hammer is also
standardized. Base assembly, hammer guide assembly, hammer, anvil, ladder, tripod, drill rod, split spoon sampler are essential components of automatic free fall hammers. Dynamic cone and cone adapters are optional items for use in case the hammer is to be used to conduct DCPT also.
Standard Penetration Test Apparatus: SPT apparatus is an essential item for geotechnical investigations. This equipment is put to maximum use during soil investigations. The N values given by it help in determining the degree of compactness of sandy soils and consistency of cohesive soils. These can even be used for design of foundations when plate load test results are not available. For earthquake resistance design also, N values are needed as these help determine the resistance of soils to liquefaction under dynamic loads and ground vibrations.
An SPT apparatus consists of a guide pipe assembly, the tripod with pulley arrangement and ladder incorporated, the drill roads, the main body which is extendable through split lengths, the weight, the shoe with a cutting edge and a head and the split spoon sampler. The split spoon sampler should conform to IS 9640 and the test should be conducted as per procedure laid in IS 2131.
DCPT apparatus: Dynamic Cone Penetration Test is another important test to be conducted for having complete information of soil characteristics. DCPT tells the resistance of various types of soil when a 50 mm cone is penetrated through them. These resistance values indicate the relative strength and relative densities of soils. The
equipment consists of a dynamic cone of 50 mm base dia and cone angles of 60 degrees, a cone adapter for coupling of the cone with the drill rod, a stand to keep the rods vertical, a guide assembly, a tripod with built in ladder, a drive weight of 65 kg as is required for SPT and drill rods. The cone used can be of threaded or plain type. For deep
depths, plain cone should be used while for shallow depths, threaded cone should be used. Plain cone is used for deeper depths as it sometimes become difficult to pull out the cone after the test. The test should be conducted as per IS 4968 Part I when bentonite slurry is not used and as per Part II when bentonite slurry is used.
SCPT apparatus: Static Cone Penetration Test (SCPT) readings provide bearing capacity profile and shear strength profile of the soil under test. SCPT also helps in finding the LCC (Load Carrying Capacity) of piles as the penetration resistance to its cone almost matches the actual load carrying capacity of pile per unit area of pile tip. SCPT cone has a base area of 10 sq.cm only. The equipment consists of a penetration cone of base area of 10 sq. cm and 60 degree cone angles, mantle tubes, load measuring head, pressure gauges, screw anchors, trusses, extension pipes for handles and extraction tubes. Pump driven SCPT is preferred these days for constant and quicker soil penetrations. Hydraulic pump used to drive the engine can be attached to the trolley on which the complete SCPT assembly is mounted. The test should be conducted as per procedure laid in IS 4968 Part III.
CBR Test equipment: California Bearing Ratio (CBR) test provides information on suitability of soils for use in pavements, their sub-grades and sub-bases. All materials that can pass through 20 mm sieve can be tested under this test. The equipment consists of a load frame, rammers of specified weight and drop, a mould, a perforated base plate for mould, a penetration piston, dial gauge, stand for dial gauge, proving ring and annular and slotted metal weights. These days, electronic and motorized equipment is available to conduct CBR test. Digital display in this equipment gives accurate results. The CBR mould and its accessories should conform to IS 9669 and the test should be conducted in the laboratory as per method explained in IS 2720, Part XVI and in the field as per IS 2720, Part 31. Load Vs penetration curves should be drawn for each test.
Electrical Resistivity Test equipment: Electrical resistivity of soil should be known for use to limit the potential of current carrying conductors during the earthing of electrical systems. Resistivity meters, electrodes and porous pots are used to conduct the test. Resistance readings should be noted in all directions like North-South, East West, North East-South West and North West-South East. Readings should be recorded by varying
electrode spacing a number of times. Resistivity in Ohm-Meter for each electrode spacing should be worked out. Tests should be conducted as per four electrode method, also called Wenner’s method.
Permeability test equipment: Permeability of soil plays a significant role in making right decisions related to dewatering of foundations, selection of soils for use in embankments, pumping of ground water and design of dams and reservoirs. Permeameters are now used to check the permeability of soils. These provide accurate results and need small samples and less time. Permeameters can be constant head type or falling head type. The former type are suitable for cohesion-less or sandy soils and the latter ones are fit for cohesive soils. The test is conducted as per IS 2720 Part XVII. The mould assembly for permeability test should be as per IS 11209.
Water sample containers: During geotechnical investigations, ground water samples should be collected from different locations of the site and sent to the laboratory for analysis after containing them in air tight containers.
Sample extractors: Sample extractors extract soil core specimens from the sampling tubes with minimum disturbance to the soil. These can also extract small size specimens from large size samples. Sample extractors can be manual or hydraulic.
Laboratory tests: The soil and water samples collected from the bore-holes and trial pits are to be subjected to laboratory tests as per procedure laid in the IS codes to know the characteristics of soil and water, specially to find the grain size distribution of soil, its shear strength, modulus of elasticity, liquid limit, plastic limit. Chemical analysis of the samples also needs to be done. Following tests to be done in the laboratory can be listed:
2. Tri-axial shear test of soil.
3. Particle size analysis of soils.
4. Unconfined Compression test of soil.
5. Liquid limit of soil test.
6. Plastic Limit of soil test.
7. Moisture content of soil.
8. Bulk density and dry density of soil.
9. Specific gravity of soil.
10. Shrinkage limit of soil.
11. Chemical test of soil.
12. Chemical test of water.
Particle size analysis is an important test and shows the percentages of gravel, sand, silt and clay and brings out soil classification for each sample. Tri-axial and direct shear tests determine the modulus of elasticity of the soil. Shear tests bring out the ‘c’ and ‘phi’ values of the soil in kg/sq cm and degrees. All these laboratory tests shall be discussed in a subsequent episode.
Geotechnical Investigation report: Care should always be taken that all these tests and investigations are made by using standard and calibrated equipment. All the observations along with laboratory investigations should be well presented in a report. Copies of the report should be preserved as these often need reference during the entire construction of the project. It should be tried to tabulate maximum amount of test results and the information obtained for easy reference in future.
