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Guest Column | November 2010

Geotechnical Investigation Equipment

Jagvir Goyal highlights the equipment that is required for geotechnical investigations at a site which involves considerable fieldwork and laboratory experiments.


 

A country cannot think of becoming a real economic power unless it has built a strong infrastructure for itself. That’s why India has been concentrating on this important sector for many years now. However, a real increase in the infrastructure index is achieved only if the structures, be they flyovers or expressways, high-rise structures or multi-purpose buildings, ports or terminals, are built on a sound foundation and are able to meet the challenges that the soil strata pose during the service of these structures. The failure of a structure may plummet a country’s image to a new low and the development plans, too, suffer a big blow. Unfortunately, geotechnical investigations is one area that has often been assigned low priority. Elaborate reports are developed but are rarely based on facts and actual tests, and so, become mere paperwork and eyewash.

 

Geotechnical investigations

 

All mega infrastructure projects need elaborate geotechnical investigations at the site of construction before the design of various structures involved are evolved. A number of tests are required to be carried out for which a variety of equipment is required. These geotechnical investigations at a site involve considerable fieldwork and laboratory experiments. The aim is to find the nature of sub-strata at a sufficient number of points so that the soil parameters and sub-surface conditions for each individual structure are available after the finalisation of the layout of the project. The design of foundation for various structures should not be left waiting on or needing any additional data. A thorough geotechnical investigation is therefore required.

 

Scope of investigations

 

The scope of geotechnical investigations to be carried out at a project site depends on the quantum of work involved, the area involved, the type of structures, their height and likely depth of their foundations below ground level. For example, a thermal project will essentially have a tall chimney of 220 m or 275 m height. The foundations for such a structure shall most likely be pile foundations, extending to 20 m or so below the ground level. A hydro project may have its power house base 20 m or more below the ground. A bridge may have pile or well foundations for itself. The investigations are to be done in such a manner that the site is suitably covered for erecting all kinds of structures. Tests should generally involve borehole investigations, trial pits, plate load tests, cone penetration tests, permeability tests, CBR tests, electrical resistivity tests, water sample tests and laboratory tests.

 

Information provided

 

The tests noted above provide valuable information that help in evolving the design of structures and to carry out field activities like dewatering and compaction. The information provided by the tests are as under:

 

Borehole investigations: These provide the following data and information—
1. N-values for each stratum, at the final depth and at every change of strata.
2. Undisturbed samples of soil at the founding level and at every change of strata for preservation and further tests.
3. Disturbed samples of soil at the 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 the ground water table.
6. Types of soil in each stratum as per IS classification, standard penetration curve and all properties of soil.

 

Trial pits: Trial pits provide essential information—undisturbed samples of soil, disturbed samples of soil, position of water table, if encountered, N-values at change of strata and base of pit, and the density of the soil.

 

Plate load tests: Nothing is more reliable than a field test for bearing capacity of foundations, preferably at or near the founding level. The plate load tests are therefore very important and give an idea of the actual bearing capacity of soil at a level where most of the foundations are most likely to be rested.

 

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 tests help in plotting a curve between moisture content in per cent 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.

 

Permeability test: This test when conducted under the constant head and falling head methods, gives the coefficient of permeability of various soil strata.

 

Field vane shear test: This test when conducted in various locations and depths, reveals the shear strength of cohesion-less soil.

 

CBR tests: The California Bearing Ratio (CBR) tests 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.

 

Electrical resistivity tests: These tests determine the electrical resistivity of soil.

 

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. They are done to check if the water is fit for drinking purposes or for concrete production.

 

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 are known. Grain size analysis shows percentages of gravel, sand, silt and clay and soil classification for each sample.

 

Geotechnical investigation equipment—The following equipment is required to carry out above investigations at a site:

 

Plate load test equipment: Conducting field tests for bearing capacity are most essential to know the exact values as soil behaviour 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. 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 anywhere on the 30 cm, 45 cm, 60 cm, 75 cm side. Plates are supplied by the equipment suppliers in customised sizes and shapes also. The 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 eases a geotechnical investigator’s job to a great extent. Depending upon their capacity, drilling rigs can drill up to medium and deep depths, expedite soil sampling, make the conducting of dynamic cone penetration test and static cone penetration test faster, facilitate vane shear tests in field and help in many other applications. These can be installed on most of medium and heavy duty vehicles. Reputed firms supply many accessories for these rigs. Drill rods, casings, bits, SPT samplers, sampling tubes, monkey weights, cone test attachments, to name a few. Hydraulically operated drilling rigs are more efficient and must figure on the fleet list of geotechnical investigators.

 

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. The height of the fall of a hammer is also standardised. Base assembly, hammer guide assembly, hammer, anvil, ladder, tripod, drill rod and 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.

 

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 determine 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 designs also, N values are needed as these help determine the resistance of soils to liquefaction under dynamic loads and ground vibrations. The split spoon sampler should conform to IS 9640 and the test should be conducted as per the 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 diameter and cone angles of 600, 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. For deep depths, a plain cone should be used while for shallow depths, the threaded cone should be used. A 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 the 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 600 cone angles, mantle tubes, load measuring head, pressure gauges, screw anchors, trusses, extension pipes for handles and extraction tubes. The pump-driven SCPT is preferred for constant and quicker soil penetrations. Hydraulic pumps 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 the procedure laid in IS 4968 Part III.

 

CBR test equipment: The 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 the 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 motorised equipment is available to conduct the 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 the 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 the four electrode method, also called the 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. The test is conducted as per IS 2720 Part XVII. The mould assembly for the 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-sized specimen from large-sized samples. Sample extractors can be manual or hydraulic.

 

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 able to tabulate maximum amount of test results and the information obtained for easy reference in future.

 

Laboratory tests

 

The soil and water samples collected from the boreholes 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 and plastic limit. Chemical analysis of the samples also needs to be done. The following tests ought to be done in the laboratory:

 

1. Direct shear test of soil
2. Tri-axial shear test of soil
3. Particle size analysis of soil
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 ÂcÊ and ÂphiÊ values of the soil in kg/sq cm and degrees.

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