Geotechnical Investigations

What is Geotechnical Investigation?

Geotechnical investigation refers to the process of assessing the properties and conditions of the soil, rock, and groundwater at a construction site or a specific area. It is conducted to gather crucial information about the subsurface materials and their behavior, which is essential for designing and constructing various types of structures, such as buildings, bridges, dams, roads, and tunnels.

Geotechnical investigations are typically carried out by geotechnical engineers or geotechnical consultants who specialize in assessing the geotechnical aspects of construction projects. These investigations involve a combination of fieldwork and laboratory testing to obtain comprehensive data on the site's geology, soil composition, bearing capacity, slope stability, groundwater conditions, and other relevant parameters.

Soil testing plays a crucial role in tunnel and underground construction projects. It helps engineers and geotechnical experts understand the soil properties, stability, and behavior, allowing them to design appropriate support systems and foundations. Here are some common soil tests conducted for tunnel and underground construction:

• Standard Penetration Test (SPT): This test measures the resistance of soil to penetration using a standard sampler driven into the ground. It provides information about soil stratification, density, and the number of blows required for penetration. The SPT test is useful for assessing soil strength and determining the bearing capacity.
• Cone Penetration Test (CPT): The CPT involves pushing a cone-shaped penetrometer into the ground while measuring the resistance. It provides data on soil type, friction angle, pore pressure, and stratification. The CPT is particularly useful for assessing soft soils and determining their stability.
• Shear Strength Testing: Various laboratory tests, such as the direct shear test or triaxial test, are conducted to determine the shear strength parameters of the soil. These tests help in evaluating the stability of tunnel walls and assessing the potential for soil movement.
• Permeability Testing: Permeability tests, such as the constant head or falling head tests, measure how easily water can flow through the soil. It helps determine the potential for groundwater seepage, which is crucial for assessing the need for dewatering measures during tunnel construction.
• Consolidation Test: The consolidation test evaluates the rate at which soil consolidates under load, providing information about settlement and time-dependent behavior. This test is essential for understanding the long-term stability of tunnel foundations and predicting settlement patterns.

These are just a few examples of the soil tests commonly performed in tunnel and underground construction. The specific tests required may vary depending on the project's geotechnical conditions, regional regulations, and the complexity of the tunnel design. It is recommended to consult with a geotechnical engineer or a geotechnical testing laboratory for a comprehensive assessment based on your project's specific needs.

When conducting tests on rock samples for tunnel design, the following tests are commonly performed to evaluate the rock's properties:

• Uniaxial Compressive Strength (UCS) Test: This test determines the maximum compressive stress that a rock sample can withstand before failure. It involves applying axial force to a cylindrical rock specimen until it fractures. The UCS value is crucial for designing the tunnel support system.
• Brazilian Tensile Strength Test: This test measures the tensile strength of a rock sample. A cylindrical or disc-shaped rock specimen is subjected to diametrical compression until it breaks. The Brazilian tensile strength is used in stability analysis and determining the rock's resistance to spalling.
• Point Load Index Test: The point load test provides a quick and simple assessment of rock strength. It involves applying a concentrated load on the rock specimen and measuring the resulting force required for failure. The point load index helps estimate the uniaxial compressive strength and rock mass quality.
• Triaxial Compression Test: This test evaluates the response of the rock sample under different confining pressures and axial loads. It provides information about the rock's strength and stress-strain behavior under more realistic conditions, considering lateral confinement.
• Direct Shear Test: The direct shear test measures the shear strength and frictional properties of a rock sample. It involves sliding two portions of a rock specimen along a pre-defined plane under normal load. The test helps determine the rock's shear strength parameters for tunnel stability analysis.
• Permeability Test: This test assesses the rock's ability to allow fluid flow. Different permeability tests, such as constant head or falling head tests, are conducted on rock samples to determine their hydraulic conductivity. This information is crucial for assessing groundwater inflow and drainage considerations in tunnel design.
• Porosity and Density Measurements: These tests determine the rock's porosity and density, which affect its mechanical properties and water-holding capacity. Porosity is measured by determining the volume of void space in the rock, while density is typically measured using core samples.
• Petrographic Analysis: Petrographic analysis involves examining thin sections of rock samples under a microscope to identify mineral composition, grain size, texture, and other characteristics. This analysis provides insights into the rock's structural features, alteration, and potential weaknesses.

It's important to note that these tests are typically performed in a laboratory setting on core samples extracted from the tunnel site. The results of these tests, combined with in-situ geotechnical investigations, geological mapping, and engineering judgment, contribute to the overall understanding of the rock's properties and help guide tunnel design and support system selection.

Tools that’s are used commonly

These tools are used to collect soil samples for laboratory testing. They include:

• Hand augers: Used for shallow soil sampling.
• Power augers: Used for drilling deeper holes in the ground.
• Soil coring devices: Used to obtain undisturbed samples for detailed analysis.
• Shelby tubes: cylindrical tubes used to extract soil cores.
• Sounding Equipment: These tools measure the depth of various subsurface layers and detect the presence of rock or soil interfaces. Examples include:
• Sounding rods: Simple metal rods used to measure depth by probing the ground.
• Dynamic cone penetrometer (DCP): Measures the resistance of the soil to penetration.
• Cone penetration test (CPT) equipment: Measures soil properties during cone penetration.
• Borehole Logging Equipment: These tools are used to gather data about the soil and rock encountered during drilling. They include:
• Borehole cameras: Used to visually inspect the condition of the borehole.
• Mechanical samplers: Collect samples of soil or rock at specific depths.
• Borehole extensometers: Measure ground movements or deformations.
• Groundwater Monitoring Equipment: These tools help assess groundwater conditions and levels. Examples include:
• Piezometers: Measure groundwater pressure and levels.
• Inclinometers: Measure the angle of groundwater flow and monitor slope stability.
• Groundwater pumps and monitoring wells: Used for groundwater sampling and monitoring.
• Geophysical Instruments: These devices use geophysical methods to investigate subsurface conditions. They include:
• Ground Penetrating Radar (GPR): Uses electromagnetic waves to detect subsurface features.
• Seismic refraction equipment: Measures the velocity of seismic waves to determine soil and rock properties.
• Electrical resistivity meters: Measure the electrical properties of the subsurface to identify soil types and water content.

It's important to note that the specific equipment used in geotechnical investigations may vary depending on the project requirements, site conditions, and the depth and complexity of the investigation. Consulting with a geotechnical engineer or geologist would provide further guidance on the appropriate equipment for a specific investigation.

In India, the primary reference code for geotechnical investigations is the Indian Standard Code of Practice for Subsurface Investigation of Soils (IS:1892-1979). This code provides guidelines for conducting geotechnical investigations, including the methods of sampling, testing, and reporting of soil and rock properties.

The international reference codes for geotechnical investigations are primarily developed by organizations such as the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE) and the American Society for Testing and Materials (ASTM). Some commonly referenced international codes include:

• ASTM D1586-14: Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils
• ASTM D3550-18: Standard Practice for Thick Wall, Ring-Lined, Split Barrel, Drive Sampling of Soils
• ASTM D4429-16: Standard Test Method for CBR (California Bearing Ratio) of Soils in the Laboratory
• ASTM D5874-16: Standard Test Methods for Determination of the Soil Water Characteristic Curve for Desorption Using Hanging Column, Pressure Extractor, Chilled Mirror Hygrometer, or Centrifuge
• ASTM D7012-14e1: Standard Test Methods for Compressive Strength and Elastic Moduli of Intact Rock Core Specimens under Varying States of Stress and Temperatures
• ISSMGE International Reference Testing Procedures for Geotechnical Laboratory Testing

It's important to note that geotechnical investigation practices may vary depending on the specific project, location, and engineering requirements. Therefore, it's recommended to consult the relevant local authorities, geotechnical engineering organizations, or project-specific guidelines for the most up-to-date and region-specific codes and standards.

Here is a list of some Indian codes related to geotechnical laboratory testing:

• IS 2720 (Part 1 to 31): Methods of Test for Soils=This series of codes provides detailed methods for testing various geotechnical properties of soils, including tests for determination of moisture content, specific gravity, grain size analysis, compaction characteristics, shear strength, consolidation, permeability, and more.
• IS 4434: Methods of Ring Shear Test for Determination of the Shear Strength Parameters of Cohesive Soils -This code specifies the method for conducting ring shear tests to determine the shear strength parameters of cohesive soils.
• IS 4978: Methods for Calibration of Pressure Transducers-This code provides guidelines for calibrating pressure transducers used in geotechnical testing equipment.
• IS 6403: Determination of Permeability Characteristics of Soils in the Laboratory - This code outlines the procedure for determining the permeability characteristics of soils using laboratory tests.
• IS 9198: Determination of Unconfined Compressive Strength of Cohesive Soil -This code describes the method for determining the unconfined compressive strength of cohesive soils in the laboratory.
• IS 9669: Determination of One-Dimensional Consolidation Properties of Soils -This code provides guidelines for determining the one-dimensional consolidation properties of soils using laboratory tests.
• IS 12287: Methods of Determination of California Bearing Ratio (CBR) of Soils in the Laboratory -This code specifies the method for determining the California Bearing Ratio (CBR) of soils in the laboratory.
• IS 13030 (Part 1 to 6): Methods of Test for Soils - Part 6: Direct Shear Test (Reaffirmed in 2014) -Although primarily focused on soil testing, Part 6 of this code covers the direct shear test, which can also be applied to rock specimens with appropriate modifications.
• IS 9143: Method for the Determination of Uniaxial Compressive Strength of Rock Materials -This code specifies the procedure for determining the uniaxial compressive strength of rock materials, which can be applied to intact rock specimens.
• IS 9144: Method for the Determination of Modulus of Deformation of Rock Materials in Uniaxial Compression - This code provides guidelines for determining the modulus of deformation of rock materials under uniaxial compression.
• IS 9145: Method for the Determination of Shear Strength Parameters of Rock Materials - This code outlines the procedure for determining the shear strength parameters of rock materials, including intact rock specimens