Site investigation is a critical aspect of geotechnical engineering, providing important information about the geological and geotechnical properties of a site. It involves the collection of data to characterize the subsurface conditions of a site, such as the geology, soil and rock properties, groundwater conditions, and other factors that may affect the design and construction of a project.
Site investigation is an important part of the construction process, as it helps engineers and designers to understand the site conditions and develop appropriate foundation designs and construction methods that are safe, economical, and sustainable. It is also used to identify potential hazards or risks that may affect the safety or performance of a project.
Site investigation is typically carried out in several stages, including a preliminary investigation, a site reconnaissance, and a detailed investigation. The type and scope of the investigation will depend on the size and complexity of the project, as well as the level of risk associated with the site conditions.
Techniques for Site Investigation
There are various techniques that can be used for site investigation, depending on the specific project requirements and the nature of the site. Some common techniques include:
- Desk study: This involves gathering and reviewing all available data and information related to the site and the surrounding area, including geological maps, aerial photographs, historical records, and previous site investigation reports.
- Site reconnaissance: This involves visiting the site to make observations and take measurements, such as mapping surface features, assessing the condition of existing structures, and identifying potential hazards.
- Geophysical surveys: These involve using various techniques to measure and map physical properties of the subsurface, such as electrical resistivity, magnetic susceptibility, and seismic waves. Examples of geophysical techniques include ground-penetrating radar, seismic reflection and refraction, and electrical resistivity imaging.
- Drilling and sampling: This involves drilling boreholes or excavating test pits to obtain samples of soil and rock, which can be analyzed in the laboratory to determine their physical and mechanical properties.
- In-situ testing: These tests are carried out on site to determine the properties of soil and rock in their natural state. Examples of in-situ tests include Standard Penetration Test (SPT), Cone Penetration Test (CPT), and Pressuremeter Test.
- Laboratory testing: This involves analyzing soil and rock samples in a laboratory to determine their physical, mechanical, and chemical properties. Examples of laboratory tests include grain size analysis, triaxial compression test, and direct shear test.
- Environmental testing: These tests are carried out to evaluate the environmental conditions of the site, including the presence of contaminants in soil, groundwater, and surface water. Examples of environmental tests include soil and groundwater sampling, and analysis for heavy metals, hydrocarbons, and other pollutants.
- Remote sensing: This involves using satellite or aerial imagery to map and analyze the site and surrounding areas. Remote sensing can be used to identify land use patterns, vegetation cover, topography, and other features that can affect site suitability and development.
These techniques can be used in combination to provide a comprehensive understanding of the site and its characteristics.
Geophysical techniques are a set of methods used in site investigation to provide information about subsurface geological conditions, without the need for excavation or drilling. These techniques involve measuring various physical properties of the subsurface, such as density, magnetic susceptibility, electrical conductivity, seismic velocity, and others. The data collected from geophysical surveys are used to create images and models of the subsurface, which can help in the identification of geological structures, such as faults, fractures, and changes in lithology. Some of the commonly used geophysical techniques in site investigation include:
- Seismic reflection: This technique involves generating seismic waves using a source and measuring the reflected waves using sensors. The data collected can be used to create a 2D or 3D image of the subsurface.
- Electrical resistivity tomography (ERT): This technique measures the electrical resistivity of the subsurface materials by passing an electrical current through the ground and measuring the voltage difference. The data collected can be used to create a model of the subsurface.
- Ground-penetrating radar (GPR): This technique involves transmitting and receiving electromagnetic waves to and from the subsurface. The data collected can be used to create a cross-sectional image of the subsurface.
- Magnetic survey: This technique measures the magnetic properties of subsurface materials using a magnetometer. The data collected can be used to identify magnetic anomalies associated with certain geological structures.
- Gravity survey: This technique measures the gravitational field of the subsurface using a gravimeter. The data collected can be used to identify changes in the density of subsurface materials, which can indicate the presence of geological structures.
- Electromagnetic survey: This technique involves transmitting and receiving electromagnetic waves to and from the subsurface, which can be used to identify changes in subsurface conductivity. This can help in the identification of certain geological structures.
These geophysical techniques can provide valuable information for site investigation, and can be used in conjunction with other methods, such as drilling and sampling, to obtain a comprehensive understanding of the subsurface geology.
Drilling and sampling
Drilling and sampling are critical techniques in site investigation that allow geotechnical engineers and geologists to obtain information about the subsurface conditions of a site. The techniques involve drilling boreholes into the ground and collecting samples of soil or rock at various depths.
The information obtained from drilling and sampling is used to determine the physical and engineering properties of the soil and rock, such as their composition, strength, permeability, and deformation characteristics. This information is then used to design foundations, excavations, tunnels, and other structures.
There are several types of drilling techniques, including:
- Auger drilling: This involves using a helical screw to penetrate the soil and collect samples. It is commonly used for shallow investigations and for soil types that are not too hard.
- Rotary drilling: This involves using a rotary drill bit to penetrate the soil or rock. It can be used for both shallow and deep investigations.
- Percussion drilling: This involves using a hammer to drive a drill bit into the soil or rock. It is commonly used for rock formations.
Once the borehole is drilled, samples of soil or rock can be collected using various sampling techniques, such as:
- Standard penetration testing (SPT): This involves driving a split spoon sampler into the soil using a hammer and counting the number of blows required to drive the sampler a specified distance. This information is used to determine the soil density and strength.
- Shelby tube sampling: This involves using a thin-walled tube to collect undisturbed soil samples from the borehole. This technique is commonly used for sampling cohesive soils.
- Rock coring: This involves using a diamond-tipped drill bit to collect rock samples. The samples can be either continuous or discontinuous, depending on the drilling technique used.
The samples collected from the borehole are then sent to a laboratory for testing and analysis. The results are used to develop a geotechnical report that provides information on the subsurface conditions of the site and recommendations for foundation design and construction.
In-situ testing refers to the methods used to measure soil and rock properties at the location where the materials are found, without removing them from their natural setting. In-situ testing can provide important information for engineering and construction projects, as it allows for a more accurate understanding of the properties of the soil and rock materials that will be encountered during excavation, construction, or other activities.
There are several different types of in-situ testing techniques, including:
- Standard Penetration Test (SPT): This is a widely used method for determining the strength and density of soils. A cylindrical sampler is driven into the soil using a hammer, and the number of blows required to penetrate the soil a certain distance is recorded.
- Cone Penetration Test (CPT): This test involves pushing a cone-shaped penetrometer into the soil at a constant rate, while measuring the resistance to penetration. The data can be used to determine soil strength, density, and other properties.
- Pressuremeter Test: This test involves inflating a cylindrical probe inside a borehole and measuring the pressure required to expand the probe. The data can be used to determine the in-situ stress-strain properties of the soil or rock.
- Shear Wave Velocity Test: This test involves measuring the speed of shear waves traveling through the soil or rock using seismic methods. The data can be used to determine the stiffness and density of the material.
- Crosshole Seismic Test: This test involves creating seismic waves at one location and measuring the waves at another location using sensors installed in boreholes. The data can be used to determine the shear wave velocity and other properties of the soil or rock.
- Thermal Conductivity Test: This test involves measuring the rate of heat flow through the soil or rock using a heat source and temperature sensors. The data can be used to determine the thermal properties of the material.
- Electrical Resistivity Test: This test involves measuring the electrical resistance of the soil or rock using probes inserted into the ground. The data can be used to determine the moisture content and other properties of the material.
These are just a few examples of the many different types of in-situ testing techniques that can be used in geotechnical engineering and site investigation. The choice of technique depends on the specific properties of the soil or rock materials, the site conditions, and the objectives of the investigation.
Laboratory testing is a crucial part of site investigation and is used to determine the physical and mechanical properties of soil and rock samples obtained from the site. These tests are important in determining the engineering properties of soil and rock, such as strength, permeability, compressibility, and deformation characteristics, which are used in the design and construction of engineering structures.
There are several laboratory tests that can be conducted on soil and rock samples, including:
- Grain size analysis: This test is used to determine the particle size distribution of soil. The test involves sieving the soil through a series of standard sieves and measuring the weight of soil retained on each sieve.
- Atterberg limits: This test is used to determine the plastic limit, liquid limit, and shrinkage limit of soil. The test involves measuring the moisture content of the soil at different stages of the test to determine the boundaries of the different states of consistency.
- Compaction test: This test is used to determine the maximum dry density and optimum moisture content of soil. The test involves compacting soil in a standard mold using a standard compaction energy and measuring the resulting density.
- Shear strength test: This test is used to determine the shear strength of soil or rock. The test involves applying a shear force to a soil or rock sample and measuring the resulting deformation.
- Permeability test: This test is used to determine the permeability of soil or rock. The test involves measuring the rate of flow of water through a soil or rock sample under a known hydraulic gradient.
- Consolidation test: This test is used to determine the rate and magnitude of settlement of soil. The test involves applying a load to a soil sample and measuring the resulting deformation over time.
- Rock mechanics testing: This includes tests such as uniaxial compression, triaxial compression, and direct shear tests, which are used to determine the strength and deformation characteristics of rock samples.
These laboratory tests are typically performed in accordance with standard testing procedures established by organizations such as ASTM International and the International Society for Rock Mechanics.
Environmental testing is the process of analyzing environmental samples to determine the presence and concentration of pollutants or contaminants. This type of testing is important for assessing the impact of human activities on the environment, and for ensuring compliance with environmental regulations.
Some common types of environmental testing include:
- Water testing: This includes the analysis of surface water, groundwater, and wastewater to determine the presence and concentration of pollutants such as heavy metals, organic compounds, and pathogens.
- Air testing: This involves the collection and analysis of air samples to measure the levels of pollutants such as particulate matter, volatile organic compounds (VOCs), and air toxics.
- Soil testing: This includes the analysis of soil samples to determine the presence and concentration of pollutants such as heavy metals, pesticides, and petroleum hydrocarbons.
- Sediment testing: This involves the analysis of sediment samples from lakes, rivers, and other bodies of water to determine the presence and concentration of pollutants such as heavy metals and organic compounds.
- Biological testing: This includes the analysis of biological samples such as fish, shellfish, and other aquatic organisms to determine the presence and concentration of pollutants that may be harmful to human health.
Environmental testing is typically performed by environmental consulting firms, government agencies, and other organizations that specialize in environmental monitoring and remediation. The results of environmental testing can be used to identify potential health risks, develop remediation plans, and ensure compliance with environmental regulations.
Remote sensing is the process of collecting information about a target or area from a distance, typically from aircraft or satellites, without making physical contact with the target or area. Remote sensing data can provide information about the physical, chemical, and biological properties of the Earth’s surface and atmosphere, as well as human activities and environmental changes over time.
There are two main types of remote sensing: passive and active. Passive remote sensing measures the energy that is naturally emitted or reflected by a target or area, such as sunlight, and records it using sensors. Examples of passive remote sensing instruments include cameras that capture visible light, infrared sensors that detect heat, and radiometers that measure the amount of energy emitted by a target. Active remote sensing, on the other hand, involves emitting energy towards a target or area and measuring the reflected or emitted energy using sensors. Examples of active remote sensing instruments include radar and lidar.
Remote sensing can be used in a variety of fields, including geology, agriculture, forestry, and urban planning. In geology, remote sensing can be used to identify and map geological structures, such as faults and folds, as well as to detect mineral deposits and changes in land use. It can also be used to monitor natural hazards, such as landslides and earthquakes.