geotechnical investigation or geotechnical studies is a specialized branch of civil engineering that aims to determine the properties of soil and rock engineering and how they interact with future civil development.
The purpose of these geotechnical investigation or geotechnical studies, also called site investigation, is to determine the variables for humans and the environment in order to:
1) Design:
- Foundation design
- Infrastructure design
- Underground structures design
2) Risk estimates:
- Geotechnical
- Environmental
- Hydrogeological
- Geological
By conducting geotechnical investigationor geotechnical studies in construction projects, higher safety and optimal performance can be achieved.
site investigation can be done for different purposes:
- Locating the project
- Earth retention or Stabilization of the excavation
- Foundation design
- Ground improvement
- Project effects on the environment
- Checking the safety conditions and stability of the existing structure
- Structural reinforcement
Geotechnical investigation Method (site investigation)
Geotechnical investigation are done in 2 parts:
1) In-situ tests: Detailed study of the project site and collection of required data, as well as drilling of boreholes, in-situ tests such as standard penetration test (SPT), cone penetration test (CPT), Vane shear test, persiometer, plate loading and in situ cutting, permeability measurement (Logan and Lofran).
2) Laboratory studies: Performing the required tests, including physical, chemical and mechanical tests depending on the type and composition of soil, such as:
- Particle Size Analysis:
Due to the importance of understanding the distribution and size of soil particle, particle size testing is performed in most laboratories.
Particle size testing is done in two main ways:
- Sieve: In the sieving method, the weight percentage of different soil particles is obtained by using a set of sieves and shaking them. This test is often used to separate particles larger than 200 sieves and determine the percentage of sand in the soil.
- Hydrometer Analysis: Hydrometric testing is also used in fine-grained soil to evaluate the distribution of particles in the soil and to determine the percentage of silt and clay.
Hydrometer Analysis equipment
- Atterberg limits test:
Fine-grained soils take on different modes as the amount of water absorbed increases. Increased water causes the particles to be covered with a layer of surface adsorbed water. By adding more water, the thickness of the water layer around the particles is added, making it easier for the particles to slide on top of each other. Therefore, soil behavior practically depends on the amount of water in the complex.
Fine-grained soil can be classified into solid, semi-solid, pasty and liquid according to its humidity content. The distance between these cases is shown in the following graph:
Liquid Limit (LL) (Boundary between pasty mode and soil liquid mode): It is the percentage of humidity in which at this humidity and higher humidity, the soil acts as a viscous fluid.
Plastic Limit (PL) (Boundary between pasty and semi-solid mode): It is the percentage of humidity in which the soil behaves in a pasty form before the liquid limit.
Shrinkage limit (SL) (boundary between semi-solid and solid mode): It is the percentage of humidity that if the soil humidity is less, there is no change in soil volume.
Plasticity index (PI) (difference between the liquid limit and the soil limit): The percentage of humidity that must be added to the soil to reach the liquid limit from the pasty limit.
Liquid index (LI): It is equal to the ratio of the difference between the percentage of soil humidity in the site with dividing plastic limit on the paste index.
- Permeability Test
Due to the empty spaces inside the soil, it is possible for water to flow from high energy points to low energy points. Therefore, in general, soil can be considered permeable. Therefore, it is necessary to estimate the amount of groundwater seepage under different hydraulic conditions, to investigate issues related to water pumping in underground construction operations, and to analyze the stability of earth dams and soil retaining structures under seepage forces. High permeability indicates that water flows rapidly through soil cavities. The permeability coefficient in the laboratory is done in 2 ways:
- Constant head test
- Falling head test
- Triaxial Test
The soil specimen is placed in a chamber and then all-round pressure is applied to it. In this case, if the water outlet valves are left open and the test is slow enough, the specimen will consolidate and the pore pressure will reach zero. After this stage, the specimen is failed by applying deviator stress. If at this stage, the water outlet valves are kept open and the test speed is low enough, the specimen will be drained. This test is repeated under different lateral pressures, then the mohair circles and their cover are drawn, the slope of the failure line, the angle of internal friction of the soil and the width of its origin give the soil adhesion. This test is performed in three ways:
1) Consolidated Drained (CD)
Stage 1: First, all-round pressure is applied and the specimen is then allowed to drain completely until the pore water pressure drops to zero.
Stage 2: Deviator stress is applied slowly (at low speed) and drainage is allowed to reduce the pore water pressure to zero. At the moment of failure, the pore water pressure will be zero.
Therefore, for the CD test at the moment of failure, we have:
2) Consolidated un-drained (CU)
Stage 1: First, all-round pressure is applied and the specimen is then allowed to drain completely until the pore water pressure drops to zero.
Stage 2: Deviator stress is applied slowly (at low speed) and drainage is not allowed, so at this stage the amount of pore water pressure will not be zero.
Therefore, for a CU test at the moment of failure, we have:
3) Un-consolidated Un-drained test (UU)
Stage 1: First, all-round pressure is applied but drainage is not allowed to the specimen. Therefore, at this stage, the amount of pore water pressure will not be equal to zero.
Stage 2: Deviator stress is applied slowly (at low speed) and drainage is not allowed.
Therefore, for the UU test at the moment of failure, we have:
- Direct Shear Test
In direct shear test, a soil specimen is placed inside the device, which consists of two separate parts. The lower part of the device is fixed and cannot be moved, but the upper part of the device can be moved on the lower part and its displacement is measured during the test. In the direct shear test, a vertical load is first applied to the specimen. Then a shear load is applied to the upper half of the machine until the specimen fails. The shear and vertical stress at the moment of failure is equal to:
Where, A is the cross section of the failure. A number of tests are repeated with different vertical stresses. The internal friction angle of the test specimen will be equal to the slope of the line obtained:
- Consolidation Test
One-dimensional consolidation tests are usually performed on fine-grained soils. In this experiment, the soil specimen is contained in a metal chamber and placed between two porous rocks in a cylindrical container filled with water. A steel loading plate placed on a porous rock inserts a vertical load into the specimen. This load causes the soil to settle. The load is gradually increased and loading and unloading sequences are applied to the specimen and specimen settlements are harvested under each loading stage at specific times. At each stage, the loading continues until the specimen settling is negligible and the overpressure created under that load is removed.
- · Unconfined Compression Test
Uniaxial compressive test (Unconfined compressive strength) is a special mode of UU triaxial test in which the overall stress is zero. In this test, the deviator stress applied to the specimen causes it to failure.
- Chemical tests
- California bearing ratio test(CBR)
This test was used in 1929, before World War II, at the California Department of Roads. With the help of this test, the shear strength of the soil at a certain humidity and specific gravity is determined.
This variable is very important in:
- Foundation to express the relative quality of the soil under the foundation
- In road construction to obtain soil shear resistance to traffic
By definition, CBR test is the load required to penetrate a standard piston with a certain shape, speed and depth in a test specimen to the force required to di in a piston with the same characteristics in standard materials.
The more resistant the soil surface, the higher the CBR rate. The standard material for this test is crushed California limestone with a CBR of 100, indicating that observations of more than 100 in dense soils are not unexpected. The following equation shows the method of obtaining this value where p is the pressure measured at the site and ps is the pressure required to achieve equal penetration in standard soil.
- Compaction Test
In many earthen structures, such as dams, retaining walls, highways and airports, soil compaction is essential.
Soil compaction is the reduction of soil volume by the reduction of air using force. In this case, the friction between the particles increases and its unit weight increases. This weight is a measure of soil compaction.
Compaction is done to increase the shear strength and decrease the permeability. Factors affecting soil compaction include:
- Soil type
- Energy
- Humidity
In 1933, Proctor invented a method for determining the maximum specific gravity of dry soil. This test is known as the Proctor compaction test.
Proctor test is done in two ways:
1) Standard method (manual compaction): This method uses a hammer weighing 5.5 pounds and a fall height of 13 inches (one foot) and 56 strokes in three layers of compaction and a diameter of 6 inches.
2) Modified method (advanced compaction): This method uses a hammer weighing 10 pounds and a fall height of 18 inches (one and a half foot) and 25 strokes in five layers of compaction and a diameter of 4 inches.
- Compressive strength of concrete test
Compressive strength is defined as the strength to failure under the compressive load. Compressive strength of concrete is one of the important characteristics of concrete. In different projects of concrete parts, at least 3 specimens are prepared. Concrete specimens made under compressive force are tested for compressive strength. Compressive strength is calculated by dividing the fracture load on the cross section of the specimen, and it is usually calculated after 28 days of concrete storage and curing. The lower the water to cement ratio, the compressive strength is higher.
Finally, after collecting the results of field and laboratory tests, by reviewing the above results and eliminating invalid results, the existing conditions were interpreted and based on it in the form of geotechnic studies report, technical and executive recommendations related to the project based on valid domestic and foreign standards are provided, which will be approved by the Tehran Province Engineering Organization if necessary.
Applications of geotechnical investigation or geotechnical studies
In addition to the importance of geotechnical studies in urban, residential and commercial constructions, the following applications can also be enumerated:
- Oil and gas projects and construction of specialized structures
- Projects related to the construction of power transmission lines
- Construction of dams and hydropower plants
- Project consulting and water supply projects and industrial and municipal sewage systems
- Urban, residential and commercial construction projects
- Mine exploration and drilling
The purposes of geotechnical investigation and geotechnical studies
The most important purposes and results that can be obtained from geotechnical investigation and geotechnical studies:
- Complete identification of underground layers
- Determining the physical and mechanical properties of the layers
- Specifying the type of foundation of the building and how to install it
- Suitable depth for foundation
- Estimating the bearing capacity of the foundation and determining the relevant coefficient
- Soil specific gravity
- Effective internal friction angle
- Cohesion coefficient
- Modulus of elasticity
- Land type classification according to Iran Regulations 2800 (type 1, 2, 3 or 4)
- The most suitable type of cement
- Concrete testing
- Survey of groundwater in the area
- Determining the seismicity of the area
- Extracting detailed information from bedrock
- Geological and seismic studies
- Location of possible previous earthquakes
- Determining the distance of faults close to the project
- Hydrometry
- Soil particle size test
- Atterberg limits
- Density determination
- Soil humidity
- Determining the type and composition of soil in the area
- Consulting on selecting and designing the best type of foundation
- Soil stability tests against natural disasters such as earthquakes
- Correct estimation of the bearing capacity of the foundation of the structure
- Calculation of soil stress and lateral pressure
- Excavation consulting
- Providing a solution to optimize the foundation of the structure
- Determining the results of soil and rock mechanics tests
- Soil and concrete quality control tests
- Studies and execution of reinforcing operations
- Weld control
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