Triaxial Shear Test on Soil | Procedure | Apparatus | Pros & Cons | Formula

This article is about Triaxial Shear Test on Soil, triaxial test procedure pdf, triaxial shear test pdf, triaxial shear test apparatus, triaxial test lab report, triaxial shear test formula, triaxial compression test and application of triaxial test.

Triaxial Shear Test on Soil

The triaxial shear strength test is utilized to determine the mechanical characteristics of soil. During the test, soil samples are subjected to stress that varies in perpendicular directions, allowing for measurement of different stresses resulting from each direction. By analyzing the resulting stresses, the soil’s properties such as shear resistance, cohesion, and dilatancy stress can be determined. This test is highly versatile and can be applied to all types of soils.

Triaxial Shear Test Apparatus

The triaxial shear test requires certain essential apparatus to be used. Firstly, a triaxial testing machine complete with a triaxial cell is required. This unit can accommodate a cylindrical soil specimen that is sealed using a rubber membrane to prevent any lateral fluid entry. The machine is capable of applying radial fluid pressure and vertical stress through a piston arrangement. Additionally, it is equipped with a system to prevent any drainage of the specimen. The pressure gauge is used to measure fluid pressure in the cell.

Secondly, equipment is needed for loading the specimen.

Finally, there must be equipment in place to measure load and deformation during the test. For measuring the load application by the piston, proving is used. On the other hand, deformation of the specimen is measured by a dial gauge.

What is Triaxial Test?

A triaxial test is a laboratory test used to determine the mechanical properties of soils, rocks, and other deformable materials. The test is called “tri”axial because it involves three independent stresses acting on the sample simultaneously. These stresses are the confining stress, the axial stress, and the pore pressure.

In a typical triaxial test, a cylindrical sample of the material is placed inside a cell filled with water or another suitable fluid. The cell is then pressurized with water to apply a confining stress on the sample. The axial stress is applied to the sample by compressing it using a hydraulic or mechanical system. During the test, the axial stress is gradually increased until the sample fails or reaches the maximum allowable stress.

Triaxial Shear Test Principle

The test specimen used in this experiment is cylindrical in shape and typically has a length to diameter ratio of 2. The test is conducted under conditions of axial symmetry, which means that the stress is applied along the axis of the cylinder.

The figure that accompanies this description likely illustrates the setup of the test, including any equipment used to apply stress to the specimen. However, without more information about the specific experiment being conducted, it is difficult to provide further details about the figure itself.

The sample size for the triaxial shear testing is either 76mm x 38mm or 100mm x 50mm. The sample is exposed to three principal stresses, out of which two are due to water pressure that exists within the confining cell. These two stresses are of equal magnitude. The third stress is caused by the load applied on top of the cell through a ram, and it differs from the other two stresses. The sample being tested is fitted with rubber O-rings at the bottom pedestal and top cap to seal the ends. Pore pressure measurement is conducted using porous stones placed at both ends of the specimen. Pressure transducers are employed to measure the pressure developed within the sample.

To conduct the triaxial shear testing, two steps are involved. The first step involves preparing the soil sample and setting it in the triaxial cell. Then, the confining pressure is applied. In the second step, an additional axial stress called deviator stress is applied to induce shear stresses within the sample. The axial stress is gradually increased until the soil sample fails. The stresses applied, axial strain, and the pore water pressure are measured in both steps.

Triaxial Shear Test Procedure

The preparation of soil specimens for triaxial shear testing can be done in two ways: remoulded or undisturbed. Undisturbed soil samples are suitable for testing when there is sufficient cohesion present. On the other hand, cohesive soils are collected and compacted properly to create remoulded specimens. Special care is taken during the preparation of cohesionless soils to ensure their integrity.

The triaxial shear test, which is commonly used for soil mechanics testing, has several variations. The most commonly employed types are the Unconsolidated Undrained Test (UU), Consolidated Undrained Test (CU), and Consolidated Drained Test (CD). These variations are used to test the soil samples under different conditions and to obtain different types of data.

The Unconsolidated Undrained Test (UU) involves testing an undisturbed or remoulded soil specimen without any prior consolidation. This type of test is useful for determining the shear strength of soft, saturated, and cohesive soils.

The Consolidated Undrained Test (CU) involves consolidating the soil sample under a specified load before shearing the sample without any drainage. This test is suitable for determining the shear strength of cohesive soils that are capable of withstanding some degree of consolidation.

The Consolidated Drained Test (CD) involves consolidating the soil sample under a specified load before shearing the sample while allowing drainage. This test is useful for determining the shear strength of granular soils and cohesive soils that are susceptible to significant consolidation.

1. Unconsolidated Undrained Test (UU)

The quick test is a method of soil testing where a soil sample is subjected to cell pressure without any provision for drainage. In this test, the cell pressure is kept at a constant value while the deviator stress applied to the sample is gradually increased until the sample fails. The purpose of this test is to determine the strength characteristics of the soil sample and to evaluate its behavior under stress.

By maintaining a constant cell pressure and increasing the deviator stress, researchers can observe how the soil sample responds to different levels of stress and identify its failure point. The quick test is a valuable tool for engineers and geologists to understand the properties of soil and make informed decisions about construction projects and other applications that involve soil mechanics.

2. Consolidated Undrained Test (CU)

The process involves applying cell pressure on the sample while allowing for drainage. Once this drainage is completed, the deviator stress is then applied while maintaining a constant cell pressure. However, there is no provision for further drainage during this stage of the process.

3. Consolidated Drained Test (CD)

The drained or slow test is a type of triaxial test used to determine the cohesion and angle of internal friction of a soil sample. The test involves gradually increasing the deviator stress while keeping the cell pressure constant and allowing drainage to occur. The loading rate is also applied slowly to prevent excess pore pressure from developing within the sample.

To perform the test, the soil specimen is enclosed in a membrane and placed in a triaxial cell. A desired lateral pressure is applied to the specimen, and this pressure is maintained until the specimen fails. During the test, measurements are taken of the vertical deformation and load readings.

To determine the cohesion and angle of internal friction values, the sample must undergo testing with three different lateral pressure values. The objective of the test is to obtain accurate measurements of these parameters to provide insight into the soil’s mechanical properties.

Triaxial Shear Test Formula

The test specimen is subjected to all around lateral pressure (σ3). The deviatoric stress applied be σd. Then total vertical stress is _σ1.

${\mathit{\sigma }}_1\mathbf{=}{\mathit{\sigma }}_d+ {\mathit{\sigma }}_3$

Mohr’s circle is a graphical method used to determine the stress state of a material. It involves plotting the normal stress and shear stress on the x-axis and y-axis, respectively. By drawing a circle based on these values, a visual representation of the stress state can be obtained. The Mohr’s rupture envelope is then determined by drawing a tangent to the circles obtained, which intercepts at the y-axis.

To obtain the value of cohesion (C), the y-intercept of the tangent line is determined. The angle of internal friction of the soil (ø) can be obtained from the slope of the failure plane or the tangent line.

If the loading increases the cross-section of the soil specimen, a correction for the deviatoric stress must be applied. This correction assumes that the volume of the specimen remains constant, but the area varied. The corrected deviatoric stress can then be obtained through this method.

${\mathit{\sigma }}_d\mathbf{=}\mathbf{(}{\mathit{P}}_1/A_0\mathbf{)} \times (1–\mathbf{\left(}{\mathit{I}}_1/I_0\mathbf{\right)})$

The given context describes the formula for calculating the shear resistance of a sample. The formula is dependent on several variables, including the applied load (P1), the original area of cross section (Ao), and the original length of the specimen (I0). Additionally, the formula takes into account the deformation of the specimen (l) in order to determine the shear resistance.

To rewrite the given context into paragraphs, we can start by introducing the formula and its main components. The shear resistance formula is used to calculate the ability of a sample to resist shearing forces. This formula relies on four key variables: the applied load (P1), the original area of cross section (Ao), the original length of the specimen (l0), and the deformation of the specimen (l).

The applied load, or P1, refers to the amount of force that is applied to the sample during testing. This force is measured in units of Newtons or pounds, and it plays a crucial role in determining the shear resistance of the sample.

The original area of cross section, or Ao, refers to the area of the sample that is perpendicular to the direction of the applied load. This area is measured in units of square meters or square inches, and it helps to determine the strength of the sample.

The original length of the specimen, or l0, refers to the length of the sample before any deformation occurs. This length is measured in units of meters or inches, and it serves as a baseline for determining the amount of deformation that the sample undergoes during testing.

Finally, the deformation of the specimen, or l, refers to the amount of lengthening or shortening that the sample experiences during testing. This deformation is measured in units of meters or inches, and it helps to determine the shear resistance of the sample.

Overall, the shear resistance formula is a complex equation that takes into account several key variables in order to determine the ability of a sample to resist shearing forces. By understanding the components of this formula, researchers and engineers can gain insights into the strength and durability of different materials and structures.

${\mathit{\tau }}_f\mathbf{=}\mathit{C}\mathbf{+} {\sigma }_1 \tan \left(\varphi \right)$

Fig.3: Determination of Stress parameters by Mohr’s Circle

Effective Stress Conditions

The figure displays the effective stresses exerted on a soil specimen during testing. It is important to note that the effective minor principle stress can be calculated by subtracting the pore pressure from the cell pressure (fluid pressure). On the other hand, the effective major principle stress can be determined by adding the deviator stress to the cell pressure.

Understanding these effective stress values is critical in soil mechanics, as they can greatly impact the behavior and stability of soil structures. By analyzing the effective stresses, engineers and geologists can make informed decisions regarding soil compaction, foundation design, and slope stability, among other applications.

Overall, the effective minor and major principle stresses play a significant role in soil mechanics and geotechnical engineering, as they provide insight into the behavior and characteristics of soil under various loading conditions.

The question appears to be asking for a clarification or expansion on a specific concept related to stress and pore pressure on a failure plane.

Overall, the context suggests that the question is related to the calculation of effective stress on a failure plane, which is an important concept in geotechnical engineering and soil mechanics.  Let the stress components on the failure plane MN be σ and τf and the failure plane is inclined at an angle α to the major principle plane.

Fig.4.Effective Stress Determination by Mohr’s Circle

Let the envelop DF cut the abscissa at angle ϕ’, C be the centre of the Mohr’s circle. From

$△ADC and △FDC$

 we get, 

$\alpha = \frac{\angle DCB}{2}=\frac{{90}^{\mathit{\circ }}\mathit{+}\varphi ‘}{2}={45}^{\circ }+\frac{\varphi ‘}{2}$

 Principle stresses relationships at failure

$DC=CB=Radius of Mohr‘s Circle=\frac{1}{2}({{\sigma }_1}^{‘} – {{\sigma }_3}^{‘})$ $OC=\frac{1}{2}\left({\sigma }_1 + {\sigma }_3\right)$ $OF=c cot\varphi ‘$

 Again from △CFD, 

$\sin \left(\varphi \right)=\frac{\mathrm{DC}}{\mathrm{FC}}=\frac{\mathrm{DC}}{\mathrm{OC}+\mathrm{FC}}=\frac{\frac{1}{2}\left({{\sigma }_1}^{‘} – {{\sigma }_3}^{‘}\right)}{\frac{1}{2}\left({{\sigma }_1}^{‘} + {{\sigma }_3}^{‘}\right)+c cot{\varphi }^{‘}}$ $=\frac{\left({{\sigma }_1}^{‘} – {{\sigma }_3}^{‘}\right)}{\left({{\sigma }_1}^{‘} + {{\sigma }_3}^{‘}\right)+c cot{\varphi }^{‘}}$

On solving this equation, we get

${{\sigma }_1}^{‘} ={\sigma }_3{\tan }^2 \left(45+\frac{\varphi }{2}\right)+2c^{‘} tan \left(45+\frac{\varphi }{2}\right)$ But $(45+\frac{\varphi }{2})={\alpha }^{‘}$ Therefore, ${{\sigma }_1}^{‘}\mathbf{=}{\sigma }_3{\tan }^2 {\alpha }^{‘} +2c^{‘} \tan {\alpha }^{‘} = {\sigma }_3{N_{\varphi }}^{‘} + 2 c^{‘} \sqrt{{N_{\varphi }}^{‘}}$

Mohr’s circle is a graphical representation used in soil mechanics to determine the shear strength parameters of soil. The circle is drawn by plotting the normal and shear stresses acting on a plane in the soil. From this circle, the straight line represents the failure plane of the soil, and the intercept created is known as the effective strength parameter c’. This parameter is an essential value used in determining the shear strength of the soil.

Furthermore, the angle formed by the failure plane is known as ø’, and this angle represents the angle of friction. By knowing the effective strength parameter c’ and the angle of friction ø’, one can calculate the effective shear strength of the soil. This effective shear strength parameter is crucial in designing and analyzing various geotechnical structures such as retaining walls, foundations, and slopes.

In conclusion, Mohr’s circle is a valuable tool in determining the shear strength parameters of soil. The effective strength parameter c’ and the angle of friction ø’ can be calculated from the circle, and these parameters are crucial in designing and analyzing geotechnical structures.

${{\mathit{\tau }}^{\mathit{‘}}}_f=C^{‘}+{{\sigma }_1}^{‘} \tan \left(\varphi ‘\right)$

Pore water pressure measurement

To obtain an accurate measurement of pore water pressure, it is crucial to ensure that there is no flow either in or out of the specimen. This is because any flow can alter the actual pressure. One way to maintain the no-flow condition is by measuring the pore water pressure at one end of the specimen while drainage occurs at the other end.

To achieve this, a null indicator can be utilized, which is essentially a U-tube partially filled with mercury. The null indicator helps to maintain the no-flow condition during the measurement process. By measuring the pore water pressure in this way, a more precise and accurate reading can be obtained, ensuring that the test results are reliable.

Advantages of Triaxial Test

The given context describes a specific testing scenario that involves several factors. Firstly, the stress distribution on the failure plane is uniform. This indicates that the stress is evenly distributed across the plane where failure is likely to occur. Secondly, the specimen is free to fail on the weakest plane, which means that the failure will occur on the plane that can withstand the least amount of stress.

Thirdly, there is complete control over the drainage. This means that any changes in the pore pressure can be accurately measured and controlled during the testing process. Fourthly, pore pressure changes and volumetric changes can be directly measured. This enables precise observations of any changes that may occur during the testing process.

Moreover, the state of stress at all intermediate stages up to failure is known. This means that the stress levels can be accurately measured at different stages of the testing process, providing detailed information about the behavior of the specimen. Additionally, the Mohr circle can be drawn at any stage of shear, allowing for visual representation of the stress levels and their relationship to the failure plane.

Finally, the test is suitable for accurate research work, and the apparatus can be adapted to special requirements, such as extension tests and tests for different stress paths. This indicates that the testing process can be customized to suit the specific needs of the research being conducted.

Disadvantages of Triaxial Test

The equipment used for this testing procedure is quite intricate, bulky, and expensive. As a result, it may not be easily accessible to all individuals or organizations that may wish to conduct these tests. Furthermore, the drained test, when compared to a direct shear test, tends to take a longer period to complete.

A significant limitation of this test is the inability to determine the cross-sectional area of the specimen accurately, particularly at larger strains. This is due to the fact that the assumption that the specimen remains cylindrical is not accurate. Therefore, the accuracy of the test results may be compromised.

Another drawback of this testing procedure is the non-uniformity of the strain conditions within the specimen. The loading cap and pedestal disc produce frictional restraint that leads to the formation of dead zones at each end of the specimen. This means that the strain conditions are not uniform, and the test results may not be reliable.

Lastly, the consolidation of the specimen in this test is isotropic. In contrast, the consolidation that occurs in the field is typically anisotropic. This difference in consolidation conditions can affect the accuracy of the test results, as the behavior of the specimen in the lab may not be representative of its behavior in the field.

Triaxial Test Procedure [PDF]

The procedure for performing a triaxial shear test involves several steps. Here is a general overview of the procedure:

1. Sample preparation: A cylindrical sample of the material to be tested is prepared with a diameter typically ranging from 50 mm to 100 mm and a height to diameter ratio of 2:1 or 1:1. The sample is trimmed to a uniform height and its surface is smoothed and perpendicular to its axis.
2. Sample saturation: The sample is saturated with water to eliminate any air voids, and the sample is then allowed to drain to the desired degree of saturation. The saturation can be performed using various methods such as vacuum saturation or back-pressure saturation.
3. Specimen installation: The sample is placed into the triaxial cell with a porous stone or filter paper on either end to allow water to drain out.
4. Application of confining pressure: The triaxial cell is filled with water, and the confining pressure is applied to the sample through the water pressure in the cell. The confining pressure is usually increased gradually until the desired pressure is reached.
5. Axial loading: An axial load is applied to the sample through a piston, which is typically applied at a rate of 0.01 to 0.1 mm/min. The load is increased until failure occurs.
6. Pore pressure measurement: During the test, the pore water pressure within the sample is measured using a pore pressure transducer. This measurement is important to assess the drainage characteristics of the sample.
7. Data collection: Throughout the test, axial deformation and confining pressure are measured, and the axial and radial stresses are calculated. These data are used to plot a stress-strain curve and determine the shear strength parameters of the material.
8. Post-test analysis: After the test, the sample can be dissected to examine the internal structure and confirm the failure mechanism.

The triaxial shear test can be performed in different variations, such as drained and undrained tests, consolidated-undrained and consolidated-drained tests, cyclic triaxial tests, and dynamic triaxial tests. The test variation depends on the specific requirements of the test and the material being tested.

Triaxial Test Lab Report

A triaxial test lab report typically includes several sections that describe the test procedure, results, and conclusions. Here are the general sections that can be included in a triaxial test lab report:

1. Introduction: This section describes the purpose and background of the triaxial test, including the material being tested, the test objective, and the significance of the test results.
2. Specimen description: This section provides a detailed description of the sample used in the test, including its dimensions, physical properties, and any relevant information about its origin and preparation.
3. Test procedure: This section outlines the steps taken to perform the triaxial test, including the equipment used, the test conditions, and the measurement techniques.
4. Test results: This section presents the data collected during the triaxial test, including the stress-strain curve, the shear strength parameters, and any other relevant test results such as pore pressure measurements or deformation characteristics.
5. Discussion: This section interprets the test results and relates them to the objectives and significance of the test. The discussion may include a comparison of the results to expected values or other relevant data, and an explanation of any deviations from expected results.
6. Conclusion: This section summarizes the key findings of the triaxial test and their implications, including any recommendations or future work that may be necessary based on the results.
7. References: This section lists any sources cited in the lab report, such as relevant literature, standards, or other resources.

It is important to follow the lab report format provided by your instructor or institution, as there may be specific requirements or guidelines for your report. Additionally, the content and structure of the lab report may vary depending on the specific material being tested and the objectives of the test.

FAQs about Triaxial Shear Test

1. What is the advantage of triaxial test in soil?

The triaxial test is a type of mechanical test used to determine the shear strength of soils. The test involves the application of an axial load and an external confining pressure on a cylindrical soil sample. The advantage of the triaxial test over other shear strength tests lies in its ability to provide more accurate and reliable results, as well as its ability to simulate the actual stress conditions experienced by soils in the field.

Here are some specific advantages of the triaxial test in soil mechanics:

1. Ability to measure pore water pressure: One of the primary advantages of the triaxial test is that it allows for the measurement of pore water pressure during the test. Pore water pressure can significantly affect the shear strength of soils, so being able to measure it allows for a more accurate determination of shear strength.
2. Control of drainage conditions: Another advantage of the triaxial test is that it allows for greater control of the drainage conditions of the soil sample. This can help to ensure that the test conditions more closely match the in-situ conditions that the soil will experience.
3. Ability to test both drained and undrained conditions: The triaxial test can be performed under both drained and undrained conditions. This allows for a more comprehensive assessment of the shear strength of soils and can provide valuable information for design and analysis.
4. Ability to simulate different stress states: The triaxial test can simulate different stress states, such as isotropic, uniaxial, and triaxial stress conditions, which are commonly encountered in the field. This allows for a more comprehensive understanding of the behavior of soils under different stress conditions.

2. What is the importance of triaxial shear test?

The triaxial shear test is an important method for measuring the mechanical properties of soils, rocks, and other granular materials. It involves subjecting a cylindrical sample of the material to a controlled axial load and confining pressure, while measuring the deformation and stress responses of the sample. Here are some of the key importance of triaxial shear test:

1. Accurate measurement of shear strength: The triaxial shear test provides a more accurate measurement of the shear strength of soils and rocks than other laboratory tests. It allows for the measurement of both drained and undrained shear strengths, and shear strength parameters such as the angle of internal friction and cohesion.
2. Simulation of real-world conditions: The triaxial shear test can simulate different stress states, such as isotropic, uniaxial, and triaxial stress conditions that are encountered in real-world situations. This allows for a better understanding of the behavior of soils and rocks under realistic stress conditions.
3. Prediction of deformation and stability: The results of the triaxial shear test can be used to predict the deformation and stability of geotechnical structures, such as foundations, retaining walls, and slopes. This information is crucial for designing and analyzing these structures.
4. Study of cyclic loading behavior: The triaxial shear test can also be used to study the behavior of soils and rocks under cyclic loading conditions, which are common in geotechnical applications such as earthquake engineering and offshore structures.
5. Evaluation of environmental effects: The triaxial shear test can be performed under different environmental conditions, such as different temperatures and moisture content, allowing for the evaluation of the effects of these conditions on the behavior of soils and rocks.