Agricultural Engineering Exam  >  Agricultural Engineering Notes  >  Soil Mechanics Notes- Agricultural Engineering  >  Types of Triaxial Tests - Strength Parameters of Soil, Soil Mechanics

Types of Triaxial Tests - Strength Parameters of Soil, Soil Mechanics | Soil Mechanics Notes- Agricultural Engineering PDF Download

Various Types of Tri-axial Tests

In triaxial test, tests are conducted in two stages, Stage I: under all round cell pressure (s3) and Stage II: under shearing or loading (as shown in Figure 12.1). During all round cell pressure, if drainage is allowed the consolidation takes place in the sample. This type of sample is called consolidated sample. However, if drainage is not allowed then the sample is called unconsolidated sample. During shearing or loading (Deviatoric stress, Δσd = σ1 - σ3), if drainage is allowed the loading is called drained loading. However, if drainage is not allowed then the loading is called undrained loading. Figure 12.2 shows a triaxial test setup. The drainage in the sample is controlled by closing or opening the drainage valve. The all round cell pressure is applied by using water inside the triaxial cell.  



Types of Triaxial Tests - Strength Parameters of Soil, Soil Mechanics | Soil Mechanics Notes- Agricultural Engineering

Fig. 12.1. Stages of Tri-axial tests.

Types of Triaxial Tests - Strength Parameters of Soil, Soil Mechanics | Soil Mechanics Notes- Agricultural Engineering

Fig. 12.2. Tri-axial tests test setup

The usual sizes of the samples are: 76 mm (length) x 38 mm (diameter) or 100 mm (length) x 50 mm (diameter). Thus, the length/diameter ratio of the cylindrical sample is 2. This test is suitable for both sand and clay. Depending on whether drainage is allowed or not during all round cell pressure and shearing, three types of triaxial tests are conducted:

(i) Unconsolidated Undrained (UU) test

(ii) Consolidated Undrained (CU) test

(iii) Consolidated Drained (CD) test

Unconsolidated Undrained (UU) test

In this type of test, pore pressure is developed during shearing. However, the pore water pressure is not measured. Thus, effective stress value is unknown. The parameters are determined in terms of total stress (cu). It is a very quick test. The determined parameters are used for the analysis under undrained condition such as short term stability.

Consolidated Undrained (CU) test

In this type of test, pore pressure is developed during shearing and it is also measured. Thus, effective stress value is known. The parameters are determined in terms of effective stress (c' and Ø' ). It is faster than CD test, but slower than the UU tests. This test is preferred to determine c' and Ø'.

Consolidated Drained (CD) test

In this type of test, no excess pore pressure is developed during the whole test. Shearing is done very slowly to avoid build-up the excess pore water pressure. The parameters are determined in terms of effective stress (c' and Ø'). It is a very slow test (can take few days). The determined parameters are used for the analysis under fully drained condition such as long term stability.

In general, during the test, deviator stress (σ- σ3), all round pressure (σ3) and pore water pressure is measured (for example in CU test). From the test data, (σ1)f value at failure is determined for different cell pressure (σ3). After subtracting the pore water pressure, (σ1)'f  and σ'3  are determined. Mohr-Circles are drawn for different (σ1)'f  and σ'3 values. Failure envelop is drawn by drawing a common tangential line for all the circles (as shown in Figure 12.3). From that line, strength parameters c' and Ø are determined. Figure 12.4 shows typical deviator stress-axial strain and volume change-axial strain plot of soils obtained from CD trai-axial tests.

Types of Triaxial Tests - Strength Parameters of Soil, Soil Mechanics | Soil Mechanics Notes- Agricultural Engineering

Fig. 12.3. Shear stress-normal stress plot.

Types of Triaxial Tests - Strength Parameters of Soil, Soil Mechanics | Soil Mechanics Notes- Agricultural Engineering

Fig.12.4. Deviator stress-axial strain and volume change-axial strain plot of

               soils obtained from CD t Tri-axial tests.

The document Types of Triaxial Tests - Strength Parameters of Soil, Soil Mechanics | Soil Mechanics Notes- Agricultural Engineering is a part of the Agricultural Engineering Course Soil Mechanics Notes- Agricultural Engineering.
All you need of Agricultural Engineering at this link: Agricultural Engineering
64 docs

FAQs on Types of Triaxial Tests - Strength Parameters of Soil, Soil Mechanics - Soil Mechanics Notes- Agricultural Engineering

1. What are the different types of triaxial tests used to determine the strength parameters of soil in soil mechanics?
Ans. The different types of triaxial tests used to determine the strength parameters of soil in soil mechanics are as follows: 1. Unconsolidated Undrained (UU) test: In this test, the soil sample is subjected to an axial load without any consolidation, and the drainage of water is prevented. This test is commonly used for quick assessment of the undrained shear strength of cohesive soils. 2. Consolidated Undrained (CU) test: In this test, the soil sample is first consolidated under a vertical load and then subjected to an axial load without any drainage. This test is used to determine the undrained shear strength of cohesive soils under consolidated conditions. 3. Consolidated Drained (CD) test: In this test, the soil sample is first consolidated under a vertical load and then subjected to an axial load with drainage allowed. This test is used to determine the drained shear strength and the stress-strain behavior of soils under drained conditions. 4. Stress Path (or K0) test: This test involves subjecting the soil sample to a sequence of different stress paths, allowing for the determination of various strength parameters such as the coefficient of earth pressure at rest (K0) and the angle of internal friction. 5. Dynamic (or cyclic) triaxial test: This test involves subjecting the soil sample to cyclic loading and unloading, which simulates the stress conditions experienced by soil under dynamic or seismic loading. It is used to determine the cyclic shear strength and the soil's response to cyclic loading.
2. What is the purpose of conducting triaxial tests in soil mechanics and agricultural engineering?
Ans. The purpose of conducting triaxial tests in soil mechanics and agricultural engineering is to determine the strength parameters and stress-strain behavior of soil. These tests provide valuable insights into the mechanical properties of soil, which are crucial for designing and analyzing various geotechnical and agricultural structures. Triaxial tests help in understanding the shear strength, deformation characteristics, and stability of soil under different stress conditions. They also help in evaluating the suitability of soil for different engineering and agricultural applications, such as foundation design, slope stability analysis, and crop cultivation.
3. How are the strength parameters of soil determined from triaxial tests?
Ans. The strength parameters of soil, such as the cohesion (c) and the angle of internal friction (φ), are determined from triaxial tests using the Mohr-Coulomb failure criterion. The test involves applying a confining pressure (σ3) on the soil sample and then subjecting it to an axial load until failure occurs. The deviator stress (σ1-σ3) and the axial strain are measured during the test. By plotting the deviator stress against the axial strain, a stress-strain curve is obtained. The peak deviator stress represents the shear strength of the soil, and the angle of internal friction can be calculated using the peak deviator stress and the confining pressure. The cohesion can be calculated as the intercept of the stress-strain curve with the deviator stress axis.
4. What are the applications of triaxial tests in agricultural engineering?
Ans. Triaxial tests have several applications in agricultural engineering, including: 1. Soil compaction: Triaxial tests can be used to determine the compacted strength of soil, which is important for designing agricultural machinery and assessing the compaction effects of heavy machinery on soil structure. 2. Soil erosion control: Triaxial tests can help in evaluating the shear strength and stability of soil under different moisture conditions, which is crucial for designing erosion control measures such as retaining walls and slope stabilization techniques. 3. Crop root growth: Triaxial tests can provide insights into the mechanical behavior of soil under different stress conditions, helping in understanding the impact of soil strength on crop root growth and development. 4. Irrigation management: Triaxial tests can be used to assess the hydraulic properties of soil, such as permeability and water retention capacity, which are important for efficient irrigation management in agricultural fields. 5. Soil structure interaction: Triaxial tests can help in studying the interaction between soil and agricultural structures, such as foundations, retaining walls, and silos, allowing for the design of safe and stable structures in agricultural settings.
5. What are the limitations of triaxial tests in determining the strength parameters of soil?
Ans. Some limitations of triaxial tests in determining the strength parameters of soil include: 1. Sample disturbance: The process of sample preparation and installation in the triaxial apparatus can cause disturbance to the soil sample, leading to altered strength parameters compared to the in-situ conditions. 2. Scale effects: Triaxial tests are typically conducted on small soil samples, which may not accurately represent the behavior of large-scale soil masses. The strength parameters obtained from small-scale tests may not scale up accurately. 3. Time-dependent behavior: Triaxial tests are usually conducted under static loading conditions, whereas in real-life scenarios, soil is subjected to dynamic or cyclic loading. The time-dependent behavior of soil may not be fully captured in conventional triaxial tests. 4. Simplified stress conditions: Triaxial tests typically apply a uniaxial stress state, whereas soil in the field is subjected to more complex stress conditions. The simplified stress conditions may not fully capture the true behavior of soil. 5. Cost and time: Triaxial tests can be time-consuming and expensive, requiring specialized equipment and expertise. Conducting a large number of tests may not be practical in some situations, limiting the amount of data available for analysis.
64 docs
Download as PDF
Explore Courses for Agricultural Engineering exam
Signup for Free!
Signup to see your scores go up within 7 days! Learn & Practice with 1000+ FREE Notes, Videos & Tests.
10M+ students study on EduRev
Related Searches

study material

,

Semester Notes

,

mock tests for examination

,

Types of Triaxial Tests - Strength Parameters of Soil

,

Free

,

ppt

,

Types of Triaxial Tests - Strength Parameters of Soil

,

Previous Year Questions with Solutions

,

past year papers

,

Important questions

,

practice quizzes

,

Soil Mechanics | Soil Mechanics Notes- Agricultural Engineering

,

Viva Questions

,

shortcuts and tricks

,

Soil Mechanics | Soil Mechanics Notes- Agricultural Engineering

,

pdf

,

Sample Paper

,

Exam

,

Objective type Questions

,

video lectures

,

Summary

,

Types of Triaxial Tests - Strength Parameters of Soil

,

Extra Questions

,

MCQs

,

Soil Mechanics | Soil Mechanics Notes- Agricultural Engineering

;