Soil Elastic Modulus Measurement Using Triaxial Test - Coursework Example

Research proposal Supervisor: Name: Date: \ Contents Table of Contents 1. project title………………………………………………………………………………..3 2. Introduction(brief)……………………………………………………………………..….3 3. Project definition………………………………………………………………………….4 3.1 key objective to be investigated………………………………………………………4 3.2 project benefit…………………...………………………………………………….....5 3.3 project deliverable……………………………………………………………………..5 4. Literature review ………………………………………………………………………….6 5. Brief research strategy…………………………………………………………………...10 6. Experimental procedure………………………………………………………………….11 6.1 measurement system for the experiment……………………………………………..12 7. Tentative brief timeline…………………………………………………………………..12 8. Reference list…………………………………………………………………………….13 1. Project Title Soil elastic modulus measurement using triaxial test 2. Introduction(Brief) The triaxial test is a procedure performed in most laboratories; the test is used to determine the shear strength parameters for variety of soils types under different conditions. The conditions include drained and undrained soil property. The triaxial tests are used with other tests to make predictions for most engineering applications. The basic concept applied in performing the triaxial shear test is that when stress is applied in the vertical direction along the axis of a cylindrical sample the resulting stress is different from stresses applied in the horizontal direction perpendicular to the sides of the cylindrical sample (BISHOP and HENKEL, 1962, p.12). When performing the triaxial test on homogonous and isotropic materials, it results in a non hydrostatic stress state, with the shear stress in certain occasions leading to the failure of the sample in shear. In homogonous and anisotropic materials the resulting shear stress may lead to failure due to the bending moments. There are various types of triaxial test. They include: Consolidated Drained (CD), consolidated undrained (CU), and unconsolidated undrained (UU).In CD, the sample is consolidated and sheared while the compression results in drainage. In CU, the given sample is not allowed to drain and is compressed at a constant rate. In UU, the sample is not allowed to drain and is compressed at a constant rate (BISHOP and HENKEL, 1962, p.25). . To perform the triaxial test on soil, a cylindrical soil sample is used. The soil sample is confined within a rigid bottom and top plate which is covered in a rubber membrane. A vertical load is then placed on top of the sample. Horizontal pressure is then applied on the sides using water (Hydrostatic pressure). The rubber membrane is used to prevent water from penetrating. Throughout the experiment the water pressure is constant. By increasing vertical pressure at a constant velocity the soil sample gradually compresses until shearing occurs. From the triaxial test the following shear strength parameters are derived. They include the cohesion, angle of internal friction and dilatancy angle. Rigid parameters such as the Young modulus of elasticity can also be determined (DONAGHE, CHANEY, and SILVER, 1988, p.6). The soils elastic modulus is a soil parameter measuring the stiffness of soil. It is defined as the ratio of stress along an axis (Vertical axis) over the strain in that along that axis provided it is within the range of elastic soil behavior. The Young’s elastic modulus is useful in estimation of soil settlement and in elastic deformation analysis. In practice, the elastic modulus and soil stiffness depend on consistency and density of the given soil under investigation. There are typical values of young’s elastic modulus for the different granular materials in various scientific books and journals (HELWANY, 2007, P.36). 3. Project definition 3.1 Key objectives to be investigated The main focus of this research is using the triaxial test to determine soil elastic modulus of a given soil sample. The test will be carried out and the results collected and analyzed to find the Young elastic modulus and other parameters . 3.2 Project benefits Performing of the triaxial compression test has a number of applications in the engineering industry. It is very useful in making predictions by obtaining the strength parameter for design. It provides parameters for hydraulic fracturing and the parameters needed in constitutive modeling for numerical stability calculations. The triaxial test also provides data for determining the failure locus (Mohr envelope).The test is also an important component in wellbore stability, sand production and subsidence calculations. Obtaining the soils shear strength parameter for the various types of soils in drained and undrained conditions and finding Young elastics modulus is the largest benefit for engaging in this particular experiment (FREDLUN and RAHARDJO, 1993, p.105). . 3.3 Project deliverable In this project the following parameters are considered: coefficient of internal friction cohesion consolidations effective stress Consolidations, properties of materials shear test pole pressure Triaxial shear test Type of soils The above factors are mainly considered in the experiment. The following terms cannot be controlled they include: soil properties, strain rate and triaxial testing machine (DONAGHE, CHANEY, and SILVER, 1988, p.18). 4 Literature review Brief explanation of Soil elastic modulus measurement using triaxial test The triaxial test is constructed so as to provide complete load and deformation data. This method is used to determine the rock and soil stress –strain behavior and their shear strength. The triaxial test is designed to permit separate control of both the lateral and axial loading. This enables the triaxial test provides data on the fundamental response characteristics of soil and rock under a wide variety of controlled states of shear HELWANY, 2007, P.76). The test permits control of the loading rate, drainage conditions and the size of the specimen used. For the application of separate lateral and axial stress paths different separate pressure controlling systems are used. The application of the axial load is achieved by various methods depending on whether the result is desired to be stress controlled or strain controlled. The specimen is loaded to failure and strain control method is applied. However, when the given specimen is studied at less than failure stress levels then stress control methods are applied. The sample is loaded directly or alternatively a lever is used (TTT WORKSHOP, et al., p.56). The standard triaxial test configuration involves placing the test specimen in a cylindrical chamber. The axial load is applied using a piston, the piston enter through a sealing device that is found on top of the apparatus. The inside of the chamber is generally pressurized to provide lateral loading by confining pressure on to the test specimen. A membrane is then used to seal the cylindrical specimen from the chamber fluid. The specimen is usually sandwiched between two plates that ensure the transfer of load between the piston and the specimen. The triaxial specimen used has a cylindrical shape with a diameter to height ratio of nearly 1:2.Axial load is applied to the z axis while lateral load is applied radically along the r axis (TTT WORKSHOP, et al., p.74). During the analysis of the triaxial specimen we can safely assume that the tangential stress and strain are equal in magnitude to the radial stress strain. We also assume that the even stress and strain distribution throughout the specimen. Considering the specimen is confined inside the chamber during loading the confining pressure (ar ) acts both radially and vertically. Therefore the total axial loading of the specimen is sum of axial force of the piston and the axial force as a result of the confining pressure (HOULSBY and WROTH, 1993.p.78) Axial stress ( aa) is then determined by taking the total axial force and dividing it the perpendicular area. At this moment we assume that the specimen is elastic, isotropic, and homogenous and deforms like a cylinder. The following states of stress can be applied on the specimen with typical stress –strain relationship (NOVA, 2010, p.52). Where ar=Confining pressure and aa=Axial stress For hydrostatic state where ar =aa The mean normal pressure p=aa+2ar 3 Plotting a (pressure -volumetric) strain curve, the slope gives the bulk modulus (k). Plotting a deviator stress (aa-ar) verses the axial strain; the slope is the young modulus of elasticity. Plotting the (deviator stress verses the deviator strain); the slope is the shear modulus G. It is important to note that the yield strength can also be determined. Yield strength values obtained from such experiments usually describe lower bound yield envelope (HOULSBY and WROTH, 1993.p.65). . Figure 1: Triaxial apparatus Advantages of using the triaxial test The major benefits of performing this test over simpler procedures such as direct shear test include: The ability to the drainage of the test specimen The ability to take measurement of pore water pressure The method is more accurate and precise compared to the direct methods therefore is suitable for research work and the apparatus are adaptable to special requirements such as the extension test The test specimen used in the experiment is free to fail on the weakest plain Stress distribution of the failure plane is uniform The Mohr circle can be drawn at any point during any stage of shear Disadvantages of the triaxial test The triaxial test however has certain disadvantages. They include: The apparatus used to perform the test is expensive, bulky and very complex Performing the drained test takes longer compared to the with the direct shear method There is formation of dead zones at ends of the specimen as the strain conditions in the specimen are not constant due to frictional restraint ,produced by the loading cap and the disc at the bottom Specimen consolidation is isotropic but in the field the consolidation is mostly anisotropic The above disadvantages do not have a being influence on the outcome of the test (MURRAY and SIVAKUMAR, 2010, p.12). 5 Brief research strategies I this project there are various stages that will help us to find the soils Young modulus of elasticity. First, an extensive knowledge and research of triaxial testing is required with specific focus on finding the soils Young elastic modulus. The understanding of the vital parameter required for the experiment is necessary. Second, an experiment is set up. The aim of performing the experiment is to find the Young modulus of elasticity for sandy loamy soil using the triaxial apparatus .The results will be recorded and further analysis undertaken. The results obtained will be compared with the typical elastic moduli for sandy loamy soil (NOVA, 2010, p.32). . 6 Experimental procedure In order to find the Young elasticity modulus the triaxial apparatus shown above is used. The step of experimental procedure 1. The test specimen is placed in the triaxial testing apparatus 2. The sample is a cylinder that is wrapped in a impermeable membrane and confined by all around hydrostatic pressure 3. The vertical stress is applied in the vertical direction along the z axis 4. Lateral loading is applied in the r axis 5. The resulting graph of strain verses strain is plotted 6. In performing the triaxial test we measure the stress applied in both the axis well as the strain that applied in all the axis It is important to note that the gradient of the stress vs. strain curve is not the modulus of the soil but as associated with the modulus. The actual modulus is found through calculations (FREDLUN and RAHARDJO, 1993, p.105). 6.1 Measurement system for the experiment In the triaxial test there are no universal measurement systems because of the wide array of load and deformations that are encountered. However to perform the experiment we used the following: To measure the confining pressure felt by the specimen we used a pressure transducer. The axial load was measured directly on the soil specimen to avoid effects of friction as a result of the piston. The measurement of the specimen deformation is a bit complex (RANJAN, 2005, P.63). The devices that we used in the experiment did not influence the deformation of the soil specimen. The devices were free from pressure influences (KANIRAJ, 1989, p.23). . During the experiment there are errors as a result of the measuring devices. For any measurement device there are three forms of errors. They include: calibration error, output error and operator error. Calibration error is strongly related to the accuracy of the measuring device. Output error depends on the device and other factors such as temperature. While conducting the experiment we are was taken to in handling the apparatus to ensure that we obtained accurate results (KANIRAJ, 1989, p.38). 7. Tentative brief time line 8 Bibliography 9 Top of Form BISHOP, A. W., & HENKEL, D. J. (1962). The measurement of soil properties in the triaxial test. London, E. Arnold. 10 Bottom of Form Top of Form TTT WORKSHOP, KWAŚNIEWSKI, M., LI, X., & TAKAHASHI, M. (2013). True triaxial testing of rocks. Leiden, Netherlands, CRC Press/Balkema. http://www.crcnetbase.com/isbn/9780415687232. Bottom of Form Top of Form KANIRAJ, S. R. (1989). Design aids in soil mechanics and foundation engineering. New Delhi, Tata McGraw-Hill Pub. Co. Bottom of Form Top of Form DONAGHE, R. T., CHANEY, R. C., & SILVER, M. L. (1988). Advanced triaxial testing of soil and rock. Philadelphia, PA, ASTM. Top of Form HELWANY, S. (2007). Applied soil mechanics with ABAQUS applications. Hoboken, NJ, Wiley. Bottom of Form Top of Form RANJAN, G. (n.d.). Basic and applied soil mechanics. S.l, s.n.]. Top of Form FREDLUND, D. G., & RAHARDJO, H. (1993). Soil mechanics for unsaturated soils. New York [u.a.], Wiley. Bottom of Form Bottom of Form Top of Form HOULSBY, G. T., & WROTH, C. P. (1993). Predictive soil mechanics: proceedings of the Wroth Memorial Symposium held at St Catherines College, Oxford, 27-29 July 1992. London, Telford. Top of Form MURRAY, E. J., & SIVAKUMAR, V. (2010). Unsaturated soils a fundamental interpretation of soil behaviour. Chichester, West Sussex, U.K., Wiley-Blackwell. http://www.books24x7.com/marc.asp?bookid=36487. NOVA, R. (2010). Soil mechanics. London, ISTE. http://search.ebscohost.com/login.aspx?direct=true&scope=site&db=nlebk&db=nlabk&AN=531673. Bottom of Form Bottom of Form Bottom of Form Read More