Soil Elastic Modulus Measurement (Coursework Sample)

Soil elastic modulus measurement using triaxial test
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Soil elastic modulus measurement using triaxial test
Purpose: The main objective of giving this speech is to present my research proposal on measuring the soils elastic modulus using the triaxial test. Tests will be carried out and the results collected and analyzed to find the Young elastic modulus and other parameters. I will also highlight practical the applications of the triaxial test and the Young elastic modulus of a given soil sample.
Thesis: The triaxial test is an accurate method to determine Young elastic modulus for a given soils sample.
Good evening, ladies and Gentlemen. I am glad to stand before you to present my research proposal, on soil elastic modulus measurements using the triaxial test. The findings of my research proposal are based on an intensive research on the subject which I conducted. 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.
In my presentation I will highlight how Young Elastic modulus is determined using the triaxial test and how the triaxial tests are used with other tests to make predictions for most engineering applications. Some of the engineering applications include; making predictions by obtaining the strength parameter for design, performing the triaxial test to establish parameters for hydraulic fracturing and other parameters needed in constitutive modeling for numerical stability calculations and finally the triaxial test is also an important component in wellbore stability, sand production and subsidence calculations.
I will start by explaining how the triaxial test is performed on a given soil sample. It is imperative to note that; soil elastic modulus is a soil parameter measuring the stiffness of soil. It is defined as the ratio of stress along an axis over the strain in that along that axis provided it is within the range of elastic soil behavior. How is the procedure performed in the laboratory?
Well, a cylindrical 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 such as the Young modulus of elasticity are calculated. However, the following the following assumptions are made concerning the test specimen. We can assume that the tangential stress and strain are equal in magnitude to the radial stress strain. We also assume that there is even stress and strain distribution throughout the specimen. After conducting the triaxial test, how do we calculate the young modulus of elasticity?
In performing the calculations, it is important to note that soil does not exhibit a linear stress vs. strain curve. The slope of the stress-stain curve is not the modulus of the soil but is closely associated with the soil modulus. 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.
Axial stress ( aa) is then determined by taking the total axial force and dividing it by 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.
I have used the following abbreviations in my calculations:
Where ar=Confining pressure, aa=Axial stress and deviator stress as (aa-ar)
Plotting a deviator stress (aa-ar) verses the axial strain; the slope is the young modulus of elasticity.
Using the triaxial test to find Young’s modulus of elasticity for a given soil sample has a numbers of advantages over direct methods such as direct shear test. They include:
* The ability to control the drainage of the test specimen
* The ability to take measurements of pore water pressure
* The method is more accurate and precise compared to the direct methods there...