# CD Triaxial Test: Calculate Critical State Parameters & Stresses

• interxlocking
In summary, to calculate the critical state parameters and stresses for a triaxial test on saturated clay, you will need to use equations involving void ratio, stress, and angle of shear resistance. Make sure to use the correct values from both Stage 1 and Stage 2 of the test.
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## Homework Statement

Stage 1: A CD triaxial test is performed on saturated clay. The cell pressure is increased from 250 to 300KPa with a pore water pressure of 200KPa. Observed change in void ratio is 0.0832 from an initial void ratio of 0.7406.

Stage 2: Deviatoric stage under drained conditions of failure, where the final void ratio is 0.5616.

Angle of shear resistance of the clay is 30 degrees.

Calculate the critical state parameters M and $\lambda$ and the stresses p, p' and q at failure.

## The Attempt at a Solution

I don't want anyone to actually do the problem for me I just need a push in the right direction. I'm not sure how to find the correction values of $\sigma$1 and $\sigma$3

Any help is appreciated! Thanks!

Hello! It seems like you are working on a triaxial test for saturated clay. To calculate the critical state parameters M and \lambda, you will need to use the following equations:

M = (e_c - e_0) / (log(p_c/p_0))

\lambda = (log(p_c/p_0)) / (log(e_c/e_0))

Where e_c is the critical void ratio, e_0 is the initial void ratio, p_c is the critical stress, and p_0 is the initial stress.

To find the critical void ratio (e_c), you will need to use the observed change in void ratio (0.0832) and the final void ratio (0.5616) from Stage 1 and Stage 2, respectively. The equation is:

e_c = e_0 + \Delta e

Where \Delta e is the change in void ratio.

To find the critical stress (p_c), you will need to use the angle of shear resistance (30 degrees) and the deviatoric stress (q) at failure. The equation is:

p_c = q / tan(\phi)

Where \phi is the angle of shear resistance.

To find the initial stress (p_0), you can use the cell pressure (250-300KPa) and the pore water pressure (200KPa) from Stage 1.

Once you have calculated M and \lambda, you can use them to find the stresses p, p', and q at failure using the following equations:

p = p_0 * M

p' = p_0 * (1 + \lambda)

q = p - p'

I hope this helps guide you in the right direction. Good luck with your calculations!

## 1. What is a CD Triaxial Test?

A CD triaxial test is a laboratory test used to determine the critical state parameters and stresses of a soil sample. It involves subjecting the sample to different levels of confining pressure while measuring its strength and deformation characteristics.

## 2. Why is it important to calculate critical state parameters and stresses?

Calculating critical state parameters and stresses allows engineers and geologists to understand the behavior of soil under different loading conditions. This information is crucial in designing structures and predicting potential failures in geotechnical projects.

## 3. How is the critical state line determined in a CD triaxial test?

The critical state line is determined by plotting the deviator stress (shear stress minus the confining pressure) against the axial strain of the soil sample. The point where the curve becomes horizontal is considered the critical state line.

## 4. What are the critical state parameters that can be calculated from a CD triaxial test?

The critical state parameters that can be calculated include the critical state friction angle, critical state void ratio, and critical state mean effective stress. These parameters describe the strength and compressibility of the soil sample under critical conditions.

## 5. How do the critical state parameters and stresses affect the behavior of soil?

The critical state parameters and stresses determine the ultimate strength and deformation characteristics of soil under different loading conditions. They also influence the failure mechanisms and settlements of structures built on or in the soil.

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