In undrained DS test, how to find soil stress path to reach failure?

The stress path of a soil sample during an undrained direct shear (DS) test can be determined by following these steps:

1. Preparation of the Sample: The soil sample is prepared and placed in the shear box apparatus. The sample is usually a cylindrical or square specimen.

2. Application of Normal Stress: A normal stress is applied to the soil sample. This is the vertical stress that the soil would experience in its natural environment.

3. Shearing of the Sample: The soil sample is then sheared at a constant rate. During this process, the shear stress and the shear strain are recorded.

4. Plotting the Stress Path: The stress path is then plotted on a graph with the vertical axis representing the shear stress and the horizontal axis representing the shear strain. The path starts at the origin (0,0) and ends at the point of failure.

5. Determination of Failure Point: The failure point is determined when the soil sample can no longer withstand the applied shear stress and deforms excessively. This point is marked on the stress path.

6. Analysis of the Stress Path: The shape and slope of the stress path can provide valuable information about the soil's behavior under shear stress. For example, a steep slope may indicate a brittle failure, while a flat slope may indicate a ductile failure.

Remember, in an undrained test, no water is allowed to enter or leave the sample during shearing. This means that the pore water pressure in the soil sample may increase during shearing, which can affect the stress path and the soil's behavior.