A saturated soil sample under CU test was subjected to a confining pressure of 200 kPa with back pressure (pore pressure) of 50 kPa. What would be the value of pore pressure if the confining pressure was raised to 400 kPa without drainage? If the pore pressure parameter A is 0.5, determine the pore pressure if a deviatoric stress of 200 kPa was applied to the sample.

To answer these questions, we need to understand the principles of effective stress and pore pressure.

1. Value of pore pressure when confining pressure is raised to 400 kPa without drainage:
When the confining pressure is increased without allowing drainage, the pore pressure also increases proportionally. This is because the increase in confining pressure compresses the soil matrix, reducing the volume of the voids, and consequently increasing the pore pressure.

To calculate the value of pore pressure when the confining pressure is raised to 400 kPa, we can use the formula:

pore pressure = initial pore pressure + (confining pressure change * pore pressure parameter)

Given:
Initial confining pressure = 200 kPa
Initial pore pressure = 50 kPa
Confining pressure change = 400 kPa - 200 kPa = 200 kPa
Pore pressure parameter A = 0.5

Substituting the values into the formula:
pore pressure = 50 kPa + (200 kPa * 0.5)
pore pressure = 50 kPa + 100 kPa
pore pressure = 150 kPa

Therefore, when the confining pressure is raised to 400 kPa without drainage, the pore pressure would be 150 kPa.

2. Pore pressure when a deviatoric stress of 200 kPa is applied to the sample:
To calculate the pore pressure when a deviatoric stress is applied, we need to use the formula for effective stress:

effective stress = total stress - pore pressure

Since the deviatoric stress represents the difference between the major and minor principal stresses, we can say:

Major principal stress = (confining pressure + deviatoric stress) / 2
Minor principal stress = (confining pressure - deviatoric stress) / 2

Given:
Confining pressure = 200 kPa
Deviatoric stress = 200 kPa

Calculating the major and minor principal stresses:
Major principal stress = (200 kPa + 200 kPa) / 2 = 200 kPa
Minor principal stress = (200 kPa - 200 kPa) / 2 = 0 kPa

Now, substituting the values back into the effective stress formula:
effective stress = major principal stress - minor principal stress
effective stress = 200 kPa - 0 kPa = 200 kPa

Finally, to calculate the pore pressure, we rearrange the formula as follows:
pore pressure = total stress - effective stress

Given:
Total stress = initial confining pressure = 200 kPa
Effective stress = 200 kPa

pore pressure = 200 kPa - 200 kPa = 0 kPa

Therefore, when a deviatoric stress of 200 kPa is applied to the sample, the pore pressure would be 0 kPa.