Pore Water Pressure and Shear Strength of Soil: Understanding the Relationship

In summary, the conversation discusses the relationship between pore water pressure and shear strength of soil. The question is raised as to why the shear strength decreases as pore water pressure dissipates and why excess pore water pressure is negative during unloading. The attempt at a solution suggests that the shear strength should increase as the pore water pressure dissipates, but it is unsure if this is correct. The conversation also mentions the possibility of the negative pore water pressure being related to the zone of capillary rise, but it is unclear how this relates to a cut slope. Further clarification is needed.
  • #1
tzx9633

Homework Statement


In the notes , I can't understand that why as the pore water pressure dissipated , the shear strength of soil will decrease
secondly , why the excess pore water pressure is negative when unloading occur ?

Homework Equations

The Attempt at a Solution


I think it's wrong , as the pore water pressure dissipated , the shear strength of soil should increase , am i right ? This is because total stress = effective stress + pore water pressure stress , i assume shear stress = effective stress here . Correct me if i am wrong ...
 

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  • #2
The pore pressure dropped to negative in the beginning of the cut , does it related to zone of capillary rise ? If yes ,. i can't relate it to the case of cut slope , can someone help ?
 

1. What is pore water pressure?

Pore water pressure is the pressure exerted by the water within the pores of a soil or rock mass. It is caused by the weight of the water and any additional forces acting on it, such as groundwater flow or changes in atmospheric pressure.

2. How does pore water pressure affect the shear strength of soil?

Pore water pressure can significantly influence the shear strength of soil. When the water pressure is high, it can reduce the effective stress between soil particles, resulting in a decrease in shear strength. On the other hand, when the water pressure is low, it can increase the effective stress and therefore increase the shear strength.

3. What is the relationship between pore water pressure and soil permeability?

There is a direct relationship between pore water pressure and soil permeability. A high pore water pressure can decrease the permeability of soil by reducing the void space available for water to flow through. Conversely, a decrease in pore water pressure can increase the permeability of soil by allowing more void space for water to flow.

4. How do changes in pore water pressure affect the stability of slopes?

Pore water pressure plays a crucial role in the stability of slopes. When the water pressure increases, it can reduce the shear strength of the soil, making it more prone to sliding. On the other hand, a decrease in pore water pressure can increase the stability of slopes by increasing the effective stress and shear strength.

5. What methods are used to measure pore water pressure in soil?

There are several methods used to measure pore water pressure in soil, including piezometers, tensiometers, and pore pressure cells. Piezometers are typically used to measure static water pressure, while tensiometers are used to measure negative pore water pressure in unsaturated soils. Pore pressure cells can measure both positive and negative pore water pressure in both saturated and unsaturated soils.

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