9.1. Change in pore water pressure

Try these first

1. 1.

   What formula represents effective stress?

   Hint

   Think about how total stress and pore water pressure interact.

2. 2.

   What happens to effective stress if pore water pressure increases?

   Hint

   Consider the relationship of stress forces.

3. 3.

   What is the primary effect of downward water flow on effective stress?

   • Decreases effective stress
   • Increases effective stress
   • No effect

   Hint

   Think about how gravity and water act together.

4. 4.

   True or False: A rise in groundwater pressure always leads to an increase in effective stress.

   • True
   • False

   Hint

   Refer back to the definition of effective stress.

5. 5.

   A soil layer with a thickness of 5m has a total stress of 40 kPa and a pore water pressure of 20 kPa. What is the effective stress and what implications does this have for stability?

   Hint

   Ensure you use the correct effective stress formula.

6. 6.

   If the critical hydraulic gradient is 1 and the unit weight of water is 9.81 kN/m³, at what depth will the upward water flow cause quick sand conditions in a deposit with a unit weight of 20 kN/m³?

   Hint

   Use the concept of balancing forces to find the answer.