Increasing pore pressure on a slope would most likely:

• A. Increase the coefficient of friction
• B. Decrease the coefficient of friction
• C. No change to the coefficient of friction
• D. Increase the factor of safety

Answer: (B)

Pore pressure on a slope lowers the effective stress that holds soil grains together. As pore water pressure rises, the effective normal stress on potential slip surfaces decreases, so the frictional resistance along those surfaces drops. Since shear strength in soils is governed by c' plus σ' tan φ' (with σ' being the effective normal stress), reducing σ' weakens the overall shear resistance. That means the apparent friction per unit area is reduced, making failure more likely and lowering the factor of safety. So increasing pore pressure would decrease the frictional resistance, aligning with the idea that the coefficient of friction behaves as if it is reduced under higher pore pressure. The slope becomes less stable, not more.