Effective stress with groundwater and surcharge

Worked SI example: a pore-pressure increase at constant total stress

At a monitoring point, total vertical stress is 160 kPa and measured pore-water pressure is initially 65 kPa. Assume total stress stays at 160 kPa during the stated pressure change; the supplied measurements already represent this point. Initial effective stress is 160 − 65 = 95 kPa. When pore pressure rises to 85 kPa, effective stress falls to 160 − 85 = 75 kPa, a 20-kPa decrease.

A larger pore pressure does not increase the soil skeleton’s share of the same total stress. Do not subtract a water contribution twice or add the pressure increase to effective stress. If a changed water table also changes the total-stress profile, recompute both quantities; the constant-total-stress assumption in this exercise cannot be carried into every groundwater problem.

Change only pore pressure to 50 kPa and calculate before looking: effective stress becomes 110 kPa. This is a local stress calculation, not a complete bearing-capacity or excavation-stability determination.

The governing relationship

Vertical effective stress is the total vertical stress minus pore-water pressure: σ'v = σv - u. Total stress includes the overburden and applicable surface surcharge. Below the water table, hydrostatic pore pressure is based on the vertical depth beneath the water table and the unit weight of water.

Reliable solution sequence

• Draw the soil profile and mark the water-table elevation.
• Compute total stress from the stated surcharge and the total unit weight of each layer.
• Compute pore pressure only over the depth below the water table.
• Subtract pore pressure from total stress and report the result with units.
• Check whether a surcharge may reasonably be treated as uniform at the point of interest.

Common failure modes

Frequent mistakes include using submerged unit weight in the total-stress sum and then subtracting pore pressure again, applying pore pressure over the full depth rather than only below the water table, or assuming a local surcharge creates its full surface intensity at depth without a stated influence model.