Geotechnical Engineering
Effective Stress
Total stress minus pore water pressure: the stress carried by the soil skeleton.
Effective stress σ′ = σ − u is the part of the load carried grain to grain. Strength and compressibility both depend on it, not on total stress, which is why raising the water table weakens a slope even though nothing was added to it.
Total stress sums unit weight times thickness down the profile, using saturated unit weight below the water table; pore pressure is the unit weight of water times the depth below the water table.
How much of the FE Civil exam is Geotechnical Engineering?