Better CE Practice Test

The main idea is that soil shear strength is controlled by the stress carried by the solid grain skeleton, not by the water in the pores. That stress is the effective stress, defined as the total stress in the soil minus the pore water pressure: sigma' = sigma - u. Why this matters: when water fills the pores, some of the load is carried by the water instead of the grains, so the grains press on each other with less force. Since shear strength comes from those intergranular contacts and friction, lower effective stress means lower shear strength. In drained soils, shear strength is often expressed as c' + sigma' tan(phi'), showing how sigma' directly sets the available resistance. In undrained or rapid loading, pore pressure can rise (u increases), which lowers sigma' and can lead to failure even without a large increase in total stress. The other forms would misrepresent how pore water affects the skeleton: they either add pressures together or subtract in the wrong way, implying water pressure increases the grain contact forces or multiplies stresses, which contradicts observed soil behaviour.