Excess pore water pressure is the increase in water pressure within soil pores above the static (hydrostatic) level, generated by cyclic or monotonic loading. Its accumulation under seismic shaking reduces effective stress and can lead to liquefaction, cyclic softening, and ground failure.
Excess pore water pressure (Ξu) is the increase in pore water pressure in a saturated soil above its static (hydrostatic) value, caused by changes in the total stress state or by cyclic loading. In a saturated soil, the total stress (Ο) is partitioned between the soil skeleton (effective stress, Ο') and the pore water (pore pressure, u) through Terzaghi's principle: Ο = Ο' + u. When the soil is subjected to cyclic shear loading, the soil skeleton tends to contract (in loose soils) or dilate (in dense soils), and if drainage is restricted, this volume change tendency is accommodated by changes in pore pressure. In loose saturated sands, cyclic loading causes a progressive increase in pore pressure; when the excess pore pressure equals the initial effective stress, the effective stress becomes zero β this is the condition of liquefaction.
Excess pore water pressure is the key physical variable in liquefaction and cyclic softening. The generation of excess pore pressure during cyclic loading depends on the soil's relative density (loose soils generate pressure more rapidly), the initial effective stress (higher confinement requires more pressure to liquefy), the number of loading cycles (more cycles produce more pressure), and the drainage conditions (poorly drained soils accumulate pressure more rapidly). The rate of pore pressure generation is often modeled using a pore pressure generation curve, expressed as a function of the cyclic stress ratio and the number of cycles. This curve is a fundamental input to effective stress analysis of liquefaction and to the calibration of advanced constitutive models such as PM4Sand and UBCSAND. In the field, excess pore pressure is measured using piezometers or piezocone penetration tests (CPTu) β the dissipation of excess pressure after stopping penetration provides information about the soil's hydraulic conductivity and drainage characteristics.
Excess pore water pressure has consequences beyond liquefaction. In cyclic softening of silts and clays, partial pore pressure generation degrades stiffness and strength even when full liquefaction does not occur. In earth structures (dams, levees, embankments), excess pore pressure can develop during construction (from rapid loading) or during earthquakes, reducing stability and potentially leading to failure. In foundation engineering, excess pore pressure generated by pile driving or dynamic compaction affects the short-term behavior of the soil and requires time for dissipation before full capacity is mobilized. Post-liquefaction, the dissipation of excess pore pressure is accompanied by settlement β as the pressure dissipates, the soil consolidates, causing ground settlement that can damage overlying structures. The rate of dissipation depends on the soil's permeability and drainage path length: clean sands dissipate rapidly (seconds to minutes), while silty sands and silts dissipate slowly (hours to days). In Iran, excess pore pressure and liquefaction assessment are important for projects in the Caspian coastal plain, Khuzestan, and other regions with shallow groundwater and loose saturated deposits. Modern practice increasingly uses effective stress analysis β rather than total stress analysis β for critical projects where pore pressure generation and dissipation play a significant role.