Despite　over　six　decades　of　field　and　laboratory　investigations,　theoretical　studies,　and　advances　in　constitutive　modeling,　questions　remain　on　the　fundamental　issues　concerning　liquefaction　mechanisms,　the　collective　influence　of　multiple　factors　on　excess　pore　water　pressure　(EPWP)　generation,　and　liquefaction　triggering　criteria.　This　paper　presents　the　general　apparent　viscosity-and　average　flow　coefficient-based　methodology　for　quantifying　the　solid-liquid　phase-change　process　of　liquefiable　soil　under　undrained　cyclic　loading.　The　analysis　reveals　that　the　evolution　of　the　soil　particle-fabric　system　is　the　fundamental　physico-mechanical　mechanism　behind　EPWP　generation　in　a　liquefiable　soil,　with　the　accompanying　change　in　soil　physical　state　serving　as　the　intrinsic　mechanism　driving　EPWP　generation.　The　study　further　identifies　the　physico-mechanical　foundations　of　EPWP　generation,　as　well　as　the　inherent　causes　and　a　unified　quantitative　characterization　of　the　coupled　influences　of　multiple　factors　on　EPWP　generation.　This　work　presents　the　novel　observation　that　the　marginal　peak　excess　pore　pressure　ratio　(ru,pm)　between　the　solid-liquid　mixed　phase　and　the　liquid　phase　of　liquefiable　soil　can　be　identified　accurately　and　that　ru,pm　is　characterized　by　its　inherent　robustness.　A　ru,pm　value　of　0.90　can　be　used　as　a　liquefaction　triggering　criterion　for　soils　both　in　laboratory　element　tests　and　in　the　field.　Another　original　finding　is　that　the　liquefaction　triggering　resistance　curve　is　the　threshold　state　curve　between　solid-liquid　mixed　phase　and　transiently　liquid　phase　of　a　liquefiable　soil　and　is　unique　for　a　specific　initial　physical　state.　The　definitions　of　liquefaction　triggering　and　corresponding　liquefaction　triggering　resistance　are　clear　and　unambiguous　and　have　the　same　physico-mechanical　basis.　The　insights　obtained　in　this　paper　will　potentially　enable　the　scientific　and　engineering　communities　to　reinterpret　the　liquefaction　mechanism,　its　evaluation,　and　liquefaction　mitigation　strategies.