Fine-grained　marine　sediments　often　contain　gas　bubbles　that　can　cause　many　geotechnical　problems.　This　soil　has　a　composite　structure　with　gas　bubbles　fitting　within　the　saturated　soil　matrix.　The　gas　cavity　has　a　detrimental　effect　on　the　soil　stiffness　and　strength　when　they　are　filled　with　undissolved　gas　only.　The　gas　cavity　can　be　filled　with　gas　and　pore　water　due　to　＂bubble　flooding＂.　Bubble　flooding　has　a　beneficial　effect　on　the　soil　stiffness　and　undrained　shear　strength　because　it　makes　the　saturated　soil　matrix　partially　drained　under　a　globally　undrained　condition.　A　critical　state　constitutive　model　for　gassy　clay　is　presented　that　accounts　for　the　composite　structure　of　the　soil　and　bubble　flooding.　The　gas　cavity　is　assumed　to　have　a　detrimental　effect　on　the　plastic　hardening　of　the　saturated　soil　matrix.　Some　of　the　bubbles　can　be　flooded　by　pore　water　from　the　saturated　soil　matrix,　which　leads　to　higher　mean　effective　stress　of　the　saturated　soil　matrix.　Consequently,　both　soil　stiffness　and　strength　increase.　Only　one　new　parameter　is　introduced　to　model　the　detrimental　effect　of　gas　bubbles　on　plastic　hardening.　The　model　has　been　validated　by　the　results　of　three　gassy　days.