Soil　conditioning　of　gravel-clay　strata　using　foams　for　earth　pressure　balance　(EPB)　shield　tunnelling　is　less　well　understood　compared　to　conditioning　for　clay　or　sand　strata.　This　lack　of　understanding　can　lead　to　muck　clogging　at　the　cutting　wheel　and　increased　wear　on　cutter　tools.　To　address　these　issues,　this　study　introduces　the　response　surface　methodology　(RSM)　to　investigate　the　effects　of　various　foaming　agent　components　and　their　interactions　on　foam　conditioning　of　gravel-clay　strata　that　are　widely　distributed　in　Southeastern　China.　The　controlled　variables　selected　for　this　study　include　anionic　surfactant　sodium　fatty　alcohol　polyoxyethylene　ether　sulfate,　nonionic　surfactant　alkyl　polyglycoside,　foam　stabilizer　hollyhock　gum,　organic　dispersant　guar　gum,　and　inorganic　dispersant　sodium　tripolyphosphate.　The　responses　evaluated　are　the　foaming　agent　viscosity,　foam　volume,　foam　half-life　time,　adhesive　force,　and　undrained　shear　strength.　Fifty　groups　of　laboratory　tests,　including　apparent　viscosity,　Ross-Miles,　liquid　loss,　improved　cone　pull-out,　and　vane　shear　tests,　are　conducted　to　characterize　these　responses.　The　surface　tension　measurements　and　microscopic　observations　are　also　performed　to　interpret　foam　performance.　Ultimately,　the　optimal　ratio　of　foaming　agent　components　is　determined　using　the　desirability　approach.　The　comparison　of　in-house　and　commercial　foaming　agents　in　both　laboratory　and　in-situ　tests,　focusing　on　their　conditioning　effects　on　soil　properties　and　tunnelling　parameters,　highlights　the　applicability　of　the　proposed　RSM　models.　This　study　offers　new　insight　into　the　consequence　and　interaction　of　different　foaming　agent　components　and　serves　as　a　valuable　reference　for　soil　conditioning　of　gravel-clay　strata　during　EPB　shield　tunnelling.　©　2025　Elsevier　Ltd