Earth　pressure　balanced　(EPB)　tunneling　boring　machine　(TBM)　are　popularly　adopted　to　excavate　sand　and　gravel　layers.　Many　difficulties　are　encountered　during　tunnelling　through　the　sand　and　gravel　layers,　such　as　instability　of　the　tunnel　face,　high　torque　of　the　machine　cutter　head,　high　cutter　wear　and　cutter　head　wear.　Soil　conditioning　is　the　most　efficient　way　to　solve　these　difficulties.　This　paper　studied　the　properties　of　foam　by　using　a　new　foam-generator　which　was　designed　by　authors,　in　order　to　efficiently　use　the　foam.　The　experimented　gravel　samples　were　taken　from　Beijing　Metro　Line　9.　Different　additives,　such　as　water,　foam　and　bentonite　slurry,　were　used　to　condition　the　excavated　soil　in　the　laboratory.　The　optimum　additives　for　the　experimented　gravel　were　obtained.