The　construction　of　twin　tunnels　excavated　by　simultaneous　drive　of　two　opposite　tunnel　boring　machines　presents　a　challenge　for　the　tunneling　industry,　in　particular　the　assurance　of　tunnel　face　stability　due　to　a　mutual　effect　when　both　tunnels　are　approaching　each　other.　To　ensure　the　stability　of　the　tunnel　face　over　the　whole　construction　phase　and　along　the　complete　alignment,　it　is　important　to　correctly　assess　the　response　of　the　individual　tunnel　face　with　respect　to　the　effect　of　the　opposite　tunneling　process.　For　the　assessment　of　such　tunnel　face　stability,　a　three-dimensional　(3D)　numerical　model　that　allows　us　to　examine　the　3D　stress　and　pore　water　distribution　at　the　tunnel　face　is　used.　This　contribution　presents　a　case　study　based　on　a　real　tunneling　project　in　the　city　of　Shenzhen,　China,　in　which　twin　tunnels　are　excavated　simultaneously　in　the　opposite　directions　using　two　large-diameter　slurry　shield　machines.　The　influence　of　slurry　infiltration　as　well　as　the　clear　distance　between　the　tunnel　faces　are　investigated　through　comparison　with　simulations　in　which　a　standard　face　stability　assessment　technique　is　employed.　It　is　shown　that　the　additional　seepage　force　induced　by　the　pore　pressure　gradient　between　the　two　faces　leads　to　the　increase　of　the　limit　support　stress　ratio.　©　2025　American　Society　of　Civil　Engineers.