Two　of　four　headrace　tunnels　in　the　Jinping　II　hydropower　project　were　constructed　using　tunnel　boring　machines　(TMBs).　The　geology　along　the　tunnel　alignment　is　dominantly　massive　to　highly　fractured　marble　and　the　maximum　overburden　depth　is　2,525　m.　The　paper　discusses　the　problems　encountered　during　the　TBM　tunneling,　including　instability　of　the　tunnel　wall　and　face　induced　by　high　in　situ　stresses,　high-pressure　groundwater　inflows　and　excessive　cutter　and　cutterhead　damage.　Measures　taken　to　overcome　these　problems　involved　modifications　to　both　the　machines　and　the　mode　of　operation　as　well　as　changes　to　the　support　parameters.