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.