Abstract: The insufficient lining thickness often causes crack or damage to doublearch tunnel structure and further threatens the tunnel safety. Thus, the surrounding rock pressure characteristics, distribution laws of lining internal force and crack of tunnel structure under condition of insufficient lining thickness of doublearch tunnels with monolithiclined central walls are studied by similarity model test. The study results show that: (1) The maximum bending moment is located in the bottom of the middle wall, and cracks and crushing occur on the outer and inner face of middle wall corner with the most serious crack damage, which is the most unfavorable position. (2) The lining border with insufficient thickness is the weak zone, cracks would occur to lining inner face of right edge when the lining thickness of left arch shoulder of left line is insufficient, and those would occur to lining outer face of upper edge when the lining thickness of left arch shoulder of left line is insufficient. (3) Large tensile stress occurs to the connecting area between middle wall top and haunch, the variation of locations of insufficient lining thickness area would affect the stress state of the lining on the middle wall top, and the crack on the upper right corner of the middle wall, which is the key area of the lining failure, appears later than that at the crown on the opposite side of the lining with insufficient thickness.

Key words: doublearch tunnel, insufficient lining thicknes, structural safety, lining internal force, failure process, model test