Abstract: Baozhen tunnel is the only soft rock tunnel and the key project of Yichang-wanzhou Railway.The complex geological environments,such as high earth stress,deep-buried rock mass,rich groundwater,very weak and cracked rock masses and bias pressure along the rock seam,make the self-stability of the tunnel unfavorable.During construction,intense deformation and serious failure happened.Many serious failures happened due to large deformation,for example,roof subsidence,wall crushing,shotcrete flaking,s-shape twisted steel support,broken lining,floor upheaval,invert crazing and lots of cracks distributed everywhere.The monitoring data show that deformation is rapid with long duration,and at the same time,the deformation presents asymmetric and uneven features.For either sandy shale whose strength is higher or macker whose strength is lower,by comparing and analyzing the monitoring data of surrounding rock and the geologic sketch of work face,the weak interlayer is thought to be the key cause of induced intense deformation and serious failure.So according to the typical composite structure between weak interlayer and surrounding rock in the heaviest deforming section,the rock strength tests on surrounding rock contained weak interlayer are executed,the stress-strain complete procedure curves are described(the thickness of weak interlayer is 2.5 centimeters and confining pressure is 4 MPa),and their variation rules of system strength with both the confining pressure and the thickness of weak interlayer are analyzed.In addition,the mechanics and features,including the effect of both the confining pressure and the thickness of weak interlayer on strength,are studied.