PRESSURE DISTRIBUTION AND ITS VARYING CHARACTERISTICS OF STEEL RIB SUPPORT IN GRADE V SURROUNDING ROCK OF ROAD TUNNEL

More complicated geological conditions can be encountered in tunnel construction. Flexible support system composing of anchors, sprayed concrete and steel ribs is widely employed in road tunnels with poor surrounding rock conditions such as the grade V surrounding rock. The steel rib is the skeleton and the main bearing structure of the flexible support system. Its pressure distribution state is one of the key factors for controlling the stability of temporary support system in advance. The pressure varying characteristics between the joint areas of steel ribs and surrounding rock can greatly reflect the surrounding rock state. These areas are the crown, the jointed points and the feet of steel ribs..According to the varying characteristics of such complicated pressure states in the grade V surrounding rock after excavation of road tunnel, safety control standards of tunnel construction process are adopted as the key factor of surrounding rock deformation. The pressure sensors are used for monitoring its variation on steel ribs in the Bangchuigou road tunnel. With a large number of monitoring data analysis, the following pre-conclusions can be obtained: (1)Pressure on the steel ribs is not symmetric. The parameters of temporary support system should be optimized in time according to the surrounding rock pressure distribution.(2)Within four meters between tunnel face and monitoring section, the pressure on steel rib at arch centering increases significantly. The stiffness of the jointed steel rib as a whole should be properly kept in high state.(3)the pressure on steel rib at the joints and feet of steel ribs also greatly changes, where the weakness parts of steel rib support system are .So the stiffness and stability should be fully considered at design stage for assuring the safety of tunnel construction. These results not only are of theoretical and constructional importance for tunnel projects, but also can help us improve g our tunnel design for other similar studies.