ENGINEERING GEOLOGICAL CONDITIONS AND SPECIAL ROCKMASS STRUCTURAL CHARACTERISTICS OF JIUBAO RAILWAY TUNNEL

Jiubao Tunnel,located between Wangquan county to Shangyi county of Hebei province,is the key and controlling works of the underconstructed ZhangjiakouJining Railway Line. The tunnel passes through the granulite and the biotite gneiss of the Upper Archen Era,where faults well developed faults and the rock mass is fractured. The geological conditions disclosed during tunneling were greatly different from those of the construction design drawing. The alteration ratio of the classification of tunnel surrounding rocks even reaches 80.3%. It was due to insufficient understanding to rock mass structure. The high alteration directly caused the occurrence of many geohazards such as squeezing,collapse,water inrush during tunneling. The paper takes the section of DK30+520～DK30+910 as the example and makes the comprehensive analysis to the key geological conditions of tunnel surrounding rock. The key conditions include geology,geotechnics,earth stress and underground water. It studies the special characteristics of rock mass structure and directs that the tunnel surrounding rock is complicatedly controlled by the fault fracture zones and the alternative outcropping of weak and hard engineering geological rock masses. It is the special rock mass structural characteristics that controls the deformation and collapse of tunnel. It also summarizes the rock mass structure types which are possibly encountered during tunnel construction. The research result on the rock mass structure of Jiubao Tunnel is helpful to the tunnel design change and construction in the metamorphic and greatly tectonic disturbed areas. 