A TYPICAL CASE HISTORY OF INFILLED KARSTIC CAVES FORMED BY REVERSE SIPHONIC CIRCULATION IN DEEP PHREATIC ZONEPRELIMINARY ANALYSIS OF THE FORMATION MECHANISM OF INFILLED HUGE CAVES EXPOSED BY YUANLIANGSHAN TUNNELING IN DEEP PHREATIC ZONE OF MAOBA SYNCLINE

The deeply seated and 11.068 km long Yuanliangshan tunnel is a key project of the railway from Chongqing to Huaihua. The tunnel goes cross the Yuanliangshan syncline,a NESW trending tightly folded syncline,and cuts numerous NWNWW striking transverse faults. The core of the syncline composes of Permian and Lower Triassic carbonate rock strata,and the soluble rock terrain is enveloped on all sides by underlying Devonian and Silurian shale. The length of the tunnel drifted through the soluble carbonate rock mass is 2,2 km. On the ground surface,there are a lot of karstic depressions,dolines and sinkholes. Numerous karstic springs emerge out from the tips of the transverse faults on both wings of the syncline,where the carbonate rock strata contacts with the underlying shale. The elevations of the springs are usually higher than 850～900 m above the mean sea level. These elevations are regarded as the local discharge base level of the underground karstic water. The tunnel is situated at 400 to 450 m under this base level. During the drifting of the tunnel,three infilled huge karstic caves(Nos. 1#, 2#,and 3#)were exposed near by the axis,at the axis and at the east wing of the syncline,respectively. Water gushing and sand boiling hazards occurred many times while drifting through the sand infilled cave No. 2#. An unexpected and seldom observed blasting and shooting extrusion of clayey soil hazard occurred when the clayey soil infilled cave No.3# was just exposed． Through comprehensive analysis of the abundant data gathered during the construction of the tunnel,it is tentatively concluded: (1)an open primitive channel for the deep circulation of karstic water has been provided by the intersection of the two transverse faults(f7 and f9)with the longitudinal tension cracks nearby the axis,the bed separation void at the core of the syncline,and an interstratal shearing zone at the east wing of the syncline. A reversesiphonic circulation of karstic water was impelled by the potential difference between the underground water levels of the two transverse faults; (2)strong water conductive faults and cracks developed into caves through longtime differential dissolution and the water in these channels transformed into pipe flow; (3)by longtime strongly dissolution and erosion(and for the cave No.2 also roof collapse),the longitudinal tension crack,the bed separation void and the interstrated shearing zone were transformed into huge karstic caves; (4)caves were infilled due to the changing of hydrologic condition．