Analysis on the characteristics of small-scale shallow karst water systems in typical tight-narrow folds and tunnel water inrush in southwestern China

In southwest China, shallow karst water, under the control of typical tight-narrow folds, has the characteristics of quick cyclic-alternating pattern and abundant groundwater resource, etc. It is of great significance to describe the pattern types and their characteristics of small-scale shallow karst water systems in order to discuss karst water resource and water inrush condition in medium-shallow buried tunnels. Taking two typical tight-narrow folds (Tongluoshan anticline in Chongqing and Tongluojing anticline in Zunyi)as the study objects, in this paper, we collate the controlling factors of karst water systems characteristics in detail, and point out that the subtle differences of stratum structure lead to different water-bearing media, and the characteristics of groundwater recharge, run-off and discharge. According to these differences, shallow karst water systems are divided into four small-scale patterns in research area. In addition, based on the differences of four patterns, water inrushing conditions of medium-shallow buried tunnel are analyzed qualitatively. The research results show that， (1)There are some differences in water inrushing conditions, when tunnel cuts through different small-scale karst water systems.(2)The connection among small-scale shallow karst water systems is existed through the transformation of surface water, due to the opposite spatial structure of strata, in the calculation of water flow, the exogenous recharge from adjacent soluable karst aquifers should be taken into account, while the encapsulated spatial structure of strata makes each system relatively independent, in the calculation of the water flow, the exogenous recharge from adjacent clastic karst aquifers should be taken into account.(3) Meanwhile, the recharge share of fracture-flow dispersed discharge type from rainfall cannot be ignored when the discharge boundary is deep valley, thus in the calculation of water flow,the recharge should be less than average annual precipitation. However, if the discharge boundary is shallow valley, the recharge is nearly equal to average annual precipitation.