Discussion on the variation characteristics of water content of Quaternary soil cover on the top of Yungang Grottoes and the problem of water seepage in caves

Under the condition of flat terrain, the spatial and temporal characteristics of the water content of Quaternary soil cover on the top of Yungang Grottoes are obvious:the periodic annual change and vertical decrease with the depth layer by layer (except in winter). The soil water content increases to store water in spring and summer and decreases to lose water in autumn and winter. In spring and summer, the soil water content increases during the day and decreases at night; in autumn and winter, it decreases both during the daytime and at night. The soil water content is constantly changing, affected by many factors such as temperature, atmospheric precipitation and micro-topography, etc. The shallow soil of the mountain slope may be the infiltration channel of the lateral saturated flow. The influence depth of rainfall on soil water content is inconsistent with the actual infiltration depth. The water contents of the roof and the upper and middle parts of the rock wall is mainly controlled by rainfall infiltrating the topsoil layer of the cave and seepage water entering the cave along rock fissure channels, while the water content at the bottom of the rock wall is more affected by the unsaturated zone in front of the cave and underground capillary water. Years of waterproofing practice have also shown that the seepage water of Yungang Grottoes mainly comes from the top of the grottoes and nearby areas. Anti-seepage drainage has always been the leading idea for the management of seepage water in Yungang Grottoes. It is suggested that regional treatment be carried out according to the topsoil depth, terrain feature and site distribution, to give full play to the anti-seepage effect of soil and reduce excessive intervention in the soil cover. Under the premise of anti-seepage effect, the material for the anti-seepage layer should have certain permeability to meet the requirements of water vapor circulation between the cave, cover layer and atmospheric environment.