岩溶土洞稳定临界深度的初步研究

在岩溶土洞发育地区进行工程建设时需要确定土洞是否会发展成地面塌陷和对工程结构物稳定性有影响，分析表明土洞临界深度可作为评价土洞稳定和加固深度的依据。本文采用有限元分析方法，以土洞塑性区贯通至地面或路基表面作为土洞发展成地面塌陷和临界深度的标准。研究表明，土洞的临界深度与土洞形状、土洞尺寸、位置和路基填土厚度有关；土洞断面形状为圆形时临界深度最小，土洞尺寸越大，路基填土越厚，相应土洞临界深度越大。 

Preliminary exploration on the critical buried depth of the soil cave in karst area

The research area is situated in a karst area with shallow cover type of karst developed, where the overlying soil layer comprises cultivated soil, silty clay, muddy clay, breccia and gravel and the underlying bedrock is limestone. The soil caves are developed in the soil layer, which often cause the ground to collapse. A single soil cave’s size is 2 m to 5 m in both height and width, with a long longitudinal extension. The plane distribution of soil caves is consistent with the ground collapse. On the basis of their transverse section, the caves can be roughly as circular and tunnel types, the latter of which has the shape of semicircular upper part and rectangle lower one. The highway passing through the area normally has a road embankment of 5.5 m to 7.5 m high. In order to determine the scope of the treatment of the soil caves, it is necessary first of all to determine whether a cave will develop into ground collapse. Based on the detailed investigation of the site, the location, size and shape of the soil cave are recorded, from which a finite element analysis model is established. Taking the zone from ground surface or road embankment surface to underground plastic deformation area into account for the determination of critical buried depth of the soil caves which may develop into ground collapse, the critical buried depth of the soil cave stability in different depth, size, shape and different backfilling height is then analyzed. This research has showed that the critical buried depth of soil cave is closely related to the shape, size and location of the caves and the thickness of roadbed fillings. The critical buried depth has a minimum value at the circular cave, while it gets larger along with increasing cave size or roadbed filling thickness. The results of this research can be used as a basis for the treatment measures towards the soil cave collapse. When the buried depth of the cave is larger than the critical buried depth, there is no need to treat it. On the contrary, when the buried soil cave is shallower than the critical buried depth, it is necessary to deal with the soil cave. In order to ensure that the soil caves do not have harmful effects on the highway project, field monitoring should be carried out both during and after road construction; and it is accordingly suggested that immediate intervention measures need to be implemented for the problematic localities.