It is common to cause errors in the selection of building foundation in karst terrain. The root reason is that the practical difficulty of karst treatment in foundation cannot be reasonably reflected by the existing evaluation of karst degree. Taking New Terminal of Nanning Wuxu International Airport as an engineering case, this article analyzes the limitation of the existing evaluation indices of karst degree, demonstrates the advanced rationality of intensity dissolution layer’s thickness as the evaluation index of karst degree, and proposes the recommended standard of the evaluation of karst degree. Results show that the intensity dissolution layer, divided by dissolution ratio distribution curve with depth in foundations, comprehensively considers the main depth range of dissolution degree of rock surface and the developing of cavern dissolution, and its distribution characteristics can reflect the most complex depth range of karst foundation. As the evaluation index of karst degree, it is reasonable. Its recommended values are suggested as follows, When the thickness of an intensity dissolution layer is less than 3.0 m, more than or equal to 3.0 m and less than 6.0 m and more than or equal to 6.0 m, the karst degree can be determined as weak development degree, medium development degree and strong development degree.

The dissolution of karst foundations can be divided into rock surface dissolution and hole dissolution. The dissolution features of karst foundations at different depths must be clearly identified before taking proper karst treatment measures. In this paper, more than 200 engineering data samples are collected from typical carbonate karstic regions in southwestern China, and 12 most representative engineering site were picked out. The rock surface dissolution ratio, hole dissolution ratio, and total dissolution ratio were measured at each engineering site, and used to fit the relationship between rock surface dissolution ratio and hole dissolution ratio at different depth sections. The results show that the karst foundation can be split into three sections from top to bottom: rock surface dissolution section Ⅰ, composite dissolution section Ⅱ, and hole dissolution section Ⅲ. The three sections occur inevitably as the rock surface dissolution occurs naturally from the inside. The dissolution degree decreases exponentially with the growing depth, and the rock surface dissolution ratio drops faster than the total dissolution ratio. Moreover, the hole dissolution ratio per unit volume of rock decreases with the increase of depth. If the holes are not developed, the engineering sites will only have rock surface dissolution section; if the holes become more developed, the composite dissolution section will expand gradually and dominate site dissolution. At most locations, the hole dissolution ratios under the minimum elevation of rock surface are within 10%. Finally, it is suggested that the final elevation of the drilling holes must be controlled above the minimum elevation of rock surface. The research results shed new light on the mitigation of dissolution of karst foundations.

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