Impact of concealed karst caves on airport surface stability and treatment techniques
If an airport is built in a karst area,a concealed karst cave will pose a challenge to the airport construction because of its concealment. The adverse geological effects in the site area of the third phase expansion project of Guangzhou Baiyun international airport are significantly pronounced. The roof thickness of 132 exposed karst caves ranges from 0.1 m to 5.2 m,the height varies from 0.7 m to 13.5 m,the span measures between 0.5 m and 11.7 m,and the buried depth ranges from 13.2 m to 32.7 m. Most of the karst caves exhibit no filling or poor filling properties. The groundwater at the site consists of loose rock pore water,with a water level depth of 4.38 m. The bedrock is mainly composed of soluble limestone. Additionally,the rock mass in the engineering site exhibits significant folding. Affected by the Guangcong Fault,many secondary faults are developed,and the brittle rock mass is easily broken. Consequently,the action of external forces such as foundation construction and road rolling likely cause the breakage of cave roofs and uneven settlement of ground,which is not conducive to the safe construction of airport pavement. Therefore,the stability evaluation and treatment technique of karst foundation are very important to the smooth completeness of this project.Based on the FLAC3D numerical simulation method,a 100 m×50 m×50 m three-dimensional calculation model was established to analyze the influence of different thickness-span ratios and high-span ratios of concealed karst caves on the stability of the airport site under the filling load of the roadbed. A comparative analysis was conducted to evaluate the displacement,plastic zone,and stress of typical karst caves before and after treatment,providing a comprehensive assessment of their stability. Finally,a field test was carried out with the combination of high-pressure pouring of low-grade concrete and sleeve valve pipe grouting. The treatment effects of filling karst caves with self-compacting soil,foam concrete,and low-grade concrete were compared and analyzed through various methods,including by core drilling,borehole wave velocity logging,geophysical prospecting tests,standard penetration tests,compressibility tests,and wave velocity profiling.The results show that when the thickness-span ratio and height-span ratio of a concealed karst cave decrease and the filling height of the subgrade increases,the displacement and stress of the karst cave show an increasing trend and tend to be unstable as a whole. Compared with the span of the cave,the roof thickness is relatively insufficient,and the bearing capacity is inadequate to bear the upper load. This presents potential risks of roof breakage and cave collapse. It can be concluded that the cave will significantly affect the stability of the airport. When the thickness-span ratio (KH) is greater than 1 or the height-span ratio (KG) is greater than 2 for the concealed karst cave,the whole cave tends to be stable,indicating a high level of its stability. After addressing the typical working conditions of karst caves through a combination of high-pressure perfusion of low-grade concrete and sleeve valve tube grouting,it has been found that the displacement of the roof of the karst cave following the treatment of low-grade concrete grouting is only 0.6 mm. This represents a reduction of 99.11％ compared to conditions without treatment. The phenomenon of stress concentration is significantly reduced,resulting in enhanced foundation strength and effective grouting outcomes. Based on the comparative analysis of numerical calculation and field test,it is proposed that low-grade concrete should be preferentially used as filling material in the treatment of concealed karst caves in the third phase expansion project of Guangzhou Baiyun airport,followed by foam concrete. This study can provide reference for the selection of karst treatment measures in South China.
万方数据
维普
