Formation mechanism and prevention countermeasures for karst collapse in Ertang railway station, Guilin
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摘要:
湘桂铁路二塘车站高铁区段1996-2016年间发生岩溶塌陷险情27起，虽经过多期悬挂式帷幕注浆整治，但塌陷问题仍是行车安全的极大隐患。通过资料收集、场地野外调查、岩溶地下水气压力长期监测站的监测资料分析，细化了研究区的地质构造、岩溶水文地质条件及岩溶发育特征，从地质结构、岩溶地下水动力特征等方面分析，认为岩溶塌陷形成的成因为临桂地区房地产基坑开挖抽排水影响岩溶地下水水位变化，造成基岩面上覆土体的潜蚀，形成土洞，进而发展至地面形成塌陷坑。针对铁路运行段地下水位埋深具承压性，隐伏溶洞充填率高的复杂岩溶地质条件以及注浆压力的限制，提出加强基岩面上覆土层抗渗强度、管控岩溶地下水降深的岩溶塌陷防治对策。
Abstract:Twenty-seven karst sinkholes had occurred at Ertang railway station, Lingui district, Guilin City in China from 1996 to 2016. This section had been undergone four stages of collapse treatment by curtain grouting of suspension type from 2010 to 2020, during which the pacing of grouting holes had been reduced from 7 m to 2 m, and the grouting depth had been deepened from 10 m to 25 m. During the 10 years, the train speed had been limited at 50 km·h−1 in this section rather than the designed speed of 200 km·h−1. However, the karst collapse problem still has been posing a threat to the safety operation of high-speed railway. In this study, the geological background including geological structure and karst hydrogeological condition has been further explored according to high-precision field surveys and data collected at 5-miniute intervals by the established long-term monitoring station of karst groundwater-air pressure. Lingui anticline and Sitang Luojin syncline are developed in the area under the influence of northeast Lingchuan fault and Ludiyan fault. Lingui anticline is C shaped and overturns at Ertang railway station. The Quaternary in this covered karst area is mainly composed of clay. The thickness of the Quaternary varies greatly from 3 m to 35 m, with an average thickness of 8 m. The underlying bedrock is the limestone of Rongxian formation of Devonian system. Due to the influence of structural belt, the depth of karstification in this area is up to 100 meters below the bedrock. The percentage of cavities encountered in drilling is up to 72%, mainly concentrating at 30 m below the ground, and 88% of the karst cavities are fully or partially filled with fluidal plastic clay. The groundwater dynamics in the site is not only affected by the atmospheric rainfall and the leakage of irrigation channels, but also affected by the pumping of groundwater from the foundation pits and factories near the railway. Two groundwater cones of depression have been formed from north to south along the railway. According to the analysis of geological conditions of karst sinkholes and with the development of real estate in Lingui area, drainage from foundation pits and factories has affected the karst groundwater, causing the failure of the overlying soil on the bedrock surface.
Generally, the prevention measures for karst collapses should be considered from three aspects, control of karst groundwater, the collapse resistance strength of Quaternary and the grouting of karst cavities. Under complex conditions involving shallow buried depth of karst groundwater borne with pressure, high cavity-filling rate and limited grout pressure, low-pressure grouting of shallow karst cavities and the evaluation of grouting effect may be faced with many challenges. Countermeasures for collapse prevention are also proposed in this study: strengthening the impermeability of the overlying soil on the bedrock surface through grouting; controlling the drawdown of groundwater in a safe way. At present, 6 crust groundwater-air pressure monitoring boreholes, covering a length of 2 km along the high-speed railway, have been arranged on both sides of Ertang railway station.
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表 1 调查区地下岩溶发育特征表
Table 1. Development characteristics of underground karst in the survey area
区段 统计孔数/个 遇洞孔数/个 钻孔遇洞率/% 有水孔数/个 平均有水钻孔
出水量/L·s−1充填、半充填溶洞
充填率/%空洞率/% 临桂城区 25 18 72 18 11 70 26.3 
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表 2 岩溶发育深度特征统计表
Table 2. Statistics of karst development depth
钻孔编号 钻孔深度/m 孔口标高/m 初见水位/m 稳定水位/m 土层厚度/m 裂隙发育段/m zk2-1 80 160.93 1.16 1.98 3.7 4.8~9.9，68.9~71 zk6-1 80 171.99 1.8 1.63 1.9 40~54.3，63.2~69.8 zk7-1 80 158.21 1.1 3.66 1.2 1.2~38.4 zk7-2 32.3 161.62 2.9 2.9 4.5 4.5~14，28.3~30.1 zk9-1 80 159.91 2.76 2.8 2.5 13.7~18.4，34.6~37.2，41.9~46.6 zkD-1 80 157.1 1.8 1.67 3.8 11.5~13.8，19.7~28，50.2~57.2，77.3~80 zk9-2 80 158.22 2.33 2.33 6.4 22.6~24.9 zk10-1 80 158.11 0.7 0.7 2.3 27.3~32 zk11-1 80 158.98 1.1 1.1 3.8 7.4~12.7，17.9~31.4，39.3~41.3，48.5~70.6 zk12-3 30 160.13 0.6 0.6 4.6 15.7~17.3 zk12-4 80 151.59 1.91 1.95 3.2 20.2~22.6，24.9~29.6，32~43.7，48.3~50.7 zk13-3 80 155.56 3.03 3.03 4.4 17.2~23.4，25.5~30.2，32.8~39.6，47.5~63.4，75.8~77.5 zk14-2 80 151.13 0.7 0.7 2 2~28.1，40.1~63.6，68.3~70.9 zk15-1 30 163.52 3.5 3.5 7 7~8.5，10.9~24.9 zk15-4 80 157.85 7.05 6.9 8.4 50~80
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