| Subsurface soil-voids and sinkhole collapses are main geological hazards during linear engineering construction in the karst regions. In this research, four constructing linear engineering sites, the Guilin-Yangshuo Highway, Guangxi Province, the Extension Section of 2nd Line Subway in Guangzhou, Guangdong Province, the Guiguang Section of the 2nd West-East Gas Pipeline, Guangxi Province, and the Anshan Section of the Express Railway from Harbin to Dalian, Liaoning Province, have been selected as typical sites. Through comprehensive analysis of the results of soil-void formation mechanism test, geophysical and drilling exploration, sinkhole monitoring and treatment, a primary system of soil-void prevention and treatment has been established.1. The mechanism of soil-void formationFor mechanism analysis of soil-void formation, laboratory tests of undisturbed soil samples, including seepage deformation test, modified pinhole test, slaking test, and chemical composition test have been taken. The results show soil seepage failure and disintegration induced by groundwater flow play an important part in the process of soil-void formation, and lixiviation should not be neglected in some condition.The exploration indicates soil-voids usually exist in two modes of soil structure, multi-layer with shallow aquifer and single-layer without shallow aquifer. In the multi-layers mode, most soil-voids locates nearby bedrock surface where is under groundwater table, and in the single layer mode, soil-voids are generally at the unsaturated zone, it should be the result of soil-cave development upward.2. Exploration of subsurface soil voidsIn order to select a suitable geophysical method to detect locations of subsurface soil-voids (potential sinkholes), Ground Penetrating Radar (GPR), Multi-electrode Electric Resistivity, and Shallow Seismic Reflection were used for survey in the research sites. Comparison of the three geophysical methods indicates that GPR can detect subsurface soil voids in a more effective and timely manner.3. Monitoring of soil-voids (sinkholes) formationThrough the feasibility and reliability tests of Time Domain Reflectometry (TDR) monitoring soil-voids, it is proved that the TDR is effective to monitoring soil-voids and potential sinkholes along linear engineering, because of unique features such as distributed measurement, long distance range, high resolution, cost-effective and remote monitoring. The precise of soil-void location defined by TDR is about one meter. The coaxial cable is both the transmission line and sensor, so coaxial cable installation will influence the validity of soil-void monitoring and forecasting. The tests of three different installation methods indicate that the coaxial cable beamy covered by cement-sands is the suitable way for soil-void monitoring under the linear engineering construction.The station of long-term monitoring and forecasting of soil-void and potential sinkhole was established at Guilin-Yangshuo highway on Dec. 21, 2006. From the milestone K14+550 to K14+650, five parallel lines of coaxial cables, with a 3 meters space between, were installed on the subgrade. Additionally, a piezometer datalogger and a rain gauge were installed to measured groundwater pressure in karst opening and precipitation, and GPR was used to scan along the same lines to confirm the abnormity of TDR.4. Treatment of soil-voids (potential sinkhole)Excavation backfill and grouting are widely used to treat the soil-voids and sinkholes. This research indicates that excavation will create new soil-voids around the interface of soil and backfill block-stone if groundwater was confined, because the block-stone exists space and makes seepage deformation easily happen in circumjacent soil. So filling the soil-void with grout is a better way to treat soil-voids and potential sinkholes. For the very good reflection in the GPR image, GPR can also be used to verify grouting quality.
|
PDF Full Text Request