Systematic Study On Geological Engineering Problems Of Thick-covering Karst Subgrade In High-speed Railway

Thick-covering karst refers to the soluble rock which covered by10to30metersthick loose deposit of Quaternary. Once crossing this type of karst, the high-speedrailway may cause series of geological engineering problems which threat theconstruction and operation. Based on the research of environmental geological conditionsand development condition of karst from Shaoguan to Huadu, this paper has studied thefailure mechanism, both the response of underground hydrodynamic force and train load,physical simulation and criterion, the risk analysis of collapse by GIS, and the disposalmeasure together with quality inspection of karst subgrade. Meanwhile, a theoreticalsystem about geological engineering problems of thick-covering karst subgrade has beenset up. And it enriches the study of karst and guides the design and construction ofhigh-speed railway passing through thick-covering karst. The main achievements of thispaper are as follows:(1) Defy the notion of thick-covering karst, which means the soluble rock whichcovered by10to30meters thick loose deposit of Quaternary.(2) Based on studied of development mechanism and distribution of soil cave, karstcave, and corroded breaking zone, geological model of failure mechanism has been raised,such as silty clay+soluble rock, silty clay+land pebble+silty clay+soluble rock, siltyclay+land pebble+soluble rock, and and pebble+silty clay+soluble rock.(3) Focusing on trigger factors, the underground hydrodynamic force has beeninvestigated. And numerical simulation of the impact of loose deposit of Quaternarycompanion with underground water level change has been made. The results show, undersingle pattern, the diameter of soil cave is related to the thickness of covering layer withthe same decline depth of water level. The thinner of the covering layer, the biggerchance of subside. However, since the soil caves are located within land pebble layer intype Ⅱ under the combined pattern, the displacement of the top is bigger than normal in natural state because of lower strength. Besides, a little fluctuation of water level mayenlarge the diameter of soil cave constantly. Once man-made heavy precipitation happens,the subsidence was liable to take place. The caves are located within silty clay in type Ⅲand Ⅳ, therefore, change of displacement is similar to type Ⅰ-1to Ⅰ-3.(4) Focusing on trigger factors, the train load has been studied. Taking silty clay+soluble rock as geological model, the conceptual model and numerical simulation havebeen established. According to the simulation, the tensile stresses mainly concentrate3to5meters below the subgrade, which means soil caves over5meters’ depth could not beaffected. While the span of soil caves is relatively larger than coping thickness, plasticzone appears at the top of the roof firstly, which brings shear compression failure.Affected by the constantly dynamic load, the plastic zone extends downward. And plasticzone at the top extends upward. Finally, transfixion happens.(5) In order to explore the development criterion of the thick covering layer, a largeentity simulation has been set up. Physical simulations are made to study the formationand evolution of collapse within the silty clay. Results did not show an obvious relationbetween water level change of Quaternary and the deformation. Under the same soil layerstructure, the regions, where the pore aquifer is close to karst aquifer, are liable tocollapse. The deformation of soil has related to water decline speed and permeategradient of fissure water system within karst conduits, which can be recognized as onecriterion of collapse. At the former Xin Shakou station, the critical speed of karst waterdecline is about0.225m/min and the critical hydraulic gradient is form0.4to1.59.(6) Using AHP and GIS, the forecast model of karst collapse has been raised. Andsix factors are chosen as index, such as lithology, rate of linear karstification, soilthickness, soil formation, underground water level, relation between underground waterlevel and bedrock surface. The analysis based on this model has defined12sections to beeasy to collapse which are focusing remediation zone.(7) Reasonable remediation design principles and engineering measures are made.Also further study is made to the parameters, for example, reinforcement depth andgrouting pitch. A maximum of30meters is considered to be the best reinforcement depth.While grouting pressure is about0.2to0.5MPa, the grouting pitch is3.5to7.0meters.The electrometric method and surface wave method are chosen to detect the remediation.(8) Aimed at thick-covering karst collapse, a combined system, includingreconnaissance, prediction and evaluation, and prevention and cure construction, has setup to study the collapse in Wuguang passenger dedicated line from Shaoguan to Huadu.