| In order to study the stability of karst roof and expressway subgrade in complex karst area under dynamic and static loads,taking a complex karst subgrade of Guangdong-Shaogan Expressway and the pile foundation construction site of a bridge on Shaozhou Avenue in Shaoguan City on both sides of the subgrade as the research object,drilling and geophysical exploration were used to find out the development of the lower karst cave in the study area,and on the basis of analyzing the evolution process of subgrade fractures,combined with the measured monitoring data,the monitoring data of horizontal displacement,settlement amount and deep part shift of the karst subgrade in this section were analyzed.On this basis,the minimum safety thickness of the submerged karst roof and the instability mechanism of karst subgrade in this section are theoretically studied,the stability development and evolution of the karst roof in the complex karst development area under dynamic and static loads are simulated by using FLAC3 D software.Then the stability of subgrade under static load and dynamic load in karst area is simulated and analyzed.The main achievements of this paper are as follows:(1)A roadbed on the Shaogan Expressway operated normally within seven years after its completion without any diseases.However,after the construction of the pile foundation of a bridge K,there were four cracks,the longest crack was 40.15 m,the widest crack was 16 mm,the height difference between the north and south edges of the crack was 5mm,and the dike lane had a sinking trend.(2)The hole rate of the subgrade drilled is 75%,and the hole rate of pile foundation drilled is 88%.There are four karst zones under the subgrade,namely Ⅰ1 zone,Ⅱ1 zone,Ⅱ2 zone,Ⅲ1zone.Karst caves A and B are located under pile foundation pier 4#.The maximum height of karst caves is 19 m,which is the largest karst cave in the study area.The 7 karst caves under pile foundation are all in irregular shape.The karst caves are the main factors affecting the stability of Shaogan-Jiangxi expressway.(3)By collating and analyzing the monitoring data,the displacement change curve and settlement change curve of karst subgrade under the condition of pile foundation construction are obtained: On the 15 th day of monitoring,the horizontal displacement of P01,P02,P03,P04 and P05 appeared abnormal.In particular,the cumulative horizontal displacement monitoring value of P01 reached a maximum of 7.1mm.In the 32 nd ～ 44 th day of monitoring,the horizontal displacement monitoring value appeared abnormal again,and the horizontal displacement monitoring value showed an overall upward trend.It is speculated that it is related to the pile foundation construction of 4# bridge pier on the 31 st ～ 36 th day of monitoring.The settlement of C01,C04 and C05 is small.The settlement value of C02 and C03 increases obviously during pile foundation construction,but decreases gradually after the stoppage.In the deep displacement monitoring,the shallow displacement changes significantly,while the deep displacement changes little.Before the construction,the deep displacement does not change significantly.After the construction of 4# pile foundation,the deep horizontal displacement changes for a single time up to 6.5mm,and the monitoring data is obviously abnormal.Based on the monitoring results of horizontal displacement,settlement and deep displacement,we can conclude that pile foundation construction affects the stability of Shaogan-Jiangxi expressway subgrade.(4)Using the semi-quantitative analysis method,the karst roof stability analysis was carried out on the karst roof cave in the I.1 area under the subgrade and the karst roof cave in the submerged A area(the largest height,the cave height of 19m)exposed by drilling,and it was found that: under static load conditions(normal traffic and traffic jam),the karst roof of the karst roof in area I1 remained basically stable,and in the absence of any construction,the karst roof of the karst roof cave in area A under the pile foundation was also stable,and the roadbed was in a stable state,and under the dynamic load conditions,the thickness of the roof of the cave in area I1 and area A did not meet the minimum safety thickness,karst cave instability leads to roadbed instability;The possibility of cracks in the pavement caused by the collapse of the cave due to the load was ruled out,and it was presumed that the collapse was caused by the construction vibration,which led to the instability of the karst roadbed in this section.(5)By modeling 7 irregular caves in the study area and subgrade above the caves,the whole process of subgrade and subgrade deformation under static load(normal driving,traffic jam),dynamic load(seismic wave,elastic wave)and superimposed static and dynamic load is simulated,and the displacement stress of the caves and subgrade under static and dynamic load is obtained.It is concluded that the subgrade and subgrade have minor deformation under static load.But all are in stable state;Under dynamic load,the wave affects the karst cave,and the subgrade becomes unstable after the cave collapses.(6)The monitoring results during the construction period are compared with the numerical simulation results,and it is found that the general rule is the same,but the numerical difference is slightly larger.The analysis reason is: the waves generated by construction vibration are not as strong as the seismic wave and sine wave,so the displacement changes will be different.(7)According to the actual monitoring and numerical simulation monitoring,once the wave is introduced into the karst foundation,the subgrade will produce obvious settlement,and the horizontal displacement will also change,but the change value is far less than the settlement value.Therefore,it is presumed that the construction vibration is mainly caused by the collapse of karst cave,which makes the roadbed produce obvious vertical displacement and makes the roadbed unstable.In the future,if the construction of the 4# and 5# pile foundations of the Bridge is continued,the underlying karst must be treated to prevent the cave from completely collapsing,resulting in complete instability and damage of the roadbed. |
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