Research On Long-term Mechanical Properties And Design Method Of Geosynthetics On Treatment Of Karst Collapse

Karst collapse have been identified widely in south China,causing serious damage to the safety of traffic.The application of geotextile with proper anchorage length and buried depth during the traditional backfilling has been validated to a novel and effective method in the treatment of karst collapse.The geotextile buried in soil are subjected not only to horizontal tensile action but also to different normal stresses at different buried depths due to the long-term function reinforcement and filtration.Additionally,the plunger creep caused by long-term plunger pressure will appear at the maximum tension position at the edge of the subsidence area.Therefore,the strength design and reinforcement selection different from that of conventional reinforcement are required to discuss.In this thesis,the tensile properties under different confinements and plunger creep properties of geosynthetics were studied by the tensile tests under confinement and plunger creep tests for geosynthetics,respectively.The corresponding reduction coefficient of plunger creep and method of strength design for the reinforcement were proposed.The research achievement provides an insight into the design and selection of geosynthetic applied for the treatment of karst collapse and the other reinforced soil structures,which can reduce the cost of geosynthetics under safety and has practical application value in engineering.The following conclusions can be drawn:(1)It was found by the tensile tests under confinement that improvement ratios of tensile strength for some geosynthetics were more than 100% under greater normal stress.The confinement of the normal stress was the major factor to improve tensile properties of geosynthetics rather than reinforcement-soil friction.The equations express the corresponding relationship between tensile properties of geosynthetics under confinement and in-air were proposed.This provides a method to predict the tensile properties of geosynthetics buried at different depths in the treatment of karst collapse by the in-air tensile properties,which could also be applied to the design of the reinforcement layers with different buried depths in other reinforced soil structures.(2)It was found by the plunger creep tests that the plunger area strain of geosynthetic increases with creep time as a logarithmic function.The equation for predicting the reduction coefficient of plunger creep from the failure strain was obtained.The reduction coefficient of plunger creep increases from 65.340% to 148.496% when comparing to that of conventional tensile creep.The application of reduction coefficient of plunger creep to the reinforcement strength design of karst collapse treated by geosynthetics could better ensure the safety of the whole structure,which could also be applied to the design of the reinforcement subjected to the continuous plunger pressure in other reinforced soil structures.(3)An improved design method of geosynthetic considering the tensile strength under confinement and the reduction coefficient of plunger creep was proposed,which was validated by the geosynthetic studied in the existing researches.The minimum tensile strength calculated by the improved design method was obviously smaller than that calculated by the standard method due to the enhancement of the tensile strength of geosynthetics caused by the confinement of soil.The improved design method could be more suitable for the strength optimization design of geosynthetics in the treatment of karst collapse in the state of security.(4)To reduce the cost of geosynthetics used in the treatment of karst collapse in the state of security,the design method proposed in this thesis was suggested to apply to the design strength of reinforcement when the improvement ratio of tensile strength under confinement was not less than 50% compared to the in-air tensile strength.Otherwise,the standard design method was still recommended.