Numerical simulation of bearing mechanism of steel casing group in complex karst area

GUO Hui; MOU Yunzhen; ZHANG Wengang; CHEN Fei; ZHANG Yanmei; WANG Shuo; LIN Sicheng

doi:10.11932/karst2024y012

Abstract：The construction of pile foundations in karst areas presents unique challenges due to the unpredictable spatial distribution of underground cavities,which may significantly increase the associated risk of such projects.Karst topography is characterized by the presence of soluble rocks,such as limestone,which have been gradually dissolved by groundwater,resulting in the formation of crevices and caves.This geological feature poses significant difficulties for engineers,as the unpredictability of these subsurface formations can lead to complications during construction,especially when steel casings are installed for pile foundations.In karst areas,the risk to the construction process tends to be exacerbated by the possibility of encountering covered caves. When steel casings are built,the structural load may exceed the side friction of the casings,potentially causing them to sink.This situation escalates the construction risks,making the project more complex and challenging.The project for pile foundation construction in the complex karst area of Yinshawan,Jiujiang,Jiangxi(hereinafter referred to as the Yinshawan Project),exemplifies such challenges associated with this type of environment.This area is highly karstic,with caves constituting 76.2％ of the karst topography and the largest cave reaching a height of 17.3 m.These characteristics require an improved approach to steel casing construction to prevent sinking and collapse. To solve these issues,this study introduces a three-dimensional numerical model that incorporates the presence of caves,informed by preliminary drilling data and exploration conditions specific to the Yinshawan Project.This model is a critical tool for understanding the subsidence behavior of steel casings in karst areas.This study classifies subsidence patterns into three distinct categories:single-row steel casing groups,triangular steel casing groups,and rectangular steel casing groups.It examines the mechanics of steel casing subsidence in areas affected by karst development.The method involves simulating specified displacements to retroactively analyze the sinking process of steel casing groups. Through numerical simulations,this study examined the load-bearing characteristics of steel casing groups in various sinking modes.The findings reveal that the sinking modes differed significantly in terms of the maximum longitudinal and transverse bending moments experienced by the casings.For instance,in the single-row steel casing group,the maximum longitudinal bending moment was found to be 1,620 kN·m,while the maximum transverse bending moment was 664.6 kN·m.In the sinking mode of the steel casing group within the rectangular area,the maximum transverse bending moment in the non-sinking steel casing was 637.8 kN·m,and the maximum longitudinal bending moment was 2,144 kN·m;both of these values were found in the same steel casing.In the sinking mode of the triangular steel casing group,the maximum longitudinal bending moment in the non-sinking steel casing was 2,090 kN·m,and the maximum transverse bending moment was 922.2 kN·m. The study offers detailed analysis of each sinking mode,highlighting the stress characteristics and potential risks of subsidence.Given these risks and the specific stress characteristics identified through simulations,a construction method in areas with significant covered caves,such as the project site in Yinshawan,should be prioritized.This method should focus on peripheral piling while selectively placing piles in the interior.This study provides valuable insights into the field of geotechnical engineering,offering guidance for future projects in similarly challenging environments.