Numerical simulation of bearing mechanism of steel casing group in complex karst area
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摘要:
岩溶区桩基施工过程中，隐伏溶洞空间分布的不确定性导致施工过程风险性提高，尤其是搭设钢护筒时，结构荷载超过钢护筒侧摩阻力，可导致钢护筒下沉对施工工作面带来安全风险。文章以江西九江银沙湾复杂岩溶区桩基施工工程为例，模拟研究了岩溶区钢护筒群的承载特性。结果表明：整个施工区域岩溶非常发育，溶洞占比为76.2%，溶洞的最大高度为17.3 m，且存在大溶洞串洞现象。按照大溶洞区空间分布特点，钢护筒群下沉模式分为三种，三角形区钢护筒群下沉、矩形区钢护筒群下沉和单排型钢护筒群下沉。提出预设指定位移来反分析钢护筒群下沉过程的模拟方法，模拟结果显示，隐伏大溶洞区施工方式应该按照外侧优先成桩，内侧选点成桩方式进行。
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.
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Key words:
- karst area /
- steel casing /
- bearing characteristics /
- numerical simulation /
- failure analysis
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表 1 钢平台面荷载统计表
Table 1. Statistics of steel platform surface load
荷载类型 面荷载/kN·m−2 恒载 钢面板自重 0.69 横梁自重 0.38 纵梁自重 0.64 栏杆自重 0.06 钢管柱及钢横撑自重 20.50 钢横撑自重 1.27 活载 施工荷载 2.00 机械荷载 1.65 水流荷载 1.17 
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表 2 岩土材料参数
Table 2. Geotechnical material parameters
重度/kN·m−3 弹性模量E50/
kN·m−2黏聚力C/
kN·m−2摩擦角Φ/° 抛石 22.0 1.00×105 27.5 45 粉质黏土 18.8 6.37×103 19.0 11 粉细砂1 19.0 1.17×104 3.0 21 粉细砂2 20.0 1.56×104 0 28 粉细砂3 20.0 2.21×104 0 31 灰岩 21.8 1.00×105 2.5×104 60
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表 3 梁与钢护筒参数
Table 3. Beam and steel casing parameters
重度/kN·m−3 弹性模梁E50 /N·m−2 截面面积/m2 惯性矩I2 /m4 极限侧摩阻力标准值/kN 极限端阻力标准值/kN 梁 78.5 2×1011 9.5×10−3 7.19×10−6 \ \ 钢护筒 78.5 2×1011 5.0×10−3 2.01×10−6 70~350 \ 混凝土桩 25.0 3×1010 1.65 2.17×10−1 100~600 4500
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