Study on softening characteristics of mudstones in shallow buried strata in Xinjiang and countermeasures for tunnel support
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摘要: 新疆输水隧洞围岩为浅埋泥岩地层，吸水后强度软化，支护难度高，对工程进度影响较大。本文以新疆某输水隧洞泥岩为研究对象，在对隧洞的泥岩进行XRD测试分析的基础上，开展不同含水率状态下泥岩单轴压缩和电镜扫描分析实验研究，揭示了岩样内部裂隙、孔隙结构的变化规律及强度软化特征。结果表明：该隧洞泥岩黏土矿物含量高达50 %，易吸水软化。随着含水率的增加，岩样内部将经历微裂隙发育、发展及贯通的破坏过程，同时伴随内部原有矿物流失。通过分析其饱和吸水曲线，将其吸水过程划分为三个阶段：急速吸水、减速吸水及匀速吸水阶段；采用玻尔兹曼预测函数对吸水后岩样强度变化曲线进行拟合，可将其强度变化分为三种状态：强度无损、急速软化及软化休止状态，获得该类型泥岩吸水强度软化的临界点为含水率6 %；综合分析得出其吸水软化规律：泥岩中以蒙脱石为主的黏土矿物吸水能力强，随着吸水量逐渐增加，泥岩孔隙中自由水含量显著提高，骨架强度逐渐降低，形成不稳定态，在受到外力作用时极易发生变形破坏。据此提出了此类软岩工程稳定性控制原则：确定软岩类型及变形力学机制，制定有针对性的稳定性控制对策，采用具有恒阻、大变形及高预应力等力学性能的支护材料。研究成果对深入了解输水隧洞泥岩强度软化特征、隧洞围岩变形破坏机理及支护设计具有重要意义。Abstract: The Xinjiang water conveyance tunnel is a shallow buried mudstone stratum. After the water absorption, the strength of the surrounding rock softens and large deformation occurs. The support is difficult and the risk is high, which has a great impact on the progress of the project. In this paper, the mudstone of a water conveyance tunnel in Xinjiang is taken as the research object. Based on the XRD test analysis of the mudstone in the tunnel, the uniaxial compression and SEM analysis of the mudstone under different water content conditions are carried out, and the internal fissures of the rock sample are revealed. The variation of the pore structure clarifies the strength softening characteristics. The results show that the clay mineral content of the tunnel is as high as 50 %, and the proportion of montmorillonite is close to 90 %, which is easy to absorb water and soften. As the water content increases, the interior of the rock sample will undergo the process of micro-crack development, development and penetration, accompanied by the loss of pore water and internal minerals. By analyzing its saturated water absorption curve, the water absorption process is divided into three stages: rapid water absorption, decelerating water absorption and uniform water absorption stage. The Boltzmann prediction function is used to fit the rock sample strength variation curve after water absorption, and its strength can be changed. It is divided into three states: non-destructive strength, rapid softening and softening and rest state. The critical point for obtaining the softening of water absorption strength of this type of mudstone is 6 % water content; the comprehensive analysis shows that the water softening law is: the clay minerals in mudstone, mainly montmorillonite, have strong water absorption capacity. With the increase of water absorption, the free water content in the mudstone pores is significantly increased, the skeleton strength is gradually reduced, and an unstable state is formed, which is easily deformed and destroyed when subjected to external force. Based on this, the principles of stability control of such soft rock engineering are proposed, which is to determine the soft rock type and deformation mechanics mechanism, formulate targeted stability control strategies, and adopt the support of mechanical properties such as constant resistance, large deformation and high prestress material. The research results are of great significance for understanding the strength and softening characteristics of the mudstone in the water conveyance tunnel, the deformation and failure mechanism of the tunnel surrounding rock and the support design.
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Key words:
- softening law /
- softening critical point /
- microstructure /
- moisture content
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图 4 岩样饱和吸水特征曲线
Figure 4. Saturated water absorption characteristic curve of rock sample
图 5 不同含水率下岩样应力-应变曲线
Figure 5. Stress-strain curves of rock samples with different water content
图 6 强度软化百分比曲线及玻尔兹曼拟合
Figure 6. Strength softening percentage curve and boltzmann fitting
图 7 不同含水率的泥岩扫描电镜结果(1 000倍)
Figure 7. Scanning electron microscopy results of mudstones with different water contents (1 000 times)
图 8 不同含水率的泥岩扫描电镜结果(5 000倍)
Figure 8. Scanning electron microscopy results of mudstones with different water contents (5 000 times)
表 1 泥岩全岩矿物组成及黏土矿物含量
Table 1. Mineral composition and clay mineral content of mudstone
% 岩样编号 矿物种类和含量 黏土矿物相对含量 混层比 石英 钾长石 钠长石 方解石 白云石 黏土矿物 蒙脱石 伊/蒙混层 伊利石 高岭石 绿泥石 绿/蒙混层 1号 25.1 0.8 23.8 — — 50.3 90 — 2 3 5 — 2号 26.6 1.2 22.6 — — 49.6 84 — 4 6 6 — 
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表 3 岩样压缩实验结果
Table 3. Experimental results of rock sample compression
岩样编号 吸水率/% 直径D/mm 高度H/mm 单轴抗压强度 平均值 弹性模量 平均弹性模量 泊松比 平均泊松比 σc/MPa σc/MPa E/GPa E/GPa ν ν B-1 10 49.48 102.00 0.70 0.81 0.24 0.22 0.38 0.37 B-2 10 49.43 101.20 0.92 0.20 0.36 B-3 10 49.17 99.87 破坏 — — — — — C-1-1 7 49.45 101.20 12.38 12.12 2.99 2.94 0.27 0.29 C-1-2 7 49.32 99.65 12.19 2.94 0.26 C-1-3 7 49.28 98.66 11.80 2.89 0.34 C-2-1 9 49.40 98.30 1.20 1.30 0.20 0.19 0.34 0.36 C-2-2 9 49.46 98.95 1.39 0.19 0.38 C-2-3 9 49.87 98.65 破坏 — — — — — C-3-1 8 49.65 100.23 2.25 3.25 0.80 0.84 0.34 0.34 C-3-2 8 49.23 100.12 3.96 0.86 0.37 C-3-3 8 49.62 99.96 3.47 0.86 0.31
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