Research on creep characteristics and pore structure evolution characteristics of freezing-thawing rocks
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摘要: 寒区露天岩体面临着循环冻融和长期荷载共同作用引起的时效性损伤的考验。为探究寒区环境对岩体稳定性的影响，以陕北某工程的红砂岩为研究对象，从冻融岩石的宏观蠕变特性及细观结构的演化特征着手，通过冻融岩石的加卸载蠕变试验，并配合核磁共振检测，对冻融红砂岩蠕变过程中的宏观力学指标及细观参数的演化进行定量分析。结果表明：应力水平在0.3σucs～0.5σucs时，孔径分布在一个较小范围内波动，当应力水平增高至0.5σucs～0.6σucs时，小孔（横向弛豫时间T2<10 ms）占比下降，大孔（T2>10 ms）占比上升；冻融加剧了蠕变阶段孔隙度的增长，高应力水平下冻融对孔隙度增长的影响更为显著。引入分形理论表征孔隙结构的复杂程度，发现大孔有明显分形特征，而小孔分形特征不明显，总孔分维DT与孔隙度呈正相关；孔隙结构的复杂程度仅在孔隙度较大时才明显影响岩石的蠕变力学特性。建立了有关冻融作用影响及蠕变损伤的冻融-损伤蠕变模型，模型曲线可以很好地反映冻融岩石的蠕变破坏特征，且与试验曲线吻合良好。本研究可为冻融环境下的岩体工程建设提供理论依据。Abstract: The rock mass of open pit in cold regions is often affected by freezing-thawing cycles and long-term loads. In order to explore the influence of cold region environment on the stability of rock mass, the red sandstone of a project in northern Shaanxi is taken as the research object, starting with the macroscopic creep characteristics and meso-structure evolution characteristics of freezing-thawing rocks. The evolution of macro-mechanical indexes and meso-parameters during the creep process of freezing-thawing red sandstone is quantitatively analyzed through the loading and unloading creep test of freezing-thawing rock and the NMR detection. The results show that the pore size distribution fluctuates in a small range when the stress level is between 0.3σucs−0.5σucs, and when the stress level increases to 0.5σucs−0.6σucs, the proportion of small holes (T2<10 ms) decreases and the proportion of large holes (T2>10 ms) increases, and freezing-thawing aggravates the increase of porosity in the creep stage, and the effect of freezing and thawing on porosity growth is more significant at high stress levels. The fractal theory is introduced to characterize the complexity of pore structure. It is found that the large pores have obvious fractal characteristics, while the fractal characteristics of small pores are not obvious. The total pore fractal dimension DT is positively correlated with porosity. The complexity of pore structure only affects the creep mechanical properties of rock when the porosity is large. The freezing-thawing-damage creep model of freezing-thawing effect and creep damage is established. The model curve can well reflect the creep characteristics of the freezing-thawing rock, which is in good agreement with the experimental curve. This study can provide a theoretical basis for rock engineering construction in freezig-thawing environment.
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Keywords:
- creep test /
- freezing-thawing cycle /
- NMR /
- porosity /
- fractal dimension /
- creep model
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图 2 冻融岩石分级加卸载蠕变应力加载方式
Figure 2. Graded loading and unloading creep stress loading method of freezing-thawing rock
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图 5 蠕变速率、塑性应变分别与加载比的关系
Figure 5. Relationship between the creep rate, plastic strain and loading ratio
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图 6 冻融红砂岩蠕变过程中T2分布曲线
Figure 6. T2 distribution during the creep process of rock samples with different freezing-thawing times
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图 7 蠕变过程中红砂岩的孔径分布占比
Figure 7. Proportion of pore size distribution of rock samples during creep
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图 11 冻融红砂岩分形维数随应力水平的演化
Figure 11. Evolution of fractal dimension of freezing-thawing red sandstone with stress level
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图 13 双因素共同作用对蠕变力学行为的影响
Figure 13. Influence of two factors on creep mechanical behavior
表 1 各项物理参数的平均值
Table 1 Mean physical parameters of the rock samples
物理参数 纵波波速/
（m∙s−1）干密度/
（g·cm−3）饱和密度/
（g·cm−3）饱和含
水率/%孔隙
度/%平均值 2 781 2.21 2.67 7.05 15.59 
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表 3 单轴蠕变试验中不同冻融岩样各级应力加载值
Table 3 Stress loading values at various levels in the uniaxial creep test of sandstone under different freezing-thawing conditions
冻融循环
次数/次岩样
编号冻融后T2
谱面积加载应力/MPa 第一级 第二级 第三级 第四级 第五级 0 A-R 10 668.82 10.01 13.34 16.68 20.01 23.35 30 B-R 11 584.31 9.71 12.95 16.19 19.42 22.66 60 C-R 11 348.65 8.40 11.20 14.00 16.79 19.59
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表 4 组Ⅰ中不同冻融岩样的力学参数
Table 4 Mechanical parameters of different freezing-thawing rock samples in group I
参数 冻融次数/次 0 30 60 抗压强度/MPa 33.37 32.37 27.99 弹性模量/GPa 39.65 37.79 31.22
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表 5 蠕变模型参数
Table 5 Creep model parameters
加载等级 $ {E_1} $
/GPa$ {E_2} $
/GPa$ {\eta _1} $
/（GPa·h）$ {\eta _2} $
/（GPa·h）$ \gamma $ $ \alpha $/h−1 $ {R^2} $ 第一级 129.969 19.024 2.245 — — — 0.97 第二级 89.972 13.931 1.568 — — — 0.95 第三级 52.979 12.401 1.496 — — — 0.98 第四级 22.135 9.671 1.054 — — — 0.93 第五级 10.973 8.066 0.809 0.001 3 0.161 0.169 0.91 注：“—”表示此处为空。
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