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Rock strength and deformation characteristics under different unloading rates
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摘要: 岩体工程的开挖本质上是岩体的卸荷过程，不同的卸荷速率会显著影响岩体的强度变形特性，开展相关研究对于岩体工程的安全稳定分析具有重要意义。针对岩石开挖卸荷中各种可能的应力路径，开展了普通三轴压缩试验以及恒主应力差卸围压、恒轴压卸围压、升轴压卸围压的3种卸荷试验，重点分析了不同卸荷速率对开挖卸荷岩体力学特性的影响规律。研究得出主要结论如下：（1）不同卸荷方案、不同卸荷速率的岩样，都具有典型的脆性破坏特征，当围压降低到一定程度时，岩样突然破坏，轴压陡降，环向应变显著增大。（2）当围压卸荷速率较高，岩样临近破坏时，变形模量随围压卸荷比的变化曲线几乎成90°直线下降，泊松比随围压卸荷比的变化曲线几乎成90°直线上升；而卸荷速率较低时，变形模量和泊松比下降/增长的趋势相对较缓。这说明围压卸荷速率越大，岩样脆性破坏特征越显著。（3）3种卸荷方案岩样在不同的卸荷速率下，破坏时的应力状态基本都位于普通三轴压缩Mogi-Coulomb强度包络线的下方，即围压卸荷时的岩样比普通三轴压缩状态的岩样更容易破坏。Abstract: The excavation of rock mass engineering is essentially the unloading process of rock mass, and different unloading rates will significantly affect the strength and deformation characteristics of rock mass, which is of great significance to the safety and stability analysis of rock mass engineering. For modeling various unloading stress paths in the rock mass excavation, the ordinary triaxial compression test, constant axial pressure unloading confining pressure test, loading axial pressure and unloading confining pressure test, and constant principal stress difference unloading confining pressure test were conducted in this research. The effects of different unloading rates on the mechanical properties of rock mass were analyzed. The main conclusions are as follows: (1) Rock shows typical brittle failure characteristics under different unloading paths and different unloading rates, when the confining pressure decreases to a certain value, the rock sample suddenly fails, the axial pressure drops sharply, and the circumferential strain ε3 increases significantly. (2) If the confining pressure unloading rate is high, when the rock sample is near failure, the deformation modulus E decreases linearly almost at a 90° angle, and Poisson’s ratio μ increases linearly almost at a 90° angle. If the unloading rate is low, the decreasing/increasing trend of E and μ is relatively slower. This shows that the brittle failure of rock sample is more obvious with the increasing unloading rate of confining pressure. (3) The stress state of rock samples under the three unloading paths are basically below the strength envelope of ordinary triaxial compression Mogi-Coulomb, which shows that the rock sample under confining pressure unloading is more easily damaged than that under normal triaxial compression.
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Key words:
- rock /
- unloading rate /
- strength /
- deformation characteristics /
- unloading confining pressure
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图 1 普通三轴压缩试验应力应变曲线
Figure 1. Stress-strain curves in general triaxial compression test
图 2 普通三轴压缩试验结果及拟合
Figure 2. Test results and fitting in general triaxial compression test
图 3 不同卸荷路径及卸荷速率下岩样的应力应变曲线
Figure 3. Stress-strain curves of rock samples under different unloading paths and different unloading rates
图 4 不同卸荷路径及卸荷速率下岩样的变形模量
Figure 4. Deformation modulus of rock samples under different unloading paths and different unloading rates
图 5 不同卸荷路径及卸荷速率下岩样的泊松比
Figure 5. Poisson's ratio of rock samples under different unloading paths and different unloading rates
图 6 不同卸荷速率岩样破坏时的围压
Figure 6. Confining pressure of rock samples at different unloading rates
表 1 试验加卸荷方案
Table 1. Test loading and unloading schemes
试验
方案试样
编号卸荷时初始
围压/MPa卸荷时初始
轴压/MPa围压卸荷速
率/(MPa·s−1)轴压卸荷速
率/(MPa·s−1)试验
方案试样
编号卸荷时初始
围压/MPa卸荷时初始
轴压/MPa围压卸荷速
率/(MPa·s−1)轴压卸荷速
率/(MPa·s−1)方案Ⅰ：普通三
轴压缩试验Ⅰ-1 10 -- -- -- 方案Ⅲ：恒主应
力差卸围压Ⅲ-1 30 89.8 −0.05 −0.05 Ⅰ-2 20 -- -- -- Ⅲ-2 30 90.2 −0.10 −0.10 Ⅰ-3 30 -- -- -- Ⅲ-3 30 89.4 −0.25 −0.25 Ⅰ-4 40 -- -- -- Ⅲ-4 30 89.8 −0.50 −0.50 Ⅲ-5 30 89.1 −0.85 −0.85 方案Ⅱ：恒轴
压卸围压Ⅱ-1 30 88.7 −0.05 -- 方案Ⅳ：升轴
压卸围压Ⅳ-1 30 68.6 −0.05 0.25 Ⅱ-2 30 90.5 −0.10 -- Ⅳ-2 30 62.4 −0.10 0.25 Ⅱ-3 30 90.9 −0.25 -- Ⅳ-3 30 62.8 −0.25 0.25 Ⅱ-4 30 89.5 −0.50 -- Ⅳ-4 30 64.7 −0.50 0.25 Ⅱ-5 30 90.5 −0.85 -- Ⅳ-5 30 64.7 −0.85 0.25 
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