基于三轴强度预测白垩系冻结砂岩的长期强度

Prediction of Long-term Strength of Frozen Cretaceous Sandstone Based on Triaxial Strength

冻结法是深厚富水基岩煤矿立井的有效施工方法,白垩系砂岩具有强度低、胶结弱、遇水软化的特点,广泛分布于中国西部煤矿建井施工中,研究冻结砂岩的长期强度对立井冻结壁稳定性评价和冻结设计提供科学的理论依据。以白垩系饱和冻结砂岩为试验对象,开展不同温度(+20、-10、-20℃),不同围压(0、2、4、6 MPa)条件下的三轴蠕变力学试验。根据蠕变试验结果,基于应变等时曲线簇和稳态蠕变速率与应力水平的关系两种模型分别计算冻结砂岩的长期强度,探讨两种模型的优劣性,并将三轴强度与长期强度对比定量分析。研究结果表明:体积应变等时曲线簇的拐点比轴向应变、径向应变等时曲线的拐点更为明显,而且其对应力水平和时间的反应更敏感;稳态蠕变速率-应力曲线的拐点计算的长期强度,存在着数值偏小的误差;确定体积应变等时曲线簇法计算冻结砂岩长期强度更为合理。依据确定的计算方法,得到+20、-10、-20℃温度下砂岩的蠕变长期强度分别为同条件下三轴压缩强度的45.7%~50.7%、41.8%~53.4%和51.5%~67.1%,结果将为确定与预测白垩系富水冻结砂岩长期强度及蠕变荷载作用下可能发生破坏的时间提供科学的指导意义。

Freezing method is an effective construction method for vertical shaft in deep water rich bedrock coal mine. Cretaceous sandstone has the characteristics of low strength, weak cementation and softening when encountering water. It is widely distributed in the construction of coal mine shaft in Western China. The research on the long-term strength of frozen sandstone provides scientific theoretical basis for the stability evaluation and freezing design of shaft freezing wall. Taking the saturated frozen sandstone of Cretaceous as the test object, the triaxial creep mechanical tests were carried out under different temperature(+20 ℃,-10 ℃ and-20 ℃), different confining pressure(0, 2 MPa, 4 MPa and 6 MPa). According to the creep test results, the long-term strength of frozen sandstone was calculated based on the strain isochronous curve cluster and the relationship between steady-state creep rate and stress level. The advantages and disadvantages of the two models were discussed, and the triaxial strength and long-term strength were compared and quantitatively analyzed. The results show that the inflexion of volume strain isochronous curve cluster is more obvious than that of axial strain and radial strain isochronous curve, and its response to stress level and time is more sensitive. The long-term strength calculated by inflexion of steady-state creep rate stress curve has small error of numerical value. The long-term strength of frozen sandstone calculated by volume strain isochronous curve cluster method is more suitable Li. According to the determined calculation method, the creep long-term strength of sandstone at +20 ℃,-10 ℃ and-20 ℃ is 45.7% ~50.7%, 41.8%~53.4% and 51.5%~67.1% of triaxial compression strength under the same conditions, respectively. The results will provide scientific guidance for determining and predicting the long-term strength of Cretaceous water rich frozen sandstone and the possible failure time under creep load.