北疆侏罗系与白垩系泥质砂岩物理力学特性对比分析及其能量损伤演化机制研究

Physico-mechanical properties and its energy damage evolution mechanism of the Jurassic and Cretaceous argillaceous sandstone in Northern Xinjiang

特殊的地理、气候条件及工程地质的复杂性决定了在北疆地区软岩地层中修建引水隧洞的设计施工难度较大。隧洞成拱效应与围岩自稳能力差,围岩渗透性强,遇水软化特性显著,极易产生软岩大变形甚至坍塌失稳灾害。为进一步研究北疆地区侏罗系与白垩系泥质砂岩的物理力学性质、遇水软化特性与能量损伤演化机制,开展了二者的单轴压缩、常规三轴与单轴蠕变试验。研究结果表明:两种岩石均富含黏土矿物,白垩系泥质砂岩的粒径分配更好,但其胶结程度较差,导致其强度稳定性与地层波速相对较低。低围压条件下,两者均以环向变形与体积扩容为主,但随着围压升高,其破坏模式由体积扩张过渡到体积压缩类型。高围压加载会造成岩石内部损伤,从而导致其抗压强度的降低。遇水后,两种岩石的延塑性与应变软化特性均明显增强,白垩系泥质砂岩的遇水软化特性更为显著。白垩系泥质砂岩的蠕变特性更为显著,两者的长期强度接近其单轴压缩损伤应力值。两种泥质砂岩的能量损伤演化过程均呈现S型变化规律,侏罗系泥质砂岩的能量硬化特性更为显著,白垩系泥质砂岩会更早地进入到能量硬化与能量软化阶段。

The design and construction of the diversion tunnel are very difficult in the soft rock strata of the Northern Xinjiang, which is caused by the complexity of geographical, climatic conditions and engineering geology. The large deformation and even collapse hazards of surrounding rock are very easily to be encountered, due to its bad self-stability, strong permeability and water-softening characteristics. Hence, the uniaxial, triaxial compression tests and uniaxial creep tests were conducted to further study the physical and mechanical properties, water-softening characteristics and energy damage evolution mechanism of the Jurassic and Cretaceous argillaceous sandstones of Northern Xinjiang. Compared with the results of these two kinds of soft rocks both with rich clay minerals, the particle size distribution of the Cretaceous argillaceous sandstone was more uniform, but its cementation degree was lower. As a result, the strength, stability and the wave velocities of strata in the Cretaceous argillaceous sandstone were lower than those in Jurassic argillaceous sandstone. Under the condition of low confining pressure, two kinds of rock deformation were mainly circumferential deformation and volume expansion. With the increase of confining pressure, the failure mode changed from volume expansion to volume compression type. High confining pressure loading can cause internal damage of rock structure, resulting in the reduction of compressive strength. When these two kinds of rocks were saturated, the ductility and strain softening characteristics were significantly enhanced. The water-softening characteristic of the Cretaceous argillaceous sandstone was much more obvious. Cretaceous argillaceous sandstone had more significant creep characteristics, and their long-term strengths were both close to their damage stress values in uniaxial compression tests. The energy damage evolution processes of two kinds of argillaceous sandstones both showed the S-shaped evolution law. The energy-hardening characteristic of Jurassic argillaceous sandstone was more significant, but the Cretaceous argillaceous sandstone was earlier to enter energy-hardening and energy-softening stages.