荆门石膏矿岩遇水软化力学特性试验研究

Experimental study on mechanical properties of water-softening gypsum rock in Jingmen

针对荆门市荆花石膏矿中留作矿柱的新鲜石膏岩进行天然和饱水两种状态下的单轴和三轴压缩试验,结合微观扫描电镜试验,研究石膏岩的强度和变形软化特性及机制。结果表明:石膏岩遇水会发生显著的强度和变形软化。随溶液中SO_4^(2-)、Ca^(2+)浓度不同,强度软化系数在0.6~0.72之间,弹性模量软化系数在0.66~0.75之间,凝聚力c显著下降而内摩擦角φ基本保持不变。石膏岩具有良好的延性,天然状态下峰后应变软化转为理想塑性变形的临界围压在2.5~5.0 MPa之间,经饱水软化后临界转换围压低于2.5 MPa。石膏岩遇水软化机制与其主要矿物成分二水硫酸钙的微溶性质密切相关。在蒸馏水中,石膏岩主要因溶解造成矿物流失,弱化内部结构而软化;在硫酸钙饱和溶液中,石膏矿物溶解-重结晶的动态平衡改变了岩石内部结构,使其疏松化,造成其力学性质发生软化。试验结果加深了对石膏岩遇水软化力学特性的认识,对石膏矿采空区稳定性分析具有重要意义。

Under nature and saturated conditions, a series of uniaxial and triaxial compression tests was conducted on gypsum rock mined from Jinghua mine in Jingmen. Mechanical properties of water-softening gypsum rock and the corresponding softening mechanism were studied by the scanning electron microscope（SEM）. The results show that the strength and deformational properties of gypsum rock are tremendously weakened by water. With different concentrations of SO_4^（2-）and Ca^（2＋） in the solution, the softening coefficient of rock strength ranges from 0.6 to 0.72 and the softening coefficient of elastic modulus is in the range of 0.66-0.75. Besides, the cohesion c decreases significantly whereas the internal friction angle φ remains essentially unchanged. Since the gypsum rock has good ductility, the critical confining pressure of the elastic deformation to the ideal plastic deformation is found between 2.5 MPa and 5 MPa in nature sate, while the critical confining pressure is lower than 2.5 MPa for saturated gypsum rock. The softening mechanism of gypsum rock is closely related to the slight solubility of the calcium sulphate dehydrate, which is the major ingredient of rock. In distilled water, the rock structure is weakened by the dissolution process of gypsum mineral. In saturated CaSO_4 solution, the dynamic equilibrium state reaches when the dissolution rate is equal to the re-crystallization rate. The continuous dissolution and re-crystallization processes tend to gradually loosen the original tight structure, resulting in softening mechanical properties. In conclusion, this study provides deep insights into the transformation process of the mechanical properties of the gypsum rock in accordance with different solutions and confining pressures, which is of great significance for the stability analysis of goaf area in gypsum mines.