碱水作用下弱胶结粉砂岩孔隙结构分形特征与非线性劣化机制

Fractal characteristics of pore structure and nonlinear deterioration mechanism of weakly cemented siltstone under alkaline water effect

弱胶结岩石遇水发生强度劣化是我国西部地区煤矿开采过程中矿压显现异常、顶板事故频发的重要原因.以我国新疆哈密地区弱胶结粉砂岩和地下水赋存环境为工程背景,通过SEM电镜扫描结合数字图像处理技术,研究了不同pH值碱性溶液、不同浸泡时间对弱胶结粉砂岩微观结构和孔隙分形维数的影响规律,确定了孔隙分形维数、孔隙率与单轴抗压强度、抗拉强度之间的函数关系,建立了碱水侵蚀条件下弱胶结粉砂岩劣化非线性动力学模型.研究结果表明:1)浸泡溶液碱性越强,弱胶结粉砂岩矿物颗粒形态越趋于圆润,且颗粒间接触形式变得疏松多孔;浸泡初期孔隙率与孔隙分形维数增幅相对较大而后期趋缓.2)碱水侵蚀作用下孔隙结构复杂程度及分布形态对力学强度影响显著,弱胶结粉砂岩单轴抗压强度、抗拉强度与孔隙分形维数、孔隙率之间均具有较强的负线性相关关系;单轴抗压强度、抗拉强度与孔隙分形维数的相关性大于孔隙率;碱水侵蚀降低了弱胶结粉砂岩的脆性.3)选取孔隙分形维数、孔隙率和单轴抗压强度为描述水化学反应系统的变量,以相应参量的时间序列值为非线性动力学方程的特解,通过反演建立了碱水化学作用弱胶结粉砂岩劣化的非线性动力学模型;采用建立的模型计算的孔隙分形维数值、孔隙率和单轴抗压强度与试验值的误差范围分别为2.37%~2.64%,2.38%~4.57%,3.48%~8.12%,可用于弱胶结岩石碱水侵蚀不同时间后孔隙分形维数、孔隙率以及单轴抗压强度等参数的预测评估.

Weakly cemented rocks have significant strength decline when exposed to groundwater,frequently resulting in roof weighting anomaly and caving disasters in western coalfields in China.Based on the coal measure geology and hydrological settings of Hami coalfields in Xinjiang Uygur Autonomous Region,the paper investigates the impact of alkaline pH levels and immersion durations on the micro-structure and pore fractal dimension of weakly cemented siltstone using SEM and digital processing techniques,determines the relationship from pore fractal dimension and porosity to uniaxial compressive strength(UCS)and tensile strength,establishing a nonlinear dynamic model to characterize the strength degradation due to alkaline erosion.The results show that:1)Mineral particles become morphologically rounded and loosened with alkalinity increase,and the enlargement in porosity and pore fractal dimension gradually slows with immersion.2)The UCS and tensile strength of eroded weakly cemented siltstone are influenced by the pore structure morphology and distribution,featuring a negative correlation to pore fractal dimension,then to porosity.The rock brittleness is decreased due to alkaline erosion.3)A nonlinear dynamic model that incorporates pore fractal dimension,porosity,and UCS as descriptors is established utilizing the time series values of these descriptors as specific solutions.The pore fractal dimension,porosity,and UCS calculated via the nonlinear model show a good agreement with experimental results,and the error ranges are 2.37%—2.64%,2.38%—4.57%,3.48%—8.12%,respectively.The model is able to assess the porosity and strength parameters of weakly cemented rocks exposed to alkaline erosion over time.