基于伯格斯混杂接触的砂质泥岩加轴压卸围压蠕变细观研究

Mesoscopic study on creep of sandy mudstone under loading axial stresses and unloading confining pressures based on Burgers hybrid contact

为了探究不同轴-围增卸比下深部软岩蠕变特性和失稳规律,利用颗粒流程序(PFC^(2D))进行二次开发,并引入一种伯格斯-平行黏结混杂接触,建立了不同开采条件下砂质泥岩细观蠕变模型,基于已有砂质泥岩常规三轴压缩和加轴压卸围压蠕变试验验证了细观模型的可靠性,并在此基础上开展了不同加轴压卸围压蠕变应力路径下砂质泥岩细观蠕变特性及能量演化规律研究。研究结果表明:3种轴-围增卸比下砂质泥岩应力-应变曲线均表现为应变硬化特征,峰值偏应力与总裂纹数随轴-围增卸比的增大而增大;设计了3种不同加轴压卸围压细观蠕变应力路径,在相同围压及其卸载速率下,随轴-围增卸比的增大,加速蠕变失稳阶段历时快速减少,轴-径应变极值的差值逐渐增大,且同阶段的轴向稳态蠕变速率也增大;随蠕变时间的增长,放顶煤和无煤柱开采的伯格斯接触占比呈先增加而后减小,保护层开采则呈先缓降后快速减小,且在同蠕变时间时,伯格斯接触占比与线性接触占比均随轴-围增卸比的增加而增长,平行黏结接触占比随轴-围增卸比的增加而下降;在相同围压及其卸载蠕变速率下,卸荷前砂质泥岩总能量以弹性能为主,卸荷后耗散能占主导地位,弹性能耗比初始阶段随轴应变的增加而呈非线性快速增长,后在前3个蠕变阶段呈“锯齿状”缓慢下降至加速蠕变阶段起始点,而后以较大速率突增,且增长速率随轴-围增卸比的增大而逐渐减小。

This study investigates the creep behavior and instability mechanisms of deep soft rock under varying ratios of axial pressure increment to confining pressure decrement(AITCD).The two-dimensional particle flow code(PFC2D)was further developed to incorporate a hybrid contact model,referred to as the Burgers-parallel bond(BPB)model,to establish a meso-creep model of sandy mudstone under different mining conditions.The model's reliability was validated using results from conventional triaxial compression tests and creep tests involving axial stress loading and confining pressure unloading of sandy mudstone.Based on this model,the mesoscopic creep characteristics and energy evolution of sandy mudstone were studied under various axial stress loading and confining pressures unloading stress paths.The results show that,under the three distinct AITCD ratios,the stress-strain curves of sandy mudstone exhibit strain-hardening behavior.Both the peak deviator stress and total crack number increase as the AITCD ratio rises.Under the same confining pressure and unloading rate,an increase in the AITCD ratio significantly reduces the duration of the accelerated creep instability phase,widens the difference between peak axial and radial strain,and raises the axial steady-state creep rate.As creep time progresses,the proportion of Burgers contacts of top coal caving and non-pillar mining initially increases before decreasing,while for protective seam mining,it first decreases gradually and then drops sharply.Additionally,the proportions of Burgers and linear contacts increase with higher AITCD ratios,whereas the proportion of parallelbond contacts decreases.Energy analysis reveals that,prior to unloading,the total energy of sandy mudstone is dominated by elastic energy,while dissipative energy becomes the primary component after unloading.The elastic energy consumption ratio exhibits nonlinear,rapid growth at the initial stage of creep.It then declines in a"sawtooth"pattern during the first three creep stages until the onset of the accelerated creep stage,where it increases sharply.This growth rate diminishes progressively with higher AITCD ratios.