基于针入强度的不同含水率煤样强度弱化特征与微观机制

Strength weakening characteristics and microscopic mechanism of coal samples with different water contents based on penetration strength

利用废弃矿井采空区作为储水空间对矿区的生态环境和经济发展起着重要作用,水浸作用下煤岩体强度直接影响地下储库的稳定性,有必要研究含水率影响下煤岩体的稳定性。针对关闭矿井大台矿井煤体各向异性强的特点,研究了低损测试针入强度与煤样抗压强度和抗拉强度的相关关系。在此基础上,研究了同一煤样不同含水率强度的变化规律,并借助CT扫描分析了水浸孔隙演化与强度弱化的内在机制。研究结果表明:不同含水率煤样单轴抗压强度(UCS)和巴西劈裂抗拉强度(BTS)与煤样针入强度(NPI)分别呈线性关系和负指数关系,煤样针入强度能够很好地匹配煤样单轴抗压强度和劈裂强度。因此,可以利用针入度指标评价煤样的强度,进而实现同一煤样不同含水率的低损循环测试,降低煤样各向异性对试验结果的影响,同时也为现场快速测试煤样强度提供有效手段。随着含水率的增加,煤样的针入强度呈指数式减小。由干燥到饱和,煤样针入强度降低幅度为86.55%。浸泡前后连通性孔和孤立孔的孔隙率分别上升了1.58%和0.97%,总体孔隙率上升2.55%,而矿物成分的体积占比下降了1.96%。煤样浸水过程中连通孔内部的黏土矿物成分遇水膨胀溶解导致煤样孔隙率变化。孔隙率(孤立孔和连通孔)的增加使得损伤裂隙沿着孔隙发育,更容易形成贯通裂隙,进而导致强度降低。此外,水充满连通节理使得裂隙强度大幅度降低,造成损伤裂隙更容易沿着弱面扩张,进一步降低煤样强度。

The use of goafs in abandoned mines as water storage space plays an important role in the ecological environment and economic development of the mining area.The strength of coal and rock mass under the action of water immersion directly affects the stability of underground storage.Aiming at the characteristics of strong anisotropy of coal in the closed Datai mine,this paper studies the correlation between the penetration strength and the compressive strength and tensile strength of coal samples.On this basis,the variation law of the same coal sample strength with different water contents was studied,and the internal mechanism of water-immersed pore evolution and strength weakening was analyzed by means of CT scanning.The research results show that:The uniaxial compressive strength(UCS)and Brazilian tensile strength(BTS)of coal samples with different water contents have a linear relationship and a negative exponential relationship with the coal sample penetra-tion index(NPI),respectively.The NPI can well match the UCS and BTS of coal samples.With the increase of water content,the penetration strength of coal samples decreases exponentially.Therefore,the penetration index can be used to evaluate the strength of the coal sample,thus realizing the low damage cycle test of the same coal sample with different water content.It reduces the influence of coal sample anisotropy on the ex-perimental results,and also provides an effective means for on-site rapid testing of coal sample strength.From drying to saturation,the penetration strength of coal samples decreased by 86.55%.Before and after soaking,the porosity of connected and isolated pores increased by 1.58%and 0.97%respectively,and the overall porosity increased by 2.55%,while the volume proportion of mineral components decreased by 1.96%.During the water soaking process of coal samples,the clay minerals in the connected pores swelled and dissolved in water,resulting in changes in the porosity of coal samples.The increase in porosity(isolated pores and connected pores)makes damage fractures develop along pores,making it easier to form through-fractures,which in turn leads to a decrease in strength.In addition,the water-filled connecting joints greatly reduce the its strength,which makes the damaged fractures more likely to expand along the weak surface,thereby reducing the strength of the coal sample.