基于点荷载试验的新生代红砂岩强度软化规律研究

Study on strength softening law of Cenozoic red sandstone based on point load test

水岩相互作用对于软岩的强度特性具有很大的影响。以安徽池州市典型的新生代红砂岩为研究对象,通过单轴试验以及点荷载试验,研究了新生代红砂岩点荷载强度与单轴强度之间的换算系数以及含水率对点荷载强度软化规律的影响;同时结合矿物组分与微观结构分析以及吸水试验结果,对新生代红砂岩的软化机制进行了探讨。研究结果表明:天然状态下,新生代红砂岩点荷载强度换算单轴抗压强度的经验系数应该适当降低,建议取9.0;红砂岩点荷载强度以及贯入比指标均随着含水率的增大而降低,呈负对数衰减的变化趋势;提出了吸水软化的饱和度特征值,该特征值将吸水软化区域分为含水率敏感区与非敏感区,敏感区内强度随含水率改变剧烈变化。黏土矿物的湿胀软化效应是其出现强度软化特征值的微观机制,因此,饱和度特征值应普适于含较多亲水性黏土矿物的软岩吸水软化规律中;新生代红砂岩黏土矿物含量高,内部存在较多的微裂隙,由于黏土矿物吸水膨胀、裂隙扩展与发育导致岩石整体胶结能力减弱以及结构体系的破坏,从而使得岩石的整体强度降低。研究结果为红砂岩地区岩石强度软化机制的分析以及强度软化参数的选取提供了参考和依据。

Water-rock interaction has a significant influence on the strength characteristics of soft rock. In this paper, the uniaxial compression test and point load test were conducted on typical Cenozoic red sandstone that was taken from Chizhou City, Anhui Province, China. The empirical conversion coefficient between point load strength(PLS) and uniaxial compression strength(UCS)and the softening law of water content on PLS were studied. Meanwhile, the softening mechanism of this sandstone was also discussed based on the analyses of mineral composition and microscopic structure as well as the results of water absorption tests. The resultes show that:(1) in the natural state, the empirical coefficient for converting the point load strength to uniaxial compressive strength of the Cenozoic red sandstone should be appropriately reduced, and it is recommended to take 9.0;(2) The PLS and penetration ratio decreased with the increase of water content, showing a trend of negative logarithmic decay;(3) The characteristic value of saturation of water absorption softening is put forward. This value divides the water absorption softening area into sensitive and non-sensitive areas, and the strength in the sensitive area changes dramatically with the change of water content.(4) The swelling-softening effect of clay minerals is the micromechanism appearing strength softening. Therefore, the characteristic saturation value should be universally suitable for soft rocks containing lots of clay minerals.(5) The Cenozoic red sandstone has a high clay mineral content and more internal microfractures. Because of clay minerals’ water-absorbing expansion, crack expanding, and developing, the overall cementation ability of the rock is weakened and the structural system is destroyed, resulting in the reduction of the overall rock strength. This study provides a reference and basis for analyzing strength softening mechanism and strength softening parameters selection in the red sandstone field.