颗粒特性对第三系弱胶结砾岩抗剪强度的影响

Effect of particle characteristics on shear strength of weakly cemented conglomerate in tertiary

摘要:

第三系弱胶结砾岩是一种介于土与岩石之间的特殊岩土材料,成岩程度低,自稳能力差,颗粒级配特征和胶结程度对岩石工程特性的影响显著.以蒙华铁路中条山隧道出口侧第三系弱胶结砾岩为对象,通过对天然原状试样和重塑试样的筛分试验、颗粒粒径分形特征分析以及大型直剪试验,对砾岩的分形特征和胶结程度与抗剪强度之间的关系进行了分析.结果表明:第三系弱胶结砾岩一般以粒径值1.0 mm为界具有二重分维特性,且细粒分维值普遍小于粗粒分维值,表明砾岩在组成上呈现出细粒质量分数相对较多的特征,而具有一重分维的砾岩具有最优级配和最高抗剪强度；根据胶结程度不同,第三系弱胶结砾岩可分为弱胶结与极弱胶结,极弱胶结砾岩粘聚力极小可以忽略,其强度特征与无黏性土类似；在相同胶结程度情况下,分维数与第三系弱胶结砾岩内摩擦角正切值存在负指数关系；胶结程度是影响第三系弱胶结砾岩抗剪强度的重要因素,颗粒级配相同情况下极弱胶结砾岩的抗剪强度和内摩擦角仅为弱胶结砾岩的33.46%～40.85%和58.64%.

Abstract:

Weakly cemented conglomerate of Tertiary system is a kind of special geotechnical material between soil and rock. It has low diagenesis degree and poor self-stability, and its characteristics of particle gradation and cementation degree have significant impact on engineering characteristics of the rock. The relationship among fractal characteristics, cementation degree, and shear strength of conglomerate was analyzed by screening test, particle size fractal characteristics study, and large-scale direct shear test on undisturbed and remolded samples taken from the weakly cemented conglomerate of Tertiary system in the tunnel exit of Menghua Railway of Zhongtiao Mountain. Research results show that weakly cemented conglomerate generally had binary fractal features in the boundary of particle size 1.0 mm, and the fractal dimension of fine particles was mostly less than that of coarse particles, which indicates that there are relatively more fine particles in weakly cemented conglomerate of Tertiary system, and the conglomerate with 1D fractal has optimal gradation and maximum shear strength. According to the degree of cementation, weakly cemented conglomerate can be divided into weak cementation and extremely weak cementation. The cohesion of extremely weak cemented conglomerate was negligible and its strength characteristics were similar to those of non-cohesive soil. Under the condition of similar cementation degree, the relationship between fractal dimension and tangent of internal friction angle of the conglomerate could be described by negative exponent function. The shear strength and internal friction angle of extremely weakly cemented conglomerate were only 33.46%-40.85% and 58.64% compared with weakly cemented conglomerate without considering the difference in particle gradation, which shows that the cementation degree is an important factor affecting the shear strength of weakly cemented conglomerate of Tertiary system.