土石混合体的剪应力波动和跌落行为机制

土石混合体在剪切过程中存在剪应力跌落的现象，基于该现象，本文采用大型直剪试验的方式，考虑不同含石量(0%、30%、50%、70%)、上覆垂直压力(50，200，300，400 kPa)、块石尺寸(9.5～19.0 mm、19.0～31.5 mm、31.5～53.0 mm) 3个主要控制因素，进行室内剪切变形试验，研究直剪过程中发生的剪应力跌落现象。同时，通过在试样内部钻孔、埋置细铝丝与干灰的方法获取剪切带变形厚度，结合其大小理解剪应力的脆性跌落特征和剪切带块石变形特征。基于试验分析表明，具备骨架结构且所含块石尺寸大于剪切带厚度的土石混合体试样在高垂直应力下的剪切过程中易出现块石应力集中，形成锁固体块石，该类块石往往控制着试样一定时空下的整体剪切强度，其受剪切作用翻滚、滑移甚至咬合棱角破碎是导致剪应力瞬间大幅度跌落的直接原因。高含石量、大尺寸块石、高垂直应力是形成块石应力锁固体的必要条件。低含石量状态(<50%)，剪切带块石多顺剪切方向翻滚，越靠近剪切面边缘，变形越明显，块石相对空间位置变化较小。高含石量状态(>70%)，剪切带块石可见相互滑移、攀爬，块石相对空间位置变化明显。块石尺寸小于剪切带厚度时，剪应力多呈现波动特征，而块石尺寸接近剪切带厚度时，剪应力波动加剧，出现明显的应力跌落，对应垂直位移出现突变。满足含石量高于70%、块石尺寸大于剪切带厚度的试样在相对较大的上覆垂直应力作用下易形成块石应力锁固体。

Mechanism of shear stress fluctuation and dropping of the soil-rock mixture

The shear stress-dropping (jump) of the soil-rock mixture (S-RM) exists under direct shear conditions. In this paper the main factors affecting the stress-dropping are explored with the large scale direct shear test. Artificial soil-rock mixture specimens are prepared for different rock block proportions (0%, 30%, 50% and 70%), normal pressures (50, 200, 300 and 400 kPa), and rock sizes ranging from 9.5 to 19.0 mm, from 19.0 to 31.5 mm and from 31.5 to 53.0 mm. Meanwhile, the thickness of the shear band is monitored by putting the aluminum wires and dry ash into the hole inside the specimens. The thickness of the shear band will contribute to further understand the stress jump and rock blocks deformation. The experimental results show that the soil-rock mixture sample with skeleton structure and larger rock blocks is inclined to form the interlocking rock block. Furthermore, the shear strength of the sample is dominated by interlocking rock blocks until the breakage of the rock block. The rotation, slippage, and breakage of interlocking rock block are the direct causes of the stress-dropping. The high content of the rock blocks, rock blocks of oversize, and high normal pressure are the necessary conditions for the formation of the stress locked patch. When the sample rock content is 50%, the rotation of shear band rock blocks in the shear direction is observed, and the deformation of rock blocks are more obvious close to the sidewall. However, the change of relative spatial arrangement of rock blocks is not more obvious, when the rock content of the sample is 70%, the behaviors of sliding, climbing, and the change of relative spatial arrangement of rock blocks are more notable than those of the sample with rock content of 50%. When the rock block size is less than the shear band thickness, the shear stress curve shows a wave shape. However, when the rock block size is close to the shear band thickness, the shear stress curve shows obvious brittle stress-dropping, and the vertical displacement will change sharply accordingly. Under the large-scale direct shear test of soil-rock mixture with high-level normal stress, the specimen with the rock content of above 70% and the size of rock block larger than the thickness of shear band will be apt to form the stress locked patch.