砂土可视化直剪试验与剪切带形成机制研究

A STUDY ON FORMING MECHANISM OF SHEAR BANDS AND VISUAL SHEARING TESTS OF SAND

通过对常规直剪仪进行可视化改造,并结合数字图像变形量测技术,探究了不同尺寸砂土试样在剪切过程中剪切带的形成机制和扩展过程,归纳了试样尺寸对砂土强度指标的影响,并针对典型尺寸试样总结了不同剪切时刻、不同法向应力下剪切带(尤其是副剪切带)的演化规律,同时对试验中发现的拱形破坏区域进行受力分析,对主/副剪切带的强度参数进行了对比。结果显示:试样尺寸影响强度指标,当长边与短边之比大于2时,试验结果偏离平均值,当比值大于4时,会造成严重误差;剪切破坏区域不是始终沿水平剪切缝发展,而是在上盒中下部逐渐形成一条向上凸起的弧形副剪切带,剪切破坏区域沿着主、副两条剪切带而展开;随着剪切位移的增加,以及法向应力的减小,拱形破坏区域会逐渐减小;拱形副剪切带上的正应力和剪应力均大于水平主剪切带;而主/副剪切带的强度指标基本一致,间接证明了副剪切带存在的合理性。

By visual reformation on traditional shear instrument, and combining digital image deformation measurement technology, it studies the forming mechanism and expansion processes of shear bands of sand samples with different sizes in shearing tests. The effects of sample sizes on sand strength are concluded. With the typical sample size, the evolution laws of shear bands(especially for secondary shear bands) in different shearing courses and various normal stresses are summarized. And the force analyses of arch damage areas discovered in tests are carried out. The strength parameters of primary and secondary shear bands are compared. The results show that sample sizes have influences on strength parameters. When the ratios of long to short side are greater than 2, test results deviate from average. When the ratios of long to short side are greater than 4, serious errors are caused. The shear failure areas are not always developed along horizontal shearing gap. The upward arch secondary shear bands are gradually formed in middle and lower part of upper boxes, and shear failure areas are developed along the primary and secondary shear bands. The arch failure areas decrease with the increasing shear displacements and decreasing normal stresses. The normal and tangential stresses on secondary shear bands are larger than those on primary shear bands. The strength indexes of primary and secondary shear bands are basically the same, which indirectly proves the rationality of the existence of secondary shear bands.