三轴试样绝对体积测量方法研究

Measuring absolute volume of triaxial soil specimens

三轴试验过程中的土样体积是用于分析土体应力应变特性所需的一个基本参数。近年来,基于图像分析的方法越来越广泛地应用于土样在三轴试验过程中的体积测量。然而,基于图像分析方法测量所得结果仅仅是试样的相对体积变化,并非试样绝对体积。为了测量三轴试验过程中试样的绝对体积,综合运用摄影测量原理、光线追踪、和最小二乘优化技术,测定了覆盖试样表面点云的三维位置;通过对点云进行三角网格划分、端部截断、插值、并扣除橡皮膜体积计算得到土样在三轴试验过程中的绝对体积;同时,还针对性地开发出了用于实现上述分析步骤以及相关测量结果后处理的配套软件,GeoTri3D。为验证该绝对体积测量方法的可行性,开展了一个标准钢柱试验、一个饱和砂土和三个非饱和粉土试样的三轴试验。钢柱与饱和砂土的三轴试验结果表明该方法能够成功实现三轴试样在试验过程中任意时刻的绝对体积测量;此外,通过对粉土试样表面三维点云的进一步分析所得的土样应变云图能够真实再现三轴试验过程中的土样全局变形过程。

The soil volume in triaxial tests is an essential parameter to characterize stress and strain behaviors of soils.In recent years,more and more image-based methods have been developed for the triaxial tests on deformation measurement of soils.However,the measured results are only the relative volume change of specimens,not their absolute volume.In order to measure the absolute soil volume in triaxial tests,the photogrammetry principle,optical ray tracing and the least square optimization are used to measure the three-dimensional position of point clouds on the soil surface.The absolute volume of the soil specimens during triaxial tests is then obtained through triangulation,end cut and interpolation based on the obtained point clouds.Meantime,a software package,GeoTri3 D,is developed to implement the above-mentioned steps and post processing of the measured results.In order to validate the proposed method,triaxial tests are conducted on a standard steel cylinder,a saturated sand specimen,and three soil specimens.The results obtained from the validation tests indicate that the proposed method can successfully and accurately capture the absolute volume of the soil specimens during triaxial tests at different time of interests.In addition,based on the analysis on the triangular meshes,the strain contour plots can be generated,which facilitates the visualization of the deforming process of soils during triaxial tests.