Triaxial tests are used extensively to evaluate stress-strain behavior for both saturated and unsaturated soils. A literature review indicates that all conventional triaxial test methods measure the relative volume of soil; however, between the initial measurements and the start of the triaxial tests, there are unavoidably disturbances during installation that cause deviation of soil volume from that at the initial condition. Recently image-based methods have been developed to measure the absolute volume of soil specimens. However, these methods still have a major limitation in their inability to determine top and bottom boundaries between the soil specimen, and the top and bottom caps. This paper proposes a photogrammetry-based method to overcome this limitation by developing a mathematically rigorous technique to determine the upper and lower boundaries of soil specimens during triaxial testing. The photogrammetry technique was used to determine the orientations of the camera, and the shape and location of the acrylic cell. Multiple ray-tracings and least-square optimization techniques were also applied to obtain the coordinates of any point inside the triaxial cell, and thus back-calculate the upper and lower boundaries. With these boundaries and the side surface, a triangular surface mesh was constructed and the specimen volume was then calculated in both unconfined compression tests and triaxial tests. The calculation procedures are presented in detail with validation tests performed on a cylindrical specimen to evaluate the accuracy of the method. Results indicate that the accuracy of the proposed method is up to 0.023% in unconfined compression tests and 0.061% in triaxial tests.

三轴试验广泛用于评估饱和土和非饱和土的应力应变行为。文献综述表明，所有常规的三轴试验方法都可测量土壤的相对体积；因此，但是，在初始测量和开始三轴试验之间，安装过程中不可避免地会出现干扰，这些干扰会导致土壤体积与初始条件下的体积发生偏差。最近，已经开发了基于图像的方法来测量土壤标本的绝对体积。但是，这些方法仍无法确定土壤样本与顶盖和底盖之间的顶部和底部边界，仍然存在很大的局限性。本文提出了一种基于摄影测量的方法，通过开发一种数学上严格的技术来确定三轴测试期间土壤样品的上下边界，从而克服了这一限制。使用摄影测量技术确定相机的方向以及丙烯酸电池的形状和位置。还应用了多种射线追踪和最小二乘最优化技术来获取三轴单元内部任何点的坐标，从而反算上下边界。利用这些边界和侧面，构造了一个三角形表面网格，然后在无边压缩试验和三轴试验中计算了样品体积。详细介绍了计算程序，并在圆柱试样上进行了验证测试以评估该方法的准确性。结果表明，该方法的精度在无边压缩试验中高达0.023％，在三轴试验中高达0.061％。