CIVIL ENGINEERING

Triaxial tests have been widely used to evaluate the soil behavior. In the past few decades, several methods have been developed to measure the volume changes of unsaturated soil specimens during triaxial tests. Literature review indicates that until now it remains a major challenge for researchers to measure the volume changes of unsaturated soil specimens during triaxial testing.
This paper presents a noncontact method to measure the total and local volume changes of unsaturated soil specimens using a conventional triaxial test apparatus for saturated soils. The method is simple and cost-effective, requiring only a commercially available digital camera to take images of an unsaturated soil specimen during triaxial testing from which accurate 3D model of the soil specimen is reconstructed. In this proposed method, the photogrammetric technique is utilized to determine the orientations of the camera where the images are taken and the shape and location of the acrylic cell, multiple optical ray tracings are employed to correct the refraction at the air-acrylic cell and acrylic cell-water interfaces, and a least–square optimization technique is applied to estimate the coordinates of any point on the specimen surface. The paper first discusses the theory of proposed method. An image analysis on a caliper was then used to evaluate the accuracy of photogrammetric analysis in the air. A series of isotropic compression tests on a stainless steel cylinder were used to demonstrate the procedure and evaluate the accuracy of the proposed method, while triaxial shearing tests on a saturated sand specimen were used to exam the capacity of the proposed method for measuring the total and localized volume changes during triaxial testing. Based upon the analysis of the test results, some conclusions are drawn.

When characterizing an unsaturated soil using the triaxial test apparatus, it is required to
measure the soil deformation during loading. Recently, a photogrammetry-based method has
been developed for total and localized volume change measurements on unsaturated soils during
triaxial testing. In this study, more in-depth discussions on the photogrammetry-based method
are addressed such as system setup, measurement procedure, accuracy self-check, data postprocessing,
and differences from the conventional image-based methods. Also, an application of
the photogrammetry-based method on unsaturated soil deformation measurements is presented
through a series of undrained triaxial tests with different loading paths. After testing, threedimensional
(3D) models of the tested soils at different loading steps were constructed based on
the 3D coordinates of measurement targets on the soil surface. Clear barreling processes for soils
during deviatoric loading were observed through the constructed 3D models at different axial
strain levels. Soil volume changes and volumetric strain non-uniformities during isotropic and
deviatoric loadings were extracted based upon detailed analyses on different soil layers. Through
a full-field strain distribution analysis, a shear band evolution process was captured for the soil
during deviatoric loading at a low confining stress. The photogrammetry-based method proved to
be very powerful for in-depth soil deformation characteristics investigation.