Digital Image Measurement System for Soil Specimens in Triaxial Tests

Abstract

In order to measure the deformation distribution over the entire surface of soil specimens in triaxial tests, a digital image technique is developed by use of a complementary metal-oxide-semiconductor (CMOS) camera and two mirrors to obtain the deformation field and the strain field of the whole surface of the specimen. Two sets of circular light-emitting diode (LED) lights are placed on the top and the bottom of the pressure chamber. A shading box seals the camera and the plate glass in front of the chamber to ensure an unchanged lighting environment. The process of data processing involves the error check, the expanding and the splicing of the images, and the calculation of the strain in each element. The measurement precision of the strain is approximately 10^–4 ~ 10^–5. And then, the application of this digital image processing technique is introduced in the study of shear band problem, membrane embedding problem, the heterogeneous soil, the end contact and end constraint, and so on. At last, based on the digital image measurement system, a series of geotechnical test instruments have been developed, including: (1) Advanced triaxial compression test apparatus; (2) Bidirectional dynamic triaxial apparatus; (3) High pressure triaxial compression apparatus; (4) Plane strain compression apparatus; (5) Unsaturated soil triaxial compression apparatus; (6) Frozen soil triaxial apparatus; (7) Asphalt concrete stress-strain apparatus; (8) Hydrate triaxial apparatus; (9) Soft material testing machine.