Suction-controllable triaxial test system for studying the micro-hydromechanical behavior of unsaturated soils with in-situ X-ray micro computed tomography scanning

A suction-controllable triaxial test system comprising:

• a triaxial loading unit that is miniature in size, less than 400 mm in height, not larger than 120 mm in diameter, and configured and adapted to inhibit collision between the assembly and an X-ray CT scanner during a scanning process, the triaxial loading unit comprising a strain-controlled axial loading unit;
• an integrated cell pressure and suction-control unit;
• a sample preparation and installation unit;
• one or more data acquisition modules;
• associated software configured and adapted to operate at least the data acquisition modules;
• an axial loading component comprising a high resolution servo-controlled stepping motor operably connected to a mechanism configured and adapted to convert rotation of the stepping motor to linear motion, wherein the motor and the mechanism are configured and adapted to produce a resolution better than 1 micron per step of the stepping motor;
• a high accuracy miniature load cell configured and adapted to monitor sample stress at a resolution better than 0.25 N;
• a high accuracy LVDT to monitor sample strain at a resolution better than 5 microns;
• a loading rod assembly configured and adapted to transfer a load from the axial loading component to a soil sample;
• an X-ray transparent chamber;
• an air tight chamber cap that allows vertical movements while inhibiting horizontal movements of the loading rod assembly;
• and
• a sample stage comprising an embedded high Air-Entry-Value (AEV) ceramic and a water reservoir;
• and
• a high accuracy water pressure transducer configured and adapted to provide negative and positive water pressure monitoring at a resolution better than 0.5 Pa;
• wherein the loading rod assembly is connected to a first set of air pressure controlling components and the X-ray transparent chamber is connected to a second set of air pressure controlling components, and wherein the first set of air pressure controlling components and second set of air pressure controlling components are configured and adapted for independent regulation and control of the magnitude of a confining pressure applied to a soil sample and a pore air pressure applied to a soil sample;
• wherein the loading rod assembly comprises a cap connected to an external air pressure source, a hollow loading rod with a smooth exterior surface, and a connector embedded with an upward radial seal and a downward radial seal;
• wherein the connector is connected to the soil sample during testing by a sample membrane;
• wherein the connector is configured and adapted to move freely in a vertical direction to separate the cell pressure in the chamber from the cell pore air pressure in the sample;
• and
• wherein the upward radial seal and the downward radial seal each are configured and adapted to seal to the smooth exterior surface of the hollow rod and maintain a first air pressure in the sample, the first air pressure different from a second air pressure in the X-ray transparent chamber.

A method for capturing in-situ micro-structure evolution of a soil sample during triaxial load testing, the method comprising:

• preparing and installing, by a suction-controllable triaxial test system, a soil sample into a triaxial loading unit of a suction-controllable triaxial test system, wherein the triaxial loading unit includes a strain-controlled axial loading unit and two or more independent modules, wherein each module, respectively, has a different technique than the other module, and wherein each independent module is configured and adapted to more precisely apply a range of matric suction or hydraulic loading either above or below a specified threshold pressure;
• conducting X-ray scanning and imaging simultaneously with conducting triaxial hydro-mechanical loading of the sample;
• and
• reconstructing one or more images of the tested sample obtained at two or more specified points in a ((p-ua), q, s) space during the X-ray scanning and imaging, to capture the in-situ micro-structure evolution of the soil sample during triaxial load testing.