Cyclic Triaxial Systems

The feedback controlled cyclic triaxial system applies cyclic or dynamic loading to the soil specimen. The system is a digitally controlled, servo-pneumatic (closed loop) system, which controls three parameters: axial load or displacement, confining pressure and back pressure.

The base system incorporates an integrated Multi-Axis Control System (IMACS). a digitally controlled 5 kN actuator, TRITECH 50 load frame, auxiliary air receiver complete with two servo-valves for cell and back pressure control, two air filters and associated cables. The system is supplied complete with cyclic and stress path software.

A double acting digitally controlled pneumatic actuator applies axial load. This cyclic load can be applied in load (N), stress (kPa), displacement (mm) or strain (%). Digitally controlled pneumatic valves apply confining and back pressures. The dynamic tests are generally carried out in conjunction with static imposed conditions of stress to the soil samples.

This means that the sample, in addition to the application of dynamic actions, can be saturated, consolidated upon different stress levels (isotropic, anisotropic, K₀, etc.) and also carried out to failure under static monotonic conditions.

The Static and Dynamic Cyclic Triaxial system has been designed to perform the following tests: 

• Standard triaxial tests (UU, CU, CD) including saturation, isotropic and anisotropic consolidation with pore pressure and volume change measurements 
• Stress path tests, including Ko consolidation 
• Cyclic loading 
• Dynamic shear strength and deformation 
• Liquefaction potential 
• Shear modulus and damping ratio

The system can work with different models of triaxial cells for specimen sizes of 50, 70, and 100 mm.
Each of the two systems includes:

• Load frame (50 or 100 kN capacity)
• 5 kN actuator with coaxial displacement transducer
• IMACS for control and data acquisition
• The software modules described above.

Transducers and triaxial cells (Tn-Cell Plus models) are not included with the base system and have to be ordered separately depending on the specimen characteristics.

Load frames
The basic standard triaxial machines Tritech 50 and Tritech 100 have been modified to include the pneumatic actuator mounted on the crosshead. The actuator has a specially designed locking mechanism, which will lock it in any position within its travel. Then shearing to load higher than the actuator capacity can be achieved using the Tritech load frame. This allows the specimen to be sheared at the end of the dynamic stage. When dynamic test is performed, the pneumatic actuator is free to move vertically; for monotonic tests the actuator is locked to prevent it taking load.

Actuator
This is a digitally controlled, double acting pneumatic actuator. It requires a minimum air supply of 800 kPa for operation.

Signals applied to the voltage / pressure converter from the IMACS enable computer controlled adjustment of the loads applied to the sample. The actuator has an integral displacement transducer, which allows tests to be run under load and displacement control.

The actuator can generate frequencies up to 70 Hz. But the testing speed is really dependent on the type of sample being tested and test conditions. Soft materials will have greater displacement under load, which means the actuator has further to travel, stiff samples can require higher loads, which will reduce the frequency. This is not a problem when testing soils, especially if the testing is for liquefaction. In this case the loads and frequency will be very low.

Integrated Multi-Axis Control System (IMACS)
The IMACS is a compact self contained unit that provides all critical control, timing and data acquisition functions for the test and the transducers. The IMACS is linked to a personal computer through the USB communications link.

The data acquisition module has 13 normalised (±10 V range) transducer input channels. These channels are digitised by accurate, high speed 20 bit (AID) converters for data analysis and presentation.

The control module has three channels for feedback control. One is dedicated to the actuator for vertical load/displacement, the second is dedicated to cell pressure, the third is for back pressure.

The feedback control module and the data acquisition module have their own dedicated high speed USB 10 b/s or RS232 interface.

This allows uninterrupted, simultaneous communication enabling increased speed of operation and flexibility.

Supervised by the PC, the IMACS automatically controls the operation of loading for individual types of test. The IMACS directly controls the servo-valve to apply the requested loading rate or waveform, cell and back pressure. While the specimen is being subjected to loading forces, the IMACS captures data from the transducers and transfers these, via the USB or RS232 link, to the PC for processing, display and storage.