Standards: BS 1377:8, ASTM D2850, ASTM D4767, ASTM D7181, NF P94-070, NF P94-074, CEN-ISO TS 17892-8.9
A knowledge of stress-strain behaviour and the shear strength parameters of soils is required when soil is interacting with structures or when soil is used as a construction material in many engineering purposes such as:
1. Excavations
2. Shallow foundations
3. Piles and deep foundations
4. Earth retaining structures, diaphragm walls, anchors
5. Slope stability
6. Ground improvement
7. Design of embankments, earth dams
The most widely used testing apparatus for investigating the stressstrainbehaviour and the strength parameters of soils is the triaxial apparatus.
Triaxial tests are typically performed with two stages: an isotropic loading followed by shear loading which is carried out up to failure.
A cylindrical saturated soil sample, undisturbed or reconstituted,is placed in a rubber membrane in order to isolate it from direct contact with the surrounding water with which the testing cell is filled, and which is pressurized. The sample sits in the cell between a rigid base and a rigid top cap and is loaded by means of a ram, at a constant speed. The water drainage in or out the sample can be allowed by means of opening or closing a valve. The two phases are carried out under different combinations of drainage conditions and give rise to 3 different standard triaxial tests. Each test is usually performed on three saturated specimens at three different confining pressures.
“UU” unconsolidated undrained test
This test is used in order to estimate the undrained shear strengthparameters (undrained shear strength cu and undrained friction angle φu = 0). It is usually performed on fine grained soils. In this test, both phases are carried out with the drainage valve closed. No volume change is allowed during the test and distortions during the shear phase occur up to the failure. This test gives a unique value of undrained shear strength, as the envelope of the Mohr circles plotted in total stresses is horizontal. The angle φu has to be zero or it is an error in the test, e.g. poor saturation. The stress-strain behaviour and the strength parameters are then used to model engineering problems when undrained conditions are present, typically in short term design.
“CU” consolidated undrained test
This test is used in order to estimate the drained and undrained shear strength parameters (effective cohesion c’, effective friction angle φ’, and undrained shear strength cu). It is usually performed on fine grained soils. In this test, the first phase is carried out with an open drainage valve in order to allow the consolidation and volume change to occur. During the second phase, the drainage valve is closed and the change of pore water pressure is measured; there is no volume change and distortions occur up to failure. It gives three values of undrained shear strength, which are the radii of the Mohr’s circles, and the values of the effective cohesion and of the effective friction angle, which are derived from the envelope of the effective stress Mohr’s circles. The stress-strain behaviour and the strength parameters are then used to model engineering problems when undrained conditions are present, after consolidation processes or in long term applications.
“CD” consolidated drained test
This test is used in order to estimate the effective shear strength parameters (effective cohesion c’, effective friction angle φ’). It is usually performed on coarse grained soils. In this test, both phases are carried out with the drainage valve open. Volume change occurs in both phases and during the second phase, distortions occur up to failure. The CD test gives the values of the effective cohesion and of the effective friction angle, which are derived from the envelope of effective stress Mohr’s circles. The stress-strain behaviour and the strength parameters are then used to model engineering problems when drained conditions are present, typically in long term design.
S301M Triaxial Test Devce, 50 kN Capacity, Digital, Touch Screen
This versatile, compact, heavy duty load frame has been designed for routine tests, for central laboratories, but also for research purposes. The frame is of rigid chromed steel twin column construction. The electronic color digital touch-screen display with microprocessor control system allows to perform tests within a speed range of 0.00001 to 12 mm/min. The maximum load capacity is 50 kN, and it is suitable either for cells S305 (max. specimen size 70x140mm) and S306 (max. specimen size 100x200mm). The system guarantees high resolutions in real time. The load plate is foreseen of electric end of stroke, to save the machine from wrong manipulations.
Frame Specifications:
- Maximum load capacity: 50kN
- Infinitesimal testing speed: from 0.00001 to 12 mm/min.
- Minimum vertical clearance: 400 mm (140 mm with ring)
- Maximum vertical clearance: 1100 mm (840 mm with ring)
- Horizontal clearance: 380 mm
- Platen diameter: 177 mm
Firmware:
- Electronic control unit “Cyber-plus Evolution” with Touch-Screen color graphic display ¼ VGA, that runs like a standard PC based on Windows operating system for the management of the data. (Analysis of the data, test results, graphs with S335-10N software; optional accessory).
- The Touch-Screen icon interface allows an easy set-up of the parameters and immediate execution of the test.
- The machine can perform the tests without any external PC, because of the “Cyber-Plus” grants performances like a PC.
- Direct connection to Intranet (connection to a LAN network) and Internet to estab lish a remote communication and receive an immediate diagnosis from technicians, or for upgrades of the Firmware.
- Unlimited memory storage with: 2 USB ports, 1 SD card.
- Possibility to select different languages.
- The machine is equipped with 8 connectors for the acquisition and data processing system up to 8 analogical/digital channels (that is activated with the S301-05 optional firmware) for load cells and transducers. Extra slot available to expand the on-board channels to 16 (with S301- 06)
The frame is supplied with loading ram, sphere, load rings, dial gauges, electric load cells or displacement transducers.
Power supply: 230V 1ph 50-60Hz 600W
Dimensions: 490x510xh1800 mm
Weight: 115 kg approximately