Automatic Triaxial Testing System
Automatic Triaxial Testing System
Product Code
Triaxial UU-CU-CD Test Systems
Standards
ASTM D2850, D4767, D7181; AASHTO T-297; BS 1377-7, BS 1377-8
Determination of mechanical properties of soils is an important step in designing foundations, embankments, and other soil structures. The Automatic Triaxial Testing System by UTEST is intended to simulate the impacts of building constructions, excavations, tunneling, and similar applications on subsoil structures in an effective manner. The system conducts Triaxial Tests to study the stress-strain relationship of undisturbed soil specimens subjected to different levels of stresses and drainage conditions.
Key Features of the Automatic Triaxial Testing System
The Automatic Triaxial Testing System offers automated triaxial compression tests on cylindrical undisturbed and remolded soil samples. It can perform Unconsolidated Undrained (UU), Consolidated Drained (CD), and Consolidated Undrained (CU) compression tests with high accuracy. Each test is automatically controlled and reported, making it a leading choice in soil testing equipment.
Unconsolidated Undrained (UU) Test
In the UU test, specimens believed to be saturated before running the test are placed into a triaxial chamber and subjected to fluid confining pressure. Cell pressure is applied to some preset value, while the specimen breaks due to constant axial strain rates. This technique maintains the intact structure and water content of the sample, with which interpretations are made entirely based on total stress only.
Generally, three specimens of the same sample are tested under varying confining stresses, thereby showing identical shear strength due to saturation. The data obtained are presented in the form of curves of principal stress difference against strain, in which Mohr circles have been drawn to calculate the undrained cohesion intercept as well as the angle of shearing resistance.
Consolidated Undrained (CU) Test & Consolidated Drained (CD) Test
The Automatic Triaxial Testing System permits the measurement of peak effective strength parameters, c' and φ', from consolidated undrained (CU) triaxial compression tests with pore pressure measurement or consolidated drained (CD) triaxial compression tests. The tests are usually carried out in several stages on three specimens, which include saturation, consolidation, and shearing.
Saturation Process
Saturation involves ensuring that pore fluid within the specimen is air-free. The same effect can be brought about through applying an elevated back pressure which may dissolve air present in pores. The back pressure, imposed pore pressure is applied through a volume change gauge at the top of the specimen with a little bit higher cell pressure. All pressures are incrementally increased and adequate time is provided for equalization. The degree of saturation can be expressed using Skempton's pore pressure parameter (Skempton, 1954):
Δ u
B=__________
Δ σ3
Where Δu is the change in pore pressure for an applied cell pressure change of Δσ₃. Ideally, for fully saturated soil, B equals 1. Standard test methods recommend achieving a value of B greater than or equal to 0.95 before proceeding to the consolidation stage.
Consolidation Stage
The consolidation stage of the effective stress triaxial test serves two purposes. Firstly, three specimens are consolidated at different effective pressures and yield widely spaced effective stress Mohr circles. Secondly, the consolidation results determine the minimum time to failure in the shear stage.
A major increase in effective consolidation pressure is usually brought about between two samples; again the pressure for this test phase shall typically have reached approximatley the same value as vertical effective stress in the soil during test phase. During the consolidation period, volume changes are also measured using a volume change monitor connected to the backpressure line. Measuring of pore pressure occurs at the base of the specimen whereas drainage takes place through porous stone placed at the top of the specimen.
Coefficient of Consolidation
Volume change may be plotted against the square root of time to estimate the coefficient of consolidation for the clay. It can be indicated through the theoretical basis that volume loss for 50% of the consolidated one would appear as a straight line on this plot. The straight line thus formed is extended to cross the horizontal line representing the time taken for 100% consolidation. The time taken at this point (designated as "t₁₀" by Bishop and Henkel) can thus be used to calculate the coefficient of consolidation.
Consolidated Undrained (CU) Test
After consolidation, the specimen is drained of the back pressure. The rate of movement in the vertical direction for the compression machine platen is determined by the result obtained from consolidation. For the sear stage, a further increase in the vertical stress is provided by the loading ram, and readings of deformation, ram load, and pore pressure are obtained at intervals.
These measurements are converted into graphs of principal stress difference (σ₁ - σ₃) and pore pressure as functions of strain. Failure is defined at the point of maximum principal stress difference.
Stress Mohr circles are drawn for the failure conditions of the three samples with different levels of consolidation. The slope and intercept of the tangent drawn to these circles represent the effective strength parameters, c' and φ'.
Consolidated Drained (CD) Test
The Consolidated Drained (CD) triaxial compression test is similar in sequence to the CU test. However, during the shear stage, the back pressure remains connected to the specimen, and loading occurs slowly enough to prevent the development of excess pore pressures. The shear stage of a drained triaxial test typically takes 7 to 15 times longer than that of an undrained test with pore pressure measurement.
After shearing is complete, the results are plotted as principal stress difference and volume change as functions of strain. Failure Mohr circles are then plotted to depict the drained failure envelope in terms of the parameters c'd and φd'.
Advance Data Management
The Automatic Triaxial Testing System by UTEST is computer-controlled, thus making it easy to transfer test values into a computer. Data processing can be done using the specialized triaxial software on the Windows operating system, and all data can be transferred to Excel for easy analysis.
Data of loading and axial displacement are transferred to the software through the UTouch Control Unit. The software captures three pressure data points on the triaxial cell, which include backpressure, pore pressure, as well as volume change. These data are collected from the interface unit and captured using a 4-channel data acquisition system known as UTCU-0020.
Typical Configuration of System for Triaxial Tests ( UU-CU-CD)
Product Code | Description | UU | UU-CU-CD |
|---|---|---|---|
UTM-0108.SMPR | Multiplex Universal Electromechanic Test Machine* | 1 | 1 |
UTGM-1180 | Load Cell 5 k N, S Type | 1 | 1 |
UTGM-1190 | Load Cell 10 kN, Pancake Type** | 1 | 1 |
UTS-2400 | Triaxial Cell*** | 1 | 1 |
UTS-2401 | Triaxial Cell*** | 1 | 1 |
UTS-2406 | Block with one connection line and pressure transducer | 1 | 3 |
UTS-2408 | Oil and Water Constant Pressure System | 1 | 2 |
UTS-2415 | Automatic Volume Change Unit | - | 1 |
UTCU-0020 | Interface Unit with 4 Channel for Data Acquisition | - | 1 |
USOFT-2419 | Software to Perform UU Triaxial Tests | 1 | 1 |
USOFT-2420 | Software to Perform CU-CD Triaxial Tests | - | 1 |
UTS-1330 | De-Airing Water Tank, 7 L. | 1 | 1 |
UTGP-1145 | Plastic Hose, Ø8mm OD, 10 m | 1 | 1 |
- Has a 50 kN frame and supplied complete with 50 mm linear potentiometric transducer (UTGM-0064) with holders (UTM-0114 and UTAS-1060) and a lower compression platen.Load cell and other test equipment should be ordered separately.
- May be need for 70-100 mm dia. samples with high strength.
- Choose the suitable cell for the specimen size (UTS-2400: 38-50 mm dia. samples / UTS-2401: 70-100 mm dia. samples).
Optional Apparatus which should be ordered seperately for de-airing water see the page of De-Airing Water Systems”.
Authorized Distributor
Kadeer Brothers Lahore, is the authorized distributor of UTEST in Pakistan and grants the client the access to the Automatic Triaxial Testing System, among other advanced testing equipment on soils.