The Relevance of the Triaxial Test to the Solution of Stability Problems Review
Triaxial Testing - an Introduction
Oct thirty 2017
one Introduction
This blog is designed to give a brief introduction to the theory of triaxial testing for a technician new to this test. This includes why the test is performed and how information technology is performed. The paper will wait at systems for triaxial tests, the stages of a triaxial test, some of the theory backside triaxial tests and besides automation of the test process.
It is recommended that this support document is read in conjunction with the glossary of terms that can be constitute in the 'Support' section of VJ Tech's website
two Triaxial testing what is it?
A typical triaxial test involves confining a cylindrical soil or stone specimen in a pressurised cell to simulate a stress condition and then shearing to failure, in order to determine the shear strength backdrop of the sample. Nigh triaxial tests are performed on high quality undisturbed specimens. The samples usually range from 38 mm to 100 mm samples, although samples considerably larger can be tested with the correct equipment. The test specimen most commonly has a pinnacle to diameter ratio of ii:1.
The sample will usually exist saturated, then consolidated and finally sheared, about normally only in compression - but extension tests may be undertaken with the correct equipment.
During the exam the sample is subjected to stress conditions that attempt to simulate the in-situ stresses. Figure 1 shows the stress conditions practical to a sample during a typical examination; Figure two shows the bones principles on constructive stresses.
Figure 1 – Stress Conditions in a Typical Triaxial Test
To summarise for a triaxial pinch test:
σone - Vertical (axial) Stress (remember of this as the vertical load applied to the sample)
This also known equally the Major Principle Stress
Can also be call σfive
σ3 - Confining Pressure (think of this as cell pressure)
This is also known every bit the Modest Principle Stress
Can too exist called σh
U - Pore Pressure
Also known as U w (Pore Water Pressure level (P.Due west.P))
σane – σ3 – Deviator Stress (the stress due to the axial load applied to the specimen in excess of the circumscribed pressure)
Figure 2 – Constructive Stresses
σone' – Effective Vertical (axial) Stress
σthree' – Constructive Confining Pressure level
U - Pore Force per unit area
3 Triaxial Testing – Why we do it
Triaxial tests are 1 of the virtually widely performed tests in a geotechnical laboratory. The advantages of the exam over other test methods used in the geotechnical laboratory used to decide shear strength (such equally direct shear) is that specimen drainage can exist controlled and pore force per unit area can be measured. The triaxial test enables parameters such equally cohesion (c'), internal angle of friction (φ ') and shear force to exist adamant.
The triaxial exam tin can besides be used to determine other variables such equally stiffness and permeability with the correct equipment.
A list of some of the mutual engineering issues that triaxial tests can be used for can be seen in tabular array 1, with the type of test described in the adjacent section.
| Field Problem | Type of Analysis | Blazon of Examination |
| Commencement Time Slope Failure | Effective Stress | CU or CD Triaxial |
| Cut Slope Failure | Effective Stress | CU Triaxial |
| World Dams | Total Stress Effective Stress | UU triaxial CU Triaxial Triaxial permeability |
| Tunnel Linings | Full Stress Effective Stress | UU triaxial CU Triaxial |
Table i – Triaxial Tests used in common Engineering Problems
It is common for UU triaxial tests to be used for curt term engineering problems and CU and CD triaxials to look at long term applied science problems.
4 Triaxial Testing – The Dissimilar Types
There are 3 mutual types of triaxial test:
· UU Triaxial - Unconsolidated Undrained (this can also be termed QU for Quick Undrained)
· CU Triaxial - Consolidated Undrained triaxial
· CD Triaxial - Consolidated Drained triaxial
UU triaxial tests are termed total stress tests and CU/CD triaxial tests are termed effective stress tests. Figure 1 shows the stresses involved with a full stress test and Figure ii summaries the stresses in an effective stress test.
CU and CD Triaxial tests normally outset with a saturation stage, which then leads onto a consolidation stage. The consolidated stage can be either;
a) isotropic; the stress surrounding the sample is equal in all directions or
b) anisotropic; the stress surrounding the sample is not equal in all directions.
The tests are commonly abbreviated to CIU (Consolidated Isotropic Undrained) or CAU (Consolidated Anisotropic Undrained). In the terminal phase the sample is sheared to failure.
UU triaxial tests commonly exercise not take a saturation or consolidation phase performed; the exam unremarkably simply consists of a shear phase. UU triaxial tests are quick tests, taking under 30 minutes to perform. The CU and CD triaxial tests take significantly longer to perform and can have weeks or fifty-fifty months to consummate. The CD Triaxial normally has the longest test duration, equally during the shear stage of the test, pore pressure is not allowed to build up, resulting in very depression strain rates. CU triaixals are quicker to perform every bit PWP is immune to increment and the backlog PWP is measured throughout the shear stage.
An additional test called unconfined compressive forcefulness (UCS) tin can also be undertaken with a triaxial system. This shears a specimen nether axial load with no confining pressure level.
v Typical Triaxial System – Typical Triaxial Cell
A typical triaxial jail cell is shown in Figure 3 of this document showing a sample setup for testing. The setup shown is used for CU and CD triaxial tests. UU triaxial test can likewise be undertaken with this system or a simpler cheaper system tin can be purchased if but UU triaxial tests are required.
The sample is sealed within a safe membrane then surround with water (jail cell pressure). The prison cell force per unit area is then used to apply a stress to the sample (σ3) Dorsum pressure tin exist added to saturate the sample if required. The sample is axially loaded during shearing with the strength measured by the load jail cell, the deformation measured by the deportation transducer and if required the pore pressure level in the sample tin be measured. Volume change of the sample is measure from the dorsum pressure line using a volume modify unit of measurement or an automatic pressure controller.
6 Typical Triaxial Organisation – Compressed Air
A typical triaxial organisation for triaxials that uses compressed air can be seen in figure 4. This system consists of the post-obit components that used for the following:
Pressure Console – This enables water and compressed air to be correctly distributed to the correct equipment to undertake a triaxial test. Prison cell Pressure (σthree - Circumscribed Pressure) and Dorsum force per unit area are controlled from hither. A force per unit area gauge is included so that known pressures can be applied to the examination specimen using the pressure regulators fitted to the panel.
Load Frame – The load frame is used to use deformation to the triaxial specimen. The load frame can exist controlled to a loftier level of accuracy (these tests normally require a slow speed). Load frames come up in a range of capacities, VJ Tech can supply frames from 10kN to 250kN capacity. The one in Figure 4 is a 50kN load frame and can apply loads of up to 50kN to a specimen.
Figure four – Compressed Air triaxial system
Air/Water Cylinders – These provide an interface between compressed air and h2o in your triaxial system. They comprise a rubber float which is filled with compressed air to pressurise the water inside the cylinder, this in plow provides the pressure level used in the cell and back pressure for our triaxial test. The pressure level the system tin apply to samples is limited by the compressor.
Load Prison cell – The load cell or other load measuring device such as a load ring, provide the loads required to shear a triaxial specimen.
Displacement transducer – This could exist a mechanical dial approximate, digital dial gauge or LSCT blazon transducer and is used to accurately mensurate the deformation applied to the triaxial specimen as it is sheared.
Triaxial Prison cell – The Triaxial jail cell comes in a range of sizes and pressure ratings. The cell contains the triaxial specimen and is pressurised throughout the test.
Pore Pressure level Transducer – In CU and CD triaxial tests, the pressures inside the sample are measured (U - Pore Pressure). This is done using a pressure transducer attached to the base of the triaxial cell.
Automated Volume Change unit – The automatic volume alter unit is used in some triaxial tests to measure the volume of water going into the specimen and also the book alter of the specimen through the test.
With a organization similar this the pressures are set manually by the technician undertaking the test. It is common for the outputs from load cell, displacement transducers and pressure transducers to be information logged in some way and data analysed using VJ Tech's Clisp Studio software.
7 Typical Triaxial System – Hydraulic Pressure level Controller
A Pro Triaxial system for Triaxial testing using hydraulic pressure can be seen in Figure 5. The system consists of the following components:
Effigy v – Pro Hydraulic Pressure Controller Triaxial system
Distribution Panel – The distribution panel is used to connect the Dual pressure level controller to the triaxial cell. It also has the de-aired water system connected to it. It allows for the easy movement of water to the required location in the system without the need to disconnect lines. The pressure gauge on the panel is used for calibration, but is non used during testing.
Automatic Pressure/Volume Controller – The Dual automatic pressure level controller (DAPC) is used to generate pressure for the triaxial test. It uses stepper motors to pressurise each cylinder of h2o to generate cell force per unit area (σ3 - Confining Pressure) and back pressure. No compressed air supply is required. The DAPC is too able to measure volume change during the test. VJ Tech APCs come in a range of force per unit area capacities and Volumes, the DAPC pictured has a maximum pressure of 3500 kPa and Volume 250 cc for each Aqueduct
Load Frame – The Pro load frame is used to apply deformation to the triaxial specimen. The load frame can be controlled to a high level of accuracy. Load frames come in a range of capacities, VJ Tech can supply frames from 10kN to 250kN capacity. The i in Figure five is a Pro 50 kN load frame and tin utilize loads of up to 50 kN to a specimen. The load frame has a built-in data logger to log transducer data during the test and can also be used in standalone fashion using the touchscreen.
Load Cell – The load cell or other load measuring device such as a load band, provide the loads required to shear a triaxial specimen.
Deportation transducer – This could exist a mechanical dial gauge, digital dial gauge or LSCT type transducer and is used to accurately measure the deformation practical to the triaxial specimen as it is sheared
Triaxial Cell – The Triaxial cell comes in a range of sizes and pressure rating. The jail cell contains the triaxial specimen and is pressurised throughout the test.
Pore Pressure level Transducer – In CU and CD triaxial tests the pressures within the sample are measured (U - Pore Pressure), this is done using a pressure transducer attached to the base of the triaxial cell
eight Additional equipment needed
For both CU/CD triaxial systems a de-aired water supply is likewise required for testing. The UU system tin can employ a mains water supply if required.
Automatic systems require a computer and software packet such as VJ Tech's Clisp Studio, to control the test and record transducer outputs.
A data logger (either inbuilt or external) will exist used to store transducer readings taken during the test.
Sample preparation equipment volition besides exist required, such as: sample extruder, sample moulds, membranes, 'o' rings, porous discs, ruler, balance and filter papers.
ix Examination Process – Standards for Triaxial Tests
Triaxial tests are document in a range of international standards. These include:
British Standard 1377 part 7 and 8
ASTM D2850, D4767 and D2166
Australian standard AS1289 6.iv.1 and half-dozen.iv.2
Hong Kong Geospec 3
German language DIN 18137
Russian GOST 12248-96
Additional tests can also be performed with triaxial systems such equally Stress Path assay and anisotropic consolidation. Although the examination procedures are not detailed in the standards above the procedures are fairly mutual and details can be institute in either academic papers or in the Manual of Soil Laboratory Testing Vol. 2 and vol. three.
With boosted equipment, a standard triaxial system can be upgraded to undertake the following tests:
Bender Element assay (to make up one's mind the Gmax of a sample)
Circadian/Dynamic triaxial testing (to make up one's mind the cyclic shear strength of a sample and also used for modulus and damping properties)
Small-scale Strain assay (used for stiffness assay) using on-sample transducers measuring axial and radial deformation can also be performed.
10 Examination Procedure – System Preparation
Before any triaxial examination is undertaken it is very important that checks are undertaken to verify the performance of the system. Failure to do this may hateful either an extended exam menstruum or a sample beingness destroyed and no results obtained.
The following list is a suggested check list of the organisation earlier any test takes place. It is causeless the system has already been installed. Boosted checks may be required depending on your local testing standard.
- Create enough de-aired water for the grooming and test (this can take a few hours to do)
- Clean the triaxial prison cell, paying special attention to the base and groove holding the 'o' ring.
- Make clean 'o' ring in cell base of operations and ensure it is complimentary from defects
- Flush the de-ambulation block that the pore pressure transducer is connected to. Pressurise and check the block for leaks
- Check the pore pressure transducer calibration and recalibrate if necessary
- Flush the back pressure line
- Check the back pressure line for leaks (following the process in almost standards will accept at least 24 hours)
- Check load cell and deportation transducers are accurate.
11 Test Procedure – Sample Setup
Virtually triaxial tests are performed on loftier quality undisturbed samples; local standards such as Eurocode 7 give details on what is considered to be a loftier quality undisturbed sample; information technology is important to realise that sample disturbance (from collection, ship to lab and storage) will touch the results obtained from a triaxial test. Remoulded specimens can be tested and preparation procedures can be found in relevant standards and texts on this subject field.
Samples are commonly prepared to give a sample height to bore ratio of 2:1. The sample ends are trimmed to ensure they are level and apartment. This is ordinarily and most easily washed using a sample mould or former for the diameter of sample that is existence tested and a straight border.
Figure 6 - Sample grooming equipment
The sample is then weighed so the bulk density tin can exist determined and measured (both length and diameter) so the volume and expanse can be calculated. It is disquisitional that the sample dimensions are accurate and so that stress and strain being applied to the specimen during testing tin be calculated accurately.
A procedure for placing the sample into the jail cell and covering with a membrane tin can be seen in the video on this link;
A number of critical checks should be made during this procedure:
one. Employ a new membrane and brand sure there are no holes in the membrane.
2. Ensure the porous discs are clean and non clogged with soil particles.
3. Place the 'o' rings using an 'o' ring tool, this will minimise sample disturbance.
four. Clean whatsoever loose material from the sample as yous get. This loose material can cause leaks.
5. Accept care not to hit the sample when placing the pinnacle of the jail cell over the sample.
During this preparation process care must exist taken to limit sample disturbance. Sample disturbance can significantly affect the results obtained from the test.
The procedure to prepare up a sand specimen can exist seen in this video link;
12 Test Procedure – Saturation
Effective stress triaxials (CU and CD tests) require that the sample is saturated for testing. The reason for this is so that reliable measurements of pore pressure can exist made. This is made possible by removing the air from the voids inside the sample.
The saturation procedure tin exist performed in a number of means details of which can be found in the bachelor Standards and text books. The most common though is incremental increment of cell and back pressure into the sample.
Figure 7 – B Bank check Instance
Effigy 8- Back Pressure level Step Example
This procedure gradually increases both the cell and back pressure so to dissolve any air that is in the voids of the soil sample. During this procedure the pore pressure coefficient B is calculated to determine the degree of saturation.
Figure 7 shows the process that happens during a B check to determine the level of saturation. Most standards country that a B value greater than 0.95 indicates that a sample is sufficiently saturated.
The B value is calculated using the equation in effigy 9.
Figure nine – B Value Equation
∆U = Alter in pore pressure level
∆σ3 = Change in Confining Pressure level
Figure viii shows the process that happens during a back pressure stride. This footstep pushes water into the sample to attempt and saturate it if the B check shows the sample isn't saturated. These steps are repeated until the sample is saturated
13 Test Procedure – Consolidation
Subsequently saturation, the specimen is ordinarily consolidated to a stress condition representative of its in-situ condition. The consolidation is unremarkably isotropic (the stresses applied vertically and horizontally to the sample are the same).
In effective stress tests the sample will be consolidated to an constructive force per unit area. This is the difference between the confining (cell) pressure level and the back pressure. If this is compared to the equation in Figure 2 defining effective stress you will meet this is incorrect.
At this point in the test pore pressure is replaced by dorsum pressure to ascertain our effective stress, to help simplify setup of this phase of the test. When the confining pressure level and back pressure have been prepare to the right values, there will be an excess of pore pressure in the sample (the pore pressure volition be higher than the dorsum pressure).
In one case the consolidation is started the backlog pore pressure in the sample will start to dissipate as the consolidation process drains of water from the sample decreasing its book. In one case this process is complete (the sample has stopped changing in volume) the pore pressure will exist at or very near the back force per unit area level and pore force per unit area will exist used to summate the effective stress conditions the sample is under.
100% dissipation of excess pore pressure level is non always possible and so most standards advise a minimum dissipation of 95% is accomplished before the consolidation process is stopped.
Note during consolidation σiii is the same as σi.
14 Examination Procedure – Shear
In the shear stage the axial strength (σ1) is gradually increased whilst the confining pressure (σ3) is maintained until failure happens. This is the maximum shear force the soil can accept.
This stage of the exam is besides commonly referred to as load stage, pinch stage or shearing to failure phase.
During the shear phase the drainage conditions that are applied to the sample will determine if a drained or undrained test is performed. If the sample is non allowed to drain this is an undrained test, the sample will not modify in volume during this test but will change in shape. If drainage is allowed, h2o will drain from the sample during the exam, allowing the volume and the shape of the sample to modify. In a CU (undrained) examination, pore pressure changes (U) are immune to take place and are measured. In a CD (Drained) test drainage is allowed to forestall pore pressure changes happening. In this type of test the book change of the sample is measured during the shear stage.
From this phase of the exam, as well as the shear strength being determined, the values of cohesion (c') and phi (φ ') tin be adamant using Mohr circle and stress path plots.
Figure ten - Mohr Circumvolve Plot with Cohesion and Phi Results
15 Advantages of a VJ Tech Automatic Triaxial System
An automatic triaxial organisation offers the following advantages:
· Tests run 24 hours a day 7 days a week
· Average time for a test to exist complete is significantly reduced
· Can exist monitored and controlled remotely
· Express staff supervision required (the technicians can perform other activities)
· Less staff required to perform more tests
· Increased quality of results and minimise human error
· Easier to maintain
· Easier to calibrate
· No compressed air required (safer)
· Software provides updates on testing via email
· Piece of cake report generation
· Easy analysis of data
· Easy to upgrade to undertake more advanced tests
Delight take a look at this case study to come across some of the advantages of the system and what VJ Tech tin offer.
Video Link 2 – Automation Case Study
xvi Further Reading on Triaxial Tests
The following text books helped put this certificate together.
Manual of Soil Laboratory Testing Vol. II: Permeability, Shear Strength and Compressibility Tests 3rd Edition by G H Head and R L Epps
Manual of Soil Laboratory Testing Vol. 3: Effective Stress Tests 3rd Edition by Grand H Caput and R J Epps
VJ Tech would recommend these books to whatever technician or laboratory undertaking triaxial tests. The books provide an in-depth understanding behind the theory of the examination and likewise the testing procedures including quality control and assay of the results.
To purchase this book please contact the VJ Tech Sales department (sales@vjtech.co.u.k.).
VJ Tech too offers a training course and/or boosted data on the subject field matter.
For more than information delight contact service@vjtech.co.united kingdom of great britain and northern ireland or visit our triaxial testing play list on YouTube.
Video Link 3 – Triaxial Testing Playlist
Source: https://www.vjtech.co.uk/blog/triaxial-testing-an-introduction
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