Consolidation Testing - An Introduction
This support document is designed to give a brief introduction to the theory of consolidation testing, for a technician new to this test. This includes why the test is performed, and how it is performed. The paper will look at the differing standard systems that are available for this test, some of the theory and will look at the advantages that the VJ Tech equipment and software can offer.
2. Consolidation testing what is it?
Consolidation tests are typically performed on a saturated cylindrical soil specimen, and are designed to measure the amount, and the rate that a sample will change in height when subjected to load. The sample is constrained radially (normally by a steel mould or cell wall) so that when a vertical load (stress) is applied to the sample the vertical height will change but the diameter of the sample will remain constant. If you consider the example below in figure 1, when a stress is applied vertically to the sample, the sample reduces in height. This reduction in height happens as the particles of the soil are forced closer together; in general terms, it is the particles that will dictate how much the sample changes height. As the particles move closer together, the voids in the soil sample are reduced. This reduction in void space causes water to be forced out of the sample. The rate at which the water can come out of the sample dictates how fast the sample will change in height (see Figure 1).The sample can be a range of diameters, commonly 38 mm to 100 mm in diameter for one dimensional consolidation tests, and up to 250 mm for a Rowe cell test (see section 6). The sample is normally significantly thinner (vertically) than one used for triaxial testing; commonly a sample 20 mm to 40 mm high will be tested. Testing is normally undertaken on high quality undisturbed samples.
3. Different types of consolidation testing
There are many ways of measuring the consolidation properties of a soil in a geotechnical laboratory. The most common of these is the one-dimensional consolidation test which is commonly referred to as either: consolidation test, oedometer test, or Incremental Loading oedometer. These are commonly undertaken in laboratories due to the relative ease of the testing process and the simplicity of the equipment required. Yet this testing method has limitations, such as the small sample size tested (which can cause results to be under estimated) and no measurement of pore water pressure in the sample. Other methods exist that overcome some of these issues, such as Constant Rate of Strain (CRS) oedometer and Rowe Consolidation where larger diameter samples can be tested and pore pressure measured. These tests are detailed in international standards and with modern equipment making these tests easier to perform, they are becoming more popular.
Consolidation data is used in an investigation when the following needs to be known:
- The amount of settlement that will ultimately take place
- Non-uniform soil condition’s giving uneven settlement
- Settlement characteristics of a deep lying strata of compressible material
- To provide an estimate of the rate of consolidation
- Is the settlement short or long term?
4. Typical Consolidation System Setup – One Dimensional Consolidation
This is a traditional dead weight system. In Figure 3, a new automated system can be seen.
A typical consolidation system for one dimensional consolidation testing can be seen in Figure 2.
Consolidation Cell – The cell is used to house the prepared sample. It contains a mould or cutter made from steel. This allows the sample to be vertically loaded but will stop the sample deforming radially. There are also porous discs that allowing drainage from the sample, and a loading cap used when applying vertical stress. A simplified version of the cell can be seen in Figure 1.
Displacement Transducer (Digital Dial Gauge or LSCT Transducer) – this is used to measure the height change of the sample as the different stresses are applied.
The Manual system in Figure 2 uses the frame shown to apply a vertical stress to the sample using dead weights which are added to the hanger. The automatic system in Figure 3 uses a stepper motor to move the sample up against a load cell to apply a vertical stress to the sample.
5. Typical Consolidation Setup – Constant Rate of Strain
A system for consolidation testing using the CRS method consists of the following equipment:
Load Frame – In Figure 4, the system is shown with a VJ Tech ACONS 2 frame. This is a stepper motor controlled mini load frame that is able to provide both stress and strain control. This could also be replaced by a triaxial load frame such as a TriSCAN. With the ACONS 2, the data logger is built into the unit so no separate data logger is required. The ACONS 2 has a maximum load capacity of 20 kN.
Load Cell – The load cell is used to measure the vertical load being applied to the specimen. In a CRS system, this can be an external load cell as shown in the picture or an internal submersible load cell can be used with the correct CRS cell. The maximum load rating is normally 15kN or 20kN.
Digital Dial Gauge – A digital dial gauge or LSCT displacement transducer is used to measure the amount of height change in the sample as the vertical stress is applied.
Pore Pressure Transducer – The pore pressure transducer is used to measure the pressure change in the sample as the vertical stress is applied.
CRS Cell – The CRS cell is used to house the specimen for testing. The cell allows a Back Pressure to be applied to the sample so that it can be saturated. The sample is constrained radially by the cell wall.
Automatic Pressure Controller – the automatic pressure controller provides a Back Pressure which allows the sample to be saturated, before the vertical stress is applied to the sample. Additional equipment is also required such as a de-aired water system, software to log the data and control the test such as Clisp Studio, pipe work to connect the cell and APC together and specimen preparation equipment.
6. Typical Consolidation Setup – Rowe Cell
A typical Rowe cell system (Figure 5) consists of the following equipment:
Distribution Panel – The panel is used to distribute water and the pressure generated using the automatic pressure controllers.
Automatic Pressure Controllers – The automatic pressure controllers are used to generate Back Pressure to saturate the sample and vertical stress to consolidate the sample. These are available in a range of capacities and volumes to suit the size of the Rowe cell used for testing.
A Displacement Transducer – The displacement transducer is used to measure the sample height change when a vertical stress is applied.
Pore Pressure Transducer – The pressure transducer is used to measure the pressure changes inside the sample as vertical stresses are applied to it.
Rowe Cell – The cell contains the sample - a basic configuration of a cell can be seen in Figure 10. The wall of the cell radially constrains the sample.
MiniScanner – A MiniScanner is a data logger used for recording data during the test.
In addition to this, a de-aired water supply will be required, a software program such as Clisp Studio to run the test and sample preparation accessories.
7. Test Procedures – Standards for Testing
Consolidation tests are document in range of International standards. These include:
British Standard 1377 Part 5
BS EN ISO 17892-5:2017 ASTM D2435
Australian Standard AS1289 6.6.1
French Standard XP P94-090 1
British Standard 1377 Part 5
With additional equipment it is also possible to perform permeability analysis. It is also possible to upgrade the automatic consolidation systems to undertake unsaturated consolidation testing. Other non-standard methods exist for consolidation testing such as Constant Rate of Loading (CRL), Constant Pore Pressure Gradient (CG) and Consolidation with Control of Back Pressure (BPC) all of which are outside the scope of this document.
8. Test Procedure – One-Dimensional Consolidation
The One-Dimensional Consolidation test is the commonly used test of this type in a laboratory due to the relative ease of the test procedure and simplicity of the test equipment. The basis of this test is that a sample radially constrained is axially loaded, and the change in specimen height is measured over a period of time. A simplified setup is shown if Figure 1 and a typical Consolidation graph is shown in Figure 6.
These tests are normally scheduled with a loading sequence starting with a stress at or near the over burden pressure of the sample, then doubling the loading for each stage; this commonly consists of 4 to 5 loadings with an unloading at the end such as:
25, 50, 100, 200, 500 kPa
12.5, 25, 50, 100, 200 kPa
If no loading sequence is scheduled the British Standard offers a suggested loading sequence.
Each loading is performed until the sample has stopped decreasing in height. Then the next loading is applied to the sample. From this a number of variables and properties can be calculated such as:
Mv (Coefficient of Volume Compressibility) – How long does the consolidation take to happen
Cv (Coefficient of Consolidation – How much consolidation takes place
Degree of Saturation
The results from the test are often shown in a graph of stress vs void ratio an example can be seen in figure 7. This graph is useful in helping to determine if the sample was over consolidated and for determining pre-consolidation pressures.
9. Test Procedure – CRS
CRS testing, unlike the other tests described in this document is a strain controlled test. The other tests are stress control tests. The sample in the CRS test has vertical stress applied at a constant rate of strain, so that the vertical stress applied to the sample gradually builds up. An example diagram of a CRS Cell is shown in Figure 8.
The CRS test starts by saturating the sample so that accurate pore pressure measurements can be made. To saturate the sample, the Back Pressure APC is used to ramp the pressure to the sample and the soil is given time to absorb water (dissolving air in the voids). The level of saturation can then be tested by increasing the Back Pressure by a small increment and then measuring the pore pressure and timing how long the pore pressure takes to reach the new Back Pressure level. If a short time is taken, the sample can be assumed to be saturated; a longer response time indicates that the sample is not saturated and that the Back Pressure should be ramped to a higher pressure. This process is continued until the sample is saturated.
After saturation, the sample has vertical load applied to it by moving the load frame controller (such as the ACONS 2 or ACONS PRO) at a variable rate, to maintain a set strain rate on the sample (strain is based on sample height so to maintain a strain rate as the height changes the speed will be altered by the frame). This loading is performed in a controlled way to stop excess pore pressure being generated. The loading will continue until a target vertical stress is achieved. Unlike increment loading for oedometers there is no loading sequence; there will be a single target vertical stress. This testing process has the advantage that the sample will be loaded smoothly through the entire range of loads to get to its target; not just 4-5 single targets. This will give the engineer significantly more data and a more complete vertical stress voids ratio plot (see Figure 9).
The sample can then be unloaded to a lower vertical stress and the cycle of load/unload can be repeated. The CRS test for most materials will take less time than a standard incremental oedometer.
10. Test Procedure – Rowe Cell
Consolidation theory is based on the dissipation of excess pore water pressure. In standard oedometer tests pore water pressure cannot be measured and the consolidation that happens to the sample is measured by measuring the height change of the sample. The Rowe cell apparatus allows the technician to undertake a consolidation test based on the theory of consolidation and measure pore water pressure and the rate of dissipation.
To understand how the Rowe Cell allows this information to be recorded the user needs to have a basic understanding of the Rowe Cell and some of the other apparatus used in this test. The basic setup of a Rowe Cell can be seen in Figure 10. In this figure, you can see that the cell is divided into two, the two halves are separated by a rubber diaphragm. The diaphragm separates the soil specimen in the lower section and a top section filled with water. The cell has a pore pressure transducer fitted at the base of the cell so that the pore water pressure in the sample can be measured during the test.
The cell has two independent pressure sources attached to it. In the system setup shown in Figure 5 the two pressure sources are two automatic pressure controllers. One Pressure controller is connected to the top half of the cell and will apply pressure to the water in this half (see vertical stress to APC on Figure 10). This APC is used to generate the vertical stresses that are applied to the sample during testing.
The other APC is connected to the cell so that water can be forced into the sample (see Back Pressure to APC in Figure 10). This is termed Back Pressure and is used to saturate the sample so that reliable pore water pressure readings can be taken from the sample through the test. Note how this pressure enters the sample through the settlement drainage rod and passing through the rubber diaphragm.
The basic test process goes through 3 basic stages; the first of these is saturation, followed by consolidation undrained and consolidation drained stages.
The saturation process works in a similar way to that used in the saturation of a triaxial specimen. The sample is taken through a series of stages that check the level of saturation alternated with stages that force water into the sample to try and dissolve any air in the sample to saturate it. First, the level of saturation is measured; this is done by closing the sample off to the Back Pressure APC (sample is in an undrained condition) and then increasing the vertical stress (σv) on the sample by an increment. The pore water pressure is then monitored and a B value is then calculated.
The B value is calculated using the equation in figure11.
Where: ∆U = Change in pore pressure
∆σv = Vertical Stress applied in the increment
Most standards state that a B value greater than 0.95 indicates that a sample is sufficiently saturated.
Should the sample not be saturated, the Back Pressure is then raised to a value lower than the vertical stress (commonly 5 – 10 kPa less). The Back Pressure is then opened to the sample and water will gradually be forced into the air voids of the material. When the sample has stopped taking in water, the B check process is then repeated. If the sample still isn’t saturated these steps are repeated until the sample is.
When the sample is saturated, the next stage of the test (undrained consolidation) is started. In this stage, the sample is closed off from the Back Pressure APC. The vertical stress APC is then increased in pressure to apply the required vertical stress on the sample. As the vertical stress is increased, the pore water pressure in the sample will increase. Once the pore water pressure has stabilised, the next stage (the drained consolidation) can be started. 7093
At the start of the drained consolidation the initial vertical position is noted using the displacement transducer and the initial pore water pressure is recorded. The sample is then opened to the Back Pressure controller which is maintaining a Back Pressure. At this point, water will start to come out of the sample and go into the Back Pressure APC. The sample will start to change in height. At regular time intervals, the pore pressure and sample height will be recorded until the pore pressure has reduced to the Back Pressure level (excess pore water pressure has dissipated).
At this point additional loadings or unloadings of stress can be applied to the sample by repeating the undrained and drained consolidation stages.
11. Advantages of VJ Tech Consolidation Systems
VJ Tech consolidation systems can automate consolidation testing. Where the standard allows for tests to be stopped when the consolidation process has finished, the program will move automatically onto the next stage. This can be done without the need for a technician to add or remove weights from the sample thus significantly reducing testing times. A reduction in test time of 50% has been seen by certain customers.
The small footprint of the ACONS units, creates significant space saving in a laboratory compared to older dead weight consolidation frames.
The automated electro-mechanical ACONS 2 can also be used in settings where conventional systems using dead weights cannot be used, such as site and offshore laboratories.
12. Further Reading on Consolidation Tests
The Following test book helped put this document together:
Manual of Soil Laboratory Testing Vol. II: Permeability, Shear Strength and Compressibility Tests 3rd Edition by K H Head and R J Epps
VJ Tech would recommend this book to any technician or laboratory undertaking consolidation tests, the book provides an in-depth understanding behind the theory of the test and also the testing procedures including quality control and analysis of results.
To purchase this book please contact the VJ Tech Sales Department firstname.lastname@example.org. For more information please contact email@example.com or visit our consolidation testing play list on YouTube.