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Consolidation Testing - An Introduction

Consolidation Testing - An Introduction

May 21 2019

1 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 VJ Tech equipment and software can offer.

It is recommended that this support document is read in conjunction with the glossary of terms that can be found in the support section of VJ Tech’s website, which can be found here.

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 at which 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; the particles in general terms 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).

 

Figure 1 - Basic Principles of Consolidation

The sample can be a range of diameters commonly 38mm to 100mm in diameter for one dimensional consolidation tests and up to 250mm for a Rowe cell test. The sample is normally significantly thinner than used for triaxial testing; commonly a sample 20mm to 40mm high will be tested. Testing is normally undertaken on high quality undisturbed samples.

3 Different types of consolidation testing

 

There a number of 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: consolidation test, oedometer test, and 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 also 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?

-       Is the settlement short or long term?

4 Typical Consolidation System Setup – One Dimensional Consolidation

 

A typical consolidation system for one dimensional consolidation testing can be seen in Figure 2. This is a traditional dead weight system. In Figure 4 a new automated system can be seen.

Figure 2 - Manual Consolidation System 

Both types of system use the following components:

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 radially deforming. There are also porous discs that allowing drainage from the sample and a loading cap through which to apply vertical stress. The components of a VJ Tech consolidation cell can be seen in Figure 3.

 

Figure 3 - Consolidation Cell

They can be purchased in a range of sizes enabling sample diameters between 38mm to 100mm to be tested.

Displacement Transducer (digital dial gauge) – 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 weight which is 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. VJ Tech also supplies an automatic system that uses a pneumatic actuator to apply the vertical load to the specimen

Figure 4 - Automated Consolidation System

5 Typical Consolidation Setup – Constant Rate of Strain

A typical system for consolidation testing using the CRS method consists of the following equipment:

Load Frame – In the picture below the system is shown with a VJ Tech ACONS Pro Multi-Purpose 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 Pro Multi-Purpose frame the data logger is built into the unit so no separate data logger is required. The ACONS Pro Multi-Purpose frame has a maximum load capacity of 20kN.

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, or normally an internal submersible load cell is used, as shown in the picture. The maximum load rating is normally 15kN or 20kN.

Figure 5 - CRS System 

Digital Dial Gauge – A dial gauge or displacement transducer is used to measure the amount of height change in the sample as the vertical stress is applied.

Pore Pressure Transducer – A pore pressure transducer is used to measure the pressure changes in the sample as the vertical stress is applied.

Multi-Purpose Consolidation Cell – The cell can use 3 different sample kits, one of which is for CRS. This 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 kit.

Automatic Pressure Controller – The pressure controller provides a back pressure. This 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 modern Rowe cell system consists of the following equipment:

A Pro Dual Automatic Pressure Controller – The Pro Dual Automatic Pressure Controller is used to generate back pressure to saturate the sample and vertical stress to consolidate the sample. The Pro Dual Automatic Pressure Controller also logs the data using the spare analogue sockets on the Rear, so a data logger is not required.

A Displacement Transducer – The displacement transducer is used to measure the sample height change when a vertical stress is applied.

Figure 6 - Rowe Cell System 

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 the cell can be seen in Figure 11. The wall of the cell radially constrains the sample.

In additional 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:

Consolidation (Oedometer)

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

AASHTO T216

 

CRS

ASTM D4186

 

Rowe Cell

British Standard 1377 Part 5

 

Using the ACONS Pro Multi-Purpose Frame with a Permeability Cell also allows permeability testing to be carried out.

Using the ACONS Pro Multi-Purpose Frame and Multi-Purpose Consolidation Cell with an SD-SWCC kit also allows unsaturated consolidation testing to be carried out.

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 height of the specimen is measured over a period of time. A simplified setup is shown if Figure 1 and a full system can be seen in Figure 3.

 

Figure 7 - Time verse height change

These tests are normally scheduled with a loading sequence starting with a stress at or near the over burden pressure (the stress applied to the specimen from the weight of material over lying it) of the sample, then doubling the loading for each stage; this commonly is 4 to 5 loadings with an unloading at the end such as:

25, 50, 100, 200, 50kPa

Or

12.5, 25, 50, 100, 200kPa

The loading sequence would normally be designed and provided by the scheduling engineer based on the ground conditions and future site use, but 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

Primary consolidation – Dissipation of excess pore water pressure

Secondary Consolidation – Slow Settlement believed to be due to the movement of soil particles caused by the application of vertical stress to the sample

Permeability (in directly) – An indication of the ability of a soil to allow fluids to flow through it

Void ratio – Ratio of the volume of voids (both gas and liquid) to volume of solids

Degree of Saturation – The ratio of voids filled with liquid

The results from the test are often shown in a graph of stress vs void ratio, an example can be seen in Figure 8. This is useful in helping to determine if the sample was over consolidated (current stress applied to the soil is less than the historic maximum stress applied to the soil) and also the pre-consolidation pressure (the maximum effective vertical stress a sample has been exposed to in the past).

Figure 8 - Vertical stress verses void ratio

9 Test Procedure – CRS

CRS testing unlike the other tests described in this document is a strain controlled test. The other tests are stress controlled tests. The sample in the CRS test has vertical stress applied at a constant rate of strain, so the vertical stress applied to the sample gradually builds up.

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 takes to reach the new back pressure level. If a short time is taken, the sample can assume 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 Pro) at a variable machine 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 slow controlled way to stop excess pore pressure being generated. The loading will continue until a target vertical stress is achieved. Unlike oedometers there is no loading sequence; there will be a single target vertical stress. This testing process has the advantage that 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.

Figure 9 - Example of a CRS Cell

 

A CRS test will not provide Mv, Cv, primary and secondary consolidation data. The test will provide you with void ratio, degree of saturation and permeability data.

 

Figure 10 - Effective Axial Stress Verses Void ratio

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. Sample height change can also be measured during the test.

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 11. 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 from the 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.

Figure 11 - Example of a Rowe Cell

The cell has two independent pressure sources attached to it. In the system setup shown in figure 5 the two pressure sources are from an automatic pressure controller. One Pressure channel 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 11). This APC channel is used to generate the vertical stresses that are applied to the sample during testing.

The other APC channel is connected to the cell, so that water can be applied to the sample (see back pressure to APC in Figure 11). 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 passes 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.

Saturation

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 channel (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 figure12.

 

?U = Change in pore pressure

?σv = Vertical Stress applied in increment

Figure 12 - B Value Equation

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 saturated.

Undrained Consolidation

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 channel. 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 stage) can be started.

Drained Consolidation

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 APC channel 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 channel. 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.

The Rowe cell test allows the calculation of the following:

Mv (Coefficient of Volume Compressibility)

Cv (Coefficient of Consolidation)

Primary consolidation

Secondary Consolidation

Permeability (indirectly)

Void ratio

Degree of Saturation

11 Advantages of VJ Tech Consolidation Systems

VJ Tech consolidation systems can automate consolidation testing. This enables (where the standard allows) tests to be stopped when the consolidation process has finished, and then automatically moved onto the next stage. This can be done without the need for a technician to add or remove weights from the sample. This can significantly reduce 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 Pro can also be used in settings where conventional systems using dead weights cannot such as site and offshore laboratories.

The systems also remove the need to have weights, and to move weights within a laboratory. This significantly reduces the health and safety risks associated with traditional dead weight systems. The systems are also easier to maintain and calibrate that the traditional dead weight frames.

12 Further Reading of 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 (sales@vjtech.co.uk).

For more information please contact service@vjtech.co.uk or visit our consolidation testing play list on YouTube.

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