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Introduction to Direct Simple Shear (DSS) Testing

Introduction to Direct Simple Shear (DSS) Testing

Jan 26 2021


Introduction to Direct Simple Shear Testing

This support document is designed to give a brief introduction to the theory of Direct Simple Shear (or DSS) testing for a technician or engineer 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.

Introduction

One of the most common tests within a geotechnical laboratory is the Direct Shear test; it is used to determine the shear strength parameters cohesion (C) and internal angle of friction (φ) of soil. The test is common due to its relative simplicity. When shearing a sample in a direct shear apparatus the sample will be forced to shear in the centre of a sample (See Figure 1); this may not be its weakest point, and thus the test may overestimate the shear strength.

The Direct Simple Shear (DSS) test was designed to overcome this limitation of the direct shear test. In the direct simple shear (DSS) test the sample is uniformly deformed without the forming of a single shearing surface (see Figure 2); the sample should shear at its weakest point. This means the shear strength determined from this test should be more representative of the material and soil conditions.

The other main difference between the two test types is the direct shear test will maintain a constant vertical stress on the sample during shearing allowing the sample to change height and volume. In the DSS test the sample volume is held constant; the vertical stress applied to the sample is allowed to change and the vertical positon is fixed to simulate undrained conditions.

During consolidation both test types allow the sample to consolidate in one dimension only (vertically); horizontally the sample is constrained. In a direct shear test the sample is constrained by the shearbox assembly. In a direct shear test the sample can be constrained by 1 of three methods: a stack of metal rings, a wire reinforced membrane or a confining cell pressure. This keeps the sample in a K0 condition and during shearing for the DSS test keeps the sample in a constant volume state.

 

Why Undertake Direct Simple Shear Tests? 

Direct Simple Shear tests are commonly undertaken where the horizontal shear strength for a location is required. A common example of this is shown in Figure 3. Here an embankment is built up causing increased vertical stress to be applied to the surrounding soil. A potential failure/slipping point (shown in red) under the embankment shows an area in compression directly under the embankment. This can be modelled in a laboratory with a compression type test such as a CU/CD triaxial. At the other end the failure the soil is in extension and again could be modelled with a CU/CD triaxial in extension. The middle section is best modelled using the horizontal shear direction of the direct simple shear test.


The dynamic simple shear test can also be used to model the soil in an area where cyclic stresses are being applied such as in an offshore platform design, were the wave actions of the sea apply a cyclic condition to the structure and foundations.

 

Different Types of Simple Shear Test

The standard type of Direct Simple Shear test described above is detailed in the following international testing Standard.

      ASTM D6528-17 - Standard Test Method for Consolidated Undrained Direct Simple Shear Testing of Fine Grain Soils

Using modern test equipment fitted with active height control systems and servos the following tests are possible:

 

·         Constant normal stress (maintain vertical stress during shear)

·         Constant normal stiffness

·         Cyclic strain controlled tests

·         Cyclic stress controlled tests

·         Creep tests

Below is described a typical system that can be used for some or all of the tests above.

 

4 Typical System – Confining Rings

Typical direct simple shear tests commonly use one of two methods to stop the sample radial deforming during the test. The most common method is a stack of metal rings (Figure 4). The other common method is using a wire reinforced membrane. This document only deals with the metal rings method.

The rings will stop the sample deforming radially under vertical stress so that during the consolidation of the sample it will only deform vertically (one dimensional consolidation). During shearing where the sample is held at a constant vertical height and with no radial deformation allowed due to the rings the sample is held in a constant volume state; the test simulates shearing under undrained conditions.

Figure 4 shows a typical stacked ring DSS setup. The stacked rings will move over each other during the shear phase of the test allowing the sample to shear but remain in constant volume (see Figure 5). The rings are normally Teflon coated to reduce the amount of friction between them reducing any error these may cause to the shear strength data.

 

There a number of different DSS test apparatus available that can perform standard height controlled static shearing tests; using passive height control to active vertical controlled systems; that can perform static and dynamic shearing. Four differing systems offered by VJ Tech are described below.

 

5 Typical System – Passive Height Control

The most basic way that a DSS test can be performed is with a passive height control system. A typical static Direct Simple Shear system that uses a passive height control system is shown in Figure 6.  The system will use a dead weight system to apply a vertical load for the consolidation phase of the test. The vertical axis is then mechanically locked for the shear stage to stop the sample from changing height. The horizontal axis is sheared using a stepper motor. This will move the horizontal carriage shearing the sample. The amount of force required to shear the sample is measured by the horizontal load cell.

Vertical load cell – This is used to measure vertical load being applied to the sample during the shearing stage of the test; then the vertical height is fixed and the leaver arm loading is locked in place to maintain the sample height. Load is measured to 1 Newton.

Vertical Displacement Transducer – This is used to measure the sample height change during the consolidation phase of the test. During the shear stage the sample shouldn’t change. Displacement is measured to 1µm.

Horizontal Load cell – this is used to measure the horizontal load applied during shearing so that a shear stress can be calculated. Load is measured to 1 Newton.

Horizontal Displacement Transducer – This is used to measure the amount of horizontal displacement applied to the sample during the shear stage of the test. Displacement is measured to 1µm.

Top Cap and Base Pedestal – These are available in a number of designs. The two most common are the grooved design and the pinned design (see Figure 7). Both are designed to grip the sample so that it doesn’t slip during shearing.

 

Vertical Load Bearing – This is designed to allow maximum vertical load to be applied in a in a linear way (directly vertical to the specimen) using a linear bearing.

Horizontal Carriage – This is designed to allow the sample to be sheared in a linear direction up to the maximum load of the system with very limited friction and effect from the vertical load applied. This is usually designed with a linear bearing.

The sample height during shearing is controlled by a mechanical system that locks the vertical position. This is set manually by the user before the shear stage is started. The mechanical lock will not allow the sample to change height. The load cell that is within this mechanism allows the load applied to the sample during shearing to be measured (see Figure 8).

The main advantage of this system is the relatively cheap cost. It is much simpler than other systems that are available. The drawback with this system is that it can only perform static shearing tests that are height controlled and testing cannot be automated.

 

Typical System – Active Height Control

An active height control system allows for more testing options that a passive control system. It’s also allows the user to easily automate the test. The system will use a closed loop control system using feedback from the vertical displacement transducer to actively maintain the sample at the desired height; so not allowing the sample to change height. The system will use a stepper motor to move the vertical axis allowing the software to control it. A typical system is shown in Figure 9.

Vertical load cell – This is used to measure vertical load being applied to the sample during the consolidation and shearing stages of the test; Allowing the user to apply the required stress to the sample for consolidation and to the measure the stress being applied to the sample when maintaining a constant height during the shear stage of the test. Load is typically measured to 0.1 Newton.

Vertical Displacement Transducer – This is used to measure the sample height change during the consolidation phase of the test. During the shear stage the transducer is used as feedback to maintain the sample height. Displacement is typically measured to 1µm.

Horizontal Load Cell – This is used to measure the horizontal load applied during shearing so that a shear stress can be calculated. Load is typically measured to 0.1 Newton.

Horizontal Displacement Transducer – This is used to measure the amount of horizontal deformation applied to the sample during the shear stage of the test. Displacement is typically measured to 1µm.

The system can use the same top cap and base pedestal options (ridged or pinned) as the passive height system (see Figure 7). The system will also use a similar horizontal carriage and vertical bearing system to the static system.

This system can perform the standard height controlled test. It is also possible to undertake constant vertical stress tests as well as constant normal stiffness. These systems can also perform very slow cyclic shear tests using our Clisp Studio software and can be upgraded to undertake one dimensional consolidation tests. This system can be upgraded to undertake Direct Shear (shearbox) tests as well as one dimensional consolidation tests.

 

7 Typical System - Dynamic Confining Rings

This system differs from the static systems as each axis is fitted with a high speed servo (instead of slower stepper motors) connected to a dynamic servo controller. This means each axis is able to perform both static and cyclic control. They are rated to loads of 5kN and can operate up to 5Hz in frequency.  They can undertake a standard constant volume direct simple shear test using the vertical servo to apply the correct load for consolidation and to maintain position during shearing. The horizontal servo is used to shear the sample at a fixed strain rate. The vertical servo can also be used to perform constant normal stress tests. The horizontal servo can be used for cyclic shearing of the sample; both displacement and load control. A typical system is shown in Figure 10.

Vertical load Cell – This is used to measure vertical load being applied to the sample during the consolidation and shearing stages of the test; Allowing the user to apply the required stress to the sample for consolidation and to the measure the stress being applied to the sample when maintaining a constant height during the shear stage of the test.

Vertical Displacement Transducer – This is used to measure the sample height change during the consolidation phase of the test. During the shear stage the transducer is used as feedback to maintain the sample height.

Horizontal Load Cell – This is used to measure the horizontal load applied during shearing so that a shear stress can be calculated.

Horizontal Displacement Transducer – This is used to measure the amount of horizontal displacement applied to the sample during the shear stage of the test.

The system can use the same top cap and base pedestal options (ridged or pinned) as the passive height system (see Figure 7) and similar horizontal carriage and vertical bearing system.

The dynamic servo controllers provide high speed data acquisition and control of each servo. The controllers can apply cyclic sine, triangular, square and user defined wave forms. Displacement can be measured down to 0.1µm and load to 0.1 Newton with the controllers.

 

8 Typical System – Dynamic Confining Pressure

The dynamic simple shear with confining pressure is a much more complex system that the 3 others described above. This document will give a brief overview of the system; if you require more details please contact VJ Tech. This system, unlike the ones above, does not use the metal rings to confining the sample; here a confining cell pressure is used. An overview of the system is seen in Figure 11.

The system places the sample into an assembly similar to the other DSS systems but with a skirted top cap and base pedestal (see Figure 12). This top cap and base pedestal cause no sample disturbance. The sample is then surrounded in a standard membrane. The assembly and sample are then placed into a cell (very similar to a triaxial cell); the cell is filled with water and connected to a pressure controller. The pressure controller is used to apply a pressure around the sample. 


The system also uses another pressure controller to apply a back pressure to the sample. This allows the sample to be saturated using the same techniques as used in an effective stress triaxial tests. The test also measures the pore pressure with the sample. This allows B checks to be undertaken, and effective horizontal and vertical stresses to be measured. Each axis is fitted with an internal submersible load cell; the horizontal is used for measuring shear strength. The vertical load cell is used for measuring the vertical stress to the sample. The system uses high speed servos to control each axis. This allows loads of up to 5kN to be applied. The servos can be used for static or dynamic control and can apply frequencies of up to 5Hz. The system is capable of cyclic displacement, load and stress control. A full system from VJ Tech can be seen in Figure 13.

The system offers many advantages including:

 

-       Full saturation of sample

-       Checking of the saturation level

-       Isotropic consolidation

-       Anisotropic consolidation

-       K0 consolidation

-       Static shearing

-       Cyclic shearing

-       Liquefaction analysis

-       Measurement and control of total vertical and horizontal stresses

-       Measurement of effective vertical and horizontal stresses

-       Upgradable to perform DSS with confining rings

-       Upgradable to undertake cyclic triaxial testing

-       Drained and undrained testing

-       No friction from confinement rings

More information about the Dynamic Simple Shear system with confining pressure can be found in this publication.

 

9 Testing – Consolidation

A test specimen selected for testing is commonly of a fine grained material. The sample can be from an undisturbed sample or remoulded. Sample preparation is extremely important so as to reduce disturbance on an undisturbed sample and ensuring a remoulded sample has achieved the correct density. Sample preparation techniques are outside the scope of this document.

The first part of the direct simple shear test is the consolidation stage. This stage of the test will consolidate the sample to a chosen normal stress. The sample is radially constrained by the confining rings around the sample. The sample will normally be covered with or given access to water to allow the specimen to saturate. The sample can be consolidated in one or a number of loading stages. It’s also possible to apply a second consolidation stage of lower stress. This is commonly used with over consolidated material.

The sample should be consolidated until the end of primary consolidation is achieved at the maximum stress applied to the specimen. The stress on the sample should be maintained for at least 10 times longer than the time to achieve end of primary consolidation or 24 hours (whichever is longer).

 

10 Testing – Shear

The final stage of the testing is the shear stage. The sample is sheared horizontally until the sample has sheared by 20% strain or the shear force has dropped from the peak value by 20%. When shearing the sample isn’t allowed to change height it is kept in a constant volume condition (simulating undrained testing). If using a pasive height control system the user must remember to lock this in place.


The shear stage data is plotted graphically as shear displacement (or shear strain) against shear stress (see Figure 15) so that a clear failure can be seen. The maximum shear stress achieved can then be plotted on a graph of normal (consolidation) stress against shear stress. This combined with a number of tests on the same material at differing vertical stresses can be used to determine the angle of shearing and Cohesion for the material (see Figure 16).

 

Normally direct simple shear systems do not measure pore pressure changes. But the change in vertical load seen during the shearing as the sample height is maintained can be used to infer a pore pressure change in the sample from this equation.

The secant shear modulus (ratio of the difference in deviator stress to the corresponding axial strain applied to the soil) can be calculated from the test results using this equation:

This can be used estimate the initial settlements of embankments built on saturated cohesive soils. This is commonly plotted against shear strain as part of the results (see Figure 18).

 

11 Advantages of VJ Tech Systems

The VJ Tech simple shear systems when used with our Clisp Studio software allow easy setup of tests. With the active height control systems, tests can be automated applying vertical stress required for consolidation and then easily moved into the shear stage of the test. The software also allows the user to generate report sheets, or export data to excel for processing. The software comes in differing modules for different test requirements (CS Simple Shear, CS DYNADSS and CS DYNADSS-C)

The active height control systems supplied by VJ Tech use electro mechanical stepper motors and servos. No compressed air is required. This makes them quieter to use along with lower power consumption. They are also easier to maintain over the long term. They can also be supplied to undertake low stress testing.

The systems also offer a range of upgrades such as slow cyclic, vertical stress control, vertical constant normal stiffness and creep tests when used with the right software and equipment upgrades.

Systems can be purchased with full installation and training.

 

12 Further Information

Additional information about direct simple shear systems can be found VJ Tech (sales@vjtech.co.uk or service@vjtech.co.uk) or by visiting our website (www.vjtech.co.uk).

The systems shown above all run via our Clisp Studio software. Further information on Clisp Studio can be found on our website or via YouTube Channel which can be accessed by clicking here.

Additional information about the DSS test can be found in the  D6528-17 Standard Test Method for Consolidated Undrained Direct Simple Shear Testing of Fine Grain Soils and from the publications page of the VJ tech website.

 


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