1. Introduction

2. Direct Shear Test – what is it?

Failure in soils usually occurs on a specific surface (shear plane) in opposition to other material, like concrete or metals. The failure occurs when the shear stress, acting parallel to that surface, exceeds the shear strength. In the case of shear stress acting on a body, the deformation which is produced is called shear strain. In direct shear tests, shear strain is measured as the displacement between the two portions of the soil specimen.

Shear strength is defined as the resistance of a soil to the induced shear strain. Shear strength is not a fundamental property in soils as it depends on the in-situ conditions, such as density, moisture, stress state etc. The value measured in the laboratory is likewise dependent upon the conditions imposed during the test and in some instances upon the duration of the test. Therefore, it is important to apply similar conditions in the laboratory as in the field.

The procedure in a direct shear test consists of;

1) placing a soil specimen in the direct shear device (shear box apparatus)

2) applying a predetermined normal stress,

3) providing the necessary conditions for wetting and/or draining of the specimen,

4) consolidating the specimen under the normal stress,

5) removing the pins that lock the shear box halves holding the test specimen, and shearing the specimen by displacing one shear box half laterally with respect to the other at a constant rate of shearing deformation while measuring the shearing force, relative lateral displacement, and normal displacement. The shearing rate must be slow enough to allow nearly complete dissipation of excess pore pressure.

The strength parameters obtained from a direct shear test are the cohesion (c) and angle of friction (φ). In densely packed granular soils, two distinct values for these parameters exist, namely Peak or Maximum and Residual or Constant.

The test is usually conducted in two stages;

The first stage involves the consolidation of the specimen under the same vertical stress as that which will be applied during shearing.

When this stage finishes, the consolidation parameters can be obtained and the time needed for the end of the primary consolidation is determined. More information on the procedures followed at the consolidation stage can be found in the support document ‘Consolidation Testing – An introduction’ which can be obtained from VJ Tech.

In the second stage (i.e. shearing stage), the specimen is sheared at a constant speed which depends on the consolidation parameters determined from the Consolidation stage. The speed must be such that sufficient time is allowed for the soil to expel the excess pore water and therefore provide drained conditions.

The apparatus used to perform direct shear tests is called a shear box. i.e. The rigid metallic container in which the soil specimen is retained during testing.

This should not be confused with the term ‘shear box apparatus’, which refers to the testing system that also includes the loading devices and the sensors.

3. Advantages and Disadvantages of Direct Shear Tests

Some of the advantages of the direct shear tests are:

• Direct measurement of shear strength
• Basic principles are easily understood
• Relatively easy testing procedure
• Simple and easy sample preparation
• Quick consolidation procedure due to the small thickness of the specimen
• Almost all soil types can be tested
• Both peak and residual shear strength is determined
• Residual strength tests are applicable
• Partially saturated soil specimens can be tested with the appropriate equipment

However, the user must be also aware of the following disadvantages, regarding the direct shear tests:

• Shear strength is measured only on a predefined shear plane
• Distribution of stresses along the shear plane is not uniform
• Only total stresses are applied, except on the testing of dry granular material, as pore water pressures cannot be measured
• There is no control of the drainage, therefore only drained tests can be performed
• The continual decrease in the contact surface of the two halves during the test produces a small error on the shear and normal stress and affects the Mohr-Coulomb failure envelope.This error is generally ignored.

4. Shear Box Apparatus

Shear Box apparatus is designed for carrying out tests on soil specimens of 60 mm or 100 mm square and 20 mm to 25 mm high. The Large Shear Box apparatus is able to carry out tests on soil specimens up to 305 mm square and 150 mm high. Finally, the Unsaturated Shear Box apparatus is used to carry out direct shear tests on unsaturated soil samples under constant suction. The size of the samples tested in the unsaturated shear box ranges between 50 and 75mm and they are either round or square shape.

A typical setup for direct shear tests is shown in Figure 1 Typical setup for a direct shear The soil specimen is placed inside the shear box and the two halves are held together using two securing pins. Porous and perforated plates are placed on the top and bottom surface of the specimen which allows free drainage during the test. The whole shear box is placed inside a container (carriage box) which is filled with water and enables the test specimen to be submerged during the test.

The vertical load is applied to the specimen through a loading cap. The side of the loading cap facing downwards is grooved allowing water to move out of the soil specimen when needed. The loading yoke supports the top half of the shear box and is connected to a load transducer which reads the resistance of the soil to the horizontal loading. This loading is applied to the specimen through a motorised device which is able to provide constant rate of displacement. The shear box assembly sits on low friction bearings which allows the bottom half to move with a minimum resistance and only in the longitudinal direction. Finally, the whole setup sits on a rigid machine bed.

Shear Box apparatus is designed for carrying out tests on soil specimens of 60 mm or 100 mm square and 20 mm to 25 mm high. The Large Shear Box apparatus is able to carry out tests on soil specimens up to 305 mm square and 150 mm high. Finally, the Unsaturated Shear Box apparatus is used to carry out direct shear tests on unsaturated soil samples under constant suction. The size of the samples tested in the unsaturated shear box ranges between 50 and 75 mm and they are either round or square shape.
A typical setup for direct shear tests is shown in Figure 1 Typical setup for a direct shear The soil specimen is placed inside the shear box and the two halves are held together using two securing pins. Porous and perforated plates are placed on the top and bottom surface of the specimen which allows free drainage during the test. The whole shear box is placed inside a container (carriage box) which is filled with water and enables the test specimen to be submerged during the test.

The vertical load is applied to the specimen through a loading cap. The side of the loading cap facing downwards is grooved allowing water to move out of the soil specimen when needed. The loading yoke supports the top half of the shear box and is connected to a load transducer which reads the resistance of the soil to the horizontal loading. This loading is applied to the specimen through a motorised device which is able to provide constant rate of displacement. The shear box assembly sits on low friction bearings which allows the bottom half to move with a minimum resistance and only in the longitudinal direction. Finally, the whole setup sits on a rigid machine bed.

This was generated using calibrated dead weights placed on a weight holder while the load was applied to the specimen through a level arm.

One upgraded design of the apparatus uses an automated pneumatic controller to apply the vertical stress, thus obviating the need for dead weights.
The most recent and advanced version, uses an integrated mechanical stepper motor to apply vertical stress to the specimen. This means that neither dead weights, nor a pneumatic air supply is required.

5. Large Shear Box Apparatus

To test larger samples, the Large Shear Box apparatus is used. The principle of the Large shear box is similar to the small shear box apparatus but the maximum sample size is up to 300 mm square. This allows for testing of soils containing particles up to 37.5 mm, i.e. coarse gravel size.
Moreover, the large shear box enables testing of soil specimens with irregular shapes where the sample is placed in the middle of the shear box and the surrounding space is filled with a rapid setting filling material (Figure 2).

Finally, soil or rock samples which contain several types of discontinuities can be tested in the large shear box. This allows the determination of the shear strength on the particular surface which needs to be levelled with the shearing surface of the shear box.

The shear strength of unsaturated soils can be determined using the Unsaturated Shear Box apparatus.  In these tests, soil specimens are partially saturated while they are subjected into a Under these conditions, the specimen is sheared and the unsaturated shear strength parameters can be determined.

6. Additional Equipment

- Calibrated dead weights used to apply the vertical load to the specimen, if the older version of the shear box apparatus is used. However, the use of the dead weights is not applicable when high vertical load is desirable.  In this case, the vertical force must be applied using either a pneumatic or a motorised controller.
Calibrated dead weights used to apply the vertical load to the specimen, if the older version of the shear box apparatus is used. However, the use of the dead weights is not applicable when high vertical load is desirable. In this case, the vertical force must be applied using either a pneumatic or a motorised controller.

- Two calibrated load cells to measure the horizontal and vertical load.

- One displacement transducer, used to measure the horizontal displacement of the shear box and therefore the relative movement between the two portions of the specimen when it is sheared. Another displacement transducer is installed above the top cap to measure the vertical movement (settlement) of the specimen during testing.

- Several tools for the preparation of the specimen inside the shear box.

- Balance, readable to 0.01 g.

- Stopwatch, readable to 1 s.

- Oven and aluminium trays to dry out soil samples and determine the moisture content.

7. Test Procedure – Standards for Direct Shear Tests

The testing procedure of the direct shear tests is described in the following standards:

• BS1377 - Part 7:1990 : British Standard Methods of test for Soils for civil engineering purposes, Part 7 - Shear strength tests (total stress)

• ASTM D3080-04 : Standard Test Method for Direct Shear Test of Soils Under Consolidated Drained Conditions

• AS1289.6.2.2 – 1998 : Methods of testing soils for engineering purposes – Method 6.2.2: Soil strength and consolidation tests – Determination of the shear strength of a soil – Direct shear test using a shear box.

8. Test Procedure – System Preparation

The first step for every laboratory test is the preparation of the testing setup. In direct shear tests, the preparation of the system is relatively simple and easy. The carriage is placed on the roller bearings and is fixed onto the motor shaft on one side. The procedure to be followed is:

- Installation of two load transducers: One load transducer is installed behind the horizontal loading yoke and another above the shear box. The transducers need to be calibrated before the first use and at regular intervals to ensure accuracy of the readings.

- Installation of two displacement transducers: One transducer is installed behind the container (carriage), measuring the horizontal displacement of the bottom portion of the shear box, and another above the top cap measuring the settlement of the specimen during the consolidation and shearing stages. Displacement transducers must be calibrated before the first use and at regular intervals to ensure accuracy of the readings.

- Preparation of the shear box: The shear box should be clean and dry before the test and its dimensions should be measured as much accurate as possible. Moreover, the thickness of the porous plates and perforated grid plates must be measured and the available volume space for the soil specimen must be determined. A thin coat of silicone grease is applied to the contact between the two halves of the shear box to reduce friction during movement. The two halves are clamped together using two securing pins placed at either corners of the box. The baseplate is placed at the bottom, followed by a porous plate and a perforated grid plate. The arrangement of the individual components in the shear box is shown in Figure 3. The perforated grid plates must be placed so as the grooves are oriented vertically to the axis of the horizontal movement.

9. Test Procedure – Sample Preparation

Either cohesive (clays) or non-cohesive (sands, silts) soils can be tested in the shear box apparatus. The large shear box apparatus, however, is also able to accommodate coarser soils (like gravels) and irregular-shaped specimens.

Normally, three or more similar specimens are prepared, either from an undisturbed or remoulded sample, and tested under different normal stress values. If the soil is prepared inside the shear box by compaction, the density of the three specimens must be identical. The preparation procedure depends on the soil type and desired testing conditions. A brief description of the processes involved for each type of soil is given below. Prior to sample preparation, the base plate, and lower porous disc and perforated grid plate are installed at the bottom of the shear box.

Before placing any soil inside the shear box, the available volume must be determined and the initial weight of the whole assembly (without soil in it) must be measured using the balance.

- Dry sands: The sand pouring method is used to prepare a dry sandy specimen inside the shear box at the desired density. To do that, sand is left to fall freely inside the shear box at a constant flow rate and from a specific height. The higher the falling height of material the higher the achieved density. A funnel with an appropriate neck opening can be used in this procedure. When sand pouring is finished, the soil surface is carefully levelled and the top perforated grid is placed on the sample. The final specimen height is determined and the sample volume is calculated. The shear box is weighed again the weight of the soil is determined by the difference between the two and the final achieved density can be calculated. Several attempts may take place, pouring the sand from different heights, so as to achieve the desired density. Compaction of the dry sand soil using a dolly or another tool is not going to be effective.

- Saturated sands: Wet sandy specimens can be prepared in the same manner as the dry sands, only this time the shear box needs to be placed inside the carriage and both are filled with water up to the estimated soil level. Soil can then be poured into the water, within the shear box, up to the desired height. The overflow of the carriage removes the excess water and keeps the level just above the soil surface. This process is not capable of producing specimens of low density.

- Wet cohesionless soils: Cohesionless soils can be prepared inside the shear box by using the moist tamping method. The soil is brought to a specific moisture content, usually close to its optimum value, by mixing it with water. If the optimum value is not known, a moisture content of 5-10% should be sufficient. Compact the soil in layers using a tamper; more layers produce denser samples. When preparing, avoid forming a layer at the same level as the split surface of the shear box. Place the perforated grid plate on the top of the specimen and determine its density in the same way as described above. Place the top cap and then the whole shear box into the carriage. Finally, determine the moisture content of the soil used by drying surplus material into the oven.

- Undisturbed cohesive soils: Undisturbed specimens can be produced from large intact samples of the soil by trimming at the right dimensions. The specimens need to be handled with care to minimise their disturbance and loss of water content, especially where sensitive soils are concerned.

- Large specimens (large shear box): Large undisturbed specimens to be tested in the large shear box, are prepared by trimming larger undisturbed soil blocks. The specimen needs to be oriented correctly at the desired direction with respect to the soil stratum in-situ. Large disturbed specimens are prepared within the shear box by compaction or moist tamping.Any particles of a size larger than the 1/10 of the specimen height must normally be removed.

10. Test Procedure – Consolidation

The first stage of the testing process is consolidation. The procedure is identical to the one dimensional consolidation test and is described in detail in the support document SUP0112 -Consolidation testing - an introduction by VJ Tech. The purpose of the consolidation stage is to ensure that effective stresses have been applied to the specimen before the shearing stage starts. This practically means that after the application of the vertical stress, the excess pore water needs to drain out of the soil. Therefore, the consolidation stage takes more time to finish with finer soils with a lower coefficient of permeability. The procedure is the following:

- The normal force is applied to the specimen to give the desired vertical (normal) stress.

- The vertical deformation of the specimen is measured using the vertical displacement transducer. Readings are taken at appropriate time intervals that allow a graph to be drawn of the settlement against the square root of the elapsed time (√𝑡), or the settlement against the logarithm of the elapsed time (log 𝑡). The stage is continued until the readings indicate that the primary consolidation has ended.

- From the settlement plots, the values 𝑡100 and t50 can be determined, which then allow the determination of the maximum shearing speed to be used in the next stage (shear).

11. Test Procedure – Shear

At the end of the consolidation stage, the securing pins MUST BE removed from the shear box before the specimen is ready for the shearing stage. A horizontal force is applied to the bottom part of the shear box at a constant speed, which produces a gradual displacement. The maximum speed used is determined from the consolidation stage. The resistance of the soil to this movement is measured by the horizontal load cell and reflects the strength of the soil to shearing, i.e. the shear strength. The readings of the horizontal displacement, horizontal reaction force, vertical displacement and vertical (normal) stress are recorded at suitable intervals. Shearing is continued until the readings indicate that the shear stress has reached its maximum value. Usually, shearing is left to continue until a drop in the shear stress is observed, followed by a period of time at which it remains constant.

The shear box apparatus can also be used to determine the residual shear strength parameters, i.e. cohesion (cR) and angle of friction (φR). These tests are performed by returning the lower part of the shear box back to its initial position, after the first shearing stage, and reshearing the specimen under the same normal stress. Repeated loading cycles are applied until the soil exhibits the same residual shear stress at subsequent cycles. This indicates that the residual strength has been achieved.

12. Test Procedure – Results

The typical behaviour of a sandy specimen to direct shearing is shown in Figure 4. In dense soils, shear stress increases together with the horizontal displacement and reaches a maximum value before it starts reducing. This is the maximum stress that the soil can withstand and is called the Peak shear stress. As shearing progresses and horizontal displacement increases, shear stress decreases until it reaches an almost constant value which is known as the Residual shear stress. When the shear stress in a soil exceeds the peak shear strength, then the maximum stress that the soil can withstand would be equal to the residual one. This behaviour is not seen on loose samples, at which the maximum shear stress is equal to the residual value from the beginning.

Typical stress-strain curves of a soil specimen under different normal stresses are shown in Figure 5. The peak and residual stress values from each curve are plotted then on a Normal Stress – Shear Stress graph (Figure 6). The relationships between the normal stress and peak shear stress and between normal stress and residual shear stress are linear. Each one of these lines cross the Shear Stress axis (y-axis) to a value which is read as the Cohesion (c) of the soil. The angle of each line to the x-axis is giving the value of the Angle of Friction or Angle of Shearing Resistance (φ). Two angles of friction are determined from this graph, i.e. the Peak angle of friction and the Residual angle of friction.

Another useful plot is the volume change graph which defines the change of the specimen volume during shearing (Figure 7). Dense sandy samples, at low normal stress, tend to show an increase in volume while they are sheared. This phenomenon is called dilatancy and occurs mainly due to the relocation and rolling of soil grains during shearing. If the specimen is submerged, water may enter into the expanding pore space and saturate it.

13. Residual strength tests

The residual strength of the soil specimen can also be obtained by performing multiple direct shear tests on a single specimen (under the same normal stress) where the Shearbox apparatus returns back to its initial position every time it completes its travel. This process can be repeated a number of times until the residual shear stress remains constant at subsequent shearing stages. Typical stress-strain curves obtained from a residual direct shear test are shown in Figure 8.
The residual strength of the soil specimen can also be obtained by performing multiple direct shear tests on a single specimen (under the same normal stress) where the Shearbox apparatus returns back to its initial position every time it completes its travel. This process can be repeated a number of times until the  residual shear stress remains constant at subsequent shearing stages. Typical stress-strain curves obtained from a residual direct shear test are shown in Figure 8.

14. Range of VJ Tech Automatic Direct Shear Systems

VJ Tech provides a range of direct shear testing apparatus capable to meet the needs of every commercial or academic laboratory.

 The VJ Tech Direct/Residual Shear Testing System (Figure 9) is based on a microprocessor controlled apparatus which shears the sample using a controlling stepper drive motor. This device utilises a lever loading piston for applying the vertical load to the sample. The device is a self-contained floor mounted model that requires weights for applying vertical load. The advanced version of the ShearTest apparatus has inbuilt data logging and is supplied complete with a lever loading device, hanger and Shearbox carriage all mounted on a floor standing frame. The user friendly Clisp Studio csSHEAR software provides the necessary test configuration, control and data acquisition and conforms to BS1377-7, AS1289.6.2.2 & ASTM D3080-04. This testing system is capable for running either direct and/or residual tests.

Automated Direct/Residual Shear Testing System – ShearSCAN 2

The VJ Tech ShearSCAN 2 utilises the pneumatic loading concept for applying the vertical load to the sample when carrying out automated shear testing. The self-contained table top model eliminates the need for the numerous weights used in dead weight systems. The microprocessor based ShearSCAN 2 system incorporates a stepper motor drive, a large graphical display, a touch sensitive keypad and forward/reverse travel limit switches. Built in data acquisition automates the testing by setting the starting and stopping conditions. The ShearSCAN 2 is designed for harsh lab environments and is ruggedly constructed. The Clisp Studio csSHEAR software enables easy test setup and control with live tabular and graphical display of measured and calculated parameters and easy data export to standard packages. It conforms to BS1377-7, ASTM D3080-04 & AS1289.6.2.2.

Automated Direct/Residual Testing System - ShearSCAN pro

The ShearSCAN Pro Motorised Loading Instrument utilises stepper motors for applying vertical and horizontal loads to the sample, which means that a compressed air supply is not necessary. The device is a self-contained table-top model that eliminates the need for the numerous weights used in dead weight systems, and is capable of cyclic shearing, in addition to Direct and Residual Shearing.

The ShearSCAN Pro can also carry out Oedometer testing using the supplied adaptor (requires csOEDO). A 7” integrated touchscreen display enables easy operation and even standalone usage. It is normally connected to a PC or laptop via USB or Ethernet, and is controlled using our renowned Clisp Studio software (csSHEAR module).

Large Direct/Residual Shear Testing System

VJ Tech’s new Electro-Mechanical Large Shear Box Pro is specifically designed for testing large specimen sizes up to 300 mm (11.8") square. It is ideal for testing samples with large particle sizes, such as aggregates.
Both horizontal and Vertical loads are applied electro-mechanically, with both axes capable of applying 100 kN (22480 lbf). The machine has a built-in four channel data logger for automatic data acquisition and features an integrated 7” touchscreen display for data input and standalone control.
VJT-csSHEAR Software is used for automated operation and control via PC (Ethernet or USB), providing live data views, graphs and tables, with results output to Excel if required. The software conforms to BS1377-7, S1289.6.2.2, ASTM D3080-04 & Q181C.

Unsaturated Shear Testing System

The VJ Tech Unsaturated Shear Testing System is used to carry out unsaturated shear tests on soil samples up to 75 mm (2.95 inches) (square or round) in a sealed sample chamber. The Unsaturated Shearbox is bench mounted and is of rugged construction designed for lab environments. The microprocessor based system incorporates a stepper motor drive for horizontal load, a large graphical display, a touch sensitive keypad and forward/reverse travel limit switches.
The pneumatic loading concept is utilised for applying the vertical load to the sample (compressed air supply required). When testing, suction within the specimen is maintained using a Pneumatic APC at the top of the sample and a Hydraulic APC at the base applying the axis-translation technique. Built in data acquisition automates the testing by recording the horizontal and vertical loads and displacements. Clisp studio csUnsatShear software enables easy test setup and control with live tabular and graphical display of measured and calculated parameters and easy test and data export and import.

15. Advantages of a VJ Tech Automatic Direct Shear System

VJ Tech automatic direct shear systems offer the following advantages:

- Tests run 24 hours a day 7 days a week

- Tests can be set up and controlled through the user-friendly software Clisp studio

- Test procedure can be automated and limited staff supervision required (the technicians
can perform other activities)

- Average time for a test to be complete is significantly reduced

- Can be monitored and controlled remotely

- Increased quality of results and minimise human error

- Effortless maintenance

- Simple calibration processes

- Easy report generation

- Easy analysis of data

The advantage of VJ Tech Direct Shear systems to automate test procedures is for valuable time to be saved, especially in very busy laboratories. Automation allows for tests to be stopped when one stage of the process has finished, and then move automatically onto the next one, with minimum supervision by the technicians.

VJ Tech automatic direct shear systems offer the following advantages:
- Tests run 24 hours a day 7 days a week
- Tests can be set up and controlled through the user-friendly software Clisp studio
- Test procedure can be automated and limited staff supervision required (the technicians
can perform other activities)
- Average time for a test to be complete is significantly reduced
- Can be monitored and controlled remotely
- Increased quality of results and minimise human error
- Effortless maintenance
- Simple calibration processes
- Easy report generation
- Easy analysis of data
The advantage of VJ Tech Direct Shear systems to automate test procedures is for valuable time to be saved, especially in very busy laboratories. Automation allows for tests to be stopped when one stage of the process has finished, and then move automatically onto the next one, with minimum supervision by the technicians.

16. Further Reading on Direct Shear Tests

The following text book helped put this document together.

K. H. Head and R. J. Epps. 2011. Manual of Soil Laboratory Testing, Vol. II: Permeability, Shear Strength and Compressibility Tests. Whittles Publishing, Caithness, Scotland, 3rd edition.

VJ Tech would recommend this book to any technician or laboratory undertaking direct shear tests. The books provide an in-depth understanding behind the theory of the test and also the testing procedures including quality control and analysis of the results.

To purchase this book please contact the VJ Tech Sales department (sales@vjtech.co.uk).

VJ Tech also offers a training course and/or additional information on the subject matter.

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