Comparisons between Bromhead and Bishop Ring Shear Testing
Bromhead & Bishop Ring Shear devices
Two main Ring Shear devices are generally available on the market, Bromhead Ring Shear and Bishop Ring Shear. In this blog post, a comparison between these two devices is presented.
Bromhead Ring Shear
The Bromhead Ring Shear is the oldest type of Ring Shear (RS) device that is widely used, mainly in commercial laboratories, for the determination of the residual shear strength of soils. The soil specimens are prepared inside an annular specimen container to a height of 5 mm, with the upper surface exposed.
A residual test on a Bromhead RS includes an initial stage of vertical loading, to consolidate the soil specimen to the required stress (Consolidation stage). Vertical stress is applied through a ring-shaped top cap with the same dimensions as the specimen. The older types of Bromhead RS were equipped with a lever arm and dead weights which were used to apply the vertical load. Newer versions of this type of RS include electromechanical motors and load cells to apply the vertical stress automatically and very accurately.
Once consolidation is completed, the bottom half of the specimen container starts rotating at a fast speed, while the top cap is restrained by a pair of load cells which determine the torque induced by the resistance of the soil mass to rotation. The fast rotation (Fast Shear stage) is important so that the shear plane is formed within the soil mass.
At the final stage (Shear stage), the base of the specimen container starts rotating at a slow speed while the torque is measured by the pair of the load cells. Vertical stress is kept constant throughout the whole test procedure. Once shear stress reaches a constant value, the test can be considered complete.
Specimens tested in the Bromhead RS are usually remoulded or reconstituted, as it is difficult to form an undisturbed sample on the required dimensions and transfer it to the specimen container. Grain sizes should not normally exceed 1/6 of the specimen’s height, resulting on soil samples with a maximum grain size of 0.83 mm. However, this type of Ring Shear is widely used to test soils with coarser grains compared to the Bishop Ring Shear. This is, mainly, due to tendency of fine soils to be squeezed out of the specimen container during consolidation and fast shearing.
The shearing plane in the Bromhead RS is not defined, as the specimen is completely constrained and therefore is allowed to fail at any point as it rotates. This, sometimes, might cause the formation of the shearing plane exactly on the interface between the soil specimen and the porous disc of the annular top plate. In these cases, the reliability of the residual shear strength can be questioned. To limit this problem, some top caps include small indents that are pushed inside the soil mass to increase the interface friction.
The sensors that are used in a typical Bromhead RS include a set of load rings or usually load cells, located at the opposite sides of a lever-arm that is measuring the torque induced by the resistance of the soil to rotate under the applied vertical stress. This force is used to calculate the shear stress that develops to the specimen during rotation. Moreover, a displacement sensor is used to determine the settlement of the specimen. In the motorised versions of the Bromhead RS, an additional load cell is used to measure the vertical load that is applied to the specimen through a stepper motor.
Two types of tests can be performed on a Bromhead RS: Soil-Soil and Soil-Steel Interface tests. In Soil-Soil tests, the residual strength of the soil mass is determined. In soil-steel interface tests, the interface friction angle between soils and displacement piles can be predicted by simulating the pile driving process in the Ring Shear device.
In Soil-Steel Interface tests, the annulus top cap is equipped with a steel disc which has similar roughness to the driving pile in question. By rotating the soil specimen against this disc, the interface friction angle can be determined and the short and long term interaction between the soil and the pile can be predicted. The non-specified shearing plane in the Bromhead RS poses, again, the question about the reliability of this device to accurately determine the friction angle of the soil-steel interface.
The simplicity of this equipment makes it very popular for commercial labs that require simple and fast testing procedures. However, the reliability of the measured parameters has been questioned by several researchers in the past, mainly due to the errors introduced from the side friction that develops through consolidation settlements. Moreover, the small specimen’s height (5 mm) makes it more sensitive to the presence of larger soil particles.
Figure 6 - Bishop Ringshear Pro
The Bishop Ring Shear is a device not widely used in commercial laboratories, mainly due to its complexity. The Bishop RS consists of a set of inner and outer confining discs that are used to hold the soil specimen in place. The discs are split into the bottom half, which is the one that rotates, and the top half, which remains stationary during the shearing process. Specimen dimensions are 150 mm X 100 mm X 20 mm (OD X ID X H), meaning that the volume of the soil specimen is considerably larger compared to the Bromhead RS. This makes it less sensitive to the presence of coarser particles into the soil mass. Even though the Bishop RS is mainly used for the determination of the shear strength in fine grained soils, the maximum grain size that can be accommodated is 3.3 mm.
Specimens tested in Bishop RS can be remoulded or reconstituted, but also undisturbed specimens can be formed using a set of specially designed concentric discs. The specimen is held inside the confining rings; therefore, no soil is allowed to be squeezed out during the consolidation stage. This makes the Bishop RS ideal for testing fine-grained soils, especially clays. The shearing plane in the Bishop RS is well defined at the point where the top and bottom halves are split during the shear stage. This allows more accurate determination of the soil resistance to shearing and the prediction of the residual shear strength, compared to the Bromhead RS.
A typical Bishop RS includes the following sensors: Two (2) load cells for the measuring the torque applied to the soil mass, one (1) load cell for measuring the vertical load, one (1) displacement sensor to determine soil’s settlement, one (1) load cell for the determination of the side friction occurred on the confining rings’ inner walls during shear and two (2) displacement sensors to measure the differential movement of the top half confining rings during the shear stage.
Figure 7 - Top Half Components Illustrated
In contrast to the Bromhead RS, in Bishop RS the side friction can be measured and accounted for by adjusting the vertical load. Therefore, the Net, instead of the Total Normal Stress can be applied to the specimen while it is sheared. Also, the use of the two displacement transducers that measure the differential settlement of the top half of the confining rings allows evaluation of the test results for their reliability.
As in the Bromhead RS, the Bishop RS is capable of performing soil-soil and soil-steel interface tests. During the rotational (Shear) stage, the two halves of the confining rings are slightly separated to form the shearing plane. That allows a more accurate determination of the soil’s residual shear strength as there is very limited impact from the top and bottom boundaries of the soil container. In a soil-steel interface test, the steel disc with the specific roughness is located exactly at the bottom half of the coil container which rotates, therefore the interface friction is better defined compared to the Bromhead RS.
The Bishop RS is considerably more complex to operate compared to the Bromhead RS and tests take longer to complete. This has, so far, made them less popular to commercial laboratories as they were mainly used in research institutes. However, there is lately an increased interest from commercial laboratories for this type of RS because of its benefits.
When combined with a software package, Bishop Ring Shear can be automated to perform soil-soil and soil-steel interface tests with minimum human intervention. More advanced types of control can be also applied to this type of RS, like the Constant Normal Stress, Constant Normal Height, or Constant Net Normal Stress.
Ring Shear devices available from VJ Tech
1. Ring Shear Pro (VJT5600A-P): The VJ Tech Ring Shear Pro is a Bromhead type torsional shear device, which produces accurate and repeatable test results to determine the residual shear strength of cohesive soils. It comes with a sintered bronze top cap as standard but can also be fitted with a Steel Top Cap of known roughness for use in the Soil-Steel Interface Tests. Rotational shear displacement is applied to the sample by a stepper motor, through a fixed ratio gearbox.
a) Applies continuous rotational shear displacement to the sample using a stepper motor. Rotational Shear is measured using a pair of Load Cells (or Load Rings if required (Junction Box is needed)).
b) Lever loading actuation of Normal Stress via dead weights.
2) Ring Shear Pro Motorised Apparatus (VJT5600M-P): The VJ Tech Ring Shear Motorised Pro is a torsional Bromhead style shear device, which produces accurate and repeatable test results to determine the residual shear strength of cohesive soils, which is particularly important when assessing slope stability.
a) The Instrument is supplied as a tabletop unit for stability and Normal load is applied to the annular ring sample through a stepper motor whilst rotational shear displacement is applied to the sample by a separate stepper motor, through a fixed ratio gearbox. This type of Ring Shear apparatus is widely used to determine the residual strength of soils as well as the interface residual friction angles between soils and piles. The ring-shaped soil specimens have a height of 5 mm. Tests are easily conducted, specimens are simple to form and both soil-soil and soil steel interface tests can be carried out.
3. Bishop Ring Shear (VJT5605): The VJ Tech Bishop Ring Shear device is a motorised Ring Shear used to determine the residual shear strength of soils, either cohesive or cohesionless. Normal stress is controlled using a stepper motor and a load cell, using closed-loop control. Shear stress is applied to the specimen by rotation of the base plate while the top part of the assembly remains stable and connected to a pair of load cells. Side wall friction is determined with the use of an S-Beam load cell mounted over the vertical load control system using a bearing mechanism. Differential settlement is determined with the use of an LSCT pair. The Bishop Ring Shear instrument can achieve maximum rotational speed of 900 degrees/min and can measure shear stress up to a maximum of 1,000 kPa. The maximum applied Normal Stress is 2,000 kPa. The capacity of the load cells can be reduced to increase the accuracy of the measurements. Using the control box, which is mounted at the side of the instrument, the user can perform manual control and bring it to the initial state before starting a test. The base plate is removable allowing the user to prepare a specimen outside the device and then transfer it to its starting position.
a) Bishop Ring Shear can be used for soil-soil testing, with the use of the csRingShear module in Clisp studio, for determining soil’s residual shear strength. Furthermore, Bishop Ring Shear can be used in soil-steel interface testing, using the csSoilSteel module, for the determination of the interface friction angle between steel surfaces of specific roughness and the soil mass.
Figure 13 - Bishop Rimgshear Pro System (VJT5605)