A shear machine for any condition: the wide Wykeham Farrance range
DETERMINATION OF SOIL SHEAR STRENGTH
If a failure occurs in the ground (for example for deep excavations performed without retaining structures), a slip circle surface is generally created within the soil. After a first immediate general failure, the soil will stabilise, since the soil can still offer a residual strength.
This residual shear strength is also defined as long term drained resistance of a soil, since large displacement movements are required.
In order to investigate both peak shear resistance and ultimate resistance, different laboratory testing methods have been developed and standardized.
Visit also Technology Communication area for deepenings on this issue.
The principle common to most of them is to measure the strength of a soil sample by shearing it along the horizontal plane. With this principle different types of shear tests are usually performed in the laboratory:
Which type of machine for which determination?
DIRECT SHEAR TESTS
In the traditional direct shear test the soil specimen (either undisturbed, remoulded or compacted) is placed in a rigid metal box and subjected to a normal constant stress. The metal box consists of two halves that can slide horizontally each other and will apply an increasing horizontal force to the lower part of the specimen while the upper part is reacting against the shearing action. From the measurement of this shearing action the shear strength of the soil is calculated.
DIRECT/RESIDUAL SHEAR TEST WITH CYCLES (forward & reverse)
The main limitation of the conventional shear box is that it is not possible to apply the shearing action for large displacement of the soil specimen and therefore the measurement of the residual strength is not correct. A first solution to overcome this problem was to repeat several time the shearing action on the same surface of the specimen already subjected to the shearing action of the traditional direct shear test.
This type of test is standardized as multi-reversal direct shear.
RESIDUAL SHEAR TESTS BY THE RING SHEAR APPARATUS
The ring shear apparatus, also known as Bromhead Apparatus, has been developed to overcome the main disadvantage of the multi-reversal shear test, where the shearing action is reversed, causing the continuous re-orientation of the soil particles.
In the ring shear apparatus the specimen is annular shaped and subjected to an unlimited rotational displacement from the lower part, while the upper part is reacting against a couple of load rings. The main advantages of this test is that large displacements make reliable the measurement of the residual strength of a soil specimen, where the area of contact on the shear plane is maintained constant. The disadvantage is that the specimen is tested only under remoulded conditions.
DIRECT/RESIDUAL SHEAR TESTS ON LARGE SCALE SHEAR BOX
For the evaluation of shear behaviour of clay and sand, the small direct shear test is generally performed using a small shear box (60 × 60 mm square).
In case of gravel, which is larger than sand, the shear strength parameters are obtained using the large direct shear test.
Coarse-grained soils are used in many fields including earth dams, harbour facilities, waste storage areas, and roadbed materials. The particle sizes of coarse-grained soils used in the sites are in the range of some cm to some dozen cm. In order to perform the shear test for the coarse grained soils with large particle sizes, a specific test and equipment have been developed, with the use of a large shear box, ranging from 100 x 100 mm to 300 x 300 mm.
Shear strength under dynamic conditions
Another field of investigation of the shear strength concerns the behavior of the soil under seismic actions. In these situations, that not necessarily lead to the collapse of the soil, it is important to investigate the stress-strain behavior and the relevant parameters of the different layers of the subsoil, information required to simulate the propagation conditions of a seismic event within the ground.
STATIC AND DYNAMIC SIMPLE SHEAR
Simple shear condition is one of the most representative strain condition of in-situ ground during seismic event and very suitable for evaluating ground response.
In the simple shear apparatus, a cylindrical sample of soil, laterally constrained by metal discs, is subjected to an axial stress and then sheared, in such a manner that the entire sample distorts without the formation of a single shearing surface.
Undrained conditions are simulated by continuously adjusting the vertical stress so that the height, and hence the volume of specimen, is kept constant. It is commonly accepted that the change in vertical stress is equal to the change in pore water pressure.
Another application example is the mode of failure encountered under the base of offshore gravity structures, subjected to cyclic wave loads.
DYNAMIC TORSIONAL SHEAR
Stress-strain behavior of the soil under seismic actions should be investigated starting from very low level of shear deformation. A specific apparatus has been developed to do that, where methods of measurements and application of stress are very sophisticated.
The Cyclic Torsional Shear (CTS), generally combined with Resonant Column (RC), can perform experiments to define the stress–strain pre-failure behavior under cyclic load of undisturbed/reconstituted soil samples. Shear modulus and damping ratio curves, as a function of shear strain generally between 0.0001% and 0.1%, are obtained. These types of results are widely used for seismic response analyses at a regional/local scale.
During CTS tests, a sinusoidal torsional force at low constant frequency (~0.1–5 Hz), for a finite number of cycles, is applied to the top of the specimen while the bottom is fixed against rotation.
Torque and deformation are continuously monitored in order to obtain relationship between average shear stress and average shear strain.