May
26
2023
Dynamic Simple Shear vs Dynamic Simple Shear with Confining Pressure
This blog aims to highlight the differences between the two types of Dynamic Simple Shear systems that VJ Tech provide, and why one might be preferred over the other:
(1) Dynamic Simple & Direct Shear (DSS)
(2) Dynamic Simple Shear with Confining Pressure (DSS-C)
(1) Dynamic Simple & Direct Shear Shear System (DSS)
Geotechnical engineers employ Dynamic Simple Shear testing to assess the shear strength and deformation characteristics of soils. This method involves subjecting a soil specimen to cyclic horizontal loading, which simulates the dynamic stresses experienced by soils in the field, as a result of natural events such as earthquakes, wind, waves, and traffic. An analytical introduction to Direct Simple Shear testing can be found here.
DSS gives a consistent and precise method of evaluating soil shear strength, which is a vital ingredient in the construction of foundations, slopes, and retaining walls. It also provides information about the soil deformation properties such as shear modulus, damping ratio, and strain-dependent stiffness. These characteristics are critical for determining the likelihood of soil to liquefy under cyclic loading.
DSS also offers the advantage of mimicking the dynamic stresses experienced by soils in the field, which is critical for the understanding of soil-structure interaction. Additionally, researchers use DSS to analyse the behaviour of soils under varied loading circumstances in order to create novel methods for soil testing and characterization.
In a cyclic DSS test, the sample is enclosed in an elastic membrane within a stack of rigid rings. These discs are usually coated with a low friction material to reduce their friction, thus allowing them to follow the sample’s deformation during the cyclic loading without any change to the sample’s diameter. The cyclic shearing is applied to the specimen either using load (stress) or displacement (strain) control after it has been subjected to one-dimensional consolidation. The cyclic shearing phase is conducted under constant volume conditions which are simulating undrained conditions in the field. Constant volume conditions can be achieved by maintaining the sample’s height with either passive or active vertical control. A wide range of frequencies can be applied with a preference to the frequency that is better simulating the field conditions.
(2) Dynamic Simple Shear with Confining Pressure (DSS-C)
The application of a confining pressure to the soil specimen is the fundamental distinction between cyclic DSS and cyclic DSS-C. The soil specimen is evaluated in cyclic DSS under a single axial force delivered to the top of the specimen, with the confining pressure provided by the stack of rings as a responding load. In cyclic DSS-C, on the other hand, the specimen is enclosed in an elastic membrane and placed in a triaxial cell. Confining fluid is used to control the lateral stress to the specimen while Pore Water Pressure (PwP) can be measured.
Without Confining Pressure With Confining Pressure
Cyclic DSS-C provides total control over a number of process, like saturation and consolidation using the same techniques as in triaxial soil testing. With this test method, it is possible to better simulate field processes and the stress history of the soil in question. This is achieved through the ability to perform anisotropic and Ko consolidation and apply specific stress paths to the soil specimen. The measurement of the sample’s pore water pressure during the cyclic loading enables a better understanding of its liquefaction potential.
This system is exceptionally well-suited for conducting tests in offshore scenarios, such as offshore cabling and offshore wind farms, where the structures must endure and adapt to the fluctuating forces of wave motion. The insights gained in comprehending the behaviour of soil under dynamic loading conditions are of paramount importance in the construction of secure and stable structures