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Factors Affecting Cyclic Behaviour of Soils. Dr. Suhad Dawood Salman Mechanical Engineering Department Faculty of Engineering, Mustansiriyah University, Baghdad, Iraq.
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Factors Affecting Cyclic Behaviour of Soils Dr. Suhad Dawood Salman Mechanical Engineering Department Faculty of Engineering, Mustansiriyah University, Baghdad, Iraq
Cyclic loading can be defined as a periodic action that when applied to a material body tends to change, and usually reverse, its stress and strain state over time. One of the most recognisable forms of a soil in a cyclic loading scenario happens during an earthquake. An earthquake is a propagation of seismic waves that radiate from an underground source, and are in most cases related to plate tectonics. These seismic waves, of which there are various kinds, impose on the soil small and large-scale movements that are both erratic and unpredictable. It is common knowledge that an earthquake can pose serious risks to population aggregates, as it can cause deaths, injuries, and property and infrastructure damage, sometimes with disastrous consequences. As such, studies that try to predict or evaluate the consequences of these phenomena are of extreme importance.
Factors Affecting Cyclic Behaviour of Soils • Cyclic strain amplitude • Void ratio • Mean effective stress • Plasticity • Overconsolidation ratio • Number of cycles • Frequency
DYNAMIC SOIL PARAMETERS The dynamic properties of soil are strain dependent and their best estimates and ranges of the variation can be obtained only by carrying out various types of field and laboratory tests. Therefore, low strain dynamic tests such as Seismic cross hole survey and high strain dynamic tests such as Block resonance tests, Cyclic plate load tests etc.
Evolution of stiffness and damping with strain For a soil under cyclic loading, as the strain amplitude increases, the shear modulus ratio, G/G0, which is equal to unity for zero shear strain amplitude, gets progressively lower. The change of the modulus ratio with shear strain is often presented visually through the use of a modulus reduction curve. The reduction of the shear modulus with strain is dependent on the soil’s material properties. Experimental results from a series of authors indicate that the soil’s plasticity has a strong effect on the shape of the modulus reduction curves. The shear modulus tends to decrease as the Plasticity Index increases, for strains over the very small range. Similarly, experimental results show a significant influence of the effective confining pressure on the shape of the shear modulus ratio curves, with soils under increasing confining pressure showing increasing stiffness for most strain ranges.
At the same time, it is known that, as the strain amplitude increases, soils tend to dissipate more energy in a loading cycle through damping, and their damping ratio, ξ, increases. As happens with the shear modulus, experimental results from a series of authors show that the plasticity of the soil has a significant effect on the evolution of damping with strain. The damping ratio tends to increase as the Plasticity Index increases, for strains over the very small range Similarly, experimental results show a significant influence of the effective confining pressure on the shape of damping ratio curves, with soils under increasing confining pressure showing decreasing damping for most strain ranges.
LABORATORY TESTS Soil element tests: Classified into two categories considering the shear strain levels at which they are able to measure accurately of these properties: • Low-strain element tests: Resonant column test Piezoelectric bender element test • High-strain element test: Cyclic direct simple shear test Cyclic triaxial test Cyclic torsional shear test. Model tests: Use a small-scale physical model of a full-scale prototype structure and aim to simulate the boundary conditions of a geotechnical problem. Shaking table tests and centrifuge tests are among the most referred model tests in these studies.
