From civil engineering view, Soil is the medium through which the structural loads are transferred safely and efficiently. Soil should be consistent enough to satisfy the requirements even under inevitable circumstances like earthquake, bomb reactions etc. It is necessary to incorporate the seismic effects into the soil properties. Like concrete or steel, engineering properties of soil cannot be found out using theory of classical dynamics and vibrations. It can be found only field and lab tests. To quench the above requirement, various techniques are employed nowadays. The most common methods are cyclic simple shear, cyclic triaxial shear and cyclic torsional shear tests. The dynamic triaxial test is the most effective method to arrive the static and dynamic properties of soil like cyclic deformation, damping ratio, liquefaction strength etc. Though it has some limitations, it is widely used for the analysis of soil under seismic forces. The fundamental parameters obtained from this test are cyclic shear stress and cyclic shear strain, through which the soil is defined. The tests can be done either by stress controlled (cyclic shear stress) or strain controlled (cyclic shear strain). The test setups are highly sophisticated and costly. It needs highly skilled labour. The measuring devices used in the system needs to be calibrated and sealed properly as it is more sensitive to disturbances. The results obtained reflect the site seismic condition to the maximum level provided the strain level is kept minimum.
Fig 1.1 Triaxial Cell Fig 1.2. A typical Cyclic triaxial apparatus
WHY DYNAMIC TRIAXIAL
The Dynamic forces are time dependent and are usually cyclic in nature i.e. they involve several cycles of loading, unloading and reloading. Earthquake is three dimensional in nature. Hence the
shear waves and body waves produced by the earthquake tend to deform the soil in all the directions (for the horizontal level ground). Dynamic Triaxial tests actually reflect the soil condition (in all round stresses) in the site. During earthquakes, the seismic waves cause the loose sand to contract and thereby increasing the pore water pressure. Under undrained loading, development of high pore pressure results in upward flow of water, thereby making the sand in liquefied condition. Pore water pressure is measured effectively in triaxial tests. Among the stress-control and strain-control condition, strain control is adopted widely. This is because; stress-control test has great sensitivity to the sample disturbance. In case of strain-control, pore pressure developed during tests is less affected by specimen fabric and density. The tests can be done on intact specimens and reconstituted specimens. While comparing the results obtained from intact and reconstituted specimens, there is much deviation in stress-control compared to strain-control. (tests done by vucetic and dobry, in 1988). Stress path control is used in the study of path dependence of soil behaviour. Stress deformation and strength characteristics depend on initial static stress field, initial void ratio, pulsating stress level and the frequency of loading. 1.2
There are variety of engineering problems which rely heavily on the behaviour of soils under dynamic conditions. These includes design and the remediation Of machine foundation, geotechnical earthquake engineering, protection against construction vibration, non-destructive characterization of the subsurface, design of offshore structures, screening of rail and traffic induced vibrations, vibration isolation etc. When it comes to dynamic triaxial test, the wide range of application is the liquefaction behaviour of soil under seismic forces.
One of the first pieces of equipment designed to test cyclic triaxial loading was the pendulum loading apparatus by Casagrande and...
References: River, NJ, 1996
Geotechnical Testing, ASTM STP 654, American Society for Testing and Materials, 1978, pp.
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