UVM Theses and Dissertations
Format:
Print
Author:
Oka, Lalita
Dept./Program:
Civil and Environmental Engineering
Year:
2012
Degree:
PhD
Abstract:
Earthquakes pose one of the grave threats to the safety of infrastructure and lives. Liquefaction is a phenomenon normally associated with earthquakes where saturated soils lose their strength and stiffness due to dynamic loading and behave like liquid. The existing guidelines for liquefaction potential evaluations of a soil are based on the cyclic stress method, also termed as the "simplified procedure". This case history-based method uses field measurements of penetration resistance from standard penetration tests and/or cone penetration tests or shear wave velocity measurements to estimate liquefaction resistance of the soil. One of the salient features of the cyclic stress method is the ease with which it can be employed in practice. Yet, there are some aspects of this method where the guidelines to the practitioners are unclear. This research investigated two specific aspects of the simplified procedure: (1) the effects of presence of large structures on liquefaction potential, and (2) the effects of non-plastic fines on shear wave velocity and liquefaction resistance of sands.
It is a well-recognized fact that the in-situ measurements such as penetration resistance and shear wave velocity are affected by the in-situ stresses. The cyclic stress method is applicable only for sites on level and gently sloping terrain and for shallow depths (>15 m). However, the procedure is commonly extended for other situations encountered in practice, including reexaminations of existing earth dams on potentially liquefiable foundations. However, the presence of an earth dam, or any other large embankment or structure, significantly alters the normal and shear stresses in the foundation. This study employed the finite element method and identified and quantified potential errors if the altered stresses near heavy structures are ignored. A methodology to incorporate these effects within the framework of the simplified procedure is proposed.
The current form of the simplified procedure may be interpreted such that for a given penetration resistance or shear wave velocity, sands with fines are expected to have greater resistance to liquefaction as compared to clean sands. The topic has become controversial in recent years because some studies have indicated that this interpretation may not be valid. A laboratory program involving cyclic triaxial tests was undertaken to investigate the effects of fines on cyclic resistance of a sand by varying non-plastic fines content. Bender elements were incorporated in the cyclic triaxial apparatus to allow shear wave velocity measurements of the specimens. The experimental results were used to assess the effects of fines on the shear wave velocity and liquefaction resistance, and their combined influence on the liquefaction potential evaluation. Current research did not support the existing form of fines correction. It was observed that that fines content does not adequately define the cyclic resistance. Concept of intergranular void ratio was used to explain the behavior of sand with non-plastic fines.
It is a well-recognized fact that the in-situ measurements such as penetration resistance and shear wave velocity are affected by the in-situ stresses. The cyclic stress method is applicable only for sites on level and gently sloping terrain and for shallow depths (>15 m). However, the procedure is commonly extended for other situations encountered in practice, including reexaminations of existing earth dams on potentially liquefiable foundations. However, the presence of an earth dam, or any other large embankment or structure, significantly alters the normal and shear stresses in the foundation. This study employed the finite element method and identified and quantified potential errors if the altered stresses near heavy structures are ignored. A methodology to incorporate these effects within the framework of the simplified procedure is proposed.
The current form of the simplified procedure may be interpreted such that for a given penetration resistance or shear wave velocity, sands with fines are expected to have greater resistance to liquefaction as compared to clean sands. The topic has become controversial in recent years because some studies have indicated that this interpretation may not be valid. A laboratory program involving cyclic triaxial tests was undertaken to investigate the effects of fines on cyclic resistance of a sand by varying non-plastic fines content. Bender elements were incorporated in the cyclic triaxial apparatus to allow shear wave velocity measurements of the specimens. The experimental results were used to assess the effects of fines on the shear wave velocity and liquefaction resistance, and their combined influence on the liquefaction potential evaluation. Current research did not support the existing form of fines correction. It was observed that that fines content does not adequately define the cyclic resistance. Concept of intergranular void ratio was used to explain the behavior of sand with non-plastic fines.