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Centrifuge, analytical and numerical modeling of normal fault propagation in uncemented and cemented soils

Authors Cai, Qipeng
Issue Date 2011
Summary Notorious earthquakes and ground deformations resulted from bedrock fault movement have often been reported to cause serious damage to infrastructures and loss of human lives. Over the last few decades, extensive research has been carried out to investigate the behavior of fault propagation in uncemented soil by physical model tests, although it is recognized that many natural soils are cemented. Although there are numerous subsurface fractures left behind by previous seismic activities, the influences of pre-existing fractures on fault propagation are not fully understood. In this study, ten centrifuge tests were carried out to investigate normal fault propagation in both of uncemented and cemented soils. A novel filter paper technique was applied to simulate the presence of a pre-existing fracture in model tests. Moreover, a new hydraulic system was developed to simulate bedrock fault movement. Soil deformations were captured by digital photographs and analyzed using a particle image velocimetry technique. Strain localization above the bedrock fault was quantified to investigate the mechanisms of ground deformation and fault propagation. Based on centrifuge test results, a simplified analytical approach was developed to estimate ground deformation in undrained clay. Numerical back-analyses were carried out to obtain fundamental understanding of the observed behavior in centrifuge tests. In addition, numerical parametric study was conducted to investigate the influences of the tip location of a pre-existing fracture and the degree of cementations (defined by cement content) in cemented clay. Results from both centrifuge tests and numerical analyses consistently show that the ground deformation mechanism in uncemented soil can be classified into a stationary zone, a shearing zone and a rigid body zone. Simple shearing failure mode can be identified. Measured data show that the presence of a pre-existing fracture produced a scarp at ground surface in sand. In contrast, for cemented clay, ground deformation mechanism consisting of a stationary zone, a bending zone and a rigid body zone can be identified from both centrifuge tests and numerical analyses. A fault rupture initiated from the bedrock fault bend over the hanging wall and increased in dip when it approached the ground surface. Different from uncemented clay, both of shearing and tensile failure modes are observed. The dominant failure mode changes from shearing failure in weakly cemented clay (i.e., 1.5% cement content) to tensile failure in strongly cemented clay (i.e., 6.0% cement content). Due to the presence of a pre-existing fracture in cemented clay, a new fault rupture initiated from the tip of the pre-existing fracture and the extent of the bending zone was reduced. Numerical analyses also reveal that the influence zone of surface ground deformation and cracking extends as the tip depth of a pre-existing fracture increases. The developed analytical solution enables researchers to estimate undrained deformation in clays with different soil properties.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2011
Language English
Format Thesis
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