Evaluation of consolidation behavior of soft cohesive soils with vertical drains by physical model tests and digital image analysis

Abstract

This dissertation investigates the consolidation behaviors of soft soils with vertical drain under equal strain condition through two approaches which focus on horizontal deformation. The first approach is based on the experimental observation and analysis of responses and deformations of cohesive soils during consolidation. For the observation of soil specimen during the experiment, a transparent and rectangular parallelepiped shape of consolidation apparatus was developed. Consolidation tests were performed under two different boundary conditions: vertical drainage condition and horizontal drainage condition. The earth pressure gauges and pore pressure transducers installed in the test apparatus measured total vertical stress and excess pore water pressure during consolidation, respectively. The images taken during the consolidation tests were utilized for digital image analysis. To obtain high quality image analysis results, a procedure determining optimum analysis condition with various global strain interval was proposed. Displacements at a number of locations of the entire specimen during the whole consolidation process were evaluated using digital image analysis under the derived optimum analysis condition. Following this procedure, strains of the defined unit elements were calculated. As a result, the deformation characteristics which varied spatially and chronologically during consolidation were understood. For the vertical drainage condition, horizontal deformation was negligible. On the other hand, distinctive horizontal deformation behavior according to the degree of consolidation under horizontal drainage condition was obtained. A relatively large displacement toward the drainage surface occurred during the early stages of consolidation and thereafter, a small displacement to the opposite direction was measured, resulting in spatial variation of void ratio according to the drainage distance. Although the horizontal drainage consolidation tests were performed under equal strain condition, the vertical strain also showed a non-uniform distribution. The second approach is the numerical assessment of the performed consolidation tests with finite element method and modified Cam-clay (MCC) model. Compared with the experimental results, the horizontal deformation behavior with time showed a similar trend but showed a difference in terms of magnitude. Stiffness anisotropy of real soil revealed a relatively small horizontal deformation. In other words, finite element analysis with isotropic MCC model overestimated the degree of horizontal deformation. However, the experimental results indicated a larger heterogeneity of the specimen at the end of consolidation. Since related studies evaluating the internal deformation of the entire specimen during horizontal-drainage consolidation are limited, the suggested testing method and analysis technique are expected to show meaningful engineering significance when predicting the consolidation behavior of soft ground with vertical drain more accurately. However, further studies are required to discover the relationship between the magnitude of horizontal deformation or retardation effect and mechanical properties such as stiffness and plasticity for practical use.