Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Timothy Newson

Abstract

The Great East Japan earthquake of 2011 generated a huge quantity of disaster waste and tsunami deposits, which required proper treatment and disposal. To effectively use these waste soils in geotechnical applications, it is essential to understand the mechanical behaviour in their native (pure) or mechanically stabilized form (amended with cement and fibre). To address these objectives, a series of monotonic stress-strain (e.g. compression and extension) triaxial drained and undrained tests were performed on pure Toyoura sand and mixtures of Toyoura sand-silt-cement-fibre. Additional laboratory studies were performed to investigate other related aspects, such as density, back pressure, small-strain elasticity, curing time and micro-fabric. Fibre and cement additives significantly enhanced the undrained and drained shear strengths of Toyoura sand. The stress-strain results showed behaviour typical of the medium dense specimens, with increasing stress ratio, peak strength and stiffness, and minimal strain softening. The secant modulus was found to be least affected by fibre additives alone, but significantly increased for cemented and fibre reinforced cemented specimens. The fibre and cement additives also increased the strength parameters (frictional angle, cohesion), dilatancy angle, slope of the critical state line, and decreased the state parameter of pure Toyoura sand. When increasing the applied back pressure, the maximum deviator stress increased, and limited increase occurred for the strength at critical state. As the curing time increased, the peak strength significantly increased, but with noticeably more brittle behaviour. The mechanical benefits of cement and fibre additives were found for all silt contents, but the most noticeable strength increase was obtained for 28% silt content. The addition of fibre or cement to Toyoura sand had no appreciable effect on the slope of the 1-dimensional normal compression line (K0 NCL), but these additives shifted the line compared to that for unreinforced sand. Fibres slightly reduced the small-strain shear modulus, but cement, and the combined effects of cement and fibre were found to provide quite significant shear modulus increases. Results obtained from local strain measurements were in close ii agreement with the results presented in the literature. The fibre orientation distribution showed that approximately 95% of fibres had an orientation that lies within ±45° of the horizontal plane. Fibre and cement additives changed the loose samples volumetric compression behaviour into more dilative behaviour. A modified form of the Severn-Trent constitutive model was also used to simulate the stress-strain behaviour of the unreinforced and reinforced Toyoura sand. Two additional parameters were introduced: a) the slope of the critical state line in the q-p' plane for reinforced specimens (𝑀∗), and b) tensile stress (𝑞0). Validation and calibration of the model was based on the laboratory tests conducted herein and results found in the literature. A close agreement of the model simulations with the experimental results was observed for many of the tests performed on pure Toyoura sand, cemented, fibre, and fibre reinforced cemented Toyoura sand for a range of mean effective stresses. The study also showed that the modified Severn-Trent constitutive model, predicted the stress-strain behaviour of specimens with varying curing duration, density, and silt content. However, it was seen that further improvements were needed to more accurately model the complex behaviour of fibre reinforced cemented Toyoura sand (i.e. higher curing duration) and fibre reinforced cemented Toyoura silty sand.

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