1.5 Scope and Limitations of Study
This research investigated the structural formation and behaviours of MICP- treated soils (i.e., residual soil and sand) by conducting the monotonic triaxial test, which is equipped with AE measurement. Isotropic consolidation and undrained shearing (either triaxial compression or extension) were conducted in the triaxial test. Figure 1.1 illustrates the key components in the scopes of work for the present research. This included the investigation of structural formation of MICP-induced soils (Chapter 4), and the progressive mechanical- AE responses (Chapter 5). The mechanical behaviour of the (MICP-treated) residual soil was compared with a poorly graded sand, in which the sand had been mechanically sieved to a MICP-favourable size range. It was followed by the investigation on the microstructural formation and the behavioural differences between the two mentioned soils. With the implementation of AE, the progressive behavioural changes (i.e., yielding, instability, and ultimate failure) based on the mobilization of soil particles and demolishment of the calcite-cemented structure were further scrutinized.
Figure 1.1: Scope of study covering structural formation and responses for MICP-treated soils
Research limitations of present experimental study were listed as followings:
I. Without local strain measurements, the obtained deformation data of soil specimens could be subjected to bedding error, end restraint effect, membrane penetration issue, and etc.
II. The AE response as triggered by grain fragmentation (> 100 kHz) could not be measured by the present AE instrumentation system, which had a resonant frequency of 32 kHz only.
III. The present AE measuring setup was only able to detect stress waves at the bottom of soil specimen and indeed a very simple system. Therefore, the stress waves that propagating to the upper zone and sideway of a soil specimen could be overlooked.
IV. The interval of isotropic consolidation pressures was too wide (i.e. ≥ 40 kPa) in which important behaviours of MICP soils could be undiscovered.
Although the anisotropy of MICP-treated residual soil was evidenced, it is still unclear over the exact pressure that the soil anisotropy phenomenon would start to launch.
V. The inter-relationships among percentage of fines, degree of calcite cementation, applied consolidation pressure, yielding resistance, and undrained shear strength were not able to be formed for the MICP residual soil in the present research.
VI. As monotonic stress-controlled (pneumatic) loading was adopted in this research, it is practically difficult to facilitate observing the apparent strain-softening phenomenon during the triaxial shearing. Accurate deformation properties at slow-rate of loading (i.e. <0.001 %/min) were also practically not obtainable.
1.6 Structure of Thesis
This thesis is divided into six main chapters: Introduction (Chapter 1), Literature review (Chapter 2), Research methodology (Chapter 3), Mechanical behaviours of MICP residual soil in comparison with sand (Chapter 4), Acoustic emission and mechanical behaviours of MICP residual soil in comparison with sand (Chapter 5), and Conclusions (Chapter 6). Introductory and closing remarks are also provided in each chapter to outline the important points.
Chapter 1 describes the background of study and the motivations behind the present research. Aim and objectives of research are clearly formulated. Scope and limitations of study are also enveloped to ensure the research objectives can be accomplished effectively.
Chapter 2 provides an extensive review on the mechanical and structural behaviours for MICP-treated soils (i.e. clean sand and soil containing fines). This chapter begins with the review on geological formation, characteristics, and mechanical behaviours of residual soils. Then, the MICP soil treatment and implementation of AE techniques are critically reviewed.
Chapter 3 covers the research framework and physical properties of residual soil and clean sand. The triaxial testing, MICP treatment and AE
measurement procedures in the laboratory are described in detail. Digital processing approaches for the mechanical and AE data are also included.
Chapter 4 discusses the microstructural formation and behavioural differences between MICP-treated residual soil and sand. Void ratio changes and anisotropy behaviours (i.e. relationship between radial and axial strains) were evaluated from the isotropic consolidation results. Consolidation properties, namely isotropic yield stress and compression index, were also evaluated for interpretation. Microscopic observation was further conducted to qualitatively justify the microstructural formation in the MICP-induced soil structure. More importantly, the undrained shearing behaviours of untreated and MICP-treated soils were investigated through the triaxial apparatus. From that, the stress-deformation and pore-water pressure responses were used to justify the observed bio-mediation effect.
Chapter 5 provides an extensive coverage on the deformation and undrained shearing behaviours through AE monitoring. In isotropic consolidation test, the mechanical measurement (as covered in Chapter 4) was correlated with the AE response. Notably, soil compressibility was correlated well to the AE rate under normal consolidation state. Progressive soil changes (namely, yielding instability and ultimate state) were traced in tandem with the mechanical responses when the soil was sheared under triaxial loading. The soil changes as determined from AE measurement was justified by referring to the previous studies and theoretical findings. Lastly, the AE-determined
yielding points were also compared with the yielding data reported by other researchers.
Finally, conclusions and research recommendations for further improvement are presented in Chapter 6 accordingly.
CHAPTER 2
LITERATURE REVIEW