Soil Fabric and Transitional Behavior in Completely Decomposed Granite: An Example of Well-Graded Soil

Unlike sedimentary soils, limited studies have dealt with completely decomposed granite (CDG) soils, even though they are plentiful and used extensively in several engineering applications. In this paper, a set of triaxial compression tests have been conducted on well-graded intact and disturbed CDG soils to study the impact of the fabric on soil behavior. The soil behavior was robustly affected by the soil fabric and its mineral composition. The intact soil showed multiple parallel compression lines, while a unique isotropic compression line was present in the case of disturbed soil. Both the intact and disturbed soils showed unique critical state lines (CSL) in both the e-log p′ and q-p′ spaces. The intact soil showed behavior unlike other transitional soils that have both distinct isotropic compression lines ICLs and CSLs. The gradient of the unique ICL of the disturbed soil was much more than that of the parallel compression lines of the intact soil. In the intact soil, the slope of the unique CSL (M) in the q-p′ space was higher than that of the disturbed soil. The isotropic response was present for both the intact and disturbed soils after erasing the inherited anisotropy as the stress increased with irrecoverable volumetric change. Soil fabric is considered the dominant factor in the transitional behavior and such a mode of soil behavior is no longer restricted to gap-graded soil as previously thought.

The behaviour of near-surface soils through ultrasonic near-surface inundation testing

Seismometers installed within the upper metre of the subsurface can experience significant variability in signal propagation and attenuation properties of observed arrivals due to meteorological events. For example, during rain events, both the time and frequency representations of observed seismic waveforms can be significantly altered, complicating potential automatic signal processing efforts. Historically, a lack of laboratory equipment to explicitly investigate the effects of active inundation on seismic wave properties in the near surface prevented recreation of the observed phenomena in a controlled environment. Presented herein is a new flow chamber designed specifically for near-surface seismic wave/fluid flow interaction phenomenology research, the ultrasonic near-surface inundation testing device and new vp-saturation and vs-saturation relationships due to the effects of matric suction on the soil fabric.

Study on the Mechanism of Stabilizing Loess with Lime: Analysis of Mineral and Microstructure Evolution

X-ray diffraction (XRD) technique was adopted to test the mineral composition of quicklime-solidified loess with different lime-adding rates at different curing periods. Scanning electron microscopy (SEM) and nitrogen adsorption were used to analyze the microporous structure of the solidified loess. The unconfined compressive strength and limit moisture content of solidified loess were combined to analyze the evolution mechanism of mineral composition and microstructure of solidified loess with the change of curing period and clarify the mechanism of quicklime-solidified loess. The results showed reduced content of clay minerals and decrease in the number of large pores due to increase of hydrates and pozzolanic products during extended curing period. The solidified soil fabric transformed from a compact structure into a mesh structure composing of acicular crystal and cementation. The main reasons for strength increase and change of liquid and plastic limits with the lime-solidified loess after extended curing are the change of the substance and the microstructure.

The Organization of Soil Fragments

Kubiëna (1938) was the first to introduce the concept of fabric in soil micromorphology, so this term has been used in soil micromorphology for a long time. The term “fabric” was initially applied to rocks by geologists and petrologists. This type of fabric is defined as the “factor of the texture of a crystalline rock which depends on the relative sizes, the shapes, and the arrangement of the component crystals” (Matthews and Boyer 1976). This definition has been adapted for soil micromorphology and its latest definition has been given by Bullock et al. (1985) as: “soil fabric deals with the total organization of a soil, expressed by the spatial arrangement of the soil constituents (solid, liquid, and gaseous), their shape, size, and frequency, considered from a configurational, functional and genetic view-point”. In conclusion, the soil micromorphologist should consider the fabric as an arrangement and∕or organization of soil constituents.

Imaging the root–rhizosphere interface using micro computed tomography: quantifying void ratio and root volume ratio profiles

Root growth alters soil fabric and consequently its mechanical and physical properties. Recent studies show that roots induce compaction of soil in their immediate vicinity, a region that is central for plant health. However, high quality quantification of root influence on the soil fabric, able to inform computational models is lacking from the literature. This study quantifies the relationship between soil physical characteristics and root growth, giving special emphasis on how roots in early stage formation influence the physical architecture of the surrounding soil structure. High-resolution X-ray micro-Computed Tomography (µCT) is used to acquire three dimensional images of two homogeneously-packed samples. It is observed that the void ratio profile extending from the soil-root interface into the bulk soil is altered by root growth. The roots considerably modify the immediate soil physical characteristics by creating micro cracks at the soil-root interface and by increasing void ratio. This paper presents the mechanisms that led to the observed structure as well as some of the implications that it has in such a dynamic zone.

Reliability assessment of slopes with three-dimensional rotated transverse anisotropy in soil properties

The influence of soil variability on three-dimensional (3D) probabilistic slope stability analysis has been previously investigated for soils that display isotropic spatial variability features or anisotropic horizontal fabric patterns. However, due to various soil deposition processes, weathering, filling or tectonic movements, the assumptions of isotropy or horizontal layering may not always be realistic. This study presents 3D analyses of slopes with spatially variable soils associated with rotated transverse anisotropy features. The results show that for cross-dip slopes where the strike direction of soil strata is perpendicular to the out-of-plane direction of the slope, the reliability depends on various factors including strata rotation angle and autocorrelation distances, and differs significantly from slopes with horizontally deposited soil fabric. The influence of strata orientation is also pronounced for dip slopes and reverse dip slopes, and these are presented in terms of reliability indices of the slopes and statistics of the length of sliding mass, and elaborated by considering the failure mechanism under different scenarios. Through these analyses, this paper discusses the key features of slope reliability considering rotated transverse anisotropy in soil properties, and their major differences from situations involving horizontal soil layers or two-dimensional probabilistic assessments.

Recovery of Small-Strain Stiffness Following Blast-Induced Liquefaction Based on Shear Wave Velocity Measurements

Changes in soil fabric following liquefaction have been studied using various in-situ methods, and often return inconclusive or conflicting observations. The time-rate variation of stiffness, when observed, is usually not evaluated over significant periods of time, limiting investigations about aging in post-liquefaction regain of stiffness. Even more uncommon is the application of geophysical techniques to evaluate changes in shear wave velocity (VS) as a proxy for small-strain stiffness. This study uses controlled blasting to examine long-term post-liquefaction regain of stiffness following multiple blast events. The Multichannel Analysis of Surface Waves (MASW) technique was used to observe changes in VS of aged deposits at a test site in South Carolina. Blast-induced liquefaction of the target liquefiable layer resulted in significant reduction to its initial small-strain stiffness owing to the destruction of the aged soil fabric. The time-rate variation in VS indicated that the initial small-strain stiffness was not re-established over many months following liquefaction. Following a second blast event, the small-strain stiffness reduced again, but recovered more quickly, similar to previously reported observations of young sand deposits. This study provides a significant basis for interpreting in-situ body and surface wave measurements of aged and young sand deposits densified using blast liquefaction.