Liang Bo Hu

Desiccation cracking of soils

ABSTRACT The scope of this paper is to present the global mechanisms of soil desiccation, including drying shrinkage and cracking. The paper first reviews the basic processes that are beneath the word “desiccation”. Then the results of an experimental study of desiccation are presented, in which strains, suction, water content, degree of saturation and crack geometry are investigated. The results show that cracking initiates close to the onset of de-saturation. Insights into the micro-scale are proposed to explain this observation. A scenario for the processes leading to crack initiation is further established in terms of the macroscopic variables: an assessment of the stress building up is proposed, until a critical point at which the tensile strength is met. Desiccation crack pattern formation is finally discussed.

Experimental study of desiccation of soil

Note: Sols Reference LMS-CONF-2006-007 Record created on 2006-11-09, modified on 2016-08-08

Numerical and phenomenological study of desiccation of soil

Note: Sols Reference LMS-CONF-2006-002 Record created on 2006-11-09, modified on 2016-08-08

A three-scale cracking criterion for drying soils

Cracking is a most unwanted development in soil structures undergoing periodic drying and wetting. Desiccation cracks arise in an apparent absence of external forces. Hence, either an internal, self-equilibrated stress pattern resulting from kinematic incompatibilities, or a stress resulting from reaction forces at the constraints appear as a cracking cause, when reaching tensile strength. At a meso-scale, tubular drying pores are considered in the vicinity of a random imperfection, inducing a stress concentration in the presence of significant pore suction. This approach allows one to use the effective stress analysis, which otherwise, away from the stress concentration, usually yields compressive effective stress and hence a physically incompatible criterion for a tensile crack. Recent experiments on idealized configurations of clusters of grains provide geometrical data suggesting that an imperfection as a result of air entry deep into the granular medium penetrates over 4 to 8 internal radii of a typical pore could yield a tensile effective stress sufficient for crack propagation.

Drying shrinkage of deformable porous media: mechanisms induced by the fluid removal

Mechanisms of free shrinkage strains of desiccatin g deformable porous media are studied . The roles of surface evaporation rate, surface tension, and viscosity of pore fluid, soil compressibility and permeability are investigated. For drying tests on two geomaterials with three different pore fluids the evolution of shrinkage strain and fluid content is reported. Most of the strain s occur in the saturated phase of drying prior to cracking . Simulations of this phase include evaporative fluxes at the external surface s, a consequent water transport across the sample toward the surface producing pore pressure and stress gradients, local water content change, and deformation. Biot theory is used.

Commonly Used Porous Building Materials: Geomorphic Pore Structure and Fluid Transport

Knowledge of microscopic geomorphic structures is critical to understanding transport processes in porous building materials. X-ray scans were obtained of a variety of commonly used porous building materials to both qualitatively and quantitatively evaluate their pore structures. The specimens included natural materials (two sandstones and a limestone) and engineered materials (three types of concretes and a brick). Scanned images were processed to reconstruct the geomorphic structures of these materials. Random walk analyses were performed on the reconstructed pore structures to estimate macroscopic transport properties (including tortuosity, specific surface, and permeability). The effective porosity and permeability of these materials were also experimentally determined and compared to computed values. Calibration of the threshold pixel value used in the postprocessing of X-ray images against measured effective porosity appears to be a more appropriate method of selecting this value than the typical approach, which employs selection based solely on observed histograms. The resulting permeabilities computed by using a calibrated threshold pixel value compare better with the measured permeabilities. This study also demonstrates that the relatively homogeneous and heterogeneous pore structures associated with the natural and engineered building materials under investigation can be captured by X-ray tomography.

Multi-scale Characterization of Coupling Mechanisms for Evolving Permeability in Oil/Gas Bearing Sediments Compaction

A proper understanding and eventual assessment of reservoir compaction and land subsidence is crucial for decision-making in petroleum and gas extraction industry. This paper presents a multi-scale multi-physics study of coupling mechanisms in the long-term compaction of oil/gas bearing sediments, also known as aging. The principal goal of this work is to quantify such coupling mechanisms at different scales and link phenomena occurring at micro- and meso-scales to a mathematical model formulated at a macroscopic continuum level. The interaction between mechanical and non-mechanical processes, originating from intergranular damage and dissolution was examined through modeling the involved phenomena at their respective micro- and meso-scales. Two major consequences that result from the intensified chemo-mechanical coupling were investigated: porosity reduction, and subsequent stiffness and permeability evolution. It appears that permeability is mildly affected by the contact area increase, and for most of the duration, by the precipitation of the mineral solute; stiffening of the grain system results from the mineral precipitation and the consequent redistribution of mass within the pore space.

The Influence of the Pore Fluid on Desiccation of a Deformable Porous Material

This paper aims at elucidating the influence of pore fluid properties on the shrinkage strains of deformable materials subjected to drying. Results of isothermal drying tests of two kinds of silts saturated with three different pore fluids are presented. Results show different strain amounts during drying and the final void ratios, depending on pore fluid. The rates of drying are consistent with saturated vapor pressures, while their amount with the surface tension. The shrinkage limit appears also to be controlled by a limitation in skeleton compressibility.

Evaluating the Factors Influencing Limestone-Dissolution Characteristics in the Karst Regions of Guizhou, China

Guizhou, China, is known for the abundance of subterranean drainage systems with sinkholes. Sinkholes may develop when acidic water starts to dissolve the surface of bedrock or bedding planes. The dissolution characteristics of limestone play a crucial role in the development of sinkholes. The main objective of this study was to develop a quantitative tool for evaluating the dissolution ability for sinkhole formations in the karst regions of Guizhou. The present study investigates a number of properties of limestone that may influence the dissolution in the karst regions of Guizhou. Over 200 rock samples were collected and tested to determine the CaO/MgO ratio, clast content, clastic/matrix ratio, primary porosity, permeability, and fracture density. A quantitative approach was developed to synthesize the influences of these multiple factors to establish the order of the limestone solubility in different regions of Guizhou via numerical modeling using the analytical hierarchy process (AHP) theory. It was found that the solubility of the karstic limestone from different geological formations was in the following descending order: Carboniferous Huanglong-Maping Formation, Permian Qixia-Maokou Formation, Cambrian Qingxudong Formation, and Triassic Yongningzhen Formation. As the water-bearing capacity of the geological formations in these regions is closely associated with underground cavern and sinkhole formations, the approach explored in the present study may have strong implications for the exploration and utilization of karst groundwater resources.

Modelling Ettringite Induced Heaving in Cement Stabilized Soils within a Chemo-mechanical Constitutive Framework

Several recent construction projects in Northeast Ohio have experienced significant sulfate induced heaving in cement stabilized soils. It is well known that the reaction between the sulfate and alumina in soils and calcium of a lime or cement stabilizer often leads to the formation of ettringite, an expansive sulfate mineral that may contribute to the expansion. This subject has been extensively researched in the past but most of the studies have been focused on the experimental aspects. This paper presents a numerical approach to understanding the observed heaving in stabilized soils. A chemo-mechanical constitutive model is employed to address the chemically induced swelling strain. Kinetics of gypsum water reaction is used to explore the evolutionary processes involved. A number of important studies have shown that multiple mechanisms may be involved in addition to formation of ettringite. Consequently, in the present study numerical simulations are performed to explore different scenarios characterized by different predominant mechanisms. The results have shown that ettringite formation and osmosis swelling can act as separate dominant mechanisms or play comparable roles in this heaving process.