The effect of microbial calcite precipitation on the retention properties of unsaturated fine-grained soils: discussion of the governing factors

In recent years, biogeotechnology has been introduced as a novel and environmentally friendly technique for soil improvement. The need to address global warming and the adverse environmental effects of the chemical additives have led to the emergence and development of the techniques which use calcite producing microorganisms in order to improve soil mechanical properties. While the effects of microbial induced calcite precipitation (MICP) on the hydraulics and mechanics of saturated coarse-grained soils have been well examined and studied, there is not yet much information on the effects these microorganisms would have on the unsaturated soil mechanical behaviour. The first step, in this regard, is to understand the effect of the processes involved in the MICP on the soil retention properties. Soil water suction is a key factor controlling soil hydraulic and mechanical behaviour. In this study, the influence of MICP on the soil total suction in an unsaturated fine-grained soil sample has been explored using filter paper experiment. The results of this study revealed that by increasing the amount of bacterial solution, the soil saturation-total suction curves are significantly affected. The soil water retention changes are attributed to the change in double layer thickness as well as the precipitation of calcite crystals

З книги "Дзеркало від писання божественного", Чернігів 1705 р. (Підготовка до друку, переклад з польської та церковно-слов'янської, коментарі Валерія Шевчука)

In recent years, the effective stress approach has received much attention in the constitutive modeling of unsaturated soils. In this approach, the effective stress parameter is very important. This parameter needs a correct definition and has to be determined properly. In this paper, a thermodynamic approach is used to develop a physically-based formula for the effective stress tensor in unsaturated soils. This approach accounts for the hydro-mechanical coupling, which is quite important when dealing with hydraulic hysteresis in unsaturated soils. The resulting formula takes into account the role of interfacial energy and the contribution of air-water specific interfacial area to the effective stress tensor. Moreover, a bi-quadratic surface is proposed to represent the contribution of the so-called suction stress in the effective stress tensor. It is shown that the proposed relationship for suction stress is in agreement with available experimental data in the full hydraulic cycle (drying, scanning, and wetting)

Thermo-mechanical behaviour of a compacted swelling clay

Compacted unsaturated swelling clay is often considered as a possible buffer material for deep nuclear waste disposal. An isotropic cell permitting simultaneous control of suction, temperature and pressure was used to study the thermo-mechanical behaviour of this clay. Tests were performed at total suctions ranging from 9 to 110 MPa, temperature from 25 to 80 degrees C, isotropic pressure from 0.1 to 60 MPa. It was observed that heating at constant suction and pressure induces either swelling or contraction. The results from compression tests at constant suction and temperature evidenced that at lower suction, the yield pressure was lower, the elastic compressibility parameter and the plastic compressibility parameter were higher. On the other hand, at a similar suction, the yield pressure was slightly influenced by the temperature; and the compressibility parameters were insensitive to temperature changes. The thermal hardening phenomenon was equally evidenced by following a thermo-mechanical path of loading-heating-cooling-reloading

The effect of microbial calcite precipitation on the retention properties of unsaturated fine-grained soils: discussion of the governing factors

In recent years, biogeotechnology has been introduced as a novel and environmentally friendly technique for soil improvement. The need to address global warming and the adverse environmental effects of the chemical additives have led to the emergence and development of the techniques which use calcite producing microorganisms in order to improve soil mechanical properties. While the effects of microbial induced calcite precipitation (MICP) on the hydraulics and mechanics of saturated coarse-grained soils have been well examined and studied, there is not yet much information on the effects these microorganisms would have on the unsaturated soil mechanical behaviour. The first step, in this regard, is to understand the effect of the processes involved in the MICP on the soil retention properties. Soil water suction is a key factor controlling soil hydraulic and mechanical behaviour. In this study, the influence of MICP on the soil total suction in an unsaturated fine-grained soil sample has been explored using filter paper experiment. The results of this study revealed that by increasing the amount of bacterial solution, the soil saturation-total suction curves are significantly affected. The soil water retention changes are attributed to the change in double layer thickness as well as the precipitation of calcite crystals

Seismic response of earth dams considering dynamic properties of unsaturated zone

It is conventionally assumed in the analysis and design of earth dams that the soil located above the phreatic line, i.e. the uppermost seepage flow line, is completely dry. However, there is often an unsaturated flow of water through an unsaturated zone above this borderline and variation in moisture content in this zone results in variation of matric suction throughout this region. Variation of matric suction, in turn, results in variation of effective stresses in this zone. In this research, the seismic response of earth dams in terms of the displacement and acceleration at the crown of the dam as well as the stress distribution in the dam body is investigated. Taking into account the effect of unsaturated zone, a comparison is made to investigate the effect of conventional simplification in ignoring the dynamic characteristics of the unsaturated zone above the phreatic line and the more complicated analysis which includes the unsaturated zone. A function for the soil-water retention curve (SWRC) was assigned to the soil in the unsaturated zone to determine the variation of matric suction in this zone and analyses were made using finite difference software (FLAC). Results are then compared to the conventional method for homogeneous dams. In these analyzes the soil shear modulus was assumed to vary with the mean effective stress both for saturated and unsaturated zones. Among various results, it was notable that the history of crest x-displacement, and acceleration show higher values in models accounting for the unsaturated region. It was attributed to the considerably lower values of damping ratio in the crest region in the unsaturated models

Seismic response of earth dams considering dynamic properties of unsaturated zone

It is conventionally assumed in the analysis and design of earth dams that the soil located above the phreatic line, i.e. the uppermost seepage flow line, is completely dry. However, there is often an unsaturated flow of water through an unsaturated zone above this borderline and variation in moisture content in this zone results in variation of matric suction throughout this region. Variation of matric suction, in turn, results in variation of effective stresses in this zone. In this research, the seismic response of earth dams in terms of the displacement and acceleration at the crown of the dam as well as the stress distribution in the dam body is investigated. Taking into account the effect of unsaturated zone, a comparison is made to investigate the effect of conventional simplification in ignoring the dynamic characteristics of the unsaturated zone above the phreatic line and the more complicated analysis which includes the unsaturated zone. A function for the soil-water retention curve (SWRC) was assigned to the soil in the unsaturated zone to determine the variation of matric suction in this zone and analyses were made using finite difference software (FLAC). Results are then compared to the conventional method for homogeneous dams. In these analyzes the soil shear modulus was assumed to vary with the mean effective stress both for saturated and unsaturated zones. Among various results, it was notable that the history of crest x-displacement, and acceleration show higher values in models accounting for the unsaturated region. It was attributed to the considerably lower values of damping ratio in the crest region in the unsaturated models

A study on the saturation degree dependency of the seismic behaviour of retaining walls

Retaining walls are important geotechnical structures that are often used in soil slopes and trenches to bring ground surface at appropriate level for the construction of roads, highways and buildings. It is common practice to assume that the soil behind a retaining structure is either fully saturated or completely dry. However, for the case the soil is partially saturated, mechanical behaviour of the soil above the water table is different than that of the dry soil. Thus, it is necessary to investigate the effect of the variations of degree of saturation on lateral pressure behind retaining walls. In this research, the seismic behaviour of unsaturated soils behind a retaining structure is analysed. A finite difference code was employed to conduct the necessary analyses and a series of equivalent linear analyses is performed to reveal the effect of the degree of saturation on the general response of the retaining structures. The required functions for unsaturated zone were defined and implemented in the code. For this purpose, a soil water retention function was employed and the soil shear modulus is assumed to vary with the mean effective stress for both saturated and unsaturated zones which naturally introduces the required hydro-mechanical coupling in unsaturated and saturated zone. The results of the analyses compared to the conventional methods which does not include the unsaturated mechanical properties, indicate that in the unsaturated state, the increase in the effective stress, and hence, the shear modulus considerably affects the seismic forces on the retaining wall

A study on the saturation degree dependency of the seismic behaviour of retaining walls

Retaining walls are important geotechnical structures that are often used in soil slopes and trenches to bring ground surface at appropriate level for the construction of roads, highways and buildings. It is common practice to assume that the soil behind a retaining structure is either fully saturated or completely dry. However, for the case the soil is partially saturated, mechanical behaviour of the soil above the water table is different than that of the dry soil. Thus, it is necessary to investigate the effect of the variations of degree of saturation on lateral pressure behind retaining walls. In this research, the seismic behaviour of unsaturated soils behind a retaining structure is analysed. A finite difference code was employed to conduct the necessary analyses and a series of equivalent linear analyses is performed to reveal the effect of the degree of saturation on the general response of the retaining structures. The required functions for unsaturated zone were defined and implemented in the code. For this purpose, a soil water retention function was employed and the soil shear modulus is assumed to vary with the mean effective stress for both saturated and unsaturated zones which naturally introduces the required hydro-mechanical coupling in unsaturated and saturated zone. The results of the analyses compared to the conventional methods which does not include the unsaturated mechanical properties, indicate that in the unsaturated state, the increase in the effective stress, and hence, the shear modulus considerably affects the seismic forces on the retaining wall