Experimental study of the stress-strain behavior of unsaturated silty sand

نوع: Type: thesis

مقطع: Segment: PHD

عنوان: Title: Experimental study of the stress-strain behavior of unsaturated silty sand

ارائه دهنده: Provider: Maziyar Abdi

اساتید راهنما: Supervisors: Dr. mohammad maleki

اساتید مشاور: Advisory Professors:

اساتید ممتحن یا داور: Examining professors or referees: Dr. makarchiyan & Dr. ardakani & Dr. biglari

زمان و تاریخ ارائه: Time and date of presentation: 30/9/2023

مکان ارائه: Place of presentation: Amphitheater of Faculty of Engineering

چکیده: Abstract: In many parts of the world, especially Iran, a significant portion of soils naturally or due to excessive groundwater use are unsaturated. This means that the majority of civil engineering activities and structures are performed on unsaturated soils. Unsaturated soil includes three phases water, solid particles, and air. The presence of air invalidates many saturated state equations for stress-strain behavior prediction of unsaturated soil. The interaction of these phases affects shear strength, volume change, permeability, and other behavioral aspects of the soil. Since the studies on the mechanical behavior of unsaturated soils have been developed with a delay compared to the saturated state, many aspects of the behavior of these soils have not yet been investigated. Based on the literature review the mechanical behavior of unsaturated soils is affected by diverse factors such as matric suction, confining pressure, loading path, and drying and wetting paths. However, very few laboratory studies have been conducted on how the composition of the solid part affects the mechanical behavior. In this context, silty sand soils have a special importance due to the strong sensitivity of the sand to the percentage of silt and so far, it has been less investigated in an unsaturated state. In the saturated state, silty sand soils exhibit different behaviors depending on fine content, initial compaction, initial stress state, applied stress path, particle type, and geometry. Comprehensive experimental studies have been conducted in saturated conditions on these soils, leading to a clear understanding of their behavior. The definition of concepts such as the threshold fine content and the equivalent intergranular void ratio has led to a homogenization of the behavior of these soils in the saturated state. Given the presence of the matric suction, the validity of the equivalent intergranular void ratio and the state parameter in the unsaturated state for silty sand soils has not been investigated. Therefore, the main focus of this research is to investigate the simultaneous effect of matric suction and silt content on the mechanical behavior of silty sand and homogenizing shear strength by application of equivalent intergranular void ratio and normalized mean effective stress in terms of matric suction. In the following, the application of a homogenized critical state line by considering a novel definition for state parameters was investigated for describing the stress-dilatancy relationships of the constitutive models. For this purpose, a set of drained and undrained double-cell triaxial tests were carried out in different values of fine contents, matric auctions, confining pressures, and densities. The soil used in this research was a combination of the Shooshab sand of Malayer in Hamedan province and stone powder as non-plastic fine. According to the main objective of this study, which is to investigate the effects of soil composition and suction on the shear strength behavior of sand-silt mixtures in the unsaturated state, it was tried to create different compositions by changing the silt to sand ratio. The tests were performed on the samples composed from sand and 0, 10, 20, and 30% non-plastic fines at the confining pressures of 50, 100, 150, and 200 kPa and matric suctions of 0, 100, 150, and 200 kPa. One of the main goals of this research is to investigate the application of the equivalent intergranular void ratio in unsaturated conditions. For this purpose, a set of samples with the same eeq were molded using sand and silt from the Farhangian town in Hamedan. These tests were carried out on sand with 0, 10, and 15% fines, at confining pressure of 50, 100, and 150 kPa, and matric suctions of 0, 50, 100, 150, and 250 kPa. Critical state lines were first piloted in both strength and volumetric change planes, and in both net stress and effective stress variables to achieve the research objectives. Critical state lines were parallel to those of the saturated state in the strength plane in terms of net stress. The results show that intercept increases while matric suction is increased. Also, the intercept increases with increasing fine content at a specific matric suction. The reason for this increase is that with increasing fine content and compaction, the voids in the soil decrease; therefore, the effect of suction increases. However, using an appropriate effective stress relationship, it is possible to obtain a unique critical state line at a specific fines content in the q-p' plane. This line depends on the fines content, and with increasing fines content, the slope of the lines increases. In the ecr-lnpnet plane, the critical state lines depend on matric suction. In clean sand, the slope of the critical state line increases with increasing matric suction. The intercept of these lines is also a function of suction. In sand with 10% fine content, the slope and intercept of the lines have a specific trend with changes in suction. In sand with 20% fine content, the slope of the lines first decreases with increasing suction and then increases. However, the distance from the origin always increases with increasing suction. Unlike the q-p' plane, in the ecr-lnp' plane, it is impossible to reach a unique critical state line for a specific fines content and at different suctions using the concept of effective stress. For this purpose, the mean effective stress normalization method proposed by Gallipoli et al., (2003) was used in this study. It was observed that by using this method, a unique critical state line could be defined for a specific fines content at different suctions. Since the critical state lines change depending on fine content, the concept of equivalent intergranular void ratio can be used to eliminate this dependence. In the application of equivalent intergranular void ratio expression, it was observed that the best method for calculating parameter b is to use the back analysis method. In some bounding surface constitutive models, the state parameter controls the relationship between stress and dilatancy. It was observed that the relationships proposed for the saturated state appropriately express the volume change behavior in unsaturated conditions. Therefore, the application of saturated state stress-dilatancy relationships for unsaturated state does not affect their efficiency. In the following, based on a unique critical state line obtained for different fine contents and matric suctions the efficiency of state parameters in stress-dilatancy relationships was evaluated. As a practical conclusion, by using ѱeq instead of ѱ, the effect of the percentage of fines is introduced through the equivalent intergranular void ratio, so that, the stress-dilatancy relationship of the constitutive model for different fine contents is identified by calibrating on only one percentage of fine

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