Experimental Study on Stress- Dilatancy Behavior of Sand- Silt Mixtures and Validation of Related Constitutive Models

نوع: Type: Thesis

مقطع: Segment: PHD

عنوان: Title: Experimental Study on Stress- Dilatancy Behavior of Sand- Silt Mixtures and Validation of Related Constitutive Models

ارائه دهنده: Provider: Erfan Chali

اساتید راهنما: Supervisors: Dr. Mohammad Maleki

اساتید مشاور: Advisory Professors:

اساتید ممتحن یا داور: Examining professors or referees: Dr. Ali Pak Dr. Vahid Reza Ohadi Dr. Alireza Bagherieh

زمان و تاریخ ارائه: Time and date of presentation: 2025

مکان ارائه: Place of presentation: اتاق مجازی

چکیده: Abstract: The stress-dilatancy behavior of sandy soils is a key factor in analyzing their mechanical properties and must be incorporated into constitutive models through mathematical relationships. Sandy soils often contain varying amounts of fine particles, which significantly influence their engineering properties. To better describe this effect, the equivalent intergranular and interfine void ratio parameters have been introduced to facilitate the correlation between soil properties. However, analyzing stress-dilatancy behavior in unsaturated mixtures is complex, as both fines content and matric suction impact the soil’s response. Due to the limited research in this area, the combined effect of fines and suction in these mixtures remains largely unknown. Therefore, this study aims to comprehensively analyze the stress-dilatancy behavior of unsaturated sand-silt mixtures by identifying and examining key states, including the peak state, transitional phase, and critical state. Additionally, a constitutive model has been developed to account for the simultaneous effects of fine content and suction in analyzing the behavior of these mixtures. This research consists of two main components: experimental investigations and the development of a constitutive model, with the goal of examining the behavioral and physical characteristics of sand samples under various stress and suction conditions. In the experimental phase, sand samples with different fines contents (0%, 10%, 30%, 40%, 60%, 80%, and 100%) were prepared. Firouzkuh sand was used as the coarse-grained fraction, while rock powder was used as the fine fraction. After sample preparation, a series of mechanical tests were conducted to analyze their behavior under different stress and suction conditions. These tests included pressure plate and triaxial tests performed under both saturated and unsaturated conditions to determine strength properties and nonlinear behavior under varying pressure and stress levels. The triaxial tests were conducted at confining pressures of 50, 100, 150, and 200 kPa, with suction levels of 0, 15, 30, and 60 kPa. The test results demonstrated that soil-water retention curves are highly dependent on fines content. At a given suction, the degree of saturation increases with higher fines content, indicating that fines enhance water retention capacity. Additionally, both the air entry value and residual suction increase with increasing fines content. The lowest values of these parameters were observed in clean sand, while the highest values corresponded to pure silt, demonstrating that, in addition to void ratio, soil texture directly influences the soil-water retention curve. Using the proposed relationship and considering the characteristic soil-water curve of sand with a threshold fines content as a reference curve, a unique characteristic curve was obtained for conditions where fc ≤ fcth and fc > fcth. The results of the triaxial tests were analyzed across three key states: transitional phase, peak state, and critical state. The findings revealed that critical state lines are influenced by both fines content and suction, causing data dispersion that complicates soil behavior analysis. To address this issue, various normalization approaches were applied, allowing the experimental data to converge into a reference critical state line. This enabled the development of unified relationships for analyzing the mechanical behavior of sand-silt mixtures. By utilizing normalized critical state lines based on suction and fines content, the strength values at transitional and peak states could be expressed as a function of an equivalent state parameter. Analysis of stress-dilatancy curves showed that, beyond fines content, both suction and confining pressure significantly influence volumetric changes and sample strength. The use of the equivalent state parameter in the stress-dilatancy relationship maintained its effectiveness, enabling the volumetric behavior of all sand-silt mixtures to be described using a single set of parameters. To develop the constitutive model, the Dafalias and Manzari (2004) model, originally designed for saturated conditions, was modified for unsaturated conditions. This development involved defining effective stress, idealizing the soil-water characteristic curve, and normalizing the critical state line. The performance of the developed model was then compared with both laboratory data and the Barcelona Basic Model (BBM), implemented in Plaxis software. The comparison revealed that the developed model accurately simulates strength behavior and volumetric changes under both drained and undrained conditions. While the BBM lacks the ability to predict sample failure and softening behavior, the developed model successfully captures these aspects. The maximum deviation of the developed model from experimental data was ±5% for ultimate strength variations and ±10% for final volumetric changes, whereas the BBM exhibited errors ranging from -10% to -15% for strength prediction and -120% to -30% for volumetric changes. With the applied normalizations, the model was optimized to describe the behavior of unsaturated sand-silt mixtures effectively. One of the key advantages of this model is its ability to determine base model parameters for a given sand-silt mixture and a specific matric suction, which can then be generalized to other mixtures and suction levels. This capability arises from the use of the equivalent void ratio concept and the normalization of relationships based on matric suction and fines content. Finally, the results demonstrated that the developed model accurately simulates various aspects of soil behavior, including dilatancy, peak shear strength, transitional phase, and critical state. This research provides a comprehensive approach to understanding and modeling the stress-dilatancy behavior of unsaturated sand-silt mixtures, offering valuable insights for geotechnical engineering applications.