LEE- Zemin Mekaniği ve Geoteknik Mühendisliği-Doktora
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Sustainable Development Goal "Goal 9: Industry, Innovation and Infrastructure" ile LEE- Zemin Mekaniği ve Geoteknik Mühendisliği-Doktora'a göz atma
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ÖgeA new approach in studying the engineering behavior and mechanical properties of artificial bonded soils in the laboratory(Graduate School, 2022-01-31) Ricardo, Richard Vall Ngangu ; Lav, Musaffa Ayşen ; 501142303 ; Soils Mechanics and Geotechnical EngineeringThe construction of structures on structured soils or the exploitation of such materials for construction purposes, such as in road pavement projects, has gained more importance with time. In some parts of the world, their study has become a necessity. Such soils, like residual soils, are widely encountered in tropical and subtropical regions. Even though their names may vary according to local culture or their morphology, they have all in common the bond structures. This property is a key parameter of those soils. However, to better study their behavior, the use of the artificial bonded sample in the laboratory has been adopted, offering an effective simulation. In the present study, the behavior of residual soil-like has been investigated under undrained conditions in triaxial equipment by using a large number of artificial samples made in the laboratory. The artificial bonded and unbonded samples were made from a mixture of sand, kaolin, and water. A thermal process was applied for the bonded specimens, whereas the unbonded samples were not fired. A preliminary investigation was carried out on four different particle size distribution curves. In those gradation curves, the dry ratio of kaolin/sand, and the kaolin particle size distribution paths, were kept the same, only the sand grain size distribution was varied. The study was conducted on the chosen best-fitted gradation curve of sand-kaolin. Besides the triaxial tests, direct shear box apparatus was also used, for comparative purposes. For every type of the tested material, three different initial effective confining pressures or normal stresses were applied. Throughout this process, five different bonding levels were used. Several properties of such soils were examined, among them: the stress-strains, the pore water pressure evolution, the stress ratio, other strength parameters, and so on. The equivalent artificial bonded specimens, but in an unbonded state, were used to gain a better understanding of their mechanical characteristics. A novel approach was investigated and established, based on a new parameter called bonding index (B_i). This parameter was set from the bounding surface, which is one of the most important features of bonded soils studied under triaxial tests. The proposed method was evaluated as an effective and practical one. The strength parameters of the bonded soils such as the cohesion intercept, the angle of internal friction, the peak strength, and the stress ratio, were found to be straightly related to B_i. The latter asserted well the enhancement of bonding. Furthermore, B_i would be used to define the confining stress level, from which a B_i close to zero value implies the highest stress level for the artificial bonded soils. However, independent of the stress level, all unbonded soils display a B_i equal to zero value. The coupled effect of B_i and the confining pressure was grouped in three main stages. The first stage, at lower confining stresses, where a remarkable high value of B_i is recorded. The second stage is a step of moderate stress and, the third stage, as where the smallest B_i value was observed. Every stage was associated with a particular behavior of those soils according to the bonding level in presence. It is worth pointing out that a soil sample of higher B_i was found to be less ductile. The suggested method was observed to be an appropriate alternative means for the geotechnical evaluation and analysis of the behavior of structured soil materials. Comparison from the results of both CIU tests and DST revealed a good agreement for weakly and unbonded samples, particularly for strength parameters, the cohesion intercept, and the angle of internal friction. However, for highly bonded materials important divergence was observed, with an overestimation from the DST results. A study of the debonding process was carried out through a new approach. This method was constructed from the deviatoric stress increment (∆q) against the axial strain (ε_a) curves, drawn in a natural scale. Six important features, points, were found to be typical of bonded soils, while only two of them were observed for unbonded samples. The first yield was identified at the initial point, after which the slope of ∆q decreased significantly coupled with the maximum pore water pressure increment 〖d∆u〗_max. This point revealed the debonding process starting point. The second point is at 〖∆q〗_max, at the second yield, a point of major loss of strength. The third and fourth points were at d∆u=0 and ∆q=0 (q_max), respectively; while the fifth point was identified as where 〖∆q〗_min. The last point was at the critical state or the equivalent state. Every point represented a particular behavior state of bonded soils. Throughout the study, it was observed that confining pressure influences considerably the response of bonded soils. For example, the aforementioned six features, specific to bonded soils, were found to be reduced to only two points, particularly for weakly and moderately bonded materials, with the increase of σ_3 from 30 kPa to 700 kPa. Furthermore, a bigger value of the bonding index was achieved at lower confining stress. Therefore, it is recommended, for a better understanding of the behavior of the bonded soil materials, to conduct such investigations at lower initial effective stress, especially for the analysis of the debonding process.
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ÖgeDevelopment of lateral load resistance-deflection curves for piles in cohesionless soils under earthquake excitation(Graduate School, 2023-02-16) Alver, Ozan ; Bayat, Esra Ece ; 501152305 ; Soil Mechanics and Geotechnical EngineeringPile foundations must be designed safely to withstand the lateral loads such as wave loads and seismic loads in offshore/onshore structures, seismic loads in bridges, buildings, port structures etc. The most common analysis method for the design is the Winkler spring approach. Researchers have suggested nonlinear formulations for the lateral load resistance-deflection (p-y) curves, but the contribution of the degree of soil nonlinearity was not studied thoroughly. The main drawback of the current approach is the use of a single stiffness in considering the soil nonlinearity. This study investigates the laterally loaded pile problem using the pressure-dependent hardening soil model with small-strain stiffness (HS-Small Model), where the degree of soil nonlinearity is better integrated. The numerical model was created, and parametric analyses were carried out on the verified model for various pile and soil properties. A modified hyperbolic model was proposed for static p-y relation, including the initial stiffness, ultimate soil resistance, and degree of nonlinearity parameters based on the numerical analysis results. The validity of the model was shown by simulating the field and centrifuge tests from the literature. The proposed model agrees with the test results in the variation of bending moment along the pile. Besides, a significant enhancement was provided in the estimation of pile deflections. Therefore, the proposed model with four parameters can more precisely consider the soil nonlinearity from very small to large displacements. The proposed p-y curves can be utilized in the design of piles subject to static lateral loading. The analysis of dynamic soil-pile interaction problems requires the relation of soil resistance to lateral loading that is represented by nonlinear p-y curves in the beam on the nonlinear Winkler foundation (BNWF) approach. Current methods for p-y curves are either based on static load tests or cannot accurately consider the dynamic soil nonlinearity. This study investigates the dynamic soil-pile interaction in cohesionless soils by numerical analyses to better characterize the p-y curves considering the nonlinear soil behavior under dynamic loading. A numerical pile-soil-structure model was created in FLAC3D and verified by two centrifuge tests published in the literature. The parametric analyses were performed to obtain the p-y curves for various pile diameters, soil relative densities, and degrees of nonlinearities. Based on the parametric analyses, a mathematical model was proposed for the dynamic p-y curves for cohesionless soils. The proposed model characterizes the backbone of dynamic p-y curves based on the three leading parameters (initial stiffness Kpy, ultimate resistance pu, and degree of nonlinearity n). The numerical analyses showed that the p-y curve nonlinearity mainly depends on the employed modulus reduction curves of soils. In the model, the degree of nonlinearity parameter (n) was directly related to the soil parameter "reference strain" (r), which solely represents the modulus reduction curve of soils. In this regard, the dependence on various dynamic soil parameters was diminished by correlating the dynamic p-y curves to the reference strain. The validation analyses performed in structural analysis software demonstrated that the proposed dynamic p-y model could accurately estimate the pile and structure response under earthquake loading by incorporating the hysteretic nonlinear soil behavior. Superstructure accelerations and bending moments along the single pile obtained using the proposed model under different earthquake records were closer to the 3-dimensional numerical analysis results when compared with the results calculated by API. Finally, the proposed static and dynamic p-y models will contribute to the design of piles by improving the initial stiffness, ultimate resistance and nonlinearity of the static load-displacement behavior and by integrating the dynamic soil nonlinearity and hysteretic behavior under directly applied seismic loads.