Automatic calibration of the SANISAND parameters for a granular material using multi-objective optimization strategies

peer reviewedThe parameter calibration of a constitutive model is a requisite to counter the uncertainty in the parameters and to approximate the simulation results effectively. Yielding a robust set of parameters for various test conditions is complicated as innumerable parameter combinations have to be investigated. In previous works, this calibration has been performed manually by trial and error without checking the robustness of the chosen parameters. Therefore, the present study introduces an automated calibration procedure using multi-objective optimization techniques. This assists in searching the parameter domain space extensively for better combinations that simulate the experiment results precisely. Though this approach is quite popular in various other engineering aspects, proposing the concept of calibrating the soil parameters and validating their efficiency has been always a challenge and interesting in this framework. In this research, SANISAND model parameters have been calibrated for crushed glass material under different triaxial conditions considering the barotropy, and pycnotropy effects. The results demonstrated that the optimized SANISAND parameters approximated the experiment results far better than manually calibrated results. This calibration approach facilitates in conserving the robust parameters besides dealing with time constraints and motivates the idea of adapting this automation platform to any constitutive model for significant approximations

Micro and macro mechanical assessment of small and intermediate strain properties of granular material

In dieser Arbeit wird der Einfluss des Feinkornanteils, fc, und der spannungsinduzierten Anisotropie auf die Steifigkeit und das Dämpfungsverhalten von granularen Materialien bei kleinen und mittleren Dehnungszuständen auf makroskopischer und mikroskopischer Ebene betrachtet. Ziel ist es für die unterschiedlichen Ausgangs- und Randbedingungen (fc und anisotroper Spannungszustand) Ansätze zu entwickeln, die maßgebenden Steifigkeiten aus möglichst wenig Versuchen zu bestimmen. Entsprechend gliedert sich die Arbeit in zwei Teile. Im ersten wird der Einfluss des fc betrachtet, im zweiten der Einfluss der spannungsinduzierten Anisotropie. Zu (1) werden zunächst Resonant Column Versuche an Huston Sand durchgeführt, wobei der Einfluss des Spannungszustandes und der Porenzahl auf G($\gamma$) und $\eta$($\gamma$) erfasst wird. Zu (2) wurde das vorhandene RC-Gerät zunächst um eine Belastungseinrichtung zum Aufbringen zusätzlicher Vertikalspannungen und damit anisotroper Spannungszustände auf die Probe erweiter

Plane strain shear strength of unsaturated fiber-reinforced fine-grained soils

Discrete randomly distributed fibers are commonly used to improve the engineering characteristics of the soil and thus soil properties such as shear strength, compressibility, density, and hydraulic conductivity. Most studies have so far focused on describing the behavior of soils containing randomly distributed fibers under dried or saturated conditions. However, the water table may seasonally fluctuate, thus generating unsaturated soil conditions. Therefore, a better understanding of the hydro-mechanical properties of unsaturated improved soils is of high necessity. In this research, the shear strength parameters of fine-grained soils were evaluated using the biaxial device available at Ruhr Universität Bochum. The applied device was modified to test unsaturated fine-grained soils with various degrees of saturation using axis translation and vapor equilibrium techniques. The experiments were conducted on fine soils containing 0, 0.5, and 1% fiber contents under a wide range of matric suctions. The ductile behavior was more noticeable in samples with lower suctions and higher straw contents. Furthermore, the shear strength of both unreinforced and reinforced fine-grained soils considerably increased by an increase in the suction. Finally, shear band inclination increased by the suction while decreasing by straw content.Ruhr-Universität Bochum (1007

Editorial for Special Issue “Fractal and Fractional in Geomaterials”

Geomaterials, such as clay, sand, rockfill and ballast, etc [...

Influence of surcharge on cone penetration test results and the inspection of various approaches for capturing its effect: a case study

Abstract Studies in recent decades demonstrate the significant effect of stress configuration (e.g., vertical stress and lateral confinement) on the shear strength or, in this study, the cone penetration test (CPT) results. Addition of a surcharge over the ground changes the stress condition, and consequently, the CPT tip resistance. In this study, the results of different CPTs conducted before and after backfilling with various thicknesses in a land development project were reviewed while focusing on the trend of an increase in CPT penetration resistance due to the additional surcharge. Both pre- to post-fill stress ratios and soil type affect the rise in corrected $${q}_{c}$$ q c values after backfilling. Moreover, there has always been a sudden increase in $${q}_{c}$$ q c values around the pre-fill surface in all studied cases. In this study, another approach was derived from the reanalysis of CPT data from a specific site for predicting the post-fill corrected $${q}_{c}$$ q c from pre-fill results by considering the above-mentioned factor. Likewise, post-fill results were predicted by depth-normalized pre-fill CPT results using Robertson’s normalization method. The proposed approach in this study showed a better match with the site data compared to the normalization method, especially at and around the pre-fill surface

Influence of plastic fines content on the liquefaction susceptibility of sands: monotonic loading

The paper presents an experimental study on the effect of plastic fines content on the undrained behavior and liquefaction susceptibility of sand–fines mixtures under monotonic loading. The results of undrained monotonic triaxial compression tests conducted on mixtures of Hostun sand with varying amount (0–20%) and type (kaolin and calcigel bentonite) of plastic fines are presented. The specimens were prepared with different initial densities using the moist tamping method and consolidated at two different isotropic effective stresses. The results demonstrate that for both types of plastic fines, an increase in the fines content leads to a more contractive response and lower values of mobilized deviatoric stress. Despite similar relative density and fines content, the sand–kaolin mixtures showed a more contractive behavior than the sand–calcigel specimens. The steady-state lines (SSLs) in e–p´ space generally move downwards with increasing clay content. While the slopes of the SSLs for the clean Hostun sand and the mixtures with 10 and 20% kaolin are quite similar, the SSL lines for the specimens containing 10% or 20% calcigel run steeper or flatter, respectively. The inclination of the SSL in the q–p′ plane was found independent of clay type and content. The sand–kaolin mixtures were observed to be more susceptible to instability and flow liquefaction than the sand–calcigel mixtures

Macro- and micromechanical assessment of the influence of non-plastic fines and stress anisotropy on the dynamic shear modulus of binary mixtures

Resonant column tests were carried out on Hostun sand mixed with 5%, 10% and 20% non-plastic fines (defined as grains smaller than 0.075 mm) in order to quantify the combined influence of the void ratio $\it (e)$, anisotropic stress state (defined as $\sigma_v$'/$\sigma_h$') and fines content $(f_c)$ on the maximum small-strain shear modulus $G_{max}$. A significant reduction in the $G_{max}$ with increasing $f_c$ was observed. Using the empirical model forwarded by Roesler, the influence of e and $\sigma_v$'/$\sigma_h$' on $G_{max}$ was captured, although the model was unable to capture the influence of varying fines content using a single equation. From the micro-CT images, a qualitative observation of the initial skeletal structure of the 'fines-in-sand' grains was performed and the equivalent granular void ratio $\it e*$ was determined. The e was henceforth replaced by $\it e*$ in Roesler's equation in order to capture the variation in $f_c$. The new modification was quantified in terms of the mean square error $R^{2}$. Furthermore, the $G_{max}$ of Hostun sand–fine mixtures was predicted with good accuracy by replacing e with $\it e*$. Additionally, a micromechanical interpretation based on the experimental observation was developed

Anisotropic stress state and small strain stiffness in granular materials: RC experiments and DEM simulations

The paper combines experimental and numerical analyses to study the relation between small strain stiffness and micro-structure of an idealized granular material under isotropic and anisotropic stress conditions. Results from the resonant column device on glass ballotini show that the relation between the maximum shear modulus and anisotropic stress components strongly depends on the applied stress path. Discrete element simulations (DEM) are performed to investigate the material behaviour along isotropic compression, triaxial compression and constant K deformation. The DEM analysis reveals that each stress path is associated with a characteristic evolution of the coordination number, i.e., the average number of contacts per particle. In turn, the maximum shear modulus is found to be a direct function of the coordination number. In order to include the micro-structure interpretation in the analytical description, a modified version of Hardin’s relation is proposed as a function of coordination number, void ratio and mean pressure