Selim Altun

Soil clustering by fuzzy c-means algorithm

In this study, hard k-means and fuzzy c-means algorithms are utilized for the classification of fine grained soils in terms of shear strength and plasticity index parameters. In order to collect data, several laboratory tests are performed on 120 undisturbed soil samples, which are obtained from Antalya region. Additionally, for the evaluation of the generalization ability of clustering analysis, 20 fine grained soil samples collected from the other regions of Turkey are also classified using the same clustering algorithms. Fuzzy c-means algorithm exhibited better clustering performance over hard k-means classifier. As expected, clustering analysis produced worse outcomes for soils collected from different regions than those of obtained from a specific region. In addition to its precise classification ability, fuzzy c-means approach is also capable of handling the uncertainty existing in soil parameters. As a result, fuzzy c-means clustering can be successfully applied to classify regional fine grained soils on the basis of shear strength and plasticity index parameters.

Cyclic Stress-Strain Behavior of Partially Saturated Soils

Although soils are generally assumed fully saturated below the groundwater table, they may be semi saturated near the state of full saturation under certain conditions. The situation of partial saturation may be caused by several factors, such as variation of water table level due to natural or manmade processes. It has been known in geotechnical engineering that the partial saturation of soil usually gives higher strength and lower compressibility. Additionally, it was shown in the recent studies that unsaturation of soil can also cause much greater amplification in ground motion then a fully saturated model and more resistance to liquefaction of sandy soils. Moreover, the resistance of sand to onset of liquefaction tends to increase with a reduction in the saturation ratio of soil specimens that was expressed in terms of the Skemptons pore pressure coefficient, B. Since the B-value is defined as the ratio of the induced pore water pressure to the applied effective confining stress, it can be simply obtained in the laboratory. The Skempton B-value method has been widely used to determine the state of saturation of laboratory soil specimens. In this study, in order to examine the stress-strain behavior and the strain softening response of partially saturated cohesive soil, a cyclic torsional shear apparatus were used, in which it was possible to perform cyclic shear tests on the specimens under undrained conditions. A series of the shear tests were conducted on the partially saturated-undisturbed clay samples. Therefore, the stress-strain properties were investigated to identify the saturation degree of near-saturated samples. Undisturbed clayey specimens were subjected to multi-staged cyclic torsional shear stress, and deformation characteristics of soils, such as the variations of the shear modulus and damping ratios were evaluated in terms of the saturation ratio. For the cyclic phase of torsional tests, the degradation curves were shown to move the right side with the decreasing Skempton B-value. It was also observed that initial values of the shear modulus were considerably affected the Skempton B-value.

Dependence of dynamic shear modulus of uniform sands on stress level and density

The first part of this study is a description and analysis of experimental investigations conducted to determine the nonlinear stress–strain behaviors of uniform sands when subjected to cyclic loadings at small strain levels. A series of torsional shear tests are performed under undrained conditions to analyze the variations in the stress–strain properties of saturated uniform sands consolidated to specified confining stresses and with certain relative densities. The role of effective confining pressure on the cyclic properties of isotropically consolidated sands is also considered. The second part of the study uses nonlinear multiple regression, neural network, and adaptive neuro-fuzzy inference methodologies to identify the complex mappings that range from void ratio and confining pressure input spaces to dynamic shear modulus output space. Test results and model comparisons are presented and evaluated in detail.

EFFECT OF FRACTAL DIMENSION ON THE STRAIN BEHAVIOR OF PARTICULATE MEDIA

In this study, the influence of several fractal identifiers of granular materials on dynamic behavior of a flexible pavement structure as a particulate stratum is considered. Using experimental results and numerical methods as well, 15 different grain-shaped sands obtained from 5 different sources were analyzed as pavement base course materials. Image analyses were carried out by use of a stereomicroscope on 15 different samples to obtain quantitative particle shape information. Furthermore, triaxial compression tests were conducted to determine stress–strain and shear strength parameters of sands. Additionally, the dynamic response of the particulate media to standard traffic loads was computed using finite element modeling (FEM) technique. Using area-perimeter, line divider and box counting methods, over a hundred grains for each sand type were subjected to fractal analysis. Relationships among fractal dimension descriptors and dynamic strain levels were established for assessment of importance of shape descriptors of sands at various scales on the dynamic behavior. In this context, the advantage of fractal geometry concept to describe irregular and fractured shapes was used to characterize the sands used as base course materials. Results indicated that fractal identifiers can be preferred to analyze the effect of shape properties of sands on dynamic behavior of pavement base layers.

Evaluation and use of clustering algorithms for standard penetration test data classification

Abstract The standard penetration test (SPT) is the most common test conducted in the field, and it is used to determine in situ properties of different soils. Although it is a matter of debate, these tests are also used for the determination of the consistency of fine-grained soils, whereby the test results can also be utilized to establish numerous empirical correlations to predict the strength of soils in the field. In this study, unsupervised clustering algorithms were employed to classify the SPT standard penetration resistance value (SPT-N) in the field. In this scope, shear strength and liquidity index parameters were used to classify the SPT-N values by taking the classification system of Terzaghi and Peck (1967) into consideration. The results showed that the input parameters were successful for classifying the SPT-N value to an acceptable degree of strength attribute. Therefore, in cases where the SPT tests are unreliable or could not be performed, laboratory tests on undisturbed specimens can give valuable information regarding the consistency and SPT-N value of the soil specimen under investigation. Data in this study is based on several tests that were conducted in a region; nevertheless, it is advised that the results of this study should be evaluated using global data.

NONPLASTİK SİLT KUM KARIŞIMLARINDA BOOTSTRAP YÖNTEMİ İLE BOŞLUK SUYU BASINCI OLUŞUMUNUN DEĞERLENDİRİLMESİ

The aim of this paper is to evaluate the applicability of the available three different models to predict excess pore water pressure generation in non-plastic silty sand mixtures during cyclic loading. Cyclic triaxial tests were performed to investigate the effect of silt content on the pore pressure generation in sand. These tests were carried to 200 cycles or to onset of initial liquefaction, whichever occurred first. Several stress-controlled cyclic triaxial tests were performed to measure excess pore water pressure generation at different levels of cyclic stress ratios for the specimens prepared at six different silt contents (FC=0% to 100%). The specimens were tested under 100 kPa confining pressures at two relative densities of 25% and 50%. Results of these tests were used to investigate the behavior of silty sands under undrained cyclic triaxial testing conditions. Seed et al. [3], Booker et al. [2] and Polito et al. [8] pore water pressure generation models based on test results are also presented in this paper. An attempt to estimate the pore pressure model coefficient a as a function of silt content, relative density, cyclic stress ratio was made. Keywords: Pore water pressure, non-plastic silt-sand mixtures, cyclic triaxial tests, bootstrap method

The Liquefaction Behavior of Poorly Graded Sands Reinforced with Fibers

This study focuses on the performance of fibers, improving the resistance to liquefaction in loose sands, medium sands, and dense sands in Izmir, Turkey. A systematic testing schedule consisting of cyclic triaxial tests was held under stress-controlled and undrained conditions on saturated sand specimens with and without fiber reinforcements. The major parameters having effects on the dynamic behavior such as fiber content, fiber length, and relative density on the liquefaction behavior and the excess pore water pressure developments of specimens with and without fibers were investigated. If the fiber content or the fiber length was increased in the specimens, higher number of loading cycles was needed in order to experience the liquefaction of sands. The reinforcement effect in medium-dense specimens was found to be apparently distinctive compared to loose specimens. The curves of pore water pressures and shear strains were achieved for the fiber-reinforced sands. The boundaries of pore water pressure curves presented in the literature on the clean sands were utilized in comparison with the pore water pressure curves of fiber-reinforced sands of this study. As a conclusion, the results presented in this study are useful to develop insight into the behavior of clean and fiber-reinforced sands under seismic loading conditions. Based on the test results, it was found that the number of loading cycles had a strong impact on the excess pore pressure generation.

The Variation of Pore Pressure Related with Failure in Fine-Grained Soils Under Uniform Cyclic Loadings

This paper discusses pore- water pressure and failure in fine-grained soils and the influences of some factors under cyclic loading. A systematic study was conducted on pore-water pressure development during cyclic failure of low - to - medium plasticity clays. A series of cyclic triaxial and cyclic torsional tests were performed on undisturbed samples taken below the water table from various sites in Turkey. Consolidation tests were conducted to estimate preconsolidation pressures for each specimen. Tests were conducted under isotropic conditions in both test systems. The loadings were stopped at a prescribed strain level of ±2.5-3% assumed as failure. Pore-pressure accumulation during the tests was observed and pore-pressure values at the time of failure were analyzed with respect to different soil parameters. In addition to the other factors such as cyclic shear stress and cyclic strain amplitude, pore pressure-accumulation varies significantly with the mineralogy of the soil sample.