The Limitations of Equivalent Linear Site Response Analysis Considering Soil Nonlinearity Properties

Seismic site effect has been a major issue in the field of earthquake engineering due to the large local amplification of the seismic motion. This paper presents the importance of an appropriate soil behavior model to simulate earthquake site response and gives a critical overview of the field of site response analysis. Some of the well known site response analysis methods are summarized and discussed. The objective of this paper is to investigate the influences of nonlinearity on the site response analysis by means of a more precise numerical model. In this respect, site responses of four different types of one layered soil deposit, based on various shear wave velocities, with the assumption of linear and rigid base bedrock, were analyzed by using the equivalent linear and fully nonlinear approaches. Nonlinear analyses’ results were compared with those of the linear method and the similarities and differences are discussed. As a result, it is concluded that, in the case of nonlinearity of soil under strong ground motions, 1-D equivalent linear modeling overestimates the amplification patterns in terms of absolute amplification level, and cannot correctly account for resonant frequencies and hysteric soil behavior. Hence more practical and appropriate numerical techniques for ground response analysis should be surveyed

The Prediction of Liquefaction Damages to a Large Span Bridge on Karoon River, Iran

Several damages have been imposed to bridges due to liquefaction of the soil layer in the position of their foundations. However, there is not a specific world - wide provisions for design of bridges in these conditions. In this paper the results of evaluating liquefaction potential and the consequence damages which is likely to happen for a large span bridge (Shirin-Shahr Bridge) to be constructed on the Karoon river (one of the main river in the south-western part of Iran) are presented. Since the liquefaction potential of a site depends not only on the geotechnical characteristics, but on the intensity of the ground motions as well, a comprehensive seismic hazard analysis was carried out to estimate the appropriate PGA in the area. At the end some geotechnical comments are suggested for the aseismic design of the bridge foundation accordingly

Earth Pressure Variations under Cyclic Surcharges

There are a tremendous studies in literature about the earth pressure, most of them are concentrated on the static loading conditions. The problems of earth pressure under dynamic loadings are still the case of studies. In this paper the results of an experimental studies about variations of earth pressure at rest under different vertical cyclic loadings are presented. Cubical soil samples of 15 cm dimensions were subjected to vertical cyclic stresses and induced lateral pressure behind the rigid side walls of the soil were measured accurately. A dry and uniform sand was used and the samples were prepared by the raining technique. A kind of true triaxial apparatus was used and different samples in the quite dense and loose conditions were tested. The variations of lateral pressure versus vertical stress and number of load cycles are plotted and discussed. In the concluding section a recommendation is made for designing retaining walls under cyclic surcharges

The Amplification of Seismic Waves in Tehran

Vibrations of the bedrock due to earthquakes are transferred to the ground level via different soil layers. The arrived waves in the ground surface which affect the superstructures are usually stronger than those induced in the bedrock, depending on the geological and geotechnical characteristics of the soil layers. In this paper the influence of different geotechnical parameters of a site on the amplification of earthquake waves are investigated using the mathematical equations of wave’s propagation in a visco-elastic half-space medium. A special computer software was used to analyze the geotechnical data of a site in the western part of Tehran. The obtained results are illustrated as the response spectra in the ground surface to evaluate the effect of each parameter on the earthquake amplification. Finally the amplification factor of the site was calculated by comparing the bedrock and the ground accelerations obtained from the analysis

The Crack Development Due to Liquefaction of Sand Lenses During Earthquake Loading

The failure of sand lenses during earthquake loadings has caused many damages to the ground and yet its mechanism has not been investigated extensively. In this paper the results of an analytical studies involving the mechanism of crack development of a sand lens due to liquefaction, are presented. A single loose and saturated sand lens embedded inside a stiff clay deposit, which will liquefy due to the earthquake loading, is modeled by finite element method. The principles of fracture mechanics were used and the soil behavior was considered as a non-linear elasto-plastic material. The computer package of NISA was used and the failure mechanism of the lens was analysed by this package. Finally, the crack development and the angle of developed crack from the tip of the lens to the horizontal was calculated and discussed

First-principles study of hydrogen dynamics in monoclinic TiO

The existence of intrinsic vacancies in cubic (monoclinic) TiO suggests opportunity for hydrogen absorption, which was addressed in recent experiments. In the present work, based on first principle calculations, the preferences are studied for the hydrogen absorption sites and diffusion paths between them. The oxygen vacancies are found to be primary hydrogen traps with absorption energy of -2.87 eV. The plausible channels for hydrogen diffusion between adjacent vacancy sites (ordered in the monoclinic TiO structure) are compared with the help of calculations done with the nudge elastic band method. Several competitive channels are identified, with barrier heights varying from 2.87 to 3.71 eV, that is high enough to ensure relative stability of trapped hydrogen atoms at oxygen vacancy sites. Moreover, the possibility of adsorption of molecular hydrogen was tested and found improbable, in the sense that the H2 molecules penetrating the TiO crystal are easily dissociated (and released atoms tend to proceed towards oxygen vacancy sites). These results suggest that hydrogen may persist in oxygen vacancy sites up to high enough temperatures.Comment: updated and enlarged version, accepted for publication in Journal of Physical Chemistry C (June 2023

Unconventional resource's production under desorption-induced effects

AbstractThousands of horizontal wells are drilled into the shale formations across the U.S. and hydrocarbon production is substantially increased during past years. This fact is accredited to advances obtained in hydraulic fracturing and pad drilling technologies. The contribution of shale rock surface desorption to production is widely accepted and confirmed by laboratory and field evidences. Nevertheless, the subsequent changes in porosity and permeability due to desorption combined with hydraulic fracture closures caused by increased net effective rock stress state, have not been captured in current shale modeling and simulation. Hence, it is essential to investigate the effects of induced permeability, porosity, and stress by desorption on ultimate hydrocarbon recovery.We have developed a numerical model to study the effect of changes in porosity, permeability and compaction on four major U.S. shale formations considering their Langmuir isotherm desorption behavior. These resources include; Marcellus, New Albany, Barnett and Haynesville Shales. First, we introduced a model that is a physical transport of single-phase gas flow in shale porous rock. Later, the governing equations are implemented into a one-dimensional numerical model and solved using a fully implicit solution method. It is found that the natural gas production is substantially affected by desorption-induced porosity/permeability changes and geomechancis. This paper provides valuable insights into accurate modeling of unconventional reservoirs that is more significant when an even small correction to the future production prediction can enormously contribute to the U.S. economy

Pin-point effect determination using a rigorous approach

A new method for evaluating the pin-point effect of pile yarn of carpets before weaving has been introduced. The method has been initially accomplished by presenting a standard method for bundle preparation and consequently the pin-point index is presented by image analysis technique. To this end, yarns with different twists are heat set at various times and temperatures. Comparison of the results shows that increasing the twist, time and temperature positively contribute to the pin-point index. In the last section, an adaptive neuro fuzzy model (ANFIS) and an artificial neural network model (ANN) have been designed to predict the pin-point index of the heat set yarns based on training with the experimental data.  The input parameters are twist, time and temperature, and the output is the pin-point index. The results illustrate that the learning capability of the ANFIS model is superior and its generalization ability is slightly better than that of a standalone ANN model