aliakbar golshani

Advancements in tunnelling technologies and ease of implementation of drilling methods in addition of other political and security issues made the construction of underground structures as an important alternative for answering the demands of population growth and the limitations of surface spaces in urban areas. Underground roads and highways, various types of tunnels and urban subway networks are the examples of underground structures being constructed and rapidly implemented in different countries. Meanwhile, for reducing negative effects to the environment, shortening the routes and improving traffic efficiency, urban tunnels should have high level of safety standards in design, construction and operation. Tunnels are considered major national projects and infrastructure investments, and huge costs are incurred around the world to build these structures. In countries located in highly active seismic zone, such as Iran, seismic researches for such important underground structures should not be ignored. The safety of such structures should be provided with respect to all loading demands and hazards issues associated with the site, including seismic loads. Reviewing seismic events in the past shows that underground structures have suffered less damage than above ground structures against seismic loads. However, in recent years, major earthquakes such as the 1995 Kobe earthquake in Japan, the 1999 Chi-Chi earthquake in Taiwan, the 1999 Kocaeli earthquake in Turkey, and the 2008 Wenchuan earthquake in China have caused underground structures to experience significant damage. There is evidence to conclude that the structural vulnerability of a tunnel in seismically active areas is an important issue but is either not yet well understood or not well assessed at the time of construction, emphasising that dynamic analysis of these structures against seismic loads is necessary. Earthquakes are likely to significantly affect tunnel performance by causing severe damage or excessive deformation of the tunnel structure. To understand the seismic-induced behaviour and performance of urban tunnels, this paper provides the state of the art in modelling studies of seismic design and assessment of tunnels. The review includes an investigation in seismic responses of real tunnels reported during past seismic events, the probable mechanisms caused damages in tunnels and physical and numerical methods used until now to either investigate those mechanisms or implemented in new designs. As an introduction, the seismic performance of tunnels affected by previous seismic events discusses first, emphasising the effective parameters in evaluation of tunnel seismic response and the relationship between the parameters, and the damage levels caused during earthquakes. Subsequently, the paper continues with a comprehensive literature review on the experimental methods used to investigate seismic-induced response in tunnels including physical testing, centrifuge tests, shaking table tests, and static tests. Analytical, quasi-static and numerical methods of dynamic analysis of tunnels and the accuracy of these methods are discussed then in details referring to some examples. The paper also reviews the effects of soil heterogeneity in the seismic response of tunnel and application of the random field for dynamic analysis of underground structures.  Examining the achievements and challenges remained in the field, the paper concludes with the existing gaps in the field to stimulate readers for doing more relevant researches.

Human growing demand for energy and in recent decades for clean and renewable energy, leading to the development of wind farms inshore areas and have moved to offshore areas to achieve more production. Noticed that wind farms are a series of large, expensive and same structures, their foundations are important and it’s necessary to minimize the probability of failure all of them. Many wind turbines are founded on large piles called monopiles. In European countries particularly in offshore areas, dominant environmental loading on monopiles is the wave. But some of the pioneer countries in wind energy development, such as China, India and the United States are highly seismic areas. Following the occurrence of natural events of wave and earthquake in the sea at the same time, considering the behavior of monopiles under their combined effects are required. In this study, three-dimensional modeling of the soil-monopile system using Open Sees software by finite element analysis was carried out and the effect of wave load on seismic responses of monopile and its surrounding soil was investigated. The wave and earthquake loads applied simultaneously on the soil-monopile system. In the nonlinear behavior modeling for sandy soils, effects of the stiffness, permeability, dilation, and potential of soil compaction on system responses are included. Studies have shown that wave can be affected on the seismic responses of monopile significantly, in addition to amplification of monopile, rotation, shear force and bending moment, can change the location of maximum moment and shear in monopile.

Hydraulic fracturing is a new and widely used method for extracting reserves and energy resources in the depths of the earth. In the near future, due to increase in energy consumption on the one hand and depletion of energy reserves on the other, using of this method will become a necessity. One of the most important and effective parameters in this process is the pressure and how it is applied in order to create fractures and fracture progression in rock layers. Another important parameter is the interaction between pre-existing natural fractures with different angles and hydraulic fracture. Due to the high costs of this process, the purpose of this study is to achieve the optimal state for the maximum progress of hydraulic fracture and the lowest amount of breakdown pressure at the same time. Numerical modeling was performed in two dimensions by Particle Flow Code ( PFC ) software from Itasca company on samples of Pocheon granite rocks with brittle behavior using distinct element method. PFC software uses circular disks in two dimensions to construct and make the sample using distinct element method. These particles are in contact with each other through bonds. In this program there are walls that interact with these disks to apply load on the sample. The flat joint model is used in order to create contacts between particles, and discrete fracture network is uded to to create pre-existing natural fractures for interacting with hydraulic fracture. Given that in the distinct element method (PFC software) our sample consists of a large number of particles in two dimensions that the general characteristics of the sample are formed based on the interaction between these particles, so we need parameters as input data to our software, existing disks and the link between them, to finally obtain the specifications of the same laboratory sample after modeling. These specifications and input data are referred to as micro parameters, and the final specifications, which are the same as our mechanical parameters in the laboratory, are referred to as macro parameters. To find the micro parameters of the sample we use the trial and error method. Here, our modeling is based on Brazilian and uniaxial compression experiments and … performed by Zhuang et al. laboratory investigations. Due to the limitations of PFC software for fluid flow modeling and limitations for using of CFD relationships, pressure equals to fluid pressure can be used as a new solution. In this way, by modeling a number of walls that form a complete circle with overlapping each other, and by considering the servo control mechanism, we move them in the opposite direction of their normal vectors and off-center, creating a comprehensive pressure Which is actually same as the fluid pressure. From the obtained results, it was found that with increasing the loading rate, the sample reaches the breakdown pressure and breaks in less time, but the amount of breakdown pressure increases. Also, by increasing the natural fracture angle relative to the horizon (clockwise), the specimen breaks at a lower breakdown pressure. Finally, by increasing the natural fracture distance from the center of the sample, the effect of the presence of the joint in the sample decreases and the breakdown pressure approaches the seamless state.

In order to increase the productivity of extraction of hydrocarbons reservoirs, the well is usually drilled in the direction of the minimum horizontal in situ stress and hydraulic fractures simultaneously initiate and propagate perpendicular/transverse to the wellbore. The last decade, more than 10,000 horizontal wells per year have been bored and hydraulically fractured, with up to a hundred hydraulic fractures placed in the horizontal segment of the well. The well is therefor stimulated in stages, with one stage consisting of a single pumping operation aimed at initiating and propagating simultaneously typically between3-8 cracks spaced about 10–30 m apart. When, the fluid pressure is applied on the surface of the fracture, the crack can propagate in the medium, but the pressure, induced from fluid injection, may have a negative influence on the extension of adjacent cracks which is stated as shadow or interaction stress. Certainly, an accurate estimation of interaction/shadow stress between the cracks leads to a more optimal design. In this research, the effect of the interaction between the hydraulic cracks with respect to the spacing and the number of cracks on the each other and considering position of the fractures are evaluated using the enhanced pseudo traction method. The results are shown that inner-fractures are further affected from shadow stress compared to outer-fractures. On the other hand as distance of hydraulic fractures increases, shadow stresses decrease. In the last, the results can be useful in determining the optimum number and spacing of cracks in the design of hydraulic fractures.

In the process of hydraulic fracture, various physical parameters such as; viscosity, inertia of fluid and toughness of rock do not influence the fracture propagation identically, and it is probable that one or more of the parameters be more pronounced. Therefore, it may persuade one special regime which is named base on dissipation of energy. In an impermeable rock, the two limiting regimes can be identified with the dominance of one or the other of the two energy dissipation mechanisms corresponding to extending the fracture in the rock and to flow of viscous fluid in the fracture, respectively. In the viscosity-dominated regime, dissipation in extending the fracture in the rock is negligible compared to the dissipation in the viscous fluid flow, and in the toughness-dominated regime, the opposite holds. It is supposed that the flow of incompressible fluid in the fracture is unidirectional and laminar. The contribution of this research is the evaluation of parameters effects on the propagation of hydraulic fracture an impermeable brittle rock. Here, the modified perturbation method suggested for evaluating fluid viscosity and inertia parameters interaction (FVII). The proposed method provides a good estimate of the solution in the wide range of the viscosity/inertia parameters because of the coexistence of both small parameters in the governing equations. The result shows, neglecting FVII is reasonable for very small fluid viscosity as well as small inertia. However, the influence of FVII become considerable for slightly higher values of these parameters.

Any investigative approach towards rock behavior will necessitate inherent deficiencies such as pores and cracks to be taken into consideration. One of the methodologies employed to study cracked rock is to consider an equivalent continuum as for the domain with defects which will lend flexibility to experimental and numerical schemes due to its seamless effects on the constitutive relationships, hence reducing computational costs as well as experimental restraints in the laboratory. A case in point in such approach is the crack tensor model which is based upon the idea to represent cracks’ size, orientation, and number density as one single entity through which proper geometric characterization of the in situ rock is carried out. Following the introduction of crack tensor concept and its application in the technical literature, the current work focuses on the determination of second rank crack tensor using P-wave velocity measurements on damaged granite. The benefit of such approach is emphasized via its role in boosting the degree of accuracy of the numerical analysis code developed in Matlab that implements different compliance matrices for four different stages of loading. The calculation results showed promising trends in agreement with those of the experimental data. Apparently, more experimental procedure is required to improve results’ accuracy in projects for which fulfilling more stringent regulatory requirements is a must.

The rapid growth of construction in urban areas has resulted in building structures on soft and weak soils. These soils possess the low bearing capacity and high settlements. One of the common ways to increase bearing capacity in weak soils is to use granular trench. Trenching can be built easier, faster and cheaper than deep foundations. The function of granular trench is similar to stone columns. However, the difference is that instead of a column of granular material, a large area of granular material is used. In this research, we compare the results of static bearing capacity for shallow foundation with and without trench obtained from numerical modeling using FLAC2D. Furthermore, the effect of depth of granular trench on bearing capacity has been studied and finally the optimum depth is presented.

From oil and gas engineering point of view, one of the challenges in low permeable or damaged wells is improving the productivity. There are different methods to increase the productivity of low permeable wells and one of the most efficient one is hydraulic fracturing. In this study, two-dimensional modeling of hydraulic fracturing using finite element method and cohesive element approach through traction-separation law has been performed. This approach avoids the singularity in the crack tip and the cohesive zone fits naturally into the conventional finite element method. Hydraulic fracture is assumed to propagate in a poroelastic and permeable medium with a constant injection rate and under quasi-static conditions and the criterion for fracture initiation is quadratic nominal stress criterion. Also as a propagation criterion, Benzeggagh Kenane (BK) approach has been considered. Two types of elements have been implemented in the model which are 4-node bilinear displacement and pore pressure reduced integration and 6-node displacement and pore pressure two- dimensional cohesive element. Cohesive elements have three degrees of freedom that two of them are in X and Y directions and one of them is pore pressure. Mesh size in the near fracture region is small enough to consider the stress and pressure distribution efficiently and avoid any problem in convergence. Meantime, to decrease the computation cost the mesh size gradually increases from fracture area to the boundaries. Also, to increase the accuracy of the model, the time steps for fracture propagation is 0.01 second. In addition, the effect of fracturing fluid has been directly included in the model which means that the fluid pressure would be applied along the fracture without any simplifying assumption. To validate the model, the results have been compared with KGD approach. The results indicate that in the initial steps the pressure at the wellbore wall is high which decreases with time significantly and eventually it gets a steady and uniform trend. In other words, in the initial steps, the fluid pressure should be high enough to overcome the hoop stress around the wellbore and after some injection periods, the fracturing fluid pressure would reach the breakdown pressure and the fracture starts to initiate and propagate. It is clearly observed that increasing the injection rate would lead to faster propagation of hydraulic fracture and in the models with higher injection rate the fracture tends to grow in the propagation direction. This indirectly means that increasing the injection rate would affect both opening and length of the hydraulic fracture which can result in increasing the productivity. The results reveal that the peak of the normal effective stress profiles corresponds to the fracture tip position, where the fracture opening is zero,and the peak value equals the cohesive strength of the material,as expected.Moreover,with increasing thedistance from the fracture tip,the stress decreases rapidly and approaches the initial stress value. The way that Young’s modulus affects the overall characteristics of hydraulic fracture implies that higher Young’s modulus would lead to longer fractures. In other words, formations with higher Young’s modulus can be fractured easily but the opening of the hydraulic fracture would reduce at the same time. This also indirectly means that Young’s modulus would play an important role in the productivity.

The lateral spreading of mildly sloping ground and the liquefaction induced by earthquakes can cause major destruction to foundations and buildings, mainly as a result of excess pore water pressure generation and softening of the subsoil. During many large earthquakes, soil liquefaction results in ground failures in the form of sand boils, differential settlements, flow slides, lateral spreading and loss of bearing capacity beneath buildings. Such ground failures have inflicted much damage to the built environment and caused significant loss of life. The risk of liquefaction and associated ground deformation can be reduced by various ground improvement methods, including densification, solidification (e.g., cementation), vibro-compaction, drainage, explosive compaction, deep soil mixing, deep dynamic compaction, permeation grouting, jet grouting, piles group and gravel drains or SCs. Nowadays, using pile foundation is one of the popular solution for soils vulnerable to liquefaction. the pile with enough length more than liquefiable soil depth can reduce the large deformation and unacceptble settlements. Liquefaction and lateral deformation of the soil has caused extensive damage to pile foundations during past earthquakes. Several example of significant damages in pile foundation have been reported in the literature from the 1964 Niigata,1983Nihonkai-Chubu,1989 Manjil and 1995 kobe earthquakes. These damage have been observed mainly in coastal areas or sloping ground. evaluation of liquefaction in order to develop the northern and southern ports and implement coastal and offshore structures in Iran is of particular importance due to locating in a high seismic hazard zone and Liquefactable soil in coastal areas. Although, in recent years many studies have been conducted to understand the various aspects of this phenomenon, yet a lot of uncertainties have remained about the lateral deformations of the soil and its effects on deep foundations. In this study, behavior of pile groups (2 × 1, 1 × 3, 2 × 2 and 3 × 3) were evaluated using fully coupled three-dimensional dynamic analysis. Therefore, the influence of effective parameters such as number of piles, ground slope angle on soil and pile behavior has been studied using the finite element software Opensees SP v2.4. results indicate that most of the factors affecting the behavior of the pile, soil are not considered in the current design codes (such as JRA 2002) and these issues indicate the need to revise the current design and analysis methods.Lateral Pressures compared to that of JRA regulations show that these regulations cannot exactly predict pressures on pile and pile groups. Altogether comparing the results of numerical model of this research to various laboratory observations indicate that the use of numerical method can be reliable to predict the behavior of the soil and pile qualitatively and quantitatively using appropriate constitutive model and parameters for soil and pile.

In the recent years, new techniques such as artificial neural networks were used for developing of the predictive models to estimate the needed parameters in Geotechnical Engineering such as swelling potential. If over 50% of the particles in a sample are able to pass through a number 200 screen or sieve then the sample is classified as either silt or clay or some combination of both. Regardless of the percentage of “fines” in a particular sample, a significant presence of clay minerals in a sample can indicate a possible expansive soil problem. When they absorb water they increase in volume. The more water they absorb the more their volume increases. Expansions of ten percent or more are not uncommon. This change in volume can exert enough force on a building or other structure to cause damage. Cracked foundations, floors and basement walls are typical types of damage done by swelling soils. Damage to the upper floors of the building can occur when motion in the structure is significant. Expansive soils will also shrink when they dry out. This shrinkage can remove support from buildings or other structures and result in damaging subsidence. Fissures in the soil can also develop. These fissures can facilitate the deep penetration of water when moist conditions or runoff occurs. This produces a cycle of shrinkage and swelling that places repetitive stress on structures. Determination of swell potential of soil is difficult, expensive and time consuming and also involves destructive tests. Multi-layer Perceptron model is one of the most sufficient methods of the Artificial Neural Networks in most of the research applications in engineering etc. In this research, Multi-layer Perceptron model and Radial Basis Function model of ANN (artificial neural networks) were used in order to predict expansive behavior of clayey soils (i.e., swell percent). All data have been modeled by using many types of architectural Multi-layer Perceptron network. Then, the output result of these networks are compared with each other according to the assessment indexes which has been leaded to the best architectural network selection in viewpoint of accusation and usage. It is noticeable that the parameters such as Natural Water Content, Plastic Index, Dry Density and Fine Soil Percent are considered as input parameters and swell percent (S%) is considered as output parameter. The Soils which are selected for this research is clayey soils from different areas of Iran. Consequently this ANN has the ability to predict expansive behavior of diverse types of clayey soils. To train this network, results of previous researches, geotechnical consultant engineering data and the available thesis about Expansive soils are used. It was found that the Multi-layer Perceptron (MLPst and MLPdy) models exhibited a higher performance than Radial Basis Function (RBF) model for predicting expansive behavior of clayey soils. Also, the comparison of the MLPst and MLPdyn network models indicates that their accuracies are almost the same. However, the time taken by MLPst is less than that of MLPst in this study. Since the population of the analyzed data is relatively limited in this study, the practical outcome of the proposed models could be used with acceptable accuracy at the preliminary stage of design.

Hydraulic fracturing as a method for reservoir stimulation depends on the properties of the media that fracture propagates in it. The elastic properties of layers greatly affect the geometry and propagation of hydraulic fractures. In this research study، the hydraulic fracturing propagation in multi-layered media (stiff and soft) with different elastic properties is investigated. Therefore، boundary element method based on the displacement discontinuity formulation is presented to solve general problems of hydraulic fracturing propagation in layered formations. The crack tip element and a higher order boundary displacement collocation technique are used to increase the accuracy of the method (2DFPM) in modeling of non-homogenous media. The stress intensity factor and tensile stress near the crack tip under different elastic modulus are evaluated to study the hydraulic fracture propagation. Related to the position of the fracture with interface (vertical، intersect and parallel) these factors have different values and finally، they control the fracture propagation. In comparison between the width of fractures in soft and stiff layers، the study displays that the fracture width in soft layers is greater than width of fracture in stiff layers.