حامد ملاداودی

The structure's response to the region's prevailing loading conditions guides the engineers in estimating the resilience of the structural materials and their reinforcement. One of the main concerns in designing rock structures is paying attention to the size effect phenomenon. The size effect influences the nominal strength, brittleness, load capacity, stress intensity factor, the characteristics of the fracture process zone at the crack tip, and the way and path of crack propagation. Therefore, studying the size effect law will make a guideline for correct decision-making, design, and implementation of efficient support systems. As a comprehensive review, this work investigates specimen size effect on the rock's mechanical and fracture properties. With a comprehensive look at this issue, it explains the essential points that help the engineers design rock structures. During the investigations carried out in this work, it is shown that the specimen size affects the fracture and mechanical properties of the rock. The severity of this phenomenon depends on various factors such as the brittleness index, the shape of the notch or crack length, and the size of the particles that create the rock. In concrete, it depends on the additive boosting materials in the concrete.

TBM penetration rate (PR) prediction is one of the most crucial issues for project cost and time estimation, however, its prediction has remained an important challenge for engineers and investors. Results of former investigations show that there are different methods for PR prediction, including theoretical and statistical models as classic methods and neural networks, fuzzy logic systems, and neuro-fuzzy models as intelligent and new methods. Modern methods are more capable of analyzing complex and non-linear relationships in comparison with classic methods. Accordingly, the implementation of modern methods for PR prediction will lead to a more precise outcome. In this paper, the information of 14 tunnels around the world is compiled within a database. Studied parameters in this database are a combination of machine and rock mass specifications, including, normal mean thrust force (Fn), cutterhead revolution per minute (Rpm), tunnel diameter (TD), rock mass rating (RMR), rock quality designation (RQD), and uniaxial compressive strength (UCS). By analyzing and reviewing relevant results, it was determined that omission or failure to use appropriate parameters causes a poor PR prediction. These results also show that UCS and RQD are amongst the most effective parameters. Furthermore, it has been concluded that using neuro-fuzzy networks (RMSE = 0.13 m/h) provides more accurate results than neural networks (RMSE = 0.38 m/h).

For most rock materials, there exists a coupling between inelastic deformations caused by crack displacements on micro-crack faces and damage evolution due to nucleation and growth of wing- and secondary-cracks. While rock material is subjected to dynamic loading, the interaction between micro-cracks plays an important role in materials behavior. The self-consistent homogenization scheme is implemented in this paper to consider micro-cracks interaction and determine the equivalent mechanical properties of micro-cracked rock deteriorated by damage evolution. The aim of this article is to develop a self-consistent based micromechanical damage model by taking into account the wing- and secondary-cracking mechanisms accompanied by inelastic strains caused by crack displacements under dynamic compressive loading. While stress intensity factors in tensile and in-plane shear modes at flaw tips exceed the material fracture toughness in modes I and II, respectively, wing- and secondary-cracks are sprouted and damage evolution occurs. For closed cracks, an appropriate criterion for the secondary-crack initiation is proposed in this paper. The developed model algorithm is programmed in the commercial finite difference software environment for numerical simulation of rock material to investigate the relationship between the macroscopic mechanical behavior and the microstructure. The fracture toughness parameters of the rock samples are experimentally determined.

In last decades, phenomenological constitutive damage models were used by many researchers to study for brittle failure of rock materials. Most phenomenological constitutive damage models utilize the irreversible thermodynamic principles to take into account the damage processes in brittle rock materials. Since this type of damage model do not consider the actual physical phenomena in the damage process, the micromechanical damage models are often used to consider the actual physical mechanisms in micro-scales specially in nucleation and propagation of wing-cracks from pre-existing flaws tips. Frictional sliding on closed micro-cracks surfaces leads to inelastic deformation and wing-cracks nucleation from flaw tips. Because of the different distribution of size and orientation of microflows in the rock materials, under dynamical loading, all of the pre-existing micro-flaws in the rock are activated and propagated. The interaction of micro-cracks with other and coalescence of these micro-cracks play a key role in accumulation of damage and macro-scale fracture plane formation in the rock. The various homogenization schemes e.g. Mori-Tanaka, Self-consistent and Ponte-Castandea were applied for calculation of equivalent mechanical parameters of materials. In this study, the Self-consistent scheme (SCS) was used for homogenization of rock sample under uniaxial dynamic compressive loading. The developed model was programmed and used as a separate and new constitutive model in the commercial finite difference software (FLAC).The dynamic compressive test of a brittle rock was simulated numerically and the stress-strain curves under dynamic loading were simulated and compared with one another. The proposed model predicts a macroscopic stress-strain relation and a peak stress (the materials compressive strength) with an associated transition strain rate beyond which the compressive strength of the material becomes highly strain rate sensitive. The results also show that as the strain rate increases, the peak strength increases and the damage evolution becomes slower.

Today, precast concrete lining (segmental) are used as system maintenance in the majority of tunnels excavated by TBM. On the other hand, the mechanism of the joint between two segments is not known under seismic loads. In this paper a numerical study about the effect of the earthquake on the segmental supporting system and the resultant vertical and shear forces on the contact surface between two segments is investigated. The Tehran -Karaj water conveyance tunnel (Amirkabir) was used as a case study. In this study, the UDEC software was used. At the first step, the segmental lining were simulated under no slip and full slip conditions and the normal and shear forces were studied. Finally, the effect of joint stiffness between two segments were investigated. Results showed that with increasing the interface properties, the normal and shear forces in the segmental joints increased. Also with increasing the joints stiffness, the normal and shear forces on the joints increased and the normal and shear displacement decreased. In other words, the rigidity increament of supporting system is associated with flexibility decrement of lining with respect to rock medium. So, the stresses increased and displacement decreased.

Summary : The case of the Shiraz (Iran) subway twin tunnel construction is an example of transport network extension. In this work, a three dimensional model of the shielded mechanized tunneling of the Shiraz subway was simulated using FLAC3D software to study the excavation influence of a new tunnel on an existing one. Also, the joint pattern in segmental lining is studied.
Due to the fast development of Iranian metropolises, a wide urban underground transport network is necessary. Thus, the prediction of the influence of mechanized tunneling method on the surface settlement is important in terms of design and construction. In recent decades, extensive studies around the world have focused on considering the effect of mechanized shield tunneling on the surrounding ground.
Methodology and Approaches : In the present paper, a 3D numerical model was developed to investigate the tunnel's lining behavior and the ground displacement surrounding the second line of the Shiraz subway tunnels. All the excavation phases of EPB-TBM (Earth pressure balanced, Tunneling Boring Machine) were simulated using the Finite Difference Method (FLAC3D). Furthermore, a new simple method were proposed to take account of the TBM shield conical shape. Also, to achieve an accurate simulation, the influence of the geometry of the longitudinal joints had been investigated on the lining behaviour.
Results and The numerical results indicated the necessity of using segmental lining with oblique joints pattern to obtain an accurate evaluation of the structural forces applied in segmental lining. In this work, the influence of the longitudinal distances between the tunnel faces on the surrounding ground displacement and the induced lining forces was investigated. The excavation of a new tunnel near to an existing tunnel with high lagging distance between twin tunnels leads to increase in the normal and the longitudinal displacements, in the internal lining forces and to a decrease in the ground surface settlement above the tunnels.

The rock materials surrounding the underground excavations typically demonstrate nonlinear mechanical response under high stress states. The dominant causes of irreversible behavior are plastic flow and damage process. The plastic flow is controlled by the presence of local shear stresses which cause dislocation to some preferential elements due to existing defects. During this process, the net number of bonds remains practically unchanged. The main cause of irreversible changes in quasi-brittle materials such as rock is the damage process occurring within the material.
In this paper, a coupled logarithmic damage and plastic model was used to simulate irreversible deformations and stiffness degradation of rock materials under loading. In this model, damage evolution and plastic flow rules were formulated in the framework of irreversible thermodynamics principles. To take into account the stiffness degradation and softening in post-peak region, logarithmic damage variable was implemented. Also, a plastic model with Drucker-Pruger yield function was used to model plastic strains. Then, an algorithm was proposed to calculate the numerical steps based on the proposed coupled plastic and damage constitutive model. The developed model was programmed in VC environment. Then, it was used as a separate and new constitutive model in DEM environment code (UDEC). Finally, the experimental oolitic limestone rock behavior was simulated based on the developed model. The irreversible strains, softening and stiffness degradation were reproduced in the numerical results. Furthermore, the confinement pressure dependency of rock behavior was simulated in according to experimental observations.

Analysis of stresses and displacements around circular opening excavated in rock mass has been one of the most important problems in tunneling. Plastic zone is formed around underground opening as a result of high stress magnitudes. The ground response curve is one of the best methods for understanding of tunnel stability that describes the relationship between the decreasing of inner pressure and the increasing of radial displacement of tunnel wall. In recent years, several methods have been suggested for analysis of ground response curve by many researchers, however, most of the analytical solutions that have been presented, are relevant to elastic-perfectly-plastic or elastic-brittle-plastic behavior of rock. But, the real behavior of plastic zone is strain-softening with dilation. For strain-softening rock masses the attempts at elasto-plastic analysis are limited. This may be due to the difficulty in defining the material behavior and in obtaining the closed-form solutions.
Summary: In this study, it is attempted to develop the earlier methods and present a new algorithm with considering quality of rock mass and variable dilatancy, the real behavior of rock mass be applied in analysis of ground response curve. The results based on the proposed analytical solution was in agreement with Numerical method and measured wall convergence in the Ghomroud tunnel. The results indicated effect of dilation parameter on convergence tunnel wall. With the increasing of rock mass quality (geological strength index),post-peak behavior of rock mass converge to elastic-brittle-plastic.
Due to loading of underground excavation and redistribution of stresses, the plastic zone form around tunnel. The real behavior of plastic zone is strain-softening with dilation. This paper proposes a new algorithm to calculate the distribution of displacements and stresses around tunnel excavated in strain softening rock masses with variable dilatancy. Also, Effect of various parameters including dilation, geological strength index, and critical softening parameter was studied on proposed algorithm.
Methodology and Approaches: In this study, a new proposed algorithm was used for calculating ground response curve. In order to investigate the verification of proposed algorithm, the results of this analytical solution was compared with numerical method. For showing the applicability of the proposed algorithm, part of Ghomroud tunnel radius convergence was calculated by this proposed analytical solution and was compared with measured tunnel wall convergence at site. The results based on the proposed analytical solution was in agreement with the measured tunnel convergence.
Results and In proposed algorithm, strain softening behavior, variable dilatancy and geological strength index applied through algorithm, which the results indicated that proposed algorithm was in good agreement with numerical and field results. The results of calculation via proposed algorithm shown importance of dilation in estimation of tunnel convergences. Using of constant dilation estimate the displacement of tunnel wall excessive. So, Analysis with constant dilation is conservative.

Today, with advances in technology, enable the design and construction of accurate underground structures are provided. Nevertheless, the analysis of underground structures due to interaction between the soil or rock surrounding infinite medium and tunnel supporting system, is very complex and then lower research is done.
The response of tunnel supporting system under seismic loads is a function of the ratio flexibility of the lining and rock or soil medium. This ratio represents the stiffness difference between the ground and tunnel lining. And in fact, the interaction of the tunnel lining and medium is indicated.
Methodology and Approaches :In this paper, two analytical methods for investigating the underground spaces behavior under seismic loading is presented. Then, the response of tunnel monolithic lining in no-slip and full-slip conditions with using analytical and numerical methods with considering of interaction between tunnel lining and medium is investigated.
Results and The results showed that the stress induced by seismic loads in tunnel lining in no slip condition is 2.7 times higher than in full slip condition. Also, monolithic lining strain value in full slip condition is further than no slip condition. Results of numerical method shows good agreement with analytical methods.