Journal of Engineering Geology

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Construction of asphaltic core dams is a relatively novel method especially in Iran. Iran is located in a region with high seismicity risk. Therefore, many researchers have focused on the behavior of such types of dams under earthquake loading. In this research, the behavior of asphaltic core rockfill dams (ACRD) has been studied under earthquake loading using nonlinear dynamic analysis method and a new method is presented to assess seismic stability of these types of dams in earthquake conditions. Based on nonlinear dynamic analysis, the current study attempts to provide an appropriate criterion for predicting the behavior of earth and rockfill dams considering real behavior of materials together with actual records of earthquake loading. In this method, the maximum acceleration of the earthquake record (PGA) increases until instability conditions. Finally, a new criterion is presented for evaluating seismic safety of ACRDs via demonstrating curves of the crest's permanent settlement and maximum shear strain against maximum earthquake acceleration. Results of the proposed criteria can assist designers of asphaltic core dams to predict dam stability during earthquake event

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Direct observation and experience of past earthquakes together with modeling carried out by researchers, has shown that ground motion acceleration and frequency is affected by the nonlinear behavior of site soil. In the process of assessing the seismic response of structures and lifelines, it is essential to understand the nonlinear behavior of the soil and how it can affect the results. In this paper, the nonlinear behavior of Urmia's subsurface soil is studied by performing one dimensional nonlinear site response analysis in time domain. Artificial acceleration time histories that were synthesized based on the result of seismic hazard analysis, conducted over three return periods, are used as input motion. Spectral acceleration at the ground surface is compared with those calculated for seismic bedrock, and spectral acceleration amplification curves are obtained. These curves show that, the amplification is greater in the central and eastern regions of the city than those for other regions of the city because of a deeper soil profile. The results show that the maximum amplification for higher return period is smaller because of greater soil nonlinear behavior

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The pseudo-static analysis is one of the conventional methods in embankment dams design and International Commission on Large Dams (ICOLD) suggests using this method before ultimate dynamic analyses. In this research, the static, pseudo-static and dynamic analysis of Masjed Soleyman embankment dam was performed. Using dynamic and pseudo-static analyses results, the safety factor of critical sliding surface was calculated. Permanent displacements of critical sliding surface were evaluated by New mark method and the calculated safety factor was compared. Based on the comparison results in different water levels of the reservoir and by introducing a new equation, the variable horizontal acceleration coefficients in height of the dam body were calculated. Finally, the obtained horizontal acceleration coefficients were compared with the other criteria introduced in different embankment dam's design codes. Totally, the results indicate that the proposed method leads to a larger horizontal acceleration coefficient in higher parts of the dam body.

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As usage of reinforced soil structures is highly increased in seismic active zones, the analysis of dynamic behavior of these structures begins to be of great significance.  The present paper is an attempt to study the seismic behavior of reinforced soil retaining walls with polymeric strips. The consequences of the most principal parameters counting the length of reinforcement, reinforcement arrangements (zigzag vs. parallel), maximum base input acceleration and wave frequency on the wall displacement have been investigated for sensitivity analyses. The main drawback of numerical methods in dynamic analysis is being very time consuming. Therefore, determination of equivalent coefficients is a suitable, easy and beneficent approach to converge   results of   pseudo-static and dynamic methods. In this case, a relatively accurate design is achieved by using pseudo-static method that takes less time. To this end, an earthquake equivalent horizontal acceleration coefficient is proposed by considering horizontal displacement of the wall as the basis for comparison

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Introduction
Unsystematic execution of blasting process may result in serious damages. Blasting is a very complex process and almost all of blast designs are made based on empirical relations resulting from trial and error. In recent decades, considerable development of numerical methods has been made possible to achieve high accuracy study of blast effects on surface and subsurface structures. Among these methods are boundary element method, finite difference method and finite element method. It should be mentioned that there is currently no software which might be able to completely simulate blast process. But the UDEC software is able to simulate different aspects of this phenomenon through simplification and focusing on each aspect.  Therefore, the UDEC software was selected. In the present study, the modeling  has been performed for Ghareh Changool ramp of Zehabad Zinc and Lead Mine against blast loads.
Material and methods
Zehabad Ore deposit is located around 2 km south of  Zehabad Village of Tarom Sofla County, 56 km to northwest of Qazvin at 49˚ 25' east longitude and 36˚ 28' north latitude.
The formation surrounding the ore deposit is generally made up of pyroclastics, lavas and sedimentary rocks of Eocene age (Karaj Formation) which have been divided into 22 stratigraphic units. Lithological composition of the tuff units are often rhyolithic to dacitic and the lava units are consisted of rhyolite, dacite and andesite.
To  accomplish this study, we took rock blocks from Ghareh Changool ramp. Then, the blocks were cored in the laboratory to provide cylindrical samples for doing uniaxial compressive, triaxial, Brazilian and direct shear tests. Physical and mechanical properties of the tuff samples were determined according to ISRM standards. 
In the present study, field studies were done to calculate strength parameters and properties of the joints.  Based on these studies, three major joint sets were determined. In order to obtain the shear strength parameters of the joints, the cylindrical samples of andesitic tuff were molded by concrete and direct shear test was done on all of the joints according to ASTM D 4554.
Results and discussion
To simulate the complex conditions of blast process, we used the discrete element software of UDEC for numerical modeling considering the discontinuity of the medium. To do a dynamic analysis, first the model should come to equilibrium in the static state. The space considered to be modeled in the study was a horse-shoe-shaped ramp with 4 m base, 4 m height and 1.5 m arc radius which was located in rocky medium consisting of tuff.  The height of overburden above the roof of the ramp was about 190 m. The dimensions of the model in UDEC was 20*20 m². The behavioral model considered for the rock blocks and discontinuities were the elastic isotropic and surface contact of the joint (elasto-plastic) associated with Coulomb sliding failure, respectively. After defining the absorbing boundary conditions, the dynamic loads were applied to the model based on the defined time period. In mines stability and blasting process, the dynamic load resulting from the blast is often applied to a model as a pulse. By application of dynamic load and considering the other mentioned variations with respect of static analysis, the dynamic response of underground space could be estimated under vibration load of blast or earthquake. To do this, the blast impact wave was applied to the left side of the model as exponential pulse with maximum pressure of 4.41 MPa and time width of 0.7 to 7 msec. The results of the numerical modeling in static analysis indicated that no block would fall (Fig. 1). After application of the blast load, the results showed that there was no falling around the ramp (Fig. 2).
Conclusion
1. In static condition, after initial equilibrium no block was fallen into the ramp, regarding the blocks’ magnification plots, as a result the ramp was stable in the static loading.
2. In dynamic loading case, considering the displacement plots  around the ramp and the low values of these displacements, as well as, magnification plot of  the blocks 40 msec after the blast it can be said that no block was fallen into the ramp. Therefore the ramp was stable in the dynamic loading case and there was no need to install support system. ./files/site1/files/133/1Extended_Abstracts.pdf

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Introduction
Concrete faced rockfill dams have been considered in recent years more than other types of dams due to their low dependency on the bed and the shape of the valley, as well as the simpler construction technology. In this regard, rockfill dams are a suitable substitute for embankment dams because of higher stability of the body and the availability of rock aggregates. On the other hand, because the permeability of rock aggregates is much higher than other materials, different methods are used to seal these types of dams. One of these methods is the use of non-impermeable concrete facing in the upstream of these dams. This particular type of gravel dams is called Concrete-Faced Rockfill Dams (CRFD). In this study, a contact element with a definition of elastic-plastic failure in the modeling process is proposed to simulate the surface separation and re-contact of the concrete face with the rockfill surface of the dam.
Method
In this paper, behavior of a concrete faced rockfill dam under earthquake loads is investigated. For this purpose, near-field earthquake records with focal depth lower than 15 km (for example Tabas earthquake 1978, M=7.4, and San Fernando earthquake 1970, M=6.6) are used. Moreover, to study the dam behavior under dynamic loads, interaction between concrete face and rockfill part of the dam is investigated and finally, some parameters including displacement, absorbed energy and base shear are evaluated. So, finite element method and Abaqus software is used for the study. Verification of the models is carried out using the results of previous researches by conducting modal analysis and determining natural vibration period. Then, the interaction between the concrete face and rockfill part as well as the effect of water level changes in stability of dam under dynamic load is investigated. Concrete behavior is simulated using concrete damaged plasticity. Therefore, concrete density, compressive strength and tensile strength and elasticity modulus are 2350 kg/m³, 25 MPa, 3 MPa and 29 GPa, respectively. Poisson’s ratio is assumed to be 0.2. Furthermore, 4-node shell elements are used to simulate concrete face and Drucker-Prager constitutive model is used to define rockfill material behavior.
The density and Poisson’s ratio for 2B, 3C and 3B layers are 2150 kg/m³ and 0.35, respectively. The shear modulus values for these layers are respectively 8.93, 2.89, and 3.85 GPa. In order to perform the simulation, the part of the dam structure beside the bed rock and the surrounding rock is considered as fixed bearing, and only the rockfill part and concrete face of the dam is simulated. Based on this assumption that the bed is rigid, there is no need to consider the dam foundation. This method is frequently used in literature review.
All the surfaces of the dam and bed rock are considered as fixed bearing to simulate the real condition where the dam is attached to bed rock and the surrounding rock. The interaction between dam layers is defined as tie. For defining the interaction between rockfill body and concrete face, tangential and normal contacts are defined using penalty method with friction coefficient equal to 0.5. In the next step, the model is meshed using 4-node shell elements for concrete face, 8-node brick and 4-node pyramid solid elements for rockfill body. Rayleigh damping is used to simulate the structure damping. The effective length of the dam reservoir has been determined by conducting several analyzes, so that the minimum required length for reservoir is reached in order to decrease the number of elements of the model.
Results and discussion
1. Interaction between concrete face and rockfill body
The results show that the increase of friction coefficient between concrete face and rockfill part from 0.5 to 0.7 has not affect the displacement of dam crown along the earthquake direction. However, when the concrete face is fixed to the rockfill part, significant changes are induced in dam crown displacement time history. In all cases, the deflection due to the dam weight is increased when the concrete face is attached to the rockfill body. The reason can be attributed to the tied interaction between these layers which results in similar deflection of concrete face with rockfill body and higher deflection of concrete dam crown. However, after the application of earthquake load, the displacement of the dam crown decreased in both analyses when tie interaction is defined between concrete face and rockfill body. In this study, due to the very high volume of analysis and its timeliness, it was not possible to examine the dam behavior in the free vibration regime, and therefore, it is not possible to assume the last displacement values at the end of analyses as the permanent displacement of dam. Figure 1 shows the relative displacement of the dam for the two selected earthquakes with a friction coefficient equal to 0.5 between the concrete face and the gravel body. According to Figure 1, the maximum displacement induced by the earthquake is related to Tabas and then, San Francisco earthquake. Furthermore, the high energy content of the Tabas record has been more effective in inducing greater displacement than the other record.
 
Figure 1. Lateral displacement of dam crown relative to the dam base for the selected earthquakes; Tabas and San Fernando.
The results also indicate that when the friction coefficient between concrete face and rockfill body is 0.5, the lowest damage occurs in the dam compared to that happens when friction coefficient is 0.7 or when the surfaces are tied. When the tied surfaces are used, the most damages takes place in concrete face, since all rockfill body displacement transmits to concrete face which results in much more concrete damages compared to the other interaction cases.
2. Effect of water level in reservoir on dam behavior
In this section, the effect of water level on seismic behavior of dam is investigated. For this purpose, the dam reservoir is analyzed in three cases including empty, half full and full (90% of dam height). Each study cases are examined under San Fernando and Tabas earthquakes. Figure 2 shows the relative displacement of dam crown in the three water level case for San Fernando and Tabas earthquakes.
 
Figure 2. Relative displacement of dam crown in three water level cases of empty, half and full for (a) Tabas and (b) San Fernando earthquakes
According to Figure 2, for both earthquakes, the dam crown displacement along the earthquake direction is significantly increased by increasing the water level, so that the maximum displacement in full case is 50% higher than empty case.
Conclusion
In this study, using the finite element method and simulation by Abaqus, the seismic behavior of concrete face rockfill dams has been investigated. For this purpose, the verification is firstly carried out using previous research results in literature. In the next step, nonlinear dynamical analysis is carried out, taking into account large displacements for the models under the earthquake record acceleration. The results illustrate that increasing the friction coefficient between the concrete face and the rockfill body from 0.5 to 0.7 has no significant effect on the displacement of the dam crown under earthquake load. Moreover, by using tie interaction between the concrete layer and the rockfill body, there is a substantial difference in the history of the relative displacement of the dam, and the displacement of the dam due to its weight has been increased. Furthermore, the results of this study exhibit that, with increasing the water level in dam reservoir, the deformation of the crown of the dam along the earthquake application direction has had a relatively significant increase, such that in the full state, the maximum displacement is increased by about 50% compared to that of the empty case. This is while the most damage of concrete is observed in the case when half height of dam in filled by water. Due to the more destructive power of near-field earthquakes and their impact nature, only near-fault earthquakes have been used in this research. Therefore, the results of this study are valid only for the behavior of dam under near-field earthquakes.
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