Journal of Engineering Geology, Vol.6, No.2, Autumn 2012 & Winter 2013

A New Approach to Evaluate Seismic Stability of
Asphaltic Core Rockfill Dams

*A. Ghanbari:
Department of Civil Engineering, University of Kharazmi, M.Mojezi, M. Fadaee:
International Institute of Earthquake Engineering and
Seismology & Ghods Niroo Engineering Company, Tehran,
Received: 8 May 2011 Revised 20 Jan 2013
Abstract
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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.
KeyWords: Dam, Asphaltic Core, Dynamic Analysis, Crest Settlement, Shear Strain, Safety Evaluation.

*Corresponding author [email protected]

Introduction
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The process of selection of different types of dams consist of technical and economical aspects. In some cases the rockfill dam option with asphaltic core is preferred considering different conditions such as location and characteristics of site, limitations in borrow sources, execution operations, etc. There is an increase in application of rockfill dams with asphaltic core because of their numerous advantages, including abundance of bituminous and asphaltic materials, decrease in body volume compared to the option of rockfill dam with clay core, less executive dependence on air conditions, simplicity of design and execution, not having the problem of providing material with low permeability such as clay, decrease in execution time, asphalt’s significant characteristic of self healing specially in conditions of asymmetric settlements and during earthquake occurrence, etc. On the other hand, as this method is new, there are disadvantages such as insufficient experience of contractors and also the required equipments. Moreover, more investigation is required on the behavior of these dams in different conditions especially during earthquake.
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Since many dams are built or being built in seismic regions, a safe design against earthquake is of great importance. A careful study of seismic stability of earth and rockfill dams is one of the complicated problems in geotechnical field. Factors such as variety in dynamic properties of dam body and diversity in material and thickness which can take a part in propagation, attenuation, and amplification of waves, existence of active fault near dam body, earthquake characteristics such as distance of epicenter to dam site, intensity and period, type and direction of the waves reaching the dam, and frequency content have important role in dynamic response of dams.
In the current research, initially, the existing methods and criteria related to seismic evaluation of dams are studied and then a new method is presented for safety evaluation of ACRDs in earthquake condition based on results of nonlinear dynamic analyses. The suggested method has the advantage of considering crest settlement and maximum shear strain while calculating the safety factor of slope stability. In addition, it is not necessary to assume a virtual value for each of the two mentioned parameters at beginning of analysis.

Review of existing Methods
In general, methods of dynamic analysis of earth dams can be divided in three categories:
Pseudo-static methods;
Equivalent linear methods or the methods based on analysis of
Newmark sliding block;
Methods of nonlinear dynamic analysis using numerical methods of finite elements or finite differences.
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The critical components in displacement seismic analysis include ground motion, dynamic resistance of structure, and probable dynamic response of the sliding mass. Newmark (1965) sliding block method, which is extensively used or is utilized as a base for other techniques, covers only a part of dynamic shape changes caused by shear stresses while it does not include ground displacement caused by volume compression. Duncan (1996) found that consistent and reasonable estimates of static factor of safety (FS) of slopes are calculated if a slope stability method satisfies all three conditions of equilibrium. Methods by Spencer, Janbo, and Morgenstern-Price belong to this category. Most programs allow a horizontal coefficient (Kh) in equilibrium equations for pseudo-static analysis. Bray et al. (1998) presented relationships for calculation of Ky as a function of slope geometry, weight, and strength. Studies of researchers (e.g. Bray and Rathje, 1998) have indicated that seismic displacement also depends on the dynamic response characteristics of the potential sliding mass.
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With all other factors held constant, seismic displacements increase when the sliding mass is near resonance compared to that calculated for very stiff or very flexible slopes (e.g. Kramer and Smith, 1997; Rathje and Bray, 2000; Wartman et al., 2003). Many of the existing methods for calculating displacement of slopes (e.g. Lin and Whitman, 1986; Ambraseys and Menu, 1988; Yegian et al., 1991b) use assumptions of original method of Newmark’s rigid sliding block which does not involve dynamic response of deformable potential sliding mass during earthquake. In contrast to the original method of Newmark (1965) with the mentioned limitation, Makdisi and Seed (1978) presented a method of equivalent acceleration for seismic loading of the potential sliding mass based on the works of Seed and Martin (1966). When the horizontal equivalent acceleration of time history is applied to a potential rigid sliding mass, it causes dynamic shear stresses along the potential sliding surface, similar to the case of carrying out a dynamic analysis. Several common pseudo-static techniques are used to evaluate the stability of slopes, such as the methods introduced by Seed (1979), Hynes-Grifin & Franklin (1984), which all involve simplified assumptions. For instance, the two above mentioned methods have been calibrated for seismic assessment of embankment dams with maximum displacement of one meter for an appropriate seismic performance. In sum, these methods do not present an obvious and decisive criterion for seismic evaluation.
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The Seed (1979) pseudo static slope stability method has been developed for earth dams with crest acceleration less than 0.75g and with materials that do not tolerate serious decrease in resistance. In this method, using seismic coefficient of 0.15, the dam performance is described appropriate if the safety factor exceeds 1.15. In this method, characteristics of ground motion and dynamic response of sliding mass is expressed by a constant coefficient of 0.15 in all cases. Therefore in the mentioned method, the ground motion, dynamic resistance, and dynamic response of the embankment dam are considered very simple, as well as the fact that the level of conservatism is uncertain in the estimate and expected seismic performance.
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In the method developed by Makdisi and Seed (1978), the first step is to evaluate the material’s strength losing potential. They propose their method for cases where shear strength loss in materials is less than 10% to 20% of peak undrained shear strength. This method has been of highest usage in the recent decades, but as they have proposed, this method should be updated consistent with advances of new data. Since 1989 numerous records have been recorded whereas the method of Makdisi and Seed is based on limited number of records. Furthermore, ground motion caused by earthquake in a site is defined by peak ground acceleration (PGA) in the slope crest and magnitude of the earthquake. PGA is much variable in slope crest and therein frequency content of ground motion is not considered. Also the employed analysis method (i.e. method of slices and a limited number of 2D linear equivalent finite elements analysis), is relatively simple. Bray et al.,s (1998) Method is mostly based on the work of Bray and Rathje (1998) which in turn follows the works of Seed and Makdisi (1966), Makdisi and Seed (1978), and Bray et al. (1995). The methodology of this technique is based on the results of 1D decoupled complete nonlinear dynamic analysis (Matasovic and Vucetic, 1995), i.e. D-MOD, in combination with the Newmark rigid sliding block. In recent years, numerous dams have been designed in static and dynamic conditions based on the mentioned methods. Russel (1993), Wilson (2000), and Guoxi (2001) demonstrated that the behavior of embankment dams during earthquake can be better studied using dynamic analyses by numerical methods of finite elements and finite differences. Gurdil(1999) analyzed dynamic behavior of Kupru asphaltic core dam for levels of MCL and DBL using equivalent linear method. Ghanooni and Mahin Roosta (2002) analyzed an asphaltic core dam using linear equivalent techniques and nonlinear methods. They concluded that in nonlinear analysis, materials in transition zones at both sides of the core reach plastic state and experience large deformations whilst asphaltic core materials remain in elastic condition. Besides, Mahin Roosta )2007( studied seismic behavior of rockfill dams with asphaltic core against earthquake using linear equivalent techniques and nonlinear methods. Feizi et al. (2008) evaluated dynamic behavior of embankment asphaltic core dams using method of finite differences. Variety of the presented equations and methods related to behavior assessment of earth structure, including earth and Rockfill dams, in earthquake conditions shows that there is no secure and accurate method in this case, so requires more studies in this field. For instance, in the widely used pseudo-static method only one value is yielded as safety factor, which cannot provide an accurate evaluation of stability analysis of earth structures since the employed methods are based on simplified assumptions. Cases such as failure in Saint Fernando dam and Ushimatiling dam are evidence of weakness in pseudo-static methods as well. Accordingly, it may be incorrect and accompanied by much estimation to exert the effect of dynamic loading caused by earthquake by a constant without considering the complex nature of earthquake, PGA, and frequency content of different earthquakes. According to what was mentioned above, it seems that there is no method capable of offering a simpler and more realistic evaluation of stability related to dynamic behavior of embankment dams while being comprehensive and utilizing the most complete ones of the mentioned methods so this field needs further research.

Criteria for Seismic Stability Evaluation in Earth and
Rockfill Dams
Seismic behavior of dams in passed earthquakes indicates that seismic events are not the principal cause of failure, since dams present a good behavior during earthquakes. However, there are not dams damage records of induced by intense earthquakes (Hernández et al, 2008). To control the stability of earth or rockfill dams during earthquake, different criteria can be taken into consideration. Different events may happen during earthquake which result in risks for dam performance. According to Ledbetter and Finn (1993), the effects of earthquake in failure of embankment dams are divided into four major categories, i.e. slopes instability, crest settlement, dam body cracking, and liquefaction. Regarding the existing observations and experiences, the probable causes of vulnerability in embankment dams against earthquake can be classified as following:
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Fracture and dam collapse due to a main fault under dam body;
Failure occurrence in side slopes of dam due to earthquake motion;
Decrease in free board due to crest settlement or sliding of the slopes;
Sliding of dam body on weak layers due to earthquake loading;
Water overflow due to wave creation or land slide in the reservoir;
Break of spillway or water output pipes;
Liquefaction of saturated sands with low density or strength loss in saturated clays due to earthquake (in sensitive clays);
Formation of longitudinal cracks in vicinity of crest due to large tension strains caused by lateral vibration;
Formation of lateral cracks due to tension strains caused by longitudinal vibration or different lateral response in neighborhood of lateral supports with steep slope or close to crest center.
According to the above mentioned issues, in order to consider an index or criterion for evaluation of seismic stability of dam, control of crest settlement or maximum shear strains can be chosen as parameters for investigation of appropriate seismic performance of earth and rockfill dams. In other words, in the case that the settlement of dam crest or maximum shear strains exceeds the allowable ranges, dam behavior during earthquake is assessed as unsuitable.
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-37845-7055

Fig. 1. Types of possible failure in embankment dams during earthquake [Das, 1993]

The Model under Study and its Characteristics
To explain the steps for calculation of safety coefficient in the suggested method, an asphaltic core dam is selected for case study and the procedure is presented using that.

1. Geometrical Characteristics
The model considered in this research is a Rockfill dam with height of 75m and lateral slopes of 1:1.4 with an asphaltic core concrete of 1m width and filters of 4.5m width in both sides of the core.
For correct wave propagation in the model during dynamic analysis, dimensions of the model elements are considered to be small enough so that the following criterion suggested by Kuhlemeyer and Lysmer(1973) is satisfied:
l ≤λ/10
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Where  is the wave length of maximum input frequency of earthquake, and l is the size of the elements. Fig. 2 illustrates the geometry of designed model.

Fig. 2. Model of asphaltic core Rockfill dam, used in analyses
2. Material Parameters
To determine material parameters of dam body, we have used parameters belonging to a body of several Rockfill dams with asphalt core which have been executed in Iran. Table 1 exhibits these parameters.
Table1. Properties of the materials used in analyses
Materials γ
(kg/m3) C
(kN/m2) φ
( ) E
(kN/m2) 
Asphaltic Core 2400 180 17 1.5*105 0.45
Filter 2100 0 32 4*104 0.3
Shell 2000 0 40 8*104 0.25
3. Accelerogram Selection
Several recorded data were studied in order to select the suitable input record of earthquake and three accelerograms with properties shown in Table 2 were used with the purpose of performing dynamic analyses. Fig. 3 illustrates the graph of these accelerograms.
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Table2. Properties of three used earthquake records
No Station Geology Earthquake Date Magnitude Epicentral
Distance
(km) Component PGA
(g)
1 Coralitos-
Eureka
Canyon Road Landslide Deposit Loma
Prieta,
October
17, 1989 7.1(MS) 7 90, 360 0.48
2 Santa
Cruz-
University of
California at Santa
Cruz Limestone Loma
Prieta,
October
17, 1989 7.1(MS) 16 90, 360 0.41
3 ParkfieldCholame
Shandon No.2 Rock Parkfield,
June 27,
1966 5.6(ML) 7 N65E, 21 0.48

88011-987641

2314194-915898

A. Accelerogram No. 1 B. Accelerogram No. 2

C. Accelerogram No. 3
Fig.3. Earthquake records used in dynamic analyses
4. Boundary Conditions
Regarding to lying of dam body on rock foundation, the underneath boundary is modeled as fixed. Furthermore, Rayleigh damping is employed in dynamic analyses for modeling damping of materials.
5. Methodology
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قیمت: تومان

دسته بندی : زمین شناسی

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