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DYNAMIC RESPONSE OF CONCRETE
GRAVITY DAM ON RANDOM SOIL
Atheer Zaki Mohsin
PhD Candidate, Building and Construction Engineering Department, University of Technology
Dr. Hassan Ali Omran
Asst. Prof., Building and Construction Engineering Department, University of Technology
Dr. Abdul-Hassan K. Al-Shukur
Prof., Civil Engineering Department, College of Engineering, University of Babylon
ABSTRAT
This research reports the dynamic response of a concrete gravity dam under seismic excitation including dam‒reservoir‒foundation interaction. A
peek ground accelerations PGAS of 0.6ghas been applied on a numerical
model of the gravity dam that is built by finite element method using ANSYS. In this model, the dam is considered as a rigid body, the reservoir as compressible in viscid fluid, and the foundation as a random soil. A parametric study is achieved through change of relative density (Dr) of ground soil, namely, Dr= 60% and 80%. Modal and transient analyses have been considered to achieve the results. The results are analyzed and compared with experimental ones. It is shown a significant variation in the estimated seismic response when the interaction is included in analyses.
Key words: Concrete gravity dam, Dam-reservoir-foundation interaction, dynamic response, hydrodynamic pressure, Random soil
Cite this Article: Mohsin, A. Z., Dr. Omran, H. A. and Dr. Al-Shukur, A.-H. K. Dynamic Response of Concrete Gravity Dam on Random Soil. International
Journal of Civil Engineering and Technology, 6(11), 2015, pp. 21-31.
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1. INTRODUCTION
Gravity dams form a lifeline of a country economy and their failure will create huge loss of life and properties. Some of dams are in seismically active area. The dynamic
analysis of a concrete gravity dam is a reasonably complex problem and hence its behavior under seismic actions due to earthquakes has become a matter of immense interest by the researchers.
The dynamic response of concrete gravity dam including dam-reservoir-foundation interaction problems subjected to earthquake excitation could be simulated numerically using finite element analysis software ANSYS.[1] used ANSYS computer program to simulate the interaction of reservoir water-dam structure and foundation bed rock. The analytical results obtained from over twenty 2D finite element modal analysis of concrete gravity dam showed that the accurate modeling of dam-reservoir-foundation and their interaction considerably affects the modal periods, mode shapes and modal hydrodynamic pressure distribution. [2] used finite element software ANSYS to simulate a two-dimensional model of gravity dam including dam-water-foundation rock interaction to find a dynamic response ( natural frequency and mode shape) for different shape of dams and the results are verified with other ones. In the same matter, [3] found that considering dam-reservoir- foundation rock interaction had an important role for safely designing a gravity dam. They used finite element software ANSYS to simulate a two-dimensional model of gravity dam to achieve these issues. To assess the accuracy of this modeling, the modal analysis and mode shapes are studied and the results are compared with other references results.
Numerous of concrete gravity dam need to be constructed on soft soil. In this case, a particular attention to be given to the problems of soil-structure interaction. Therefore, [4] study the effect on gravity dam where is completely resting on soil media and surrounded by soil media by using finite element analysis software ANSYS. In her research the relevant amount of soil around and bottom of the gravity dam has been modeled to simulate the in-situ conditions. Also, the dynamic loading in transient analysis is considered to carry out the influence of soil properties on the response of dam in terms of stress and deformation.
This paper aims to investigate on dynamic responses of optimized concrete gravity dam section on random soil including dam‒reservoir‒foundation interaction subjected to earthquake excitations.
To achieve this aim, the research is organized as follows: Section 2 describes the model of the dam that is built by ANSYS, the governing equations, the related parameters and materials, and the suitable elements to mesh the model and solve the problem. The dynamic response concepts and analyses are presented in section 3. The parametric studies that will depend to achieve this research are also considered in this section. The obtained results are presented in Section 4. Moreover, these results are analyzed and discussed in this section. The conclusions and recommendations are presented in Sections 5 and 6, respectively.
2. MODEL OF THE DAM BY ANSYS
The section of dam that is modeled by ANSYA is shown in Fig. (1).In this figure the secant pile are considered beneath dam to serve as a barrier to reduce seepage beneath dam, as a device to improve bearing capacity of soil, and as a key under base of dam to overcome both sliding and overturning.
Figure 1 Prototype Dam Model [After[5]]
The dam section is optimized to achieve all factors of safety and stability requirements. Table (1) gives the optimal dimensions of both dam and pile sections.
Table 1 Optimized Dam& Pile Sections[After [5]]
The properties of materials that used to build the model could be given in Table (2-a,b, and c) to represent the properties each of concrete of dam body, water reservoir, and soil foundation.
Table 2 Properties of Materials [After [5]]
The 2D finite element model of the problem is discretized by ANSYS APDL15.0 and it is shown in Fig. (2).
In this figure, the elements shall be used are: four‒nodes PLANE 42 element (structural 2D solids) plain strain, shown in Fig. 3 which available in ANSYS 15.0 is used for both dam body and soil foundation modeling. This element represents equation of a structural dynamics given in Eg. (1) [6] and [7]
(1) Where , , and are the structural mass, damping and stiffness matrices, respectively, is the nodal displacement vector with respect to ground and the term
represents the nodal force vector associated with the hydrodynamic pressure produced by the reservoir.
Figure 3 PLANE42 Element Geometry [6]
Also, the interface of the soil‒structure interaction problem that is expressed numerically by coupling equation (Eq. 1) can be discretize by making NUMMRGE command for all nodes and elements on the contact surfaces ( interaction planes ) or by CONTA172 and TARGE 169 elements which making a SURF in between them. CONTA172 is used to represent contact and sliding between 2D "target" surfaces (TARGE169) and a deformable surface, defined by this element. The element is applicable to 2D structural and coupled field contact analyses. This element is located on the surfaces of 2D solid elements with midside nodes. It has the same geometric characteristics as the solid element face with which it is connected Fig.4. Contact occurs when the element surface penetrates one of the target segment elements (TARGE169) on a specified target surface. Coulomb and shear stress friction is allowed. This element also allows separation of bonded contact to simulate interface delamination. TARGE169, Fig.5, is used to represent various 2D "target" surfaces for the associated contact elements. The contact elements themselves overlay the solid elements describing the boundary of a deformable body and are potentially in contact with the target surface, defined by TARGE169. This target surface is discretized by a set of target segment elements (TARGE169) and is paired with its associated contact surface via a shared real constant set. It can impose any translational or rotational displacement, temperature, voltage, and magnetic potential on the target segment element. Also, it can impose forces and moments on target elements.
Figure 5 TARGE169 Geometry [6]
In other hand, a four-node FLUID 29 element shown in Fig. (6) is used to discretize both fluid and coupled fluid‒structure interaction domains represented by Eq. (2).
Figure 6 FLUID29Element Geometry [6]
(2) Where: = and =
3. DYNAMIC RESPONSE
The dynamic response represents modal analyses for mode shapes and natural frequencies. Also, von misses stresses, pressure on reservoir (hydrodynamic), and displacement have been considered. In other hand, a transient analysis is also considered in the dynamic response of the dam where a PGA of 0.6g as a transient load is applied. In this type of analysis, the displacements on the top, middle and down of the dam will be calculated.
In order to include a parametric study in this research, the characteristics of soil have been changed. The modulus of elasticity of soil have been selected so could range in-between medium to densemedia of sand soil with relative densities 60% and 80% respectively as given in Table (3).
Mode1 Mode3 Mode5
Mode1 Mode3 Mode5
(a) Deformation
Mode1 Mode3 Mode5
(b) Displacement
Mode1 Mode3 Mode5
(c) Pressure
Mode1 Mode3 Mode5
(d) Stress
Mode1 Mode3 Mode5
Mode1 Mode3 Mode5
(a) Deformation
Mode1 Mode3 Mode5
(b) Displacement
Mode1 Mode3 Mode5
(c) Pressure
Mode1 Mode3 Mode5
(d) Stress
Figure 8 Mode shapes of the dam fordense sand soil Dr=80%
4. RESULTS AND DISCUSSIONS
As mentioned earlier, the parametric study is involved in the dynamic response of the dam. The results will show the effects of soil with relative density (Dr) of 60%, and 80%.The results of the dynamic response could be categorized as given below:
4.1. Modal analyses
In modal analysis where load is zero, the behavior of dam inc luding dam-reservoir-foundation interaction is represented through mode shapes and natural frequencies. The number of mode is five, but the more effective modes results will be shown. Moreover, displacement of the dam, dynamic pressure on reservoir (hydrod ynamic), and von misses stress of the dam are observed in these mode shapes as shown in Fig. 7 (a, b, c, and d) for Dr=60% and Fig. 8 (a, b, c, and d) for Dr=80%, respectively. The results of modal analyses are summarized in Table (4).
Table 4 Natural frequency of modal analyses Mode No.
Item
Mode 1 Mode 3 Mode 5
Dr=60%
Natural frequency(Hz) 144.08 216.00 239.41 Max. Displacement (m) 0.138ᵡ10-5 0.349ᵡ10-5 0.235ᵡ10-5 Max. Pressure (Pa) 729.525 1281.91 1041.28 Max. Stress (Pa) 33944.7 33367.2 13503.8 Dr=80%
Natural frequency(Hz) 858.64 1260.72 1485.55 Max. Displacement (m) 0.332ᵡ10-5 0.421ᵡ10-6 0.414ᵡ10-6 Max. Pressure (Pa) 4327.46 4942.51 5626.64 Max. Stress (Pa) 129555 14594.8 32926
To ensure accuracy of the results, the verification is made with experimental results given in research of [9] as shown in Fig.9.
(b) Test-2(Dr=60%)
(a) Test-1(Dr=80%)
Figure 9 Observations on dam after third phase-0.6g tests[After [9]]
The results show that the deformation (mode shape 3) of Fig. 7 (a) givea reasonable compatible result with observation shown in Fig. 9 (b). Also, it is showed that the behavior of dam on deformation (mode shapes) of Fig. 8 (a) gives somewhat compatible results with the experimental observation result shown in Fig. 9 (a).
4.2. Transient analysis
Transient dynamic analysis (sometimes called time-history analysis) is a technique used to determine the dynamic response of a structure under the action of any general
time-dependent loads. The transient analysis results from ANSYS for two parametric studies include dynamic response of displacement in different loca tions on dam. These locations are taken as located in the experimental work which symbolized by LVDTs as shown in Fig. 10, where other dynamic responses are not considered in this research.
Figure 10 Layout of locations of output dynamic responses transducers in the experimental work [After [9]]
The dynamic response results for Dr=60% andDr=80% from ANSYS and experimental work are shown in Fig. 11(a and b ) and12(a and b ), respectively.
(a) Displacement from ANSYS (b) Displacement from experimental work After[9]
Figure 11 Dynamic responses on dam for Dr=60% from ANSYS andExperimental Work It is shown from figs. 11 and 12 that there is almost compatible on dynamic response of dam in between numerical analysis by ANSYS and experimental work given in [9] with some differences with times especially between 3-4 sec.
5. CONCLUSIONS
A dynamic response including modal and transient analyses considering dam-reservoir- random soil foundation interaction are investigated numerically. A 2D-plain strain rigid dam model is built by using finite element computer programming APDL/ANYS 15.0 taking water reservoir as an inviscid and compressible fluid and flexible foundation of random and soil. To make a parametric study, two values of the relative density namely Dr=60% and Dr=80% are taken to represent medium
-1.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00 0 1 2 3 4 5 6 7 8 9 10 D is p la c e m e n t (c m ) Time (Sec) LVDT1 -0.50 0.00 0.50 1.00 1.50 2.00 0 1 2 3 4 5 6 7 8 9 10 D is p la c e m e n t (c m ) Time (Sec) LVDT2 -4.00 -3.00 -2.00 -1.00 0.00 1.00 0 1 2 3 4 5 6 7 8 9 10 D is p la c e m e n t (c m ) Time (Sec) LVDT3
and dense sand soil. The concrete gravity dam model is built to simulate exactly a physical model construct experimentally given through a research in the literature. The results are analyzed and compared with these of experimental work. It is concluded the following:
(a) Displacement from ANSYS (b) Displacement from experimental work [After[9]]
Figure 12 Dynamic responses on dam for Dr=80% from ANSYS and Experimental Work The model that built by using ANSYS is efficient on the dynamic analyses of concrete gravity dam under earthquake excitation.
Generally, the results are verified with other experimental work and give almost compatible results.
The natural frequency of dam constructed on dense sand soil is more than by 80% from this with medium soil due to difference in stiffens factor.
The modal analyses on dynamic response of dam gives the maximum displacements of dam in the case of Dr=60% more by 85% than of case of Dr=80% due to the flexibility of the first one.
The dynamic pressure (hydrodynamic) applied on the dam face in the case of Dr=60% is little by 80% than of case of Dr=80% as a result of absorption of the wave that impact the dam by water reservoir through earthquake by soil when it beca me more loss.
The max von misses equivalent stresses in the case of Dr=60% concentrated on the heel and toe of dam and on the position of the pile where increased by 70% more than in the case of soil of Dr=80% where the stresses distributed in different pos itions on dam and soil due to interaction in between them.
The liquefaction phenomenon is observed obviously incase of soil of Dr=60% which is behaved as a saturated loss sandy clay soil.
The concrete gravity dam is more stab le on soil of Dr=80% than on soil of Dr=60%. The sliding and overturning are more effective in medium loss san soil than this one in dense [10].
6. RECOMMENDATIONS
From the results obtained in this research, it is recommended the following: -0.50 0.00 0.50 1.00 1.50 0 1 2 3 4 5 6 7 8 9 10 D is p la c e m e n t (c m ) Time (Sec) LVDT1 -0.50 0.00 0.50 1.00 1.50 0 1 2 3 4 5 6 7 8 9 10 D is p la c e m e n t (c m ) Time (Sec) LVDT2 -1.00 -0.80 -0.60 -0.40 -0.20 0.00 0.20 0 1 2 3 4 5 6 7 8 9 10 D is p la c e m e n t (c m ) Time (Sec) LVDT3
Avoidance of construct this type of dam on random soil at region affected by seismic zone of PGA=0.6g (Category V of moderate shaking and 4.5 of Richter Scale) and more.
In the contrast, it is ability to construct this type of dam founded on random soil as located on region affected by seismic zone of PGA= (0.01‒0.4)g (Category I, II-III, and IV as Not felt, weak, and light shaking respectively that ranged 1-4 of Richter Scale) with some precautions .
Construction of blanket of clay layer about 30 cm upstream dam to reduce seepage flow through foundation.
Construct sheet of secant piles beneath dam to reduce seepage, pear the weight of dam, and to prevent both sliding and overturning.
Avoidance of construct concrete gravity dam founded on saturated loss to medium dense sand soil under effects of seismic zone.
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