ASSESSMENT OF STRESS DISTRIBUTION IN DARAB'S ROODBAL EMBANKMEND DAM USING COMMERCIAL CODE FLAC

Transcription

1 Cumhuriyet Üniversitesi Fen Fakültesi Fen Bilimleri Dergisi (CFD), Cilt:36, No: 4 Özel Sayı (2015) ISSN: Cumhuriyet University Faculty of Science Science Journal (CSJ), Vol. 36, No: 4 Special Issue (2015) ISSN: ASSESSMENT OF STRESS DISTRIBUTION IN DARAB'S ROODBAL EMBANKMEND DAM USING COMMERCIAL CODE FLAC Yashar Ostovar KASHKOOLI 1, Dr.Mohammad Mehdi JABBARI 2,* a Department of Civil Engineering, Marvdasht Branch,Islamic Azad University, Marvdasht,Iran b Department of Civil Engineering, Shiraz Branch,Islamic Azad University, Shiraz,Iran Received: ; Accepted: Abstract. Evaluation of performance and behavior of a dam is viable by monitoring of dam using instrumentation. Stress and deformation analysis during end of construction is one of the important parameters in analysis, design, and stability of an embankment dam. In this study, a comparison between various models in stress and deformation analysis of Roodbal dam during end of construction using finite element method in FLAC software was established. The results showed that Mohr-Coulomb model has good accuracy in analysis of vertical settlement of dam core. It was also known that vertical displacement of Darab's Roodbal dam is slow ascending which is slower in upstream than downstream shell. The overall stress in dam body is also slow ascending that shifted from lower level near the core to middle lower level of core during first impoundment through the end of first 10-year utilization period. The analysis over whole body showed that after completion of construction and then initial impoundment of dam pore water pressure in the core of dam is 370 kpa which was the same in middle level of core. According to analysis conducted over whole body of dam, displacement reached 96 cm after completion of construction which was also the same for one-third middle level of core. Keywords: Stress Analysis, Back Analysis, Horizontal and Vertical Deformation, Roodbal Dam, FLAC INTRODUCTION Design and Construction of embankment dams including sciences that have a few thousands years old, and many countries including Iran located in arid and semi-arid regions possesses beneficial experiences in this field. The real and remarkable mutation about embankment dams began in early twentieth century by wide studies of Karell Tarzaghy. This scientific context due to various and many effective factors especially significant effect of phenomena and natural factors on dam behavior such as geological, geotechnical, hydrological, geohydrological condition, and climate of region has been dealt with difficulties and ignoring them lead to failure of project, human and financial losses. The destruction of a significant number of reservoir dams after 1970s is an evidence for this claim [1]. In recent decades, numerical methods such as finite element, boundary element and discrete element method have been used to solve engineering problems. Today, these methods are being widely used to design dams, underground spaces such as tunnels, underground structures, slopes and etc.. Reliability of design speculations significantly depends on input data of analysis, so that it will be so much difficult to determine geomechanical properties including stresses, displacements, and settlements due to inaccuracy of input data and/or unreal values of mechanical parameters of interested continuum [2]. In the study of earthen and rock-filled dam behavior, due to lack of crucial criteria for evaluating of data, it is often relied on individual judgment and experiences. However, since the first numerical simulation of dam construction significant advances in both numerical methods and physical interpretation of problems have occurred. Although, a limited number of numerical predictions modeled precisely deformations during construction and first impoundment [3]. Many researchers have taken steps in understanding of dam behavior. Ziayi (2013) studied changes in pore water pressure of Maku dam * Corresponding author. mmjabbari@yahoo.com Special Issue: Technological Advances of Engineering Sciences Faculty of Science, Cumhuriyet University

2 KASHKOOLI, JABBARI during utilization. In his study, based on piezometric data installed into the foundation and both sides of grout curtain, significant loss in downstream level of piezometer around the grout curtain was observed, indicating acceptable performance of curtain. Also, this loss inside clay core as an impervious core in this type of dams indicates obviously proper performance of curtain. In this research, comparison between plots of instrumentation and numerical analysis showed that there is good consistency between acquired data of electrical piezometers in clay core of dam and results of software, indicating reliable performance of dam from viewpoint of seepage. Rattue (2000) investigated behavior of Sante- Marguerite 3 dam with 171m height. It was observed low pore water pressure, considering high permeability of earth material and earth humidity equivalent to optimal humidity. BY comparison numerical analysis using parameters obtained from triaxial test, it was known that values obtained from analysis are not in agreement with measured values, hence parameters used in model were modified by a trial and error method and it was observed that Young's modulus of dam is 2 to 3 times of value obtained from triaxial test [5]. Ebrahim Nezhad, Seddiq, Emami Tabrizi, and Barari presented an article as "Monitoring of foundation and body settlement of Alavian Dam during utilization". In their study, meanwhile analyzing measured displacements and vertical deformation of body and foundation of Alavian dam with 80 m height 4 years after completion of construction, interaction between gallery and foundation was also investigated [6]. Bemani Yazdi under guidance of Dr. Mir Mohammad Hosseini investigated behavior of embankment dam of Karkheh after impoundment and used Plaxis and CA2 to carry out back analysis and modeling of stress-strain behavior of dam. In his research, variation trend in dam behavioral characteristics such as pore water pressure and deformation and reproducing this trend was also widely investigated using numerical methods. Settlement, pore water pressure and marginal total normal pressure values and variation trend during construction and impoundment was investigated and analyzed. Obtained and measured values of pore water pressure coefficients and arching ratio were investigated. At the end of aforesaid study, prediction of dam behavior due to increase in reservoir level and also faster loss than reservoir was carried out [7]. In this research, behavior of embankment dam of Roodbal during construction and first impoundment (One of the most important stages in loading and investigation of static behavior) was investigated. Meanwhile, behavior of dam during construction and impoundment using measuring devices was monitored and recorded. To investigate behavior of dam during construction and impoundment, FLAC software was used and numerical analysis compared to monitoring data. FLAC can analyze displacements trend, stresses, arching during end of construction and impoundment. By back analysis, realistic parameters of dam behavior were obtained by which dam behavior under subsequent loading is analyzed. 1. BEHAVIORAL ASSESSMENT OF EARTH AND ROCK-FILL DAMS DURING CONSTRUCTION Stress during construction Total and effective stress during construction depends on geometry, resistance parameters, and compressibility of materials. Elastic analysis for a homogeneous dam on rigid foundation shows that normal stress in axis of symmetry depends on geometry, and lateral stress depends on Poisson's ratio of core and shell materials in addition to geometry of dam. In early stages of construction in which embankment width is larger than height, it is reasonable to suppose stress increase is equal to earth depth multiplied by weight. However, it doesn't seem to be much reasonable in next stages [8]. 2290

3 Assessment Of Stress Distribution In Darab's Roodbal Embankmend Dam Using Commercial Code Flac Displacement of dam during first impoundment Displacement of dam due to first impoundment has a complex nature. The water load can lead to following effects [9]: 1. Water load on upstream of core increases total lateral stress and cause to downward movement of dam. 2. The Effect of water load on upstream foundation is such that high compressibility of foundation significantly influence on deformation of dam body. Settlement difference inside foundation can lead to some issues. 3. Decrease of effective stress in upstream shell caused by buoyancy lead to uplift deformation. Therefore, it should be noted that tangent modulus of materials in unloading is much larger than loading. 4. Decrease of shear resistance parameters and elasticity modulus of materials of upstream shell due to wetting and submergence of rockfill material can lead to settlement. Fig. 1 illustrates the effects of impoundment on dam. Figure 1. Effect of impoundment in embankment dam. 2. FLAC COMMERCIAL SOFTWARE OPERATION PROCEDURE The following constitutive models are presented in FLAC commercial software including: Elasticisotropic model, Mohr-Coulomb plastic model, Drucker-Prager plastic model, Hoek-Brown plastic model, Strain-hardening/softening plastic model, Double-yield plastic model, and modified Cam-clay plastic model. However, there are more available plastic and creep models and pore-pressure generation models in dynamic analysis for FLAC software. One of the interesting features of FLAC is that users may modify existing or create their own constitutive models using C++ source code. 3. ROODBAL DAM INTRODUCTION Roodbal dam is located at 25 km from Darab on the road to Estahban. The river bed is 1310 m from sea level and surface area of basin through dam site is 910 km 2. Average annual precipitation of basin is 365 mm, average annual stream flow, minimum stream flow (water year, ), and maximum stream flow (water year, ) for 30-year statistical period were 110.9, 28.62, and million cubic metre, respectively. Surface area, and volume of reservoir in maximum level ( m) were 3.27 km 2 and 95 million cubic metre, respectively. Inflow floods to reservoir with 100, 1000, and year return periods are 1000, 1950, and 3100 m 3 /s, respectively [10]. Roodbal dam is an earth-rockfill type with inclined clay-core. Length, and width of crest are 485, and 10 m, respectively. Height (foundation), crest level, and width of largest section are 77, 1381, and 330 m, respectively. Normal level of dam is 1378 m from sea level. Reservoir volume and surface area at normal level are 82 million cubic metre and 2.7 km 2, respectively [10]. Surface area of grout curtain in lime bulk is m 2. Roodbal dam has three instrumentation section B, E, and H. Fig. 2 shows layout of these sections. Various instruments were used, including electrical and open standpipe piezometers, pressure cells, hydraulic and magnetic settlement meters, inclinometer probe, accelerograph and weather systems. 2291

4 KASHKOOLI, JABBARI Figure 2. Schematic view and Layout plan of instrumentation of Darab's Roodbal dam. 4. CONSTRUCTION AND IMPOUNDMENT MODELING PROCEDURE There are various methods such as grid, simple, block, and radial for creating element mesh. In this work, the block method was used. It was used six horizontal blocks (two block for foundation and four block for body) and eleven vertical block (upstrem and downstream shell, upstream and downstream filter-transition, forside space of dam in upstream and downstream and core) to create the elements. The block method can vary the mesh density [10]. To improve the solution accuracy, length-width ratio in definition of mesh elements was selected close to unit. For ratios larger than 10 FLAC software encounters ERROR. In general, the following code is used to create a block [10]: Gen x1,y1 x2,y2 x3,y3 x4,y4 i=1 m j =1 n. By above code a quadrilateral block is formed with coordination of x1,y1 x2,y2 x3,y3 x4,y4 in which number of created elements equal to m n. In order to model Roodbal dam, a mesh element with size was used. Total size of mesh was m. Fig. 3 indicates elements and geometry for modeling body and foundation of dam using FLAC software. Figure 3. Elements and geometry of model in FLAC software. 2292

5 Assessment Of Stress Distribution In Darab's Roodbal Embankmend Dam Using Commercial Code Flac Table 1. Initial values of materials parameters. Material Y, KN / m 3 E, MN / m 2 C, KN / m 2,deg,deg K x, m / s y, m/ s K e Foundation e-10 1e Core E-8 1E Filter E-4 1E Drainage e-3 1e Transition e-2 1e Zone Shell e-1 1e Cofferdam e-4 1e VERTICAL AND HORIZONTAL DEFORMATION OF DAM BODY Deformation of embankment dams during and after construction occurs both inside and outside of body, that may risk stability and performance of dam. In order to measure internal deformations two combined inclinometer-settlement system in cross section (E-E) of dam body were installed. Vertical deformations were read from inclinometer-settlement system. By surveying instruments, apparent deformations were measured via continuous pointing of embedded marked points on dam body face. The settlement metering instruments were used to measure vertical deformations so that magnet plates were installed inside the casing of settlement meter with 5 cm intervals. The lowest plate as a reference plate was installed on stone foundation in which minimum deformation occurs. In section (E-E) three settlement meter (I-E1, I-E2, I-E3) in upstream shell with 3, 53, and 103 m from dam axis were installed. To evaluate horizontal and vertical deformations of section (E-E) during construction, FLAC software was used to analyze settlement measured by described instruments. Figure 4. Contours of vertical deformation in embankment level of 1381 m, settlement variation for several levels in downstream shell for section (E-E) to embankment level of 1381 m. Fig. 4 shows the measured and calculated settlement from back analysis in downstream shell at the end of embankment level 1381 m. Data error was 5%. As shown in fig. 4, despite the measured settlement of downstream shell is somewhat less than of upstream shell, the variation trend is the same so that maximum measured settlement was 92 cm and maximum settlement obtained from back analysis was 95 cm. 2293

6 KASHKOOLI, JABBARI 6. ASSESSMENT 0F NORMAL PRESSURE IN SECTION (E-E) Three pressure cell were installed in different levels. The installed pressure cell at level of 1315 with maximum total stress was used in back analysis. Two of three cluster in this section were installed inside of core with and 1365 m level, respectively. As can be seen, variation trend of total measured pressure in the core is acceptable. In general, the pressure measured by pressure cells relative to results obtained from software decreases with increasing embankment level. This can be possible due to local arching as well as rotation of pressure plates. According to principles of the strength of materials, when a plane rotate in direction of horizon as θ, the stress in plane with angle of θ is defined as follows: 2 y cos (1) In the case of installed pressure cells in dam body if pressure plate rotate as θ So instead of normal stress,, less stress i.e. is recorded. y Figure 5. Contours of normal stress in embankment level of 1381 m. In order to install pressure cells, a hole was digged and pressure cells placed into it, then digged hole filled by the same materials. Since filling soil is compacted by light-weight rollers, the soil inside the hole is less compact than outside the hole. By increasing the embankment level local arching occurs in hole, and less stress is applied on pressure plate. Average error in total stress was 11%. 7. BEHAVIOR OF DARAB'S ROODBAL EMBANKMENT DAM DURING INITIAL IMPOUNDMENT Behavior of embankment dam of Roodbal with 71m height was investigated until the end of construction. Reading instruments installed in dam body after reaching level of 1355 during initial impoundment was interpreted and compared to analysis results, indicating natural and favorable behavior of dam during construction. In the following, considering results of back analysis and real parameters of materials, behavior of dam during utilization is predicted. 2294

7 Assessment Of Stress Distribution In Darab's Roodbal Embankmend Dam Using Commercial Code Flac Figure 6. Vertical displacement in impounding level of 1355 m, and contours of pore water pressure in the same impounding level. According to results of analysis after completion of construction and initial impoundment pore water pressure in core of dam reached 370kPa which was also the same in middle level of core. Based on results of analysis over whole body of dam displacement reached 96 cm after completion of construction which was also the same for one-third middle level of core. Figure 7. Contours of total stress in impounding level of 1355 m, and horizontal displacement in the same impounding level. As shown in fig. 7, the relative symmetry of horizontal displacement decreased that is due to impounding of downstream shell. Considering the conducted analysis over whole body of dam, total stress slightly decreases that is also the same in the corner portion of core level. 8. RESULTS AND DISCUSSION In this study, stability factors of Darab's Roodbal dam was investigated. First, reasonable data set using instrumentation data in construction period, finite element numerical method, and commercial code of FLAC via back analysis method obtained from initial data, then utilization period was divided into two 10-year periods. The period of utilization began in FLAC software uses finite difference method to model very large deformations. The main formulation in FLAC 2D was based on two-dimensional plane strain. The lagrangian computation method and special techniques of zoning helped FLAC software for better modeling of plastic flow and deformation of materials. Advantages of this software in comparison with other softwares is the capability in plane strain, plane stress, and axisymmetric models, in problems with large strain for dynamic analysis, modeling of multi-stage construction, solution of total and effective stress problems, analysis of static, linear and non-linear dynamic, and so on. 2295

8 KASHKOOLI, JABBARI Altogether, results of this study are as follows: 1- The effective stress increased from initial impoundment through end of first 10-year period, but from the beginning of second 10-year period had descending trend. 2- Variation trend of total stress in dam body was ascending, and during initial impoundment through end of first 10-year period shifted from lower level near the core to middle lower level of core. 3- Vertical displacement was slow ascending, and in upstream was slower than downstream shell. 4- Horizontal displacement was slow ascending as well as vertical displacement, and in downstream was faster due to water load. REFERENCES [1] Pagano, L. Sica, S. and Desideri, A. (2006), "Representativeness of measurments in the interpretation of earth dam behavior" Canadian Geotechnical Journal.Vol 43, pp [2] Zolfaghari, M. (2004), "Monitoring of Gavoshan dam using instrumentation data during construction and prediction of behavior at the end of construction period", Msc. thesis, university of shahid chamran of Ahvaz. [3] Clough, R.W. and Woodward, R.J. (1967), "Analysis of embankment stresses and deformation" Journal of the Soil Mechanics and Foundation Division, Vol.93 (SM4), pp [4] Ziayi, M. (2013), "Variation of pore water pressure in core of embankment dams during utilization", the 7 th National Congress on Civil Engineering, Zahedan, Iran. [5] Rattue, D.A. Hammamji, Y. Tournier, J.P. (2000), "Deformation of the Sante Marguerite 3 dam during construction and reservoir filling" 20th International Conference on Large Dams, vol.3. [6] Ebrahim nezhad, S. Emami Tabrizi, Barari, (2000), "Monitoring of foundation and body of Alavian dam during utilization", 4 th Conference on Dam Construction, Tehran, Iran. [7] Bemani Yazdi, P. (2002) "Monitoring of Karkheh dam using instrumentation data after impoundment", Msc. thesis, University of Amirkabir, Department of civil and environmental engineering. [8] Duncan, J.M. (1996), State of the art: Limit equilibrium and finite-element analysis of slopes. ASCE, Journal of Geotechnical Engineering, Vol 122 (7), pp [9] Zamiran, S. (2012), "Modeling and analysis of earthen and rock-fill structures in FLAC", Noavar Publication. [10] Technical reports and instrumentation of Roodbal dam, Parab Fars Consulting Eng. Co. 2296