ANALYSIS OF THE SEISMIC BEHAVIOR OF KEDDARA DAM USING STRONG MOTION RECORDS

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1 ANALYSIS OF THE SEISMIC BEHAVIOR OF KEDDARA DAM USING STRONG MOTION RECORDS S. Louadj, R. Bahar, E. Vinens, N. Laouami 4 Ph.D Student, Geomaterials and Environment Laboratory LGEA, University of Tizi-Ouzou, Algeria Professor, Geomaterials and Environment Laboratory LGEA, University of Tizi-Ouzou, Algeria Senior Leturer, Laboratoire de Tribologie et Dynamique des systèmes, Eole Centrale de Lyon, Frane 4 Professor, National Researh Center Applied in Earthquake Engineering, Algeria louadj_s@yahoo.fr, ramdane_bahar@mail.ummto.dz, eri.vinens@e-lyon.fr, nlaouami@gs-dz.org ABSTRACT : On May, the Keddara rokfill dam was shaken by Boumerdes earthquake (M L =6.8) whih epienter was loated at km from the dam. Aelerographs, installed on the rest and the right abutment by the Algerian National Centre of Applied Researh in Earthquake Engineering, have measured the motion during the main shok. Nonlinearities may have affeted the behavior of the dam during this event. In this study, we provide an analysis of what took plae during the main shok, and on the basis of the strong motion reords, we investigate the possible development of nonlinearities in the dam materials and the importane of loss ohereny effets. KEYWORDS: Boumerdes earthquake, strong motion, nonlinearity, ohereny loss, yli loading, shear modulus degradation, hystereti damping. INTRODUCTION Algeria is one of the most seismially ative regions in the world. The Northern part of the ountry is struturally rossed by numerous faults. Keddara dam loated no far from one of these faults was shaken by May, Boumerdes earthquake with a magnitude M L of 6.8. The dam was equipped with aelerometers that reorded the dam response during the main shok and the aftershoks. The aim of this work is to evaluate degree of redution in shear modulus and an inrease of damping with an inrease in shear amplitude of materials, onstituting Keddara dam. Ramberg-Osgood is seleted to simulate the behavior of a olumn of soil materials under sinusoidal exitation of variable amplitude.. DESCRIPTION OF KEDDARA ROCKFILL DAM The Keddara dam, loated on the Boudouaou River in Boumerdes region about 5 km east of Algiers, was ompleted in 985. This dam is 6 m high above its rok foundation, onsisting of shist, and 46.6 m width at the base. The rest has a width of m and a maximum length of 486 m. Figures and show respetively the plan view of the dam and the vertial ross setion of the dam at mid length, whih has an impervious inlined lay ore overed by the filters and transition zones. The dam shoulders are of limestone rokfill. The transition zones between ore and shoulders are onsisting of sand and gravel down to bed rok. The upstream and downstream slopes are of h/v. The material properties used in the dam onstrution are given in Table.. In this study, a D finite differene program, FLAC D, was used for the analysis. Table.. Material properties used in Keddara dam onstrution (monographi report). Zone Zone Zone Zone 4 Zone 5 Dry unit weight (kn/m ) Frition angle ( ) Cohesion (kpa) 55 8 Bulk modulus 6 (kpa) Shear modulus 6 (kpa)

2 Figure. Plan View of Keddara rokfill dam site. Normal water level : Shist : Clay : Sand 4: Gravely fill 5: Limestone Figure. Typial ross setion of Keddara dam.. ANALYSIS OF THE RECORDED MOTIONS DURING MAY, MAIN SHOCK The Fourier spetra of the motions reorded on May,, (time history shown in Figures and 5), are presented in Figures 4 and 6 for the left abutment and the rest respetively. The omputed Fourier spetra indiate that the seismi energy is mainly onentrated in a range of frequenies lower than Hz. It should be noted that two wavelets are present in the spetral representation of the motions at the bed rok in the transversal and longitudinal diretions. The first one orresponds to ontent lower than 5 Hz and was found highly amplified at the rest. The seond one, greater than 5 Hz, was not signifiantly amplified. However, in the vertial diretion, the spetral ontent is spread over a large range of frequenies while amplifiation only took plae for frequenies lower than 5 Hz.

3 Figure. Left abutment reords during /5/ main shok: a) transversal omponent; b) longitudinal omponent; ) vertial omponent Figure 4. Fourier Spetrum of left abutment reord motions during /5/ main shok: a) transversal omponent; b) longitudinal omponent; ) vertial omponent.

4 Amplirude Figure 5. Crest reords during /5/ main shok: a) transversal omponent; b) longitudinal omponent; ) vertial omponent. Figure 6. Fourier Spetrum of rest reord motions during /5/ main shok: a) transversal omponent; b) longitudinal omponent; ) vertial omponent.

5 4. DETECTION OF NONLINEARITY AND LOSS COHERENCY IN MOTIONS RECORDED ON MAY, MAIN SHOCK The oherene funtion is estimated to assess whether the experimental data inlude information relevant to the nonlinear behaviour of Keddara dam during the earthquake. It is well known that for the idealised ase of a linear system subjeted to a uniform exitation, the oherene funtion between exitation and response is equal to unity for all frequenies. However, when the oherene funtion is lower than unity, it an be attributed to noisy measurements, system nonlinearities, or spatially varying exitation (Bendat and Piersol 98). The spetral analysis of the reords during the main shok reveals a low ohereny between strutural input and output in the three diretions (Fig. 7). It an be attributed either to a nonlinear behaviour of the materials or to a spatially varying ground motion... Coherene funtion Coherene funtion Coherene funtion Figure 7. Coherene funtion between strutural input and output reords during /5/ mainshok : a) transversal omponent; b) longitudinal omponent; ) vertial omponent. 5. IDENTIFICATION OF G-γ AND D-γ CURVES FOR SAND AND CLAY USED FOR THE DAM CONSTRUCTION UNDER CYCLIC LOADING The purpose of this setion is to evaluate the degree of redution in shear modulus and the inrease of damping with the inrease in shear strain amplitude for sand and lay used in the dam onstrution, using a non-linear pattern proposed by Ramberg - Osgood (94). It is a relationship stress-strain with three parameters that reflets the deterioration of the module and takes into aount the onept of loading yle. We present below this formulation. One implemented in software analysis FLAC D, this model will be used on a olumn of soil subjet to a shear loading.

6 5.. Presentation of Ramberg-Osgood model 5... Shear modulus The formulation of Ramberg-Osgood hysteresis between stress and shear distortion an simulate elasti and non-linear behaviour of materials. The shear modulus depends on shear stress and position in the yle and hene the distortion. The lassi formula Ramberg-Osgood writes: r τ τ γ γ = + α τ τ Gmax nτ y ( ) (5.) where n = in the first loading and then n =, τ et γ are respetively the shear stress and shear strain at the last hange of diretion of loading, G max is the initial tangent shear modulus. r, α are the parameters of the model, τ y is the larger shear stress, γ y is related to τ y by the relationship : τ = G γ y max y (5.) The onstant r is a parameter ontrolling the rate of inrease of non-linearity of the stress-strain relationship. The onstant α is defined by the ratio between the maximum shear modulus and shear modulus at τ = τ y. The formulation of Ramberg-Osgood is bounded by the plasti Mohr Coulomb shear yield riterion. As proposed, the formulation (5.) is given in the form stress-strain model, to use it; it is interesting to transform it into a relationship of degradation G/G max. The module shear follows the Ramberg-Osgood formula (equation (5.)) and degrades as follows: dτ Gmax G = = d γ τ τ + α. n. τ y r (5.) The G max and K max modulus are depending on the average effetive stress and follow the Hertz law. The shear seant modulus depends on both the pressure and shear strain Damping ratio The damping D is defined by the energy dissipated by the material in a losed yle by the formula: D = π r r + G G max (5.4) 5.. Detetion of yles The formulation of Ramberg-Osgood requires knowledge of the "distane" between the urrent state of stress and that orresponding to the last hange of loading diretion τ τ. It is set during a half-yle. One the half-yle deteted, the updated mehanial properties of materials are taken into aount. The distane τ τ s s is defined as the projetion of the urrent stress tensor ( ij ij ) on the one, orresponding to

7 p the earlier half yle ( sij sij ). The distane τ τ, is written: τ τ =.5.5 p ( sij sij ) ( sij sij ) p p ( sij sij ) ( sij sij ) (5.5) p where: sij is the deviatori tensor stress, s ij is the deviatori tensor stress at the first peak, s ij is the deviatori tensor stress at the penultimate peak. During a half yle, the distane τ τ is inreasing, passes through a maximum when a peak is deteted. Reversing the diretion of the soliitation requires an updating of mehanial properties. The deviatori stress tensors p sij and sij must be updated. 5.. G-γ and D-γ urves 5... Modelling the shear test of a drained olumn of soil To represent shear phenomenon, FLAC D does not properly represent the boundary onditions on a single element of ground. Therefore, we modelled a olumn of soil, disretized into several elements, subjet to a shear movement. Initially, the olumn is subjet to a onfinement stress. It then applies a yli movement at the base of the olumn, whih an represent the movement aused by a seismi wave for example. The funtion, hosen to represent this movement, is a sine wave, whih an be of a varied range. The tests represented are in drained onditions. The results are presented in Figures 8 and 9 for the sand and in Figures and for the lay. a- Case of a soil olumn onsisting of sand used for filter and the transition zone in the dam G/Gmax,,8 Computed,6,4 Limit urves,,,e-6,e-5,e-4,e-,e- Shear train amplitude Damping (%) 8 4 Limit urves 6 Computed 8 4,E-6,E-5,E-4,E-,E- Shear strain amplitude Figure 8. Variation of shear modulus for sand; the limit urves are those set by Seed et al. (97). Figure 9. Variation of damping ratio for sand; the limit urves are those set by Seed et al. (97).

8 b- Case of a soil olumn onsisting of lay used for ore zone in the dam G/Gmax,,8,6 Ip=5,4 Ip=, omputed,,,,, Damping (%) 5 Ip=5 5 Ip= omputed 5,,,, Shear strain amplitude Shear strain amplitude Figure. Variation of shear modulus for lay; the limit urves are those set by Vueti and Dobry (99). Figure. Variation of damping ratio for lay; the limit urves are those set by Vueti and Dobry (99). 6. Conlusion In this paper, the reorded motions during Boumerdes earthquake on May,, at the left abutment and rest of Keddara rokfill dam are used and the spetral analysis is performed. This dam was strongly shaken by the main shok without any damage. Analysis of oherene funtion between strutural input and output reords during the main shok revealed lower values. This an be attributed either to a nonlinear behaviour of the materials or to a spatially varying ground motion. To evaluate the degree of redution in shear modulus and the inrease of damping with inrease in shear strain amplitude for sand and lay used in the dam onstrution, a non-linear model, proposed by Ramberg - Osgood is used. This relationship with three parameters reflets the deterioration of the modulus and takes into aount the loading yle onept in a more appropriate manner. 7. REFERENCES Agene Nationale des barrages (987). Monographie du barrage de Keddara, -4. Bendat, J.S., and Piersol, A.G. (98). Engineering appliation of orrelation and spetral analysis, John Wiley and Sons, New York, N. Y. Seed, H.B. and Idriss, I.M. (97). Soil muduli and damping fators for dynami response analysis. Report EERC 7-. Earthquake engineering researh enter, University of California, Berkeley. Vueti, M. and Dobry, R. (99). Effet of soil plastiity on yli response. Journal of the geotehnial Engineering Division, ASCE, 7:, 89-7.