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EARTHQUAKE SAFETY OF DAMS AND THE IMPORTANCE OF EMERGENCY PLANNING

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1 st National Symposium and Exposition on Dam Safety May 28-30, 2007 EARTHQUAKE SAFETY OF DAMS AND THE IMPORTANCE OF EMERGENCY PLANNING Martin WIELAND 1 ABSTRACT As earthquake ground shaking affects the
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1 st National Symposium and Exposition on Dam Safety May 28-30, 2007 EARTHQUAKE SAFETY OF DAMS AND THE IMPORTANCE OF EMERGENCY PLANNING Martin WIELAND 1 ABSTRACT As earthquake ground shaking affects the dam body and appurtenant structures and all hydromechanical and electromechanical components etc. of a dam project at the same time, the earthquake load case is probably the most challenging one for dam engineers. Thus all these elements have to be able to resist some degree of earthquake action. This also applies to temporary structures like cofferdams, etc. Less than ten embankment, buttress and concrete gravity dams of significant size have been severely damaged during earthquakes and no loss of life has been reported as a direct consequence of the failure of a dam. This very favourable record may be due to the fact that few modern dams have been exposed to strong ground shaking. During the 2001 Bhuj earthquake in India about 245 embankment dams most of them small dams used for irrigation and water supply were damaged. Fortunately, the reservoirs were almost empty at the time of the earthquake and thus further damage could be prevented. Little experience exists about the seismic performance of modern RCC and concrete face rockfill dams, which are being built in increasing number today. A qualitative assessment of the expected earthquake behaviour of these new types of dams is given. The work carried out by the Committee on Seismic Aspects of Dam Design of the International Commission on Large Dams (ICOLD) is presented and its role in harmonizing seismic design criteria is discussed. Finally the need for emergency planning and water alarm systems for important dam projects is discussed. By issuing a timely warning, practically all people in the flood plain can be saved. Experience with the water alarm in Switzerland is discussed where 65 dams are equipped with such a system. The first installations were already made more than 50 years ago. Keywords: dam safety, existing dams, concrete dams, embankment dams, emergency planning INTRODUCTION The 1971 San Fernando earthquake, which caused near failure of the Lower San Fernando dam, a hydraulic fill dam, had a major impact on the earthquake analysis and design of dams. Until then, concrete dams were designed against earthquakes using a pseudostatic method, which originated in the 1930s and included the inertia forces of the dam and the hydrodynamic pressures from the reservoir. In the case of embankment dams, slope stability analyses were carried out in which the pseudostatic inertia forces of the sliding mass were taken into account. This concept is even older than that mentioned for concrete dams. Significant progress has been achieved since 1971 in the linear-elastic dynamic analysis of concrete dams and such analyses are routinely carried out nowadays for dam projects. For embankment dams, the equivalent linear method of analysis has been developed, which is also very popular among dam engineers. The true nonlinear dynamic behaviour of concrete and embankment dams is still under research and development. Also dynamic concrete dam-foundation interaction is a problem, which has not yet been 1 Chairman, ICOLD Committee on Seismic Aspects of Dam Design, Poyry Energy Ltd., Hardturmstrasse 161, CH-8037 Zurich, Switzerland, solved satisfactorily as the proposed foundation models are still far from representing reality. Significant progress has also been achieved in the understanding and in the testing of the dynamic characteristics of embankment and foundation materials. Before substantial further progress is possible, additional information has to be collected from dams, which have experienced severe ground shaking similar to the one expected during the maximum credible earthquake (MCE). These events will show the actual nature of earthquake problems of dams. In the absence of such information, it is necessary to perform model tests up to dam failure. The characteristics of near-fault ground motions with high velocity pulses may also be considered. Earthquakes affect the dam, foundation, safety devices, pressure system, appurtenant structures, underground works, hydromechanical and electromechanical equipment, etc. at the same time. Therefore, they have to be designed or checked for their earthquake resistance and safety. For the safety-relevant components of dam, the same design criteria shall be used as for the dam. For the appurtenant structures, the building codes may be used if no specific guidelines exist. Generally, dam engineers, geologists, mechanical and electrical engineers, etc. are mainly looking into their specific problems. Thus it is quite likely that different seismic design criteria are used by different engineers working at the same project. Earthquake is one of the hazards, which may cause failure of a dam. If there are public concerns about the earthquake safety of a dam, then emergency planning and the installation of water alarm systems are highly recommended. This would not only minimise the loss of lives in the unlikely event of a flood wave but, more importantly, this would also contribute to the improvement of public confidence in dam safety. SPECIAL FEATURES OF EARTHQUAKE HAZARD TO BE CONSIDERED FOR LARGE DAM PROJECTS The estimate of the design earthquake ground motion is an important task for all large construction and infrastructure projects. Earthquakes are multiple hazards, which have the following main features in the case of a storage dam: (i) ground shaking causing vibrations in dams, appurtenant structures and equipment, and their foundations; (ii) fault movements in the dam foundation causing structural distortions; (iii) fault displacement in the reservoir bottom causing water waves in the reservoir or loss of freeboard; and (iv) mass movements into the reservoir causing impulse waves in the reservoir. Usually the main hazard, which is addressed in seismic codes and regulations, is the earthquake ground shaking. It causes stresses, deformations, cracking, sliding, overturning, liquefaction, etc. A hazard, which is often underestimated, is the large number of rockfalls in mountainous regions. If a major earthquake capable of damaging a well-constructed dam occurs, then it has to be expected that the buildings and infrastructure in the reservoir region are also severely damaged and that access to the (often remote) dam site and the reservoir may be obstructed. OBSERVED SEISMIC DEFORMATIONS IN CONCRETE DAMS The few observations of earthquake damage in concrete gravity dams show that ground shaking results in formation of cracks in the highly stressed central crest region along some weak planes, such as horizontal lift surfaces and grouted vertical contraction joints (Fig. 1). Once a concrete block gets separated from the rest of the dam by such cracks, it can experience substantial inelastic (nonlinear) displacements in the form of rocking and sliding without actually leading to a dam failure. As no arch dam has suffered serious damage during earthquake ground shaking, little experience exists about the damage, which can be caused by, for example, the MCE. However, based on linear-elastic dynamic analyses, it is obvious that a strong earthquake induces tensile stresses that exceed the dynamic tensile strength of mass concrete and thus joint opening and/or cracking can be expected. Fig. 1: Horizontal crack at lift joints in Sefid Rud buttress dam caused by the magnitude 7.5, 1990 Manjil earthquake in Iran (left: crack at upstream face; right: crack in web of buttress at downstream face of the dam) SEISMIC DEFORMATIONS OF EMBANKMENT DAMS The seismic safety of embankment dams is assessed by investigating the following: permanent deformations experienced during and after an earthquake (e.g. loss of freeboard); stability of slopes during and after the earthquake, and dynamic slope movements; build-up of excess pore water pressures in embankment and foundation materials (soil liquefaction); damage to filter, drainage and transition layers; damage to waterproofing elements in dam and foundation (core, upstream concrete or asphalt membranes, geotextiles, grout curtain, diaphragm walls in foundation, etc.) vulnerability of dam to internal erosion after formation of cracks and limited sliding movements of embankment slopes, or formation of loose material zones due to high shear, etc. For large embankment dams, the expected seismic deformations must be determined. The calculations of the permanent settlement of large rockfill or concrete-face rockfill (CFR) dams are rather approximate, as dynamic soil tests are usually carried out with aggregate sizes of less than 5 cm. This is a problem for dams, where the coarse rock aggregates, have not been properly compacted at the time of construction. Poorly compacted rockfill may settle significantly during strong ground shaking but may still withstand strong earthquakes. Transverse cracking as a result of deformations is an important aspect. Cracks could lead to failure of an embankment dams when the dam does not have filter, drain and transition zones, or filter, drain and transition zones do not extend above the reservoir water surface, or modern filter criteria were not used to design the dam. SEISMIC ASPECTS OF ROLLER COMPACTED CONCRETE DAMS Most roller compacted concrete (RCC) dams are gravity dams with an earthquake behaviour that is similar to that of conventional gravity dams. High seismic stresses occur in the central upper portion of gravity and arch-gravity dams. The main difference between RCC and conventional gravity dams is on the one hand the dynamic behaviour of mass concrete and on the other hand the spacing of lift joints. In RCC dams, the tensile strength in a lift joint may be a fraction of that of the parent mass concrete. This means that, in case of a strong earthquake, horizontal cracks are more likely to form along these interfaces. Besides, there will also be opening of vertical contraction joints. It has to be assumed that horizontal cracks extend from the upstream to the downstream face of a dam and thus completely separate the upper portion of the dam from the remaining part. Such cracks protect the remaining dam body from development of even higher stresses. Also, the dam deformations will be mainly due to crack opening. Thus the post-cracking dynamic behaviour of concrete blocks separated by cracks or joints can be modelled by relatively simple rigid body models. Based on a qualitative assessment, it can be concluded that the seismic safety of RCC dams under strong ground shaking is most probably satisfactory, as cracks in the highly stressed central upper portion of the dam will develop along the horizontal construction interfaces. This is favourable for the dynamic stability of detached concrete blocks during strong ground shaking. However, further studies and observational evidence are needed to support this conclusion (Wieland 2003; Wieland et al. 2004). SEISMIC ASPECTS OF CONCRETE-FACE ROCKFILL DAMS The seismic safety of concrete-face rockfill (CFR) dams is often assumed to be superior to that of conventional rockfill dams with impervious core. In general, embankment dams are analysed with a two-dimensional model of the highest dam section. In such a seismic analysis, only relatively small dynamic stresses result in the concrete face. Due to the fact that the deformational behaviour of the concrete slab, which acts as a rigid diaphragm for vibrations in cross-canyon direction, is very different from that of the rockfill and transition zone material, the cross-canyon response of the rockfill may be restrained by the relatively rigid concrete slab. This may result in high in-plane stresses in the concrete slab that may be sufficiently large to cause local buckling, shearing off of the slab along the joints or to damage the plinth. Although this is still a hypothetical scenario, it is necessary to look carefully into the behaviour of the concrete face under the cross-canyon component of the earthquake ground shaking. Therefore, it is also not so obvious that CFR dams are more suitable to cope with strong earthquakes than conventional embankment dams. As experience with the seismic behaviour of CFR dams is still very limited, more efforts have to be undertaken to study the seismic behaviour of these dams (Wieland 2003; Wieland et al. 2004). RESERVOIR-TRIGGERED SEISMICITY If a large dam has been designed so that it can safely withstand the ground motions caused by the MCE, it can also withstand the effects of the largest reservoir-triggered earthquake, as the maximum reservoir-triggered earthquake cannot be stronger than the MCE. Thus, reservoir-triggered seismicity (RTS) is not a safety problem for a well-designed dam. However, RTS may still be a problem for the buildings and structures in the vicinity of a dam, because they have a lower earthquake resistance than the dam. In the great majority of RTS events, the magnitudes are small and of no safety concern. As reservoir-triggered earthquakes have often a shallow focus and their epicentres are relatively close to the dam sites or the reservoir, the peak ground accelerations can be quite high for the strongest events. The psychological effects of RTS shall not be underestimated especially when people living close to a dam or the reservoir area experience suddenly a large number of seismic shocks. For example, in a geothermal project in Basel, Switzerland earthquakes with magnitudes up to 3.4 have occurred following pumping of water into the rock at a depth of 5 km. Although these events did probably not cause any damage, the project had to be suspended and its future is uncertain. COMPREHENSIVE EARTHQUAKE SAFETY CONCEPT The main safety concern is the failure of a dam and the uncontrolled release of the reservoir water with flood consequences (loss of life, economical damage, environmental damage etc.), which will usually exceed the economical damage to the dam. Therefore, for the seismic risk assessment of a dam, full reservoir is the critical situation. Basically, the seismic safety of a dam depends on the following factors: 1. Structural Safety: strength to resist seismic forces without damage; capability to absorb high seismic forces by inelastic deformations (opening of joints and cracks in concrete dams; movements of joints in the foundation rock; inelastic deformation characteristics of embankment materials); stability (sliding and overturning stability), etc. 2. Safety Monitoring: strong motion instrumentation of dam and foundation; visual observations and inspection after an earthquake; data analysis and interpretation; post-earthquake safety assessment etc. 3. Operational Safety: Rule curves and operational guidelines for post-earthquake phase; experienced and qualified dam maintenance staff, etc. 4. Emergency Planning: water alarm; flood mapping and evacuation plans; safe access to dam and reservoir after a strong earthquake; lowering of reservoir; engineering back-up, etc. In general, dams, which can resist the strong ground shaking of the MCE, will perform well under other types of actions. In the subsequent section, the emphasis is on the structural safety of dams, which can be improved by (i) changing the dynamic behaviour and seismic response of the dam, and (ii) by reducing the vulnerability of the dam. These measures are referred to as structural measures. Safety monitoring, operational safety and emergency planning are non-structural measures among those only emergency planning will be discussed. SEISMIC DESIGN AND PERFORMANCE CRITERIA For the seismic design of dams, abutments and safety relevant components (spillway gates, bottom outlets, etc.) the following types of design earthquakes are used (Wieland 2005): 1. Operating Basis Earthquake (OBE): The OBE design is used to limit the earthquake damage to a dam project and, therefore, is mainly a concern of the dam owner. Accordingly, there are no fixed criteria for the OBE although ICOLD has proposed an average return period of ca. 145 years (50% probability of exceedance in 100 years). Sometimes return periods of 200 or 500 years are used. The dam shall remain operable after the OBE and only minor, repairable damage is accepted. 2. Maximum Credible Earthquake (MCE), Maximum Design Earthquake (MDE) or Safety Evaluation Earthquake (SEE): Strictly speaking, the MCE is a deterministic event, and is the largest reasonably conceivable earthquake that appears possible along a recognized fault or within a geographically defined tectonic province, under the presently known or presumed tectonic framework. But in practice, due to the problems involved in estimating of the corresponding ground motion, the MCE is usually defined statistically with a typical return period of 10,000 years. Thus, the terms MDE or SEE are used as substitutes for the MCE. The stability of the dam must be ensured under the worst possible ground motions at the dam site and no uncontrolled release of water from the reservoir shall take place, although significant structural damage is accepted. In the case of significant earthquake damage, the reservoir may have to be lowered. Historically, the performance criteria for dams and other structures have evolved from the observation of damage and/or experimental investigations. As mentioned above, the performance criteria for dams during the OBE and MCE/SEE are of very general nature and have to be considered on a case-by-case basis. In view of the large uncertainties in predicting the seismic behaviour of dams it is strongly recommended to increase its resilience to earthquake loading rather than trying to reduce the uncertainties in the seismic hazard, the scatter in the material properties or to use more sophisticated methods of seismic analyses, which incorporate all kind of features such as e.g. dynamic dam-soil interaction effects etc. SEISMIC RISK CONSIDERATIONS The requirement that the dams with a large damage potential shall be able to withstand the ground motion caused by the MCE or the 10,000-year SEE is a logical and consistent requirement as the same dams have to be able to safely release the probable maximum flood (PMF) or the 10,000 year flood. Strong earthquakes are rare events, especially in regions of low to moderate seismicity, and there are great uncertainties involved in the estimation of the maximum ground motion at a dam site. The discrepancies between design accelerations used for older dams and the PGA value of the SEE can be very large. In regions of moderate seismicity, floods are generally considered as the dominant natural hazard. A recent study in Switzerland has, however, shown that about 50% of the risk from the natural environment is caused by earthquakes, and flooding is less than one third of that. The main portion of the seismic risk originates from the strongest earthquakes. Due to the very low probability of occurrence of destructive earthquakes, the earthquake risk has been underestimated and it has taken quite some time to realise this fact. This is a general problem with low probability and high impact hazards. UNIFORM APPLICATION OF DESIGN CONCEPTS BY DAM, HYDROMECHANICAL AND ELECTROMECHANICAL ENGINEERS Civil, hydromechanical and electromechanical engineers have different guidelines for the seismic design of structures and equipment. In many cases, a value of 0.1 g was used irrespective of the location of the equipment within a dam. In the nuclear industry the so-called floor response spectra have been used for a long time, which serve as the seismic input for the equipment design. In dams, the same concept has to be used. Secondary structures and equipment attached to or located in the dam have to be designed for the support motion acting at the location of these secondary structures. Depending on the safety class of the equipment they may not have to be designed for the MCE. Equipment or secondary structu
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