外文原文

73rd Annual Meeting of ICOLD
Tehran, IRAN
May 1- 6, 2005
Paper No.: 012-W4 REVIEW OF SEISMIC DESIGN CRITERIA OF LARGE CONCRETE
AND EMBANKMENT DAMS
Martin Wieland
Chairman, ICOLD Committee on Seismic Aspects of Dam Design, c/o Electrowatt-Ekono Ltd. (Jaakko Pöyry Group),
Hardturmstrasse 161, CH-8037 Zurich, Switzerland
ABSTRACT
Of all the actions that dams have to resist from the natural and man-made environment, earthquakes pose probably the greatest challenge to dam engineers as earthquake ground shaking affects all structures (dam body, grout curtains, diaphragm walls, underground structures and different appurtenant structures) and all components (hydromechanical, electromechanical, etc.) at the same time (ICOLD, 2001; ICOLD, 2002). Thus all these elements have to be able to resist some degree of earthquake action. This also applies to temporary structures like cofferdams, etc. For the specification of the degree of earthquake action these components have to be able to resist an assessment of the consequences of possible failure scenarios is needed.
Little experience exists about the seismic performance of modern concrete-face rockfill dams, which are being built in increasing number today. A qualitative assessment of these dams is given.
Keywords: concrete dams, embankment dams, concrete-face rockfill dams, CFRD, grout curtain, earthquake design criteria for dams
INTRODUCTION
Since the 1971 San Fernando earthquake in California, major progress has been achieved in the understanding of earthquake action on concrete dams. The progress was mainly due to the development of computer programs for the dynamic analysis of dams. However, it is still not possible to reliably predict the behaviour of dams during very strong ground shaking due to the difficulty in modelling joint opening and the crack formation in the dam body, the nonlinear behaviour of the foundation, the insufficient information on the spatial variation of ground motion in arch dams and other factors. The same applies to embankment dams where the results of inelastic dynamic analyses performed by different computer programs tend to differ even more than in the case of concrete dams. Also, considerable progress has been made in the definition of seismic input, which is one of the main uncertainties in the seismic design and seismic safety evaluation of dams.
It has been recognized that during strong earthquakes such as the maximum credible earthquake (MCE), the maximum design earthquake (MDE) or the safety evaluation earthquake (SEE) ground motions can occur, which exceed those used for the design of large dams in the past. Already a moderate shallow-focus earthquake with a magnitude of say 5.5 to 6 can cause peak ground accelerations (PGA) of 0.5 g. However, the duration of strong ground shaking of such events is quite short and the predominant frequencies of the acceleration time history are also rather high. Therefore, smaller concrete dams may be more
vulnerable to such actions than high dams, which have predominant eigenfrequencies that are smaller than those of such ground acceleration records.
We have to recognize that most of the existing dams have been designed against earthquake actions using the pseudostatic approach with an acceleration of 0.1 g. In regions of high seismicity like Iran the PGA of the SEE may exceed 0.5 g at many dam sites. Therefore, some damage may have to be expected in concrete dams designed by a pseudostatic method with a seismic coefficient of 0.1.
Because of the large differences between the design acceleration and the PGA, and because of the uncertainties in estimating the ground motion of very strong earthquakes at a dam site, mechanisms are needed that ensure that a dam will not fail if the design acceleration is exceeded substantially.
In the case of large dams, ICOLD recommends to use the MCE as the basis for the dam safety checks and dam design. Theoretically no ground motion should occur, which exceeds that of the MCE. However, in view of the difficulties in estimating the ground motion at a dam site, it is still possible that larger ground motions may occur. Some 50 years ago, many structural engineers considered a value of ca. 0.2 g as the upper bound of the PGA, but today with more earthquake records available, the upper bound has exceeded 1 g and some important structures have already been checked against such high ground accelerations.
1. SEISMIC DESIGN CRITERIA FOR LARGE DAM PROJECTS
According to ICOLD Bulletin 72 (1989), large dams have to be able to withstand the effects of the MCE. This is the strongest earthquake that could occur in the region of a dam, and is considered to have a return period of several thousand years (typically 10’000 years in regions of low to moderate seismicity).
Per definition, the MCE is the largest event that can be expected to affect the dam. This event can be very powerful and can happen close to the dam. The designer must take into account the motions resulting from any earthquake at any distance from the dam site, and possible movement of the foundation if a potentially active fault crosses the dam site. Having an active fault in the foundation is sometimes unavoidable, especially in highly seismically active regions, and should be considered as one of the most severe design challenges requiring special attention.
It has to be kept in mind that each dam is a prototype structure and that the experience gained from the seismic behaviour of other dams has limited value, therefore, observations have to be combined with sophisticated analyses, which should reflect reality as close as possible.
We also have to realize that earthquake engineering is a relatively young discipline with plenty of unsolved problems. Therefore, every time there is another strong earthquake some new unexpected phenomena are likely to emerge, which have implications on regulations and codes. This is particularly true for dams as very few modern dams have actually been exposed to very strong ground motions.
As mentioned earlier, the time of the pseudostatic design with a seismic coefficient of 0.1 has long passed. Of course, this concept was very much liked by designers because the small seismic coefficients being used did not require any special analyses and the seismic requirement could easily be satisfied. As a result, the seismic load case was usually not the governing one. This situation has changed and the earthquake load case has become the governing one for the design for most high risk (dam) projects, especially in regions of moderate to high seismicity.
As a general guideline, the following minimum seismic requirements should be observed:
(i)Dam and safety-relevant elements: Design earthquakes (i.e. operating basis earthquake (OBE),
and SEE) are determined based on a seismic hazard study and are usually represented by appropriate response spectra (and the PGA or the effective peak acceleration).
(ii)All appurtenant structures and non safety-relevant elements of dam: Use of local seismic building codes (including seismic zonation, importance factor, and local soil conditions) if no other regulations are available. However, the seismic design criteria should not be less than
those given in building codes. As shown in Fig. 1 the earthquake damage to appurtenant structures can be very severe.
(iii)Temporary structures and construction phase: The PGA for temporary structures and the construction phase should be of the order of 50% of the PGA of the design earthquake of the seismic building code. For important cofferdams a separate seismic hazard assessment may be needed. According to Eurocode 8 (2004), the design PGA for temporary structures and the construction phase, PGA c can be taken as
PGA c = PGA (t rc/t ro)k
where PGA is according to the building code, t ro = 475 years (probability of exceedance of 10% in 50 years), for k a value between 0.3 and 0.4 can be used depending on the seismicity of the region and
t rc = approx. t c/p
where t c is the duration of the construction phase and p is the acceptable probability of exceedance of the design seismic event during this phase, typically a value of p = 0.05 is selected.
Therefore, assuming k = 0.35 and a construction phase of an appurtenant structure of 3 years results in PGA c = 0.48 PGA, and for a cofferdam, which may stay for 10 years, we obtain PGA c = 0.74 PGA. This numerical example shows that the seismic action for temporary structures and for construction phases can be quite substantial. In many cases the effects of seismic action during dam construction has been underestimated or ignored.
In general, building codes exclude dams, powerplants, hydraulic structures, underground structures and other appurtenant structures and equipment. It is expected that separate codes or regulations cover these special infrastructure projects. However, only few countries have such regulations. Therefore, either the ICOLD Bulletins (ICOLD Bulletin 123, 2002) are used as a reference or the (local) seismic building codes (Eurocode 8, 2004). The seismic building codes are very useful reference documents for checking the design criteria for a dam. As the return period of the SEE of a large dam is usually much longer than the return period of the design earthquake for buildings, which in many parts of the world is taken as 475 years, the PGA values of the SEE should be larger than that of the design earthquake for building structures with a 475 year return period multiplied with the importance factor for high risk projects. If this basic check is not satisfied, then a building located at the dam site would have to be designed for stronger ground motions than the dam. In such controversial situations further investigations and justifications are needed.
In the past, the pseudostatic acceleration of 0.1 g was used for many dams. This acceleration can be assumed as effective ground acceleration, which is ca. 2/3 of the PGA. Therefore, the SEE for a dam with large damage potential should not have a PGA of less than say 0.15 g as this would be less than what was used in the past. However, this is mainly a concern for dam projects in regions of low to moderate seismicity (Wieland, 2003).
For the seismic design of equipment, the acceleration response calculated at the location of the equipment (so-called floor response spectra) shall be used. It should also be noted that the peak acceleration at the equipment location is normally larger than the PGA. For example, the radial acceleration on the crest of an arch dam is about 5 times larger than the PGA of the SEE (nonlinear dynamic response of the dam) and up to 8 times larger in the case of the OBE (values of up to 13 have been measured in Satsunai concrete gravity dam during the 26.9.2003 Tokachi-Oki earthquake in Japan).
2. SEISMIC SAFETY ASPECTS AND SEISMIC PERFORMANCE CRITERIA
Basically, the seismic safety of a dam depends on the following factors (Wieland, 2003):
(1)Structural Safety: site selection; optimum dam type and shape; construction materials and quality
of construction; stiffness to control static and dynamic deformations; 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;
plastic deformation characteristics of embankment materials); stability (sliding and overturning stability), etc.
(2)Safety Monitoring and Proper Maintenance: strong motion instrumentation of dam and
foundation; visual observations and inspection after an earthquake; data analysis and interpretation; post-earthquake safety assessment etc. (The dams should be maintained properly including periodic inspections).
(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.
These basic safety elements are almost independent of the type of hazard. In general, dams, which can resist the strong ground shaking of the MCE, will also perform well under other types of loads.
In the subsequent sections, the emphasis will be put on the structural safety aspects, which can be improved by structural measures. Safety monitoring, operational safety and emergency planning are non-structural measures as they do not reduce the seismic vulnerability of the dam directly.
For the seismic design of dams, abutments and safety relevant components (spillway gates, bottom outlets, etc.) the following types of design earthquakes are used (ICOLD, 1989):
•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 easily repairable damage is accepted.
•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 for countries of low to moderate seismicity. 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. 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.
3. EARTHQUAKE DESIGN ASPECTS OF CONCRETE DAMS
There are several design details that are regarded as contributing to a favourable seismic performance of arch dams (ICOLD, 2001; Wieland, 2002), i.e.:
•Design of a dam shape with symmetrical and anti-symmetrical mode shapes that are excited by along valley and cross-canyon components of ground shaking.
•Maintenance of continuous compressive loading along the foundation, by shaping of the foundation, by thickening of the arches towards the abutments (filets) or by a plinth structure to support the dam and transfer load to the foundation.
•Limiting the crest length to height ratio, to assure that the dam carries a substantial portion of the applied seismic forces by arch action, and that nonuniform ground motions excite higher modes and lead to undesired stress concentrations.
•Providing contraction joints with adequate interlocking.
•Improving the dynamic resistance and consolidation of the foundation rock by appropriate excavation, grouting etc.
•Provision of well-prepared lift surfaces to maximize bond and tensile strength.
•Increasing the crest width to reduce high dynamic tensile stresses in crest region.
•Minimizing unnecessary mass in the upper portion of the dam that does not contribute effectively to the stiffness of the crest.
•Maintenance of low concrete placing temperatures to minimize initial, heat-induced tensile stresses and shrinkage cracking.
•Development and maintenance of a good drainage system.
The structural features, which improve the seismic performance of gravity and buttress dams, are essentially the same as that for arch dams. Earthquake observations have shown that a break in slope on the downstream faces of gravity and buttress dams should be avoided to eliminate local stress concentrations and cracking under moderate earthquakes. The webs of buttresses should be sufficiently massive to prevent damage from cross-canyon earthquake excitations.
4. DYNAMIC ANALYSIS ASPECTS OF CONCRETE DAMS
The main factor, which governs the dynamic response of a dam subjected to small-amplitude ground motions, can be summarized under the term damping. Structural damping ratios obtained from forced and ambient vibration tests are surprisingly low, i.e. damping ratios of the lowest modes of vibrations are of the order of 1 to 2% of critical. In these field measurements the effect of radiation damping in the foundation and the reservoir are already included. Linear-elastic analyses of dam-foundation-reservoir systems would suggest damping ratios of about 10% for the lowest modes of vibration and even higher values for the higher modes. Under earthquake excitations the dynamic stresses in an arch dam might be a factor of 2 to 3 smaller than those obtained from an analysis with 5% damping where the reservoir is assumed to be incompressible and the dynamic interaction effects with the foundation are represented by the foundation flexibility only (massless foundation). Therefore, the dam engineer may be willing to invest more time in a sophisticated dynamic interaction analysis in order to reduce the seismic response of an arch dam. Unfortunately, there is a lack of observational evidence, which would justify the use of large damping ratios in seismic analyses of concrete dams.
Under strong ground shaking caused by the MCE or SEE tensile stresses are likely to occur that exceed the dynamic tensile strength of mass concrete. In addition, there are contraction joints and lift joints, which have tensile strength properties that are inferior to those of the parent mass concrete. Therefore, in the highly stressed upper portion of an arch dam the contraction joints will start to open first and cracks will develop along the lift joints (this coincides with the seismic behaviour of Sefid Rud dam). Along the up- and downstream contacts between the dam and the foundation rock local stress concentrations will occur, which will lead to the formation of cracks in the concrete and the foundation rock. Little information exists on this type of cracks. But such cracks are also likely to develop at the upstream heel of the dam under the hydrostatic water load. Thus, the dynamic deformations of the dam will mainly occur at these contraction and lift joints and along a few cracks in the mass concrete. The remaining parts of the dam will behave more or like as rigid bodies and will exhibit relatively small dynamic tensile stresses. Joint opening and crack formation will also lead to higher compressive stresses in the dam. However, these dynamic compressive stresses are unlikely to cause any damage and the engineer can focus on tensile stresses only. For the analysis of joint opening other numerical models are needed than for the linear-elastic dynamic analysis of a dam.
Two concepts are used for the nonlinear analysis of a cracked dam, i.e. the smeared crack approach, and the discrete crack approach. The main advantages of discrete crack models are their simplicity (only information on strength properties of joints is needed) and their ability to model the observed behaviour of dams during strong ground shaking. Further developments are still needed.
In nonlinear dam models radiation damping may play a less prominent role as in the case of a linear-elastic analysis where this issue is still controversial. Once cracks along joints are fully developed, the viscous damping in the cracked region of a dam may be replaced by hysteretic damping in the joints. As these damping mechanisms are still not well understood or are too complex to be considered in practical analyses, it is recommended to perform a sensitivity analysis in which the effect of damping on the dynamic response of a dam is investigated.
The dynamic tensile strength of mass concrete, ft, is another key parameter in the seismic safety assessment of concrete dams. It depends on the following factors:
•uniaxial compressive strength of mass concrete, f c’, different correlations between f t and f c’ have been proposed in the literature;
•age effect on concrete strength (it may be assumed that the OBE or SEE occurs when the dam has reached about one-third of the design life);
•strain-rate effect (under earthquake loading, the tensile strength increases); and
•size effect (the tensile strength depends on the fracture toughness of mass concrete and the thickness of the dam).
If the size effect is considered, the dynamic tensile strength of mass concrete in relatively thick arch-gravity dams drops to below 3 to 4 MPa.
An additional factor, which is hardly considered in arch dam analyses, is the spatial variation of the earthquake ground motion.
The criteria for the assessment of the safety of the dam foundation during strong ground shaking need further improvements. Today, the dam analyst delivers the seismic abutment forces to the geotechnical engineer, who performs rock stability analyses.
5. ASSESSMENT OF SEISMIC DESIGN OF EMBANKMENT DAMS
Basically, the seismic safety and performance of embankment dams is assessed by investigating the following aspects (Wieland, 2003):
•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 (i.e. whether they will function properly after the earthquake);
•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 (shear bands), etc.
•vulnerability of hydromechanical equipment to ground displacements and vibrations, etc.
•damage to intake and outlet works (release of the water from the reservoir may be endangered).
The dynamic response of a dam during strong ground shaking is governed by the deformational characteristics of the different soil materials. Therefore, most of the above factors are directly related to deformations of the dam.
Liquefaction phenomena are a major problem for tailings dams and small earth dams constructed of or founded on relatively loose cohesionless materials, and used for irrigation and water supply schemes that have not been designed against earthquakes. This can be assessed based on relatively simple in situ tests. For example, there exist empirical relations between SPT blow counts and liquefaction susceptibility for different earthquake ground motions, which are characterized by the number of stress cycles and the ground acceleration.
For large storage dams, the earthquake-induced permanent deformations must be calculated. Damage categories are, e.g., expressed in terms of the ratio of crest settlement to dam height. The calculations of the permanent settlement of large rockfill dams based on dynamic analyses are still very approximate, as most of the dynamic soil tests are usually carried out with maximum aggregate size of less than 5 cm. This is a particular problem for rockfill dams and other dams with large rock aggregates and in dams, where the shell materials, containing coarse rock aggregates, have not been compacted at the time of construction. Poorly compacted rockfill may settle significantly during strong ground shaking but may well withstand strong earthquakes.
To get information on the dynamic material properties, dynamic direct shear or triaxial tests with large samples are needed. These tests are too costly for most rockfill dams. But as information on the dynamic behaviour of rockfill published in the literature is also scarce, the settlement prediction involves sensitivity analyses and engineering judgment.
Transverse cracking as a result of deformations is an important aspect. Cracks could cause failure of embankment dams that do not have filter, drain and transition zones; or have filter, drain and transition zones that do not extend above the reservoir water surface; or modern filter criteria were not used to design the dam.
6. SEISMIC ASPECTS OF CONCRETE FACED ROCKFILL DAMS
The seismic safety of concrete-faced rockfill dams (CFRDs) is often assumed to be superior to that of conventional rockfill dams with impervious core. However, the crucial element in CFRDs is the behaviour and performance of the concrete slab during and after an earthquake.
The settlements of a rockfill dam caused by the MCE or SEE are rather difficult to predict and depend on the type of rockfill and the compaction of the rockfill during dam construction. Depending on the valley section, the dam deformations will also be non-uniform along the upstream face, causing differential support movements of the concrete face, local buckling in compression zones etc.
In many cases, embankment dams are analysed with the equivalent linear method using a two-dimensional model of the highest dam section. In such a seismic analysis, only reversible elastic deformations and stresses are calculated, which are small and do not cause high dynamic stresses in the concrete face. These simple models have to be complemented by models, which also include the cross-canyon component of the earthquake ground motion as well as the inelastic deformations of the dam body. For such a dynamic analysis, a three-dimensional dam model has to be used and the interface between the concrete face and the soil transition zones must be modelled properly.
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. The seismic forces that can be transferred from the rockfill to the concrete slab are limited by the friction forces between the transition zone of the rockfill and the concrete slab. Due to the fact that the whole water load is supported by the concrete slab, these friction forces are quite high and, therefore, the in-plane stresses in the concrete slab 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 CFRDs are more suitable to cope with strong earthquakes than conventional embankment dams. The main advantage with CFRDs is their resistance to erosion if water seeps through a cracked face.
As experience with the seismic behaviour of CFRDs is still very limited, more efforts have to be undertaken to study the seismic behaviour of these dams (Wieland, 2003; Wieland and Brenner, 2004).
7. SEISMIC ASPECTS OF DIAPHRAGM WALLS AND GROUT CURTAINS
7.1 Diaphragm Walls
Diaphragm walls are used as waterproofing elements in embankment dams on soils or on very pervious rock and used to be made of ordinary reinforced concrete. Today, preference is given to plastic concrete. The wall should have a stiffness of similar magnitude to that of the surrounding soil or rock in order to prevent the attraction of load when the soil deforms after dam construction. The dynamic stiffness of both the wall and the surrounding soil or rock, however, is higher than the corresponding static value.
Although earthquakes may still cause significant dynamic stresses in the plastic concrete cut-off wall, sufficient ductility of the plastic concrete will minimize the formation of cracks. The highest stresses in the wall are expected to be caused by seismic excitations in cross-canyon direction.。