大坝安全和地震风险

Why Dam Safety Is an Earthquake Priority

Dams represent a unique category of civil infrastructure where seismic failure creates 次生地震灾害由地震震动引发而非震动本身直接造成的灾害，包括海啸、滑坡、液化、火灾、水坝溃决及化学品泄漏等，其造成的损失往往超过震动本身。 of potentially catastrophic scale. A dam failure releases not just the structure itself but the entire reservoir behind it — millions to billions of cubic meters of water that inundates downstream communities with little warning. The combination of structural failure and sudden flood creates a compound disaster that can kill thousands and devastate entire valleys.

The 最大可信地震（MCE）基于地质证据，特定断层或地区合理可能发生的最大规模地震，用于水坝、核电站等重要设施的设计。 (Maximum Considered Earthquake) concept is central to dam safety regulation worldwide. Unlike most buildings where the design earthquake represents a probability of exceedance over a 50-year building life, dam design often uses deterministic maximum credible earthquake scenarios that consider the largest physically possible earthquake on capable faults that could affect the dam site. The asymmetry of consequences — dam failure releasing a catastrophic flood — justifies this extreme conservatism in hazard characterization.

Dam Types and Their Seismic Behavior

Concrete dams — gravity dams, arch dams, and buttress dams — respond to earthquakes differently from earthfill dams. Concrete gravity dams rely on their mass and base friction to resist both water pressure and earthquake forces. Their seismic vulnerability concentrates in the concrete's tensile capacity, which is limited, and in the base contact zone, where sliding or rocking under dynamic loading can develop. Arch dams transfer reservoir loads to canyon walls through arching action and are designed for flexibility, but large earthquakes can damage the canyon rock abutments that provide the reaction for arch forces.

Earthfill dams are the most common dam type worldwide and have distinctive seismic vulnerabilities. Saturated zones within the embankment or foundation can liquefy under earthquake loading, causing sudden loss of shear strength that allows the embankment to flow or slide. 液化饱和松散土壤在强烈震动下暂时失去强度、表现如液体般的现象。可导致建筑物下沉、倾斜或陷入地下坍塌。 in dam foundations caused the failure of the Lower San Fernando Dam in the 1971 San Fernando earthquake: the upstream slope of the dam slid 33 feet, leaving only a few feet of freeboard and requiring emergency evacuation of 80,000 downstream residents. Had the dam failed completely, the consequences would have been catastrophic.

Liquefaction: The Critical Dam Seismic Hazard

液化饱和松散土壤在强烈震动下暂时失去强度、表现如液体般的现象。可导致建筑物下沉、倾斜或陷入地下坍塌。 of saturated loose cohesionless soils (sands and silts) occurs when earthquake shaking increases pore water pressure to the point where effective stress drops to zero and the soil loses its strength. Within an earthfill dam or its foundation, liquefaction can trigger flow slides, lateral spreading, and large permanent deformations. Modern dam design requires thorough geotechnical investigation to identify potentially liquefiable materials and either remove them, treat them through densification or grouting, or design the embankment to remain stable despite potential liquefaction.

The 抗震设计旨在使结构物能够承受地震作用力的设计实践。现代抗震设计致力于防止倒塌、保护生命，同时在大地震中允许出现一定程度的结构损伤。 of earthfill dams for liquefaction uses several approaches. Computer-based dynamic analysis models the soil's cyclic behavior, tracking pore pressure development and identifying zones at risk. Post-liquefaction stability analysis evaluates whether the dam can remain stable with liquefied zones providing only residual undrained strength. Deformation analysis estimates total movements and determines whether the crest settles below the reservoir level, which would trigger overtopping and failure.

Seismic Safety Evaluation Programs

Most dam safety programs worldwide require periodic seismic safety evaluations that apply current methods and hazard information to existing dams. The Federal Energy Regulatory Commission (FERC) in the United States requires hydropower dam owners to evaluate seismic safety, and the Association of State Dam Safety Officials (ASDSO) promotes systematic state programs for evaluating non-federal dams.

A seismic safety evaluation typically begins with seismic hazard characterization: identifying all capable faults within a broad radius, developing probabilistic seismic hazard analysis (概率地震危险性分析（PSHA）一种量化地震危险性的方法，综合考虑所有可能的地震震源、震级及地震动水平，以超过特定震动水平的概率来表示结果。) results for the site, and defining the design earthquake as either a probabilistic level (e.g., 10,000-year return period) or the 最大可信地震（MCE）基于地质证据，特定断层或地区合理可能发生的最大规模地震，用于水坝、核电站等重要设施的设计。 deterministic scenario. The evaluation then examines the dam's structural response to this hazard through analysis appropriate to the dam type and failure modes.

For concrete dams, linear and nonlinear dynamic analysis examines stress distributions, joint opening and sliding, and stability under the design earthquake. For earthfill dams, nonlinear dynamic analysis tracks pore pressure development, identifies liquefaction potential, and estimates deformations. The evaluation identifies whether the dam meets current safety standards and, if not, what modifications are required.

次生地震灾害由地震震动引发而非震动本身直接造成的灾害，包括海啸、滑坡、液化、火灾、水坝溃决及化学品泄漏等，其造成的损失往往超过震动本身。 Downstream

Dam failure 次生地震灾害由地震震动引发而非震动本身直接造成的灾害，包括海啸、滑坡、液化、火灾、水坝溃决及化学品泄漏等，其造成的损失往往超过震动本身。 are among the most severe consequences of major earthquakes. A dam failure during or immediately after an earthquake combines the direct casualties of the earthquake with the additional casualties and destruction from the resulting flood wave. Historical dam failures — Vajont in Italy (1963), Banqiao and Shimantan in China (1975) — demonstrate that flooding casualties can far exceed those of the triggering event.

The 2008 Sichuan earthquake damaged hundreds of dams in China, including the Tangjiashan landslide dam — a natural dam created when the earthquake triggered a massive landslide that blocked a river. The resulting lake threatened 1.3 million people downstream. Emergency mechanical excavation to lower the dam crest and control the outflow required three weeks of intensive work. Managing the cascade of dam safety threats in a post-earthquake environment, where infrastructure is damaged and access is limited, is a major challenge for emergency managers.

Planning for dam failure scenarios requires mapping inundation zones, establishing warning systems, and developing evacuation plans for downstream communities. Federal guidelines in the United States require Emergency Action Plans (EAPs) for all significant and high-hazard dams, specifying the actions to be taken if dam failure becomes imminent. These plans include notification procedures, pre-calculated inundation maps, and coordination with local emergency management.

Seismic Upgrade Strategies

When seismic evaluation identifies deficiencies in existing dams, a range of remediation strategies is available. For earthfill dams with liquefaction risk in the foundation, ground improvement through dynamic compaction, vibro-compaction, or permeation grouting can densify or strengthen susceptible soils. Upstream blankets and drainage layers manage pore pressure buildup during earthquake shaking.

Raising the dam crest to increase freeboard — the distance between the water surface and the crest — provides safety margin against deformation. If the crest settles during an earthquake but remains above the reservoir level, overtopping and failure are prevented. This approach accepts some embankment deformation as tolerable while ensuring that the ultimate failure mode (overtopping) does not occur.

For concrete dams, post-tensioned anchors can improve resistance to sliding and overturning at the base. Drainage improvements reduce uplift water pressure. Detailed nonlinear analysis sometimes reveals that existing dams are more capable than simplified analysis suggested, reducing the apparent need for structural modifications.

The 抗震设计旨在使结构物能够承受地震作用力的设计实践。现代抗震设计致力于防止倒塌、保护生命，同时在大地震中允许出现一定程度的结构损伤。 of new dams incorporates lessons from historical failures and advances in analysis methods, site characterization, and construction quality control that make modern dams substantially more reliable than their predecessors. But the legacy inventory of older dams worldwide represents an enormous ongoing seismic safety challenge that will require sustained investment and systematic evaluation over many decades.