2.1 Dams in This Study

This study investigates the safety incidents at five earth-cored embankment dams in Korea, denoted as W, A, D, Y, and H dam. A summary of their dimensions, construction timelines, observed degradation, and remediation measures is provided in Table 1. Fig. 1 illustrates their typical cross- sections.

These dams, built with central clay cores serving as primary water barriers, exhibited varying degrees of core material degradation. Observed issues included sinkholes, wet zones, slope failures, and increased seepage rates. Initial remediation efforts, such as compaction and grouting, were often insufficient, necessitating further intervention. Ultimately, low-pressure permeation grouting was implemented in all cases as the definitive remediation strategy to restore functionality and safety.

Fig. 1 Typical cross-section of dams in the study

2.2 W Dam

W dam, a 55 m tall earth-cored rockfill structure, experienced significant core degradation, marked by the formation of three sinkholes near the crest during normal operation in 1998 (Fig. 2). According to ^{Park and Oh (2018)}, excessive leakage at the downstream toe, reaching 2,340 m³/day at Normal High Water Level (NHWL), was initially reduced to 860 m³/day following partial compaction grouting. However, seepage levels rose to 1,007 m³/day within 2.5 years due to diversified flow paths, prompting comprehensive remediation through low-pressure permeation grouting in 2000.

Subsequent investigations in 2013 revealed localized cracks acting as seepage paths, emphasizing the need for continuous monitoring despite the remediation efforts maintaining core impermeability.

Fig. 2 W dam sinkholes incidents

2.3 A dam

A dam, constructed with a clay core containing significant sandy soil, exhibited wet downstream zones shortly after impounding. Despite four rounds of grouting, heavy rainfall in 2003 triggered a major downstream slope failure (Fig. 3). Emergency stabilization was followed by full-length permeation grouting, addressing fluidized layers detected in geotechnical investigations.

Investigations in 2013, a decade after remediation, revealed new fluidized zones and leaks, suggesting the ongoing development of flow paths. Continuous monitoring was recommended to ensure long-term stability, and the downstream slope was reinforced by with riprap to enhance slope stability.

Fig. 3 A dam safety incident

2.4 D and Y dam

Unlike W and A dams, D and Y dams displayed no visible signs of degradation. However, geotechnical investigations revealed localized core degradation, including fluidized zones, coarse granular particles, and high water content areas (Figs. 4-5). Electrical resistivity surveys identified low-resistivity zones indicative of internal erosion (Fig. 6).

D Dam, completed in 1990, exhibited two fluidized layers at 50% and 75% of the dam height from the crest, corresponding to construction phases with material changes. Similarly, Y dam, completed in 1979, showed loose upper layers and fluidized zones at lower sections, posing long-term risks of internal erosion. Remedial measures included low-pressure permeation grouting for both dams.

Fig. 4 Locally deteriorated core samples in D dam ^{(Park and Oh, 2016a)}

Fig. 5 Locally deteriorated core samples in Y dam ^{(Park and Oh, 2016a)}

Fig. 6 Electrical resistivity survey result of D dam

2.5 H Dam

H Dam exhibited extensive wet zones on its downstream surface, with leakage observed near the berm at one-third of its height from the crest (Fig. 7). Investigations revealed high permeability and loose core material, with the permeability coefficient exceeding design values. Construction deficiencies, including inadequate core height and steep slopes, contributed to its inherent vulnerabilities.

Remediation involved permeation grouting and bentonite injection in the crest area, addressing both structural and material deficiencies. These efforts effectively restored the dam's impermeability, though continued monitoring remains crucial.

Fig. 7 Anomalies of H dam

3.1 Lesson 1: Need for Strategic Dam Assessment to Address Varied Vulnerabilities

Tailored Investigation Approaches: Each dam's safety concerns were addressed through comprehensive geotechnical investigations, including borehole logging, soil and field testing, and geophysical surveys. Matching investigation strategies to specific objectives—such as seepage path detection or material property assessment—is crucial. Techniques like Standard Penetration Test (SPT), soil mechanics laboratory tests, geotechnical site characterization, and electrical resistivity surveys proved instrumental as shown in ^{Park and Oh (2016b)}. Drone photogrammetry is another useful tool for deformation monitoring ^{(Kim et al., 2021; Park et al., 2023)}.

Swift Responses to Indicators: Signs of safety issues, such as excessive leakage, sinkholes, or slope instability, demand immediate, in-depth examination, even during operation as indicated in ^{Park and Lim (2016)}. Methods such as dry borehole drilling and no-rotary wash techniques are effective in these conditions ^{(Park and Oh, 2016a)}.

Multi-Tiered Assessments: A tiered approach, starting with basic surveys and advancing to detailed investigations, enhances efficiency. For W, A, Y, D dams in this study, initial electrical resistivity surveys provide a broad overview, while targeted drilling and 3D tomography offer in-depth insights into dam integrity.

3.2 Lesson 2: Necessity of Proactive Diagnostics Beyond Visual Inspection for Aging Dams

Proactive Diagnostics Essential: Aging dams require active investigation methods to uncover hidden risks. While W, A, and H dams exhibited visible safety concerns, issues in D and Y dams remained undetected for decades until geotechnical investigations revealed core anomalies ^{(Park and Oh, 2016b)}.

Bridging Data Gaps: Many older dams lack comprehensive construction or design records as shown in A and W dams. Proactive diagnostics—combining direct investigations and geophysical techniques—are critical for evaluating their safety and extending their operational life as shown in Table 1.

3.3 Lesson 3: Importance of Minimizing Inherent Vulnerabilities to Prevent Dam Incidents

Addressing Core Deficiencies: The studied dams exhibited vulnerabilities such as fluidized cores, saturated slopes, and sinkholes, often resulting from hydraulic fracturing. Alternative theories suggest that internal erosion and material deficiencies, compounded by inadequate compaction and saturation, amplify these risks.

Construction Practices Impact Longevity: Cases such as W Dam revealed that changes in material sourcing during construction directly influenced long-term safety. H dam's excessive leakage stemmed from insufficient core height, emphasizing the need for rigorous quality control during design and construction phases.

Minimizing Inherent Vulnerabilities: By identifying and addressing design and construction errors early, the risk of long-term failures can be mitigated. A lifecycle approach ensures that inherent vulnerabilities are minimized at every stage.

Noteworthy incidents in the dams, including unevenly distributed fluidized cores with elevated moisture content, saturated downstream slopes, sinkholes at the dam crest, and downstream slope sliding, are often attributed to hydraulic fracturing within the core zone. This phenomenon is generally observed in the core zones of embankment dams, aligning with Sherard's theory ^{(Sherard, 1986)}, which posits that dam damage or failure often results from hydraulic fracturing or dispersive clays. Hydraulic fracturing is characterized by the initiation and expansion of cracks from the upstream side of the core, driven by water pressure surpassing the total stress on the crack surface and the subsequent widening of these cracks. Despite the prevalence of Sherard's explanation, the pattern of incidents in the examined dams raises questions regarding the actual mechanics of hydraulic fracturing. The depth, thickness, and distribution of fluidized zones, alongside the occurrence and dimensions of cracks relative to dam height, suggest a deviation from the classic hydraulic fracturing model proposed by Sherard.

Alternative perspectives offered by Mesri et al. ^{(Mesri and Ali, 1988)}, Nonveiller ^{(Nonveiller, 1988)}, and Lofquist ^{(Lofquist, 1988)} highlight the impact of water pressure and flow velocity on pre-existing weak areas, leading to internal erosion, piping, water leakage, and slope destabilization. This viewpoint, suggesting the amplification of inherent vulnerabilities due to factors like saturation-induced settlement, variations in material sourcing, and inadequate compaction, offers a more convincing explanation than the traditional hydraulic fracturing theory like the Sherard's theory.

Case studies, such as the W dam, where fluidized zones and water inflow areas are predominantly found at specific depths tied to changes in material sourcing during construction, underscore the significance of construction phase decisions on long-term dam integrity. Similarly, investigations into the H dam revealed discrepancies in core height from design specifications, contributing to significant water leakage.

As demonstrated, design and construction phase errors or deficiencies serve as inherent vulnerabilities that can precipitate failures or incidents throughout a dam's operational lifespan. The identification of such inherent vulnerabilities and their impact on the dam's aging process emphasizes the critical importance of minimizing these inherent risks through meticulous quality control from the outset of dam design and construction. This proactive approach is fundamental to ensuring the long-term safety and resilience of dam infrastructure.

Fig. 8 shows the depth, thickness, and distribution of fluidized zones found in the study dams, and Fig. 9 shows crack occurrence cases investigated by Foster and Fell ^{(Foster and Fell, 1999)} and the fluidized zones considered as cracks in the 5 study dams. This shows the status of crack occurrence compared to the height of the dam. As shown in Figs. 8-9, considering the variation in location and size of the cracks, it is difficult to believe that the pure hydraulic fracturing phenomenon claimed by Sherard occurred as described in Table 2.

In fact, in the case of W dam, as shown in Fig. 10, the fluidized zone and the inflow water are concentrated at a depth of 20 to 45 m from the dam crest. According to construction records, the part where the material source was changed during the construction process corresponds to this section. In the case of the H dam, as shown in Fig. 11, as a result of the excavation investigation of the dam crest, it was discovered that the core was not built to the designed height, which can be seen as contributing to the excessive water leakage.

As confirmed in Figs. 8 to 11, incidents that occur during post-construction operation can be seen as being greatly affected by various defects or errors in the design or construction phase. As such, various defects or errors in the design or construction process exist as inherent vulnerabilities throughout the entire life cycle of a dam and can become a major cause of failures or incidents during the operation and management phases. Fig. 12 shows the general relationship between the aging process and inherent vulnerability during the entire life cycle of a dam. In Fig. 12, path O-A-A’ has no inherent vulnerability, path O-B-B’ has some inherent vulnerability and deteriorates faster and has a shorter lifespan than both paths O-A-A’ and O-C-C’. It represents a life cycle pattern in which the vulnerability is so severe that an accident such as collapse occurs during the initial freshening process.

Therefore, minimizing inherent vulnerabilities through thorough quality control from the design and construction process is the best way to ensure the safety of the dam throughout its entire lifespan.

Fig. 8 Locations and thickness of fluidized zones of the study dams

Fig. 9 Depths of cracks in core and height of dams

Fig. 10 Embanking schedule and change of borrow materials for the core zone (W dam)

Fig. 11 Status of the top of core by excavation investigation (H dam)

Fig. 12 A generic aging model of dams taking into account inherent vulnerability

3.4 Lesson 4: Lifelong Commitment Required for Dam Safety – Addressing Root Causes

Lifecycle Perspective: Dam safety incidents during operation often originate from earlier phases, such as design or construction. Following guidelines from ICOLD Bulletins 154 and 175, organizations should adopt comprehensive safety protocols that span a dam's entire lifecycle, from planning to decommissioning.

Comprehensive Monitoring Systems: Developing and maintaining robust systems for issue identification, monitoring, and response are essential. These systems must be adaptable to address both internal and external risks effectively.

All incidents that occur during the operation phase of a dam originate from the previous phase. Every incident encountered during the operational phase of a dam can be traced back to earlier stages in its lifecycle, encompassing the plan, design, construction, commissioning, operation, and eventual rehabilitation or decommissioning phases. As outlined in the International Commission on Large Dams (ICOLD) Bulletin 154, the operational phase represents the most extended period in a dam's lifecycle ^{(ICOLD, 2017)}. Organizations overseeing dam operations are tasked with the development and maintenance of comprehensive safety protocols. These protocols must be designed to effectively identify, monitor, and rectify any potential or actual issues impacting dam safety over the long haul, while also being robust against both external and internal disruptions.

ICOLD Bulletin 175 ^{(ICOLD, 2021)} further emphasizes that incidents during the operation phase are not spontaneous but have roots in the dam's previous phases. This underscores the importance of a holistic approach to dam safety, recognizing that effective incident prevention and management stem from rigorous attention to detail throughout every phase of the dam's lifecycle.

3.5 Lesson 5: Urgent Need for a Government-Level Incident Management System (IMS) for Dams

Integrated Management: Effective dam safety requires government-level incident management systems to support local authorities, especially in cases with limited expertise or resources. South Korea's initiative to classify high-risk reservoirs and provide financial support for reinforcement is an exemplary approach.

Enhanced Technical Oversight: Beyond financial assistance, governments must establish expert-driven frameworks for incident prevention, investigation, and response. This ensures comprehensive safety management across diverse dam infrastructures. The necessity of a government-level incident management system becomes evident in cases like the H dam, where local governmental management faces challenges due to limited dam safety expertise and budget constraints. These limitations hinder effective safety management and can lead to frequent catastrophic failures.

In South Korea, the government's initiative to support disaster-prone and high-risk reservoirs through various projects showcases a proactive approach to enhancing dam and reservoir safety under local government management. By 2023, out of 13,685 local government-managed reservoirs, approximately 230 were identified as high-risk through inspections and risk assessments (Fig. 13). These were subsequently classified as disaster risk reservoirs, qualifying them for government-funded reinforcement measures.

Despite these advancements, the government's role is primarily focused on designating disaster risk reservoirs and facilitating financial support. The technical aspects of incident prevention, investigation, and response largely depend on a selected group of experts. This reveals a significant area for improvement, indicating the need for a more integrated and expert-driven approach within the government-led incident management framework to ensure the comprehensive safety and integrity of dams and reservoirs.

In conclusion, for dams that have experienced incidents, as in the case studies examined, the establishment of an Incident Management System (IMS) is paramount. This system should be capable of methodically overseeing the entirety of the process—from the initial detection of an incident through to its thorough investigation and subsequent reinforcement measures.

Fig. 13 Number of designation of dams with hazards by year in South Korea