Application of Geogrid to Make Tsunami Resilient Rubble Mound Breakwaters
Introduction
Tsunamis, among nature's most formidable phenomena, strike with devastating force, leaving behind trails of destruction and human suffering. These colossal waves, often triggered by seismic events beneath the ocean floor, unleash unimaginable energy as they travel across vast expanses of water, culminating in catastrophic impacts upon coastal regions. In the wake of such disasters, the imperative to fortify coastal defense against the ravages of tsunamis becomes very clear.
Figure 1: RM breakwater protecting the coastal/port side
Among the primary structures designed to mitigate the impact of tsunamis are rubble mound (RM) breakwaters, which are strategically positioned barriers engineered to absorb and dissipate the energy of incoming waves (Figure 1). Rubble mound is defined as a flexible heterogeneous assemblage structure of natural rubble consisting of quarried rocks in the core and natural or artificial armour as a protection cover. It’s generally trapezoidal in shape with flatter slopes on its seaside and steeper slopes on the harbour side. However, the efficacy of conventional RM breakwaters in withstanding extreme tsunami conditions remains a subject of concern, spurring the quest for innovative reinforcement strategies to bolster coastal resilience. Several breakwaters have been damaged under the tsunami impact in the past (Figure 2).
Figure 2: Damage occurred to breakwater during the past tsunamis
The vulnerability of conventional RM breakwaters to the ferocity of tsunamis stems from their susceptibility to erosion, scouring, and structural failure under extreme events like tsunamis (Figure 3). The past few decades have witnessed the 2004 Indian Ocean and 2011 Great East Japan tsunamis during which coastal communities face an urgent need to develop and implement robust defense mechanisms capable of withstanding future tsunamis.
Figure 3: Breakwater damages during the past tsunamis
In response to this imperative, researchers from the Geo-Disaster Prevention Laboratory at the National Institute of Technology, Karnataka have pioneered a range of innovative reinforcement techniques aimed at enhancing the resilience and effectiveness of RM breakwaters in mitigating the impact of tsunamis. By integrating advanced materials, structural components, and reinforcing methods, these techniques seek to fortify breakwaters against tsunami-induced forces and minimize the risk of catastrophic failure.
Key among these reinforcement strategies is the incorporation of geogrid layers along the slopes of the breakwater, sheet piles in the seabed, and crown walls with shear keys into the conventional RM breakwaters. Geogrid layers, composed of high-strength polymers, serve to interlock with the rubble, enhancing the stability of the breakwater while minimizing settlement and lateral displacement of the crest. Sheet piles, strategically positioned along the seabed adjacent to the breakwater, act as impermeable barriers, preventing excess seepage and erosion of the foundation seabed. Crown walls, equipped with shear keys embedded in the core layer, provide additional protection against tsunami impact forces, anchoring the crown wall and minimizing displacement during tsunami events.
Through a combination of physical model tests and advanced numerical analyses, the research sought to validate and optimize these innovative reinforcement techniques. By subjecting both conventional and reinforced breakwater models to simulated tsunami overflow scenarios, the performance benefits of various reinforcement elements were quantified in terms of reduced settlement, minimal lateral displacement, and enhanced stability under tsunami events. The proposed reinforcing techniques were based on real-world applications, further underscoring the effectiveness of these techniques in fortifying coastal defense infrastructure and protecting vulnerable communities from the devastating impact of tsunamis.
Application of Geogrid
The proposed reinforcing technique aims to enhance the resilience of RM breakwaters against the destructive forces of tsunamis by incorporating innovative elements such as geogrids, sheet piles, and crown walls with shear keys. Geogrid layers play a pivotal role in stabilizing the breakwater by interlocking with the rubble, thereby reducing the likelihood of displacement during tsunami overflow. These geogrid layers, strategically placed along the slopes of the breakwater on both seaside and harbour sides, provide additional structural integrity while allowing water to pass through, minimizing excess seepage-induced damages (Figure 4).
Figure 4: Geogrid reinforced RM breakwater
The most effective configuration of geogrid was investigated by performing several experiments and numerical studies. The geogrid layers provided above the armour layer on the harbour side and below the armour on the seaside formed the first configuration. Owing to the limitations of the first configuration in preventing the rolling down of rubbles from the harbour side, an extension of the geogrid layer was provided through the toe rubbles in the second configuration. The damages observed from the experimental analysis on the second configuration resulted in an enhanced third configuration where double-layer geogrid was applied above and below the harbour side armour layer (Figure 5).
Figure 5: Geogrid configurations investigated
Furthermore, the inclusion of sheet piles at both ends of the RM breakwater serves as cut-off walls, effectively preventing seawater from seeping through seabed soils beneath the mound during tsunamis. This reduction in seepage helps maintain the shear strength of the foundation soils, which is crucial for preventing settlement and failure of the breakwater. Additionally, crown walls with shear keys are introduced to protect the crest of the breakwater from scouring and lateral forces exerted by tsunami waves. The shear keys enhance the stability of the crown walls, further bolstering the overall resilience of the breakwater structure.
Model Test
The model tests conducted to evaluate the effectiveness of the proposed reinforcing technique involved meticulous planning and execution to simulate realistic tsunami overflow scenarios. The physical model, developed in the Geo Disaster Prevention laboratory at the National Institute of Technology, Karnataka, Surathkal, India, comprised an apparatus designed for continuous recirculation of water to generate tsunami-like waves. The apparatus, constructed with acrylic plates and steel frames, allowed for precise control and monitoring of water flow to simulate overflowing tsunami waves accurately. The physical model incorporated layers of river sand strata to mimic seabed soils with varying densities and simulate different soil conditions. Additionally, crushed stones of various sizes were utilized to construct the RM breakwater, with coloured stones distinguishing different layers for detailed analysis. During testing, instrumentation such as displacement transducers and pore water pressure transducers were deployed to monitor lateral displacement, vertical settlement, and incremental pore water pressure within the breakwater and seabed soils. The physical model tests enabled a comprehensive analysis of the reinforced and conventional RM breakwaters' performance under tsunami overflow conditions. Data collected from the instrumentation provided insights into the effectiveness of the reinforcing technique in reducing settlement, minimizing lateral displacement, and mitigating incremental pore water pressure. These model tests served as a crucial step in validating the proposed reinforcing technique's efficacy, paving the way for its potential application in real-world coastal protection projects to enhance resilience against tsunamis.
Figure 6: Damage of conventional breakwater model after tsunami overflow testThe results from the study on reinforcing RM breakwaters against tsunami-induced damage reveal promising advancements in coastal protection strategies. Through meticulous physical model tests, analytical studies, and numerical simulations, the study evaluated the effectiveness of various countermeasure techniques, mainly focusing on the incorporation of geogrids, sheet piles, and crown walls with shear keys. One of the key findings of the study is the significant reduction in the average settlement of the RM breakwater crest during tsunami overflow with the implementation of reinforced models. The conventional RM breakwater experienced severe deformation, with rubble shifting away and settling vertically due to excess seepage and intense lateral forces from tsunami waves (Figure 6). In contrast, the reinforced models, particularly the third configuration with a geogrid double layer, demonstrated remarkable resistance to displacement, resulting in a reduction of up to 97% in crest settlement (Figure 7).
Moreover, the study assessed the lateral displacement of the breakwater crest, which is crucial for preventing exposure of the core layer to overflowing tsunami waves. Reinforced models effectively minimized lateral displacement, with the third configuration of geogrid almost entirely arresting the lateral displacements of the crown wall. This highlights the importance of incorporating reinforcing elements such as geogrids and sheet piles to enhance the resilience of breakwaters against tsunamis.
Figure 7: Damage of reinforced breakwater model after tsunami overflow test
Furthermore, the analysis of incremental pore water pressure along the breakwater length elucidated the role of reinforcement in mitigating instability and failure risks. The reinforced models exhibited an average 49% reduction in the incremental pore water pressure during tsunami overflow, indicating improved stability. The insertion of sheet piles as cut-off walls proved effective in reducing seepage through seabed soils, contributing to the overall reduction in pore water pressure.
Damage Analysis
The damage analysis conducted on the breakwater models provides valuable insights into their performance under tsunami overflow conditions. Two key parameters, the relative displacement of armour units and dimensionless eroded area, were utilized to quantify the extent of damage inflicted on the breakwaters. In the case of the conventional breakwater model, the relative displacement of armour units was remarkably high, reaching nearly 86%, indicating significant instability and vulnerability to scouring. This high level of displacement underscores the breakwater's inability to withstand the impact of tsunami waves effectively, leading to extensive damage.
The introduction of reinforcing elements in the breakwater models resulted in notable improvements in damage mitigation. The first reinforced model demonstrated a significant reduction in the relative displacement of armour units, indicating enhanced stability compared to the conventional model. The placement of geogrid layers on either side of the breakwater contributed to this improvement by providing additional structural support and preventing the dislocation of the rubble.
Figure 8: Relative displacements of conventional and reinforced models.Subsequent reinforced models showed further reductions in relative displacement, with the third reinforced model with a double-layer geogrid configuration achieving the most significant reduction of 86% (Figure 8). This underscores the effectiveness of the reinforcing technique in enhancing breakwater resilience against tsunami- induced damage.
Dimensionless eroded area analysis further corroborated the efficacy of the reinforcing technique in reducing damage to the breakwaters. The conventional breakwater model exhibited a fully damaged state after tsunami overflow, as indicated by the high DEA value. However, the introduction of reinforcements led to substantial reductions in DEA values for the reinforced models. The double-layered geogrid configuration, in particular, demonstrated exceptional resilience, withstanding tsunami overflow without undergoing significant deformations. These findings highlight the crucial role of reinforcing elements, such as geogrid layers, in mitigating damage and enhancing the overall stability of breakwater structures in tsunami-prone coastal areas.
The damage analysis underscores the importance of adopting innovative engineering solutions, such as the proposed reinforcing technique, to improve the resilience of coastal infrastructure against natural disasters like tsunamis. By quantifying the extent of damage and evaluating the effectiveness of reinforcement measures, engineers and policymakers can make informed decisions to enhance coastal protection strategies and mitigate the potential impact of tsunamis on vulnerable communities and ecosystems.
Numerical Simulation
The numerical analysis conducted in this study provided insightful results regarding the performance of both conventional and reinforced models of RM breakwaters under tsunami overflow conditions. By simulating seepage flow and deformations, the analysis highlighted the vulnerability of conventional breakwaters to failure during tsunami events. Specifically, the numerical models demonstrated that conventional breakwaters experienced significant displacement and deformation, leading to compromised structural integrity under the impact of tsunami waves.
Moreover, the numerical analysis offered valuable insights into the effectiveness of proposed reinforcement techniques in enhancing breakwater stability. Results showed that the incorporation of reinforcement elements, such as geogrid layers and sheet piles, significantly improved the performance of RM breakwaters under tsunami overflow conditions. In particular, the numerical models revealed that reinforced breakwaters exhibited reduced displacement and deformation compared to their conventional counterparts, indicating enhanced resilience against tsunami-induced forces.
Additionally, the numerical analysis provided quantitative outputs, such as displacement contours and factor of safety calculations, which further elucidated the stability and failure mechanisms of RM breakwaters. These results corroborated findings from physical model tests, validating the efficacy of numerical simulations in predicting structural behaviour under extreme loading conditions. The numerical analysis complemented experimental observations, offering a comprehensive understanding of RM breakwater performance and providing valuable insights for engineering design and disaster mitigation strategies in coastal areas prone to tsunamis.
Conclusion
In conclusion, this study presents innovative techniques for reinforcing conventional RM breakwaters to mitigate damage caused by tsunamis. Through a series of physical model tests and numerical analyses, the effectiveness of these novel reinforcement methods was evaluated under tsunami overflow conditions. The findings highlight the vulnerability of conventional RM breakwaters, with significant erosion observed in the crest when subjected to tsunami waves. However, with the implementation of reinforcement techniques such as geogrid layers, sheet piles, and crown walls with shear keys, the performance of RM breakwaters was notably improved, demonstrating better stability and resilience against tsunami-induced forces.
Furthermore, the study investigated three reinforced models, focusing on the optimal placement of geogrid layers to enhance breakwater stability. Results indicate that the provision of geogrid layers along harbour and seaside slopes, particularly in the double-layer geogrid reinforced model, significantly reduced vertical settlement and arrested lateral displacements. Moreover, the incorporation of sheet piles effectively mitigated excess seepage through the seabed, preserving seabed soil strength beneath the breakwater. Numerical analysis further supported these findings, confirming the stability of the reinforced models against tsunami overflow and highlighting the effectiveness of the third geogrid configuration with the reinforced model, in particular.
This research represents a pioneering application of geogrid in RM breakwaters to mitigate the adverse effects of tsunamis. By proposing and validating these novel reinforcement techniques, the study provided a sustainable solution for strengthening RM breakwaters and enhancing their resilience to potential future tsunamis. The outcomes of this research hold significant promise in advancing coastal engineering practices and contributing to the development of robust infrastructure capable of withstanding extreme natural events.
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