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Geotextile Sand Containers: Important Design Considerations

| Categories Coastal Development Coastal Improvement Coastal Protection | Tags #geotubes #geobags #geosynthetics #geotextile #GSC

In my opinion, the biggest influences in the scientific field of Geotextile Sand Containers (GSCs) come from K.W. Pilarczyk, Hocine Oumeraci, Juan Recio and Darshana Dassanayake. Few papers have been published about this subject so far; nonetheless, there is significant knowledge available about the behavior of these kind of structures, as a number of large and small scale tests have been conducted in recent years. 

The Complexity of Failing Mechanisms

It is important to emphasize that the main difficulty in forming mathematical models lies in the fact that many failing mechanisms can occur, because units are highly deformable, which has an important impact on behavior. That said, installation and material quality control play a crucial role too. Sand fill ratio for example is one of the most important variables and should be controlled by experienced and skilled workers only.

Many other factors can cause failure, such as: units overturning, sliding, punctures, overtopping, toe scouring, material weakening. Material specifications can influence structure durability and stability as well. UV radiation can reduce the strength. The only way to overcome this is by protecting geosynthetics from direct weathering.

Other relevant issues are still uninvestigated, like the influence on overall structure stability when placing geotextile in the back of the revetment structure. It will without doubt decrease structure permeability, and reflect back more energy.

One of the most important reasons of structure failure by a sliding mechanism is internal ‘pipe pressure’ as a result of an impermeable structure (voids between the bags act like a water-conducting pipe system, while units themselves can be treated as impermeable). A similar rule can be applied to traditional rock structures, where larger armour layer units will be needed in case of an impermeable structure (Hudson based his formulae and research on permeable cores).

Furthermore, forces applied to the structure will be affected by wave properties as well as structure slope, material roughness, stacking method, unit dimensions and positioning.

Historical Overview of GSC Theoretical Research

The beginnings of this research can be traced back to the 1950s when Hudson proposed his first formula. However, not until the works of Wouters were published at the end of the last millennium did we have proper work on this subject. Wouters' proposed formula - based on Iribarren number - provided a much more accurate estimation on structure stability.

Oumeraci et al. conducted a series of tests a few years later. Bearing in mind the works of Hudson and Wouters, he proposed formulae based on surf similarity parameters as well, however, also recognized the difference in behavior between slope and crest bags. Separate approaches were developed for these two exceptional cases.

Juan Recio dug deeper into failing mechanisms and introduced new stability formulae in 2007, observing it on a much lower bag level. His stability curves in relation to wave height are parabolic functions. He suggested that wave height has a much bigger impact than previously thought. To clarify, a small increase in wave height can basically be fatal for poorly designed and constructed structures. He also noted that a unit's deformation influences stability and offered a mathematical solution accordingly (re. sand fill ratio effect). 

Ecocoast and GSC Structures

Ecocoast - as always - strives towards perfection and tries to push the limits. As a regional leader in coastal protection, we work hard to synthesize all knowledge and scientific breakthroughs. We have developed software in-house that aids us in getting the best possible structure stability design. This also enables us to properly predict structure behavior and design structures that are not over-conservative or doomed to fail.

Based on load and structure configuration, we are able to properly design structures, which is crucial for these kind of implementations. It is critical to combine knowledge from practical, construction experience and knowledge from research i.e. the theoretical approach, especially when it comes to GSC structures.

What are your thoughts? We welcome your input!

Written by Aleksa Cavic - Technical Engineer at Ecocoast