By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Jul 18 2024The concept of underwater construction emerged from the development of underwater research stations by Jacques Cousteau’s team in the 1960s. Since then, with the advancement of technology, underwater constructions have been built for various purposes, including transportation, telecommunication, defense, and leisure.1
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With an ever-increasing population and limited land availability, the scope of underwater construction is further expanding.2 However, this requires a combination of several advanced techniques. With approximately three-fourths of the earth covered with water, underwater construction technologies can be exploited to benefit both humanity and the environment.1 This article explores prevalent techniques and materials for underwater construction and emerging technologies.
Techniques Used in Underwater Construction
All underwater construction projects typically rely on caissons and cofferdams as foundational elements. A caisson, derived from the Latin word for "box," is a pre-fabricated, hollow, cylindrical structure assembled on-site and submerged to a specified depth. Once positioned, it is filled with concrete to establish a solid, load-bearing foundation. Common types include box caissons, suction caissons, open caissons, pneumatic caissons, and others.1,2
In contrast, a cofferdam (or coffer) is a temporary enclosure constructed within or across a body of water, enabling the enclosed area to be drained. This creates a dry environment conducive to the construction or repair of structures such as dams, oil platforms, and piers. Coffers, typically made of welded steel, are removed once the project is completed. Varieties include earthen, rockfill, single-walled, double-walled, cellular, and braced cofferdams.1,2
The process of placing underwater concrete for significant structures employs various techniques to prevent mixing with the surrounding water. These methods include the tremie, pump, two-stage preplaced aggregate, toggle bags, and bag works methods.2,3
The tremie method is particularly prevalent for underwater concreting. It uses a long steel pipe, typically 15 to 30 cm in diameter, positioned vertically into the waterbed with a concrete hopper at the top, above water. The concrete mix pushes out water and air as it moves down the pipe to the bed.1,2
A more rapid and sophisticated variant of the tremie is the pumping technique, which uses mechanical pumps to transport concrete through pipes, enhancing efficiency.1,2 The two-stage preplaced aggregate method involves placing coarse aggregates at the site first, followed by the injection of grout to fill the interstices, offering excellent bonding and suitability for underwater repairs and additions.
For minimal cement applications, the toggle bags method is ideal. It involves a reusable sack, closed with a chain and secured with toggles, filled with concrete and deployed underwater. The concrete is then released from the bag's bottom to complete the structure, commonly used for repairs.1,2
Lastly, the bag work method uses robust fabric bags of 10-20 liters capacity, filled with concrete to create brick-like structures underwater, often for ballast renewal or temporary hole sealing.1,2
Ocean Exploration with Underwater Acoustic Sensor Networks
Suitable Materials
Underwater concrete is susceptible to cement washout, laitance, segregation, cold joints, and water entrapment. Thus, its characteristics, including flowability, air content, bleeding, setting time, and aggregate, are adjusted by adding retarders or admixtures.3
Ordinary Portland cement (OPC) is the most common underwater concreting material, requiring admixtures such as polycarboxylates, phosphonates, and anti-washout mixes to ensure self-compacting. However, the negative environmental effects of OPC production are well-known.3
Alternatively, geopolymers comprising pozzolanic materials such as fly ash and slag are promising underwater concrete materials with excellent anti-washout properties, strength, durability, and workability (comparable to OPC).3
Other common materials used in underwater construction are steel and acrylic. While the former is used for reinforcements due to its high strength and corrosion resistance, the latter is preferred over glass (greater durability and customizability) for transparent structures underwater.2
Challenges
Despite several technological advancements, underwater construction remains a significant challenge. The primary reason for this is the extremely high project costs, which stem from the need for specialized heavy equipment and a highly trained workforce. Additionally, finding suitable materials that can withstand water pressure, corrosion, and erosion in harsh underwater conditions is a complex task.1,2
Another major hurdle in underwater construction is the temperature variation, which ranges from warm at the surface to very cold at deeper levels. This variability affects construction activities and material performance. Post-construction, underwater structures such as resorts and transport tunnels face challenges with air circulation. Maintaining an inexhaustible air supply is crucial for respiration and for eliminating undesirable gases.1,2
Underwater oil and gas exploration activities also pose significant hazards to these constructions. The processes of locating fuel, drilling, and transporting it to ships can jeopardize underwater structures. Furthermore, the long-term environmental impacts of underwater constructions, particularly on fragile aquatic ecosystems, are not yet fully understood.1,2
Emerging Trends and Future Prospects
Emerging trends and future prospects in underwater construction are leveraging novel technologies to address existing challenges and optimize the building process beneath the sea. For instance, a study highlighted in the Journal of Marine Science and Engineering explores the use of optical image-based three-dimensional (3D) reconstruction technology. This advancement has been facilitated by the development of ready-made underwater camera systems and customizable deep-sea robots, which enhance the accessibility and accuracy of underwater images and videos for 3D reconstruction.4
This image-based 3D reconstruction approach offers a cost-effective, simple, and quick method to gather essential visual information. The technique’s inherent challenges related to range and resolution can be addressed by integrating it with other systems, such as sonar sensors.4
Additionally, innovative research has led to the development of a buoyancy-based autonomous underwater construction robot. This robot efficiently transports cement building blocks using active ballasting, which involves compressed air for buoyancy control and a battery-powered thruster system. The construction system is designed to correct placement errors through an active ballasting system that supports compliant placement and grasp behaviors. In trials, this free-floating robot demonstrated the capability to construct structures comprising 12 components, totaling 100 kg (75 kg underwater), thereby reducing the energy costs associated with transporting cement blocks.5
In conclusion, the field of underwater construction is witnessing rapid advancements that promise to enhance the durability and cost-effectiveness of submerged structures. The successful integration of innovative methods and materials is poised to further accelerate the development of underwater infrastructure.
References and Further Reading
1. Rajput, D. (2021). Underwater Construction: A Review. Journal of Emerging Technologies and Innovative Research, 8(6). https://www.academia.edu/81246704/Underwater_Construction_A_Review
2. Kurian, C., Gandhi, M. D., & Selvi, P. V. (2021). A Study on Advanced Underwater Construction and Its Challenges. International Research Journal of Engineering and Technology, 8(4). https://www.irjet.net/archives/V8/i4/IRJET-V8I4115.pdf
3. Ahmad Zaidi, F. H., Ahmad, R., Al Bakri Abdullah, M. M., Abd Rahim, S. Z., Yahya, Z., Li, L. Y., & Ediati, R. (2021). Geopolymer as underwater concreting material: A review. Construction and Building Materials, 291, 123276. DOI: 10.1016/j.conbuildmat.2021.123276
4. Hu, K., Wang, T., Shen, C., Weng, C., Zhou, F., Xia, M., & Weng, L. (2023). Overview of Underwater 3D Reconstruction Technology Based on Optical Images. Journal of Marine Science and Engineering, 11(5), 949–949. DOI: 10.3390/jmse11050949
5. Lensgraf, S., Balkcom, D., & Alberto Quattrini Li. (2023). Buoyancy enabled autonomous underwater construction with cement blocks. IEEE International Conference on Robotics and Automation, 5207-5213. DOI: 10.1109/icra48891.2023.10160589
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