Sep 25 • Machinist

Building Cross-Sea Bridges on Coral Reefs: Engineering and Equipment Challenges

Constructing a bridge between coral islands is fundamentally different from building on conventional soil or rock. Coral reef formations usually combine a loose upper layer of coral debris with a deeper layer of reef limestone. These materials can be highly porous, brittle and irregular, while open-sea conditions add waves, currents and restricted working space. A recent study of the China–Maldives Friendship Bridge examined how these conditions affect pile performance and how construction equipment can be adapted to improve safety and accuracy. 

The research combined field investigations, laboratory testing, model pile tests and full-scale pile testing. The results showed that coral sand behaves differently from ordinary quartz sand. When a steel pipe pile is driven into coral debris, particles can fracture, rotate and rearrange around the pile shaft. This process reduces the lateral pressure acting on the pile and limits the development of shaft friction. In the model tests, the ultimate shaft resistance in coral sand was approximately 17.5 kPa—only about one-fifth of the comparable value measured in quartz sand. 

These findings are important for pile design. Conventional calculation methods developed for ordinary soils may overestimate the contribution of shaft friction when applied directly to coral deposits. The researchers therefore proposed a modified approach based on the observed stress distribution and pile–soil interaction. When applied to field data from 32 test piles, the method produced an average difference of approximately 10.3% compared with measured results and remained conservative, making it more suitable for preliminary design and verification. 

Cast-in-place piles installed into reef limestone presented a different set of problems. The limestone contained pores, cavities and zones with varying degrees of cementation. These features could cause excessive borehole enlargement, local instability and deviations from the intended vertical alignment. Testing showed that the pile–rock interface could develop greater shaft resistance than is normally assumed for comparable terrestrial limestone, although the final design value still had to account for drilling damage and the quality of the borehole wall. 

The construction team developed several equipment solutions for the project. For driven steel pipe piles, a hydraulic chuck with automatic opening and closing was integrated into a large cantilever guide frame. The system helped control the pile during lifting and positioning in waves and currents. A fuse mechanism was included to protect the guide frame if a pile slipped unexpectedly during driving. Field results showed that pile positioning could be controlled within approximately 25 cm in plan, while pile inclination remained below 0.5%. 

Drilling equipment also required adaptation. Specialised drilling heads were used for the different reef layers, including extended barrel tools for circular cutting and tooth-cutting bits for harder, interbedded limestone. Rotary and reverse-circulation systems were modified to control borehole diameter and limit wall collapse. The research provided indicative drilling parameters: drilling speed decreased as the reef limestone became more strongly cemented, while drilling pressure had to be adjusted to avoid excessive damage to the borehole wall. 

The China–Maldives Friendship Bridge provided a full-scale application of these methods. The project connects Malé, the airport island and Hulhumalé across the Gaadhoo Koa Strait. Its main bridge is approximately 760 metres long, and some composite pile foundations use steel casings between 3.2 and 3.6 metres in diameter. Cast-in-place piles reach lengths of up to 110 metres. The project demonstrates how geotechnical investigation, pile testing, specialised guidance systems and adapted drilling tools must work together in an exposed marine environment. 

The main lesson is that coral reefs cannot be treated as ordinary sand or rock during foundation design. Their unusual particle behaviour, variable cementation and hidden cavities directly influence pile capacity, drilling stability and installation accuracy. For piling specialists, the study highlights the importance of site-specific testing, conservative calculations, controlled pile handling, protected guide systems and equipment selected for the actual reef structure rather than for conventional ground conditions. 

Source:
Adapted for EduPile from Zhong Y., Zhang Y., Yu C., Ji F. and Chen P., “Construction techniques for cross-sea bridges in coral reef and island environments,” Frontiers in Earth Science, 29 October 2025. 
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