A major infrastructure project is in development to connect Koh Samui to the mainland via Thailand’s longest sea bridge, spanning 22 kilometers. The Expressway Authority of Thailand (EXAT) is currently conducting a feasibility study on the economic, financial, engineering, and environmental aspects of the project. The study is expected to be completed by 2026, with construction planned to begin in 2029 and operations targeted for 2033.
The proposed expressway will begin at kilometer marker 14+500 on Highway 4142 in Don Sak district, Surat Thani province. The route will traverse agricultural land, rubber plantations, and palm plantations before reaching Highway 4142 at kilometer 35+700 in Khanom district, Nakhon Si Thammarat province. From there, it will continue eastward, crossing local Highway 4004, passing through coastal areas, and avoiding deep-sea trenches and coral reefs. The bridge will make landfall at Laem Sor Hill on Koh Samui, before connecting to Highway 4170 in Taling Ngam subdistrict. The total length of the project is approximately 37.41 kilometers, with 15 kilometers of road on land and a 22-kilometer bridge section over the sea.
The four-lane bridge will have a lane width of 3.5 meters, an outer shoulder width of 2.5 meters, and an inner shoulder width of 1 meter. A concrete barrier will serve as the median. The bridge will be designed to minimize disruption to marine navigation and control construction and maintenance costs.
The structure will incorporate three types of bridge designs. The main span will be a cable-stayed bridge, similar to Thailand’s Rama IX Bridge, with a span length of 450 meters and a clearance of approximately 50 meters above sea level to accommodate large vessels such as gas transport ships, oil tankers, ferries, and naval vessels. The secondary span will be an extradosed bridge, a hybrid between a cable-stayed and girder bridge, with shorter pylons and spans of 220-250 meters. This section will be built near Koh Samui’s shoreline, ensuring that bridge piers do not impact coral reefs. Lastly, a series of standard concrete girder bridges will facilitate local fishing boat traffic, with a clearance of approximately 15 meters above sea level and spans of 60 meters. This section will cover approximately 21.5 kilometers and will be more cost-effective than the cable-stayed and extradosed sections.
EXAT has designed the bridge to withstand harsh marine environments, strong winds, and seismic activity. Measures will also be implemented to prevent large vessels from colliding with bridge piers. Further updates on the project are expected following the completion of the feasibility study.
The latest stakeholder consultation meetings are scheduled from March 27 to March 29, 2025. Various officials and experts are expected to participate in discussions regarding the project’s potential impact and development.
