Top-notch “waterproof clothing”! “Polyurea helps to build the first tunnel of the Han River in thousands of miles!



The 5.4 km long Xiang Yang East-West Axis Project, known as the first tunnel on the Han River, is the largest inland river immersed tube tunnel in China, with a design life of 100 years. The heaviest section of the six immersed tubes in the dry dock of the East Branch was 31,200 tonnes, while the lightest was over 22,600 tonnes. The consequences of a leak would be unthinkable. Waterproofing is one of the most important aspects of the project’s quality management. How to make the sinker drip-free was a question that the project team had been thinking about before the trial float.

A waterproof coating called “polyurea” caught everyone’s attention. This waterproof coating has excellent corrosion, waterproofing and wear resistance properties and is suitable for the geological conditions and geographical environment of the Han River bottom, but the cost is high. After calculation, there are 10 sections of sunken pipe with a total length of 1011m, and it will cost nearly 6.6 million to spray polyurea waterproofing on only 6 sections of the test floating sunken pipe, and spraying polyurea on the exterior of the sunken pipe is not common in domestic sunken pipe construction. There were also different views within the project team as to whether polyurea should be used.

There are many factors involved in the waterproofing performance of the sinker, such as concrete proportioning and temperature-controlled crack resistance, and we could have put more effort into other aspects.” “With the cost of the project at stake, is it worth spending such a huge amount of money?” However, Sun Xiaowei’s words gave everyone confidence: “As long as we can make the sinker safe for 100 years, the necessary cost is worth it!” So, after reporting to higher authorities, communicating and coordinating with the owner and design units, polyurea officially became the coating for the east-west axis of the Xiangyang sunken pipe.

“Spraying polyurea, the construction steps are similar to ‘painting a wall’, which looks simple but is difficult.” Technician Hu Tao was involved in the whole process of testing the polyurea painting process, “We designed a ‘three-step’ approach to the test.” First, the concrete substrate of the sinker was polished to remove the surface floating slurry and create open holes. Then, a special polyurea primer was applied to increase adhesion; finally, polyurea was sprayed. However, seven days after construction, Hu Tao found that the surface of the polyurea coating had pinholes, bulges and other appearance quality problems because of the rough concrete surface, and the pulling force of the coating was not enough. So, the technical team added the step of scraping putty to fill the holes on the concrete surface after grinding the concrete base and then lightly sanding it. However, the side of the sinker was not sufficiently flat in places and the effect of the scraping was not obvious. The technical team added a high penetration crystalline primer before scraping. The polyurea spraying process was finalised after a number of trials and tribulations.

The process wasn’t enough, the final polyurea spraying also depended on the weather. “The polyurea spraying requirements are very strict, the air humidity is greater than 85% can not be constructed, three wind speed above can not be constructed. And the Han River is humid and windy, which hinders the progress of the paint construction.” Hu Tao was somewhat helpless, but did not sit back and wait. They played hide-and-seek with the wind at the site, choosing to work on the leeward side and setting up canvas shelters on both sides of the lift to block the wind. In addition, in order to deal with the special smell of polyurea primer material, which is prone to attracting insects, and the sticky nature of it, which tends to stick to poplar and willow, the technical team also appointed someone to clean up foreign objects before spraying polyurea.

After more than 90 days of hard work, the six sections of the test floating sunken pipe finally donned the “waterproof suit” as scheduled. After nearly a month of water injection and test floating, all six sections of the sunken pipe were floated, proving that the waterproofing effect was initially qualified. Subsequently, the technical team tested the interior of the sunken pipe and found that the waterproofing performance was watertight. “The polyurea completely covered the top and sides of the sunken pipe, spraying a total area of up to 30,000 square metres, with a coating thickness of 1.5 mm and 54 tonnes of material, just like putting a thick, seamless ‘waterproof suit’ on the sunken pipe.” Sun Xiaowei said proudly.

Post time: May-30-2022