In Hong Kong-Zhuhai-Macao Bridge project the immersed tunnel element reached up to 76 000t in mass for sea transportation. The true water resistance during towing for such a large element had never been studied to author's knowledge. By the time 30 elements were installed, the towing team had formed a good cooperation and gained confidence in safe work. The first author, taking advantage of this rare opportunity, proposed and organized the towing test to find the actual water resistance and the corresponding velocity of tunnel element. Five tunnel elements transportation were involved. The test results of the water resistance and the relevant velocity were reported in forms of table and graph. It was also found that tunnel elements, being towed in the limited navigation channel, had its velocity increased with the increased towing force. However, when the velocity reached a certain threshold value, it ceased to increase. In addition, the test results were compared to that of the physical model test of scale 1:40, it was found that the results of the physical model testing underestimated the water resistance; the main reason of the difference is the scale effect.
The immersed tunnel of Hong Kong-Zhuhai Macao Bridge (HZMB) is made of 33 tunnel elements, each has a typical length of 180m and a mass of around 76,000 t (Lin, 2017; Lin, 2018).
Four tugboats were planned to tow thirty-three immersed tunnel elements at the early stage of the HZMB project, based on the findings of the scaled physical model test. Trial towing of a large barge, which has a similar mass of the tunnel element, was conducted as a construction preparation, it was found that six tugboats were needed rather than four. Then, for the first time towing, the author used eight tugboats, even with the "redundancy", in the halfway the tunnel element and the tugboats did not advance but fell backwards by around 700~800m due to current. Obviously, the water resistance was underestimated and there is a difference of water resistance of the real towing work and that of the scaled physical model test.