A Scale Model Test on Hydraulic Resistance of Tunnel Elements during Floating Transportation

Transcription

1 Advanced Materials Research Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland A Scale Model Test on Hydraulic Resistance of Tunnel Elements during Floating Transportation Ying Zongquan 1, a*, Su Linwang 1,b and Lin Meihong 1,c 1 CCCC Fourth Harbor Engineering Institute Co., Ltd., Guangzhou, China a yzongquan@gzpcc.com, b slinwang@gzpcc.com, c lmeihong@gzpcc.com Keywords: Immersed tunnel; Floating transportation; Resistance; Scale model test; Abstract: The transportation of tunnel elements is one of the most important procedures of immersed tunnel construction. To investigate the hydraulic resistance of tunnel elements during transportation, hydrodynamic scale model tests of the were carried out in a towing tank for the tunnel element transportation under wave action. The test results show that the hydraulic resistance is linear to the square of towing speed basically. The drag coefficients of the element under different water depths and dragging angles are calculated. The dragging coefficient decreases with the increase of water depth, and increases with the increase of the dragging angle. Finally, the influence of the wave is also discussed for the tunnel dragged in the open sea. Introduction An immersed tunnel consists of one or more prefabricated tunnel elements that are floated to site, installed one by one, and connected to one another under water. Compared with a bridge, an immersed tunnel has advantages of being little influenced by big smog and typhoon, stable operation and good resistance against earthquakes. Due to the special economical and technological advantages of the immersed tunnel, more and more under water immersed tunnel are built or are being built in the word. Building an undersea immersed tunnel is generally a super-large and challenging project that involves many key engineering techniques, such as transporting and immersing, and underwater linking et al(molenaar, V.L. 1993). Eexperience has shown that a tunnel element behaves differently under tow than a vessel does. This is largely because of the deep draft and the large flat-plate areas presented by the end bulkheads of a typical element. Therefore, the immersed tunnel was widely studied with respect to transportation, in situ stability and immersion of tunnel elements (Lv, W. et al. 2013; Jensen, O.L. et al. 7; Cozijn, H. et al. 9; Pan 4), in which hydrodynamic model test was the most popular method. To establish the required tug capacity, the drag of the element under different towing speeds and directions needs to be investigated. Based on the relative motion concept, the model experiments of the tunnel element during transportation were carried out in the present study. The test results show that the resistance coefficient is sensitive to the water depth and the angle between element and current. General project information The Hong Kong-Zhuhai-Macao Bridge, being situated at the waters of Lingdingyang of Pearl River Estuary in China, is a mega-size sea crossing project linking Hong Kong, the city of Zhuhai and Macau. The project includes a 29.6 km dual 3-lane carriageway in the form of bridge-cum-tunnel structure comprising an immersed tunnel of about 6 km long; two artificial islands and associated works. the length of each typical element is 180m weighing about 75690t. The dimension of a typical elements is L W H=180m 37.95m 11.4m, and the cross-sections are symmetrical. Its half section is shown in Figure. 1. The freeboard is set as 30cm during transportation. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-18/09/16,06:18:59)

2 842 Advanced Construction Technologies The floating transportation route is 12 km long, which is divided into two sections, including the section of Rongshutou Navigation Channel and that along the foundation trench of tunnel, as shown in Figure 2. When elements are towed in the sea, the environmental conditions are more complicated than in inland rivers, consideration should be given to the combined effect of flow, wave and wind resistances.it is important to determine the hydraulic resistance of elements at different flow angles, which would provide a basis for the safety of towing and maneuverability of the towing process. 12 km Fig. 1 1/2 cross section of the element(unit::mm)fig. 2 Diagram of transportation route Preparation for model experiments Similarity criteria:during the process of tunnel element mooring, a tunnel element is mainly subject to the effect of current and wave. This effect is of the same hydrodynamic characteristic as that of the resistance against an element during floating transportation. In consideration of the low Reynolds number and Froude number of the tunnel element, the viscous resistance in the total resistance is larger than the friction resistance. The similarity criteria used in the experiment are as follows: Scale similarity: Ls = λ Lm, where λ is scale factor, and in the present experiment, λ = 40, subscripts s and m indicating prototype tunnel element and model element, respectively. Velocity similarity:vs = λ1/2 Vm. Resistance similarity:vs = λ3 Vm. The parameter comparisons presented in the paper are all based on the above similarity criteria. Testing Equipments:The experiments are carried out in a towing tank of 132m long, 10.8m wide and 2.0m deep. A flap-type wave generator with 24 elements can be used to generate the regular and irregular waves. At the other end, a wave energy dissipation device is installed, which can absorb the wave energy up to 90%. The towing tank is equipped with a towing carriage that runs on two rails on either side. The towing carriage can either tow the model or follow the self-propelled model, and is equipped with computers and devices to control, respectively, variables such as speed and towing angle etc. The range of towing speed is 0.01~5.0m/s. A wind generator is also equipped on the towing carriage. Fig.3 Models of the element and pontoon Experimental models:the physical model used for test included a tunnel element, tow pontoons, tugging lines. The tunnel element model is constructed at a model scale of 1:40 according to the similarity criteria. The two pontoons were constructed of wood model in consideration of processing, making and installation of tension sensors, as shown in Fig. 3.

3 Advanced Materials Research Vols Experimental investigations There are two parts for this experiment, such as towing testing in still water and towing under the combination of wind, wave, with different speed and towing directions. Testing in still water: Owing to the long distance of floating transportation voyage, the orientation element needs to be changed in the course, and the depth of water, current direction and velocity are likely to vary greatly. So tests were conducted for elements at a water depth of 13 m, 14 m, 15 m and 20 m respectively, and with the angle between current and towing direction assumed to be 180, 186, 198, 210, 222, 240 and 270 respectively, which is defined as shown in Figure 4, where the X direction denotes the element s longitudinal direction (vertical), and the Y direction the element s transverse direction (horizontal). The model test is shown in Figure 5. Y Wave direction (or current,wind) Positive angle X Tunnel element Fig. 4. Definition of the angles Fig. 5. Model test in still waters The resistance forces for element model were measured, in which the element model was towed with different speeds and directions. Then, based on the similarity criteria, the resistance for real element can be derived by (1): 3 Rs = ρ sλ Rm ρm (1) in which, R s, Rm is resistance for real element and model element, respectively; ρ s, ρm is the density of the water at the sea site, and the density of water used in the experiment, respectively; λ is scale factor, and in the present experiment, λ = 40. The experimental results show that the resistances are almost in linear proportion to the square of the towing speed, which can be written as: 2 Rs = 0.5Cs ρs AV (2) where, Cs is the drag coefficient (dimensionless), A is the projected areas of the element below the waterline, V is the towing speed of the element The drag coefficient can be derived by the analysis of the relation between total resistance and towing speeds and directions, as shown in Fig Drag Coefficient m 14m 15m 20m Angle ( o ) Fig. 6 Testing results of drag coefficients As shown in Fig.5, the drag coefficients increase with increasing the towing angle. Besides, the water depth also affects the value of the drag coefficient. The drag coefficients decrease with increasing the water depth. And, as noted, the element will touch the bottom of the tank, when the water depth is 13m, the towing angle is larger than 42º.

4 844 Advanced Construction Technologies Testing under wind and wave action: As mentioned above, the transportation distance of element is very long, and the current direction may be varied with the time and different procedures. In this part of the experiment, the water depth is 13m, the towing angles are 180 and 210, the wave heights (Hw) are 0.8m and 1.0m, with wave period 6s. The wind speed is 13.8m/s. Limited by the conditions of the towing tank, the directions of wind, wave, and current are same. During the towing, the element and pontoon are rigidly connected together. The model system was towed by the towing carriage, then the wave generator and wind generator started working. The tensions of the towing lines were measured until the towing carriage stopped. Then the line tension can be obtained under each load case. For the towing angle 180, the test results for are shown in Fig.7, in which only the longitudinal force acts on the tunnel element. Longitudinal resistance (t) Maximum value for Hw=1.0m Maximum value for Hw=0.8m Mean value for Hw=1.0m Maximum value for Hw=0.8m Resistance for still water Towing speed (m/s) Fig. 6 Longitudinal resistance of the element for towing angle 180 When the towing angle is 210,The longitudinal and transversal resistance of the element are shown in Fig.7 and Fig.8, respectively. Longitudinal resistance (t) Maximum value (Hw=1.0m) Maximum value (Hw=0.8m) Mean value (Hw=1.0m) Mean value (Hw=0.8m) Resistance in still water Towing speed (m/s) Transversal resistance (t) 1 Maximum value (Hw=1.0m) Maximum value (Hw=0.8m) 1 Mean value (Hw=1.0m) 1000 Mean value (Hw=0.8m) Resistance in still water Towing speed(m/s) Fig. 7 Longitudinal resistance for 210 Fig. 8 Transversal resistance for 210 Seen from Fig.6~Fig.8, the mean value of resistance under wave actions are very nearly equal to the resistance measured in still water. Meanwhile, the maximum value is times of the value of the resistance for still water. For the consideration of tugging boat capacity, the output towing force must be larger than the mean value of the resistance. During the floating transportation, the maximum value of resistance must be taken into account for the choosing of towing line. Conclusions The floating transportation of the typical elements for the Hong Kong-Zhuhai-Macao Bridge immersed tunnel was investigated by the hydrodynamic scale model tests. Based on the relative motion concept, the model experiments of the tunnel element during transportation were carried out in the present study. The results of model tests showed that the water resistances were larger in 13 m than in 20 m water depth. The drag coefficients of the element with several water depths are also determined.

5 Advanced Materials Research Vols The results of Towing testing of element under wind and wave action show that the mean value of resistance under wave actions are very nearly equal to the resistance measured in still water. The oscillation of the floating resistance is very obvious; the maximum value can be up to times of the mean value. For the design of towing line, the wave effect should be taken into account. Acknowledgments This work was supported by Key Technologies R&D Program of China. Project number: 2011BAG07B01. Reference [1] Molenaar, V.L Construction Techniques. State of the Art Report in Immersed and Floating Tunnels. Tunnelling and Underground Space Technology, 8, 2, [2] Lv W., Ying Z., Wu R. et al An analytical study on the hydraulic resistance for the immersed tunnel elements during transportation for the project of Hong Kong-Zhuhai-Macao Bridge. Proceedings of the World Tunnel Congress Geneva. p [3] Jensen, O.L., Olsen T.H., Kim C.W. Heo J.W. et al. 7. Construction of immersed tunnel in off-shore wave conditions Busan-Geoje project South Korea. IABSE Symposium, Weimar 7, 8, [4] Cozijn, H. Jin W. H.Analysis of the Tunnel Immersion for the Busan-Geoje Fixed Link Project Through Scale Model Tests and Computer Simulations[C]. Proceedings of the ASME 28th International Conference on Ocean, Offshore and Arctic Engineering, 9. [5] Pan, Y.R. 4. The Floating Transport Methods of Large Element Employed for Shanghai Out-ring Immersed Tube Tunnel. Construction Technology, 33,5, (in Chinese).