MARINE GEO-ENVIRONMENT ANALYSIS OF THE PROPOSED BRIDGE-TUNNEL PATH SYSTEM ACROSS THE BOHAI STRAIT AND SUGGESTIONS ON CONSTRUCTION SCHEMES

Surveys of earth crust stability, marine geo-environment and island geology along the Bohai Strait suggest that the Bohai Strait is feasible for cross-strait path construction, and the scheme of "bridge-south and tunnel-north" is the best and optimum one for construction among others. Construction of such a path way will certainly bring very high economic benefits to the region as well as to the country. A rough calculation for a highway with possible traffic flow of 40 000 vehicles per day suggests that 17 billion RMB of fuel charge could be saved every year with the facility, and it takes only six years to recover the investment cost in the years to come.It will certainly relieve the traffic pressure around the region and as the result reduce the haze caused by fuel emission. The optimum route for such a path is along the axis of the Miaodao islands because of more stable earth crust, thinner sediment cover, and shorter actual cross-sea distance there. From the Penglaijiao to the Tuoji Island the water depth is only 20m on average, while the water depth from Tuoji Island to Daqin Island is 35~40 m, and from the Daqin Island to the Laotieshanjiao 50-55m on average. It is absolutely feasible for construction of a bridge system in the south. Our suggested scheme of "bridge-south and tunnel-north" is completely different from the alternative proposal which prefers to take the Beihuangcheng Island as the connection point for the bridge and the tunnel though they also agree to the idea of bridge-south and tunnel-north. We prefer to have the bridge - tunnel connection at the Tuoji Island, which is 7.05 km². The Daqin Island can be the second choice of the connection place for the bridge and the tunnel. If the Tuoji Island is selected as the bridge-tunnel connection, the bridge will be 58km long and the tunnel is 55km, with the total up to 113 km.Upon the above consideration, it is estimated that 8~12 years is needed to complete the construction with a total cost of 100~120 billion RMB if the service life of the system is assumed as 100 years. The Bohai Sea, as a continental sea of China, has weaker stormy waves comparing to the East China Sea and the South China Sea. The impact of stormy wave on the bridge across the Bohai Strait is supposed to be larger than that on the Jiaozhou Bay Bridge, but almost the same as the Hangzhou Bay Bridge, even the water depth of Bohai Strait is deeper. It is estimated that there would be more than 60 windy and foggy days every year when the traffic will be suspended in the Bohai Strait, almost the same as the period and time on the two land ends nearby. Therefore, it will not bring too much traffic problems to the region. Anyway, the weather should not be the reason to deny the scheme of "bridge-south and tunnel-north". The cost for bridge building is lower in the shallower water area, and the engineering will also proceed easily and complete in a shorter time in this broad zone. Moreover, tunneling costs more, needs more complicated engineering operation and of course much longer time. In fact, the length of tunnel is the factor which will decide the time required by the whole project. The scheme of "tunnel only" claims that it has the reputation of "performance under all the weather condition", but the "tunnel", which is 125 km long totally, will cost 250 billion RMB, at least twice over the scheme of "bridge-south and tunnel-north", and need twice or even more time for construction. And the economic benefits can only be generated after the whole project completed. So "tunnel only" scheme is costly, time-consuming and inefficient, and thus unrealistic for the time being. The developing trend of this area shows that the transport by highway rises rapidly, as the transport by railway declines sharply, so highway has the priority over the railway as a more realistic consideration. In the proposed scheme of "bridge-south and tunnel-north", bridge-will be constructed only for highway, but the tunnels in the north, there might be two alternatives. If some of the technical problems, such as ventilation, can be solved, the tunnel in the north should take highway as the priority.If the problems cannot be solved, the option of railway has to be acceptable. The trains may bring vehicles through the tunnel, of course, it is less efficient than the highway tunnel which may bring vehicles running through directly. The reduced transport capacity can be compensated by ferry transportation.In conclusion, no matter which type of path is to be built, highway or railway or both, the optimum scheme is the system with bridge-south and tunnel-north, which will be economic, time-saving and efficient, with economic benefits produced during different construction stages. The south bridge should be designed and constructed according to a basic seismic intensity of Ⅷ and a typhoon attack of 12 degree and the associated stormy waves. The bridge pier should keep a distance as far as possible to faults and fault zones. The north tunnel should be designed according to the basic seismic intensity of Ⅶ. To sum up, the shallower water in the south is good for bridge construction, and the "bridge-south and tunnel-north" scheme will make a perfect cross-strait path; which will be really economic, time-saving and efficient, and benefits will be produced during each stages of construction.