Study On Construction Mechanical Behaviors And Key Technologies Of The Soft Rock Underwater Tunnels With Small Clear Spacing By The Shallow Mining Method

The shallow embedded underwater traffic tunnels with small spacing have wide application prospect in crossing the river in the city, because they can accord with the setting slope easily, link the existing roads on the banks conveniently and save land significantly, but they are seldom built by the shallow mining method in our country. Taking the Liuyanghe highway tunnel in Changsha as project background, the construction mechanical behaviors and key technologies of the underwater soft rock tunnels with small spacing by the shallow mining method were researched by field test, indoor test, analytical calculation and numerical simulation. Following are research process and main achievements:(1)At the construction site, the real-time positions of the working faces, the surrounding rock condition of the driving faces and the situation of water leakage were recorded in detail as the firsthand informations, which was helpful to describe the problems and verify the research results.(2)The instrument for fixing the earth pressure cell was designed. The installing methods and the protection measures of the testing elements were given in detail. The changing characteristics of the testing indexes were analysed, including water level, ground surface settlement, crown settlement, convergence, water pressure behind initial lining, contact pressure between surrounding rock and initial lining, axial force of anchor, stress of section steel frame and secondary lining. The construction links effects on the lining mechanics character were studied, including excavating the lower bench, closing the initial lining layer, digging out the temporary fill on the initial lining, moving the lining trolley, constructing the invert secondary lining and the arch wall secondary lining. The change of the ground water seepage path during the period of construction was analysed. The mechanics characteristics of the constructing interaction of the underwater twin caves were studied. According to the result of the field test, before constructing the secondary lining, the water pressure behind the initial lining could be calculated by the discount coefficient method, and the determination method of the discount coefficient was put forward. (3)The bench cut method was used in the Liuyanghe tunnel, when the primary support constructing was delayed after excavating the lower bench, the supporting force existed only on the upper semi-section, and unluckily, the river water level was increased at the same time. According to this situation, the calculation expression of the surrounding rock additional stress of the shallow underwater tunnel was obtained by the formula derivation.(4)By using the seepage numerical model of the underwater twin caves with small spacing by the shallow mining method, riverbed settlement, water pressure, crown settlement and principal stress of initial lining were calculated and analysed, the effect of the change of the water level on crown settlement of initial lining was studied also. The comparative analysis was carried out between the numerical simulation results and the field test results.(5)The combined advanced support technology scheme was given and applied in the strong weathering conglomerate underwater tunnel by the shallow mining method. The problems about the preventing-grout wall and the borehole orifice-pipe were discussed. The surrounding rock conditions obtained by TSP advanced geological exploration and observing after constructing the combined advanced support and excavating were compared to each other. The action mechanism of the slurry into the strong weathering conglomerate was analysed, and the forming mechanism of the surrounding rock lateral load-bearing arch under the action of the combined advanced support was studied. By the indoor test, the comparative analysis on the mechanical property was carried out among the three kinds of the slurries and the strong weathering conglomerate, and the rock sample velocity under the action of the combined advanced support was studied. The technical scheme of the blasting excavation of the upper bench was determined considering the action of the combined advanced support. By the field test and the numerical simulation, the efficacy of the arch anchor bolt under the action of the combined advanced support was studied and the related design scheme was optimized. Considering the effect of seepage, the comparative analysis on the mechanics character of the initial lining was carried out among no applying the advanced support, applying the full-sectional grouting only and the combined advanced support, the effect of the combined advanced support on the mechanics character of the initial lining was studied also by the field test. The proper construction time of the arch wall secondary lining was determined considering the action of the combined advanced support.(6)The Liuyanghe tunnel included two lines and four caves, they would transfix respectively under the river. According to the condition, the three-dimensional numerical model considering seepage was established. By analysing crown settlement and principal stress of initial lining, longitudinal displacement and principal stress of surrounding rock, riverbed settlement and vertical stress of interlaid rock in the transfixion zone, the mechanics characters of the surrounding rock and the initial lining during transfixion were revealed, and the corresponding transfixion scheme and measures were put forward and applied in practice. The field survey and test showed that the transfixion effect was good, which proved that the numerical simulation results had reference value.(7)By the longtime field investigation during the period of construction and operation, the action mechanism of the invert waterproof board and the application effect of the whole enwraped and closed waterproof technology were analysed. The water pressure and the earth pressure of the waterproof secondary lining after the longtime operation were calculated. The integral invert secondary lining was applied in the soft rock underwater tunnel by the shallow mining method, sometimes the bottom residue was not treated during the period of construction, which would bring a harmful effect on the mechanical performance of the waterproof secondary lining after the longtime operation, the harmful effect was revealed by the comparative calculation, and the corresponding measures were put forward.