| Construction of underground caverns is very complex, with multi-surface operations, including multi-process and parallel operation of drilling, blasting, loading slag, transporting, shotcrete-bolt supporting and secondary lining. Under the environment of trackless transport, construction ventilation is not only affecting the entire construction progress, but also to the lives and safety of construction workers. Taking the construction ventilation of diversion tunnels in Jinping Ⅱ hydropower project as engineering background, based on the project features of super long tunnels with large section, wide interface, no outcrop, internal combustion operation, trackless transportation, large pollution in Jinping diversion tunnels, by utilizing an integrated method of literature review, engineering analogy, theoretical analysis and numerical modeling, field tests and field monitoring, induction and summarization, etc. A series of questions were researched including:(1) The influence of wall roughness to the ventilation effect,(2) computational fluid dynamics analysis techniques of construction ventilation in tunnel,(3) forced ventilation technology supplemented jet ventilation in branch tunnel,(4) gallery ventilation technology,(5) field test of ventilation effect. The main wok done and achievements in this dissertation are as follows:The extreme ventilation distance of forced ventilation under the branch tunnel section size restricted is first obtained, the calculation of the extreme ventilation distance is as the visibility to reach20m, the minimum wind speed of the extreme ventilation distance is determined to be0.1m/s. The research results shows that the extreme distance of forced ventilation is more appropriate in the5000m. On the basis the experience of Jinping auxiliary tunnel combined with west branch tunnel section and maximum wind speed design, seven sets of75kw power jet fans can meet the construction need, fans arrangement in branch tunnel are the same as the calculation results.This study conducted a deep analysis of the influence of the rough element shape, height, spacing and tunnel section diameter under the same rough conditions on the tunnel construction ventilation, the results show that except for the sinusoidal model, the velocity distribution of the square, triangle and mixed model appeared eddy current near the rough wall, the hindering effect for air circulation and CO rapid discharge of the sinusoidal rough model is less than the other three rough model. The impediment for the fluid flow increased with increase of the rough height increases under the same roughness spacing. The larger rough spacing, the less impediment for the fluid flow under the same roughness height and the larger tunnel section diameter, the less impediment for the fluid flow in the same rough conditions(the same roughness shape, height, spacing). The average roughness height on wall surface in tunnel is first defined by the envelope area of actual tunnel excavation contour and design excavation contour divided by the sampling length. The calculation method of roughness constant is first proposed, including the model decomposition method and main roughness method.The average wall roughness height is0.231m and the roughness constant is0.46in Jinping diversion tunnels. Numerical calculation on construction ventilation system of Jinping diversion tunnels and drainage tunnel optimizes the arrangement of jet fans and axial duct in the tunnel. It is found that work area (within300m from the tunnel face) of1#to4#tunnel has reached the construction requirements after15minutes. The best layout of the jet fan is aroud the transverse channel under the direction of airflow and jet fans should not be too far away from the transverse channel and install the fans at the top of the tunnel as far as possible, or the ventilation will deteriorate. The effective range of duct in tunnel face is about30m. The jet fans at the intersection arranges at15m from the branch tunnel and diversion tunnel cross line at the right side of the tunnel will introduce fresh air effect better in the branch. The introduction strong wind caused by the high boost of the jet fans near intermediate cross-channel has influences the main flow direction of the air flow in the diversion tunnel.The jet technology in operation ventilation is introduced to construction ventilation, the required airflow of jet ventilation, the calculation formula of jet fan number and the adoption principleis of related parameters is studied by theoretical research combined with field test of ventilation. The research results shows that the difference between the number of units with the theoretical calculation and the actual fans in the number of units2, within the normal range, and the actual construction air quality can meet the requirements of the specification and the health of persons. The number of fans are related to the ratio of ventilation resistance and fan boost, the frictional resistant coefficient λ is0.1-0.3, the larger roughness, the larger λ, in the shotcrete position the λ is relatively large and in the secondary lining the λ is relatively small. Fans layout in accordance with the principle of less fans in the air inlet and more fans in the air outlet, more jet fans in the tunnel with axial flow fan and less jet fans in the tunnel with no axial flow fan. The gallery ventilation through exchanging side is conducive to the tunnel to carry out multi-step parallel operations, and achieve satisfactory ventilation effect.It is found from the field monitoring of ventilation effect that the average wind speed of polluted air is greater than0.15m/s and the visible distance greater than50m to ensure the tunnel ventilation quality before the diversion tunnel is excavated to2000m. But along with the tunnel advancing, the ventilation effect declines sharp and the worst visible distance is less than10m, especially the diversion tunnel is excavated to3300m. The air quality is good when the gallery ventilation is applied, the average wind speed is greater than0.2m/s in diversion tunnels and the average wind speed is2-4m/s in the construction branch tunnel and drainage tunnel. The visible distance at the work face is greater than50m at5minutes after ventilation and greater than150m during mucking in the diversion tunnels.These technical achievements and their application in the design and construction played a very important role in achieving a safe, fast, effective construction with high quality and in time, which can surely provide a very valuable reference for the construction of similar underground projects world wide in the future. |
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