Study Of Reasonable Minimum Spacing And Supporting On Small Clear Distance Tunnel

Due to complicated topographical and geological conditions as well as large numbers of mountainous areas, constructions of high-grade highway unavoidably meet with lots of tunnel works. As a result of special geological and topographical limitations, space of tunnels cannot satisfy with general technical codes. Therefore, such special structure shape as small clear distance tunnel, twin-bore tunnel should be applied.The small clear distance tunnel is the structure style between ordinary tunnel and multiple arch tunnel. The clear distance is smaller than the tunnel which the standard stipulated possible to be called the small clear distance tunnel. The small clear distance tunnel with high economy and project value, has been arousing the interest by its unique merit not only for replacing multiple arch tunnel but also for route selection, the land takes over for use, the environment protection and so on.Whereas many critical technical problems have not been solved in both design and construction, such as possible destruction modes of neighborhood tunnel, definition of smallest reasonable spacing, critical factors that influence stability of surrounding rocks, the function of primary lining and secondary lining, the optimal construction method for secondary tunnel, the best strengthening method of middle rock pillar, design proposal and construction technique concerning neighborhood tunnel which is at unsymmetrical pressure, and so on.This paper applies finite element numerical method to numerical modeling analysis on tunnels with different adjoining rock classes and different clear distances. Through Griffith criterion and Drucker-Prager criterion, we fit the fitting curves between different criterions and clear distances and find the clear distance which makes the two tunnels interact obviously with the clear distance reducing. As a result, we can reduce its distance between two tunnels obviously to simple the construction and reduce engineering costs.This paper found the reasonable minimum spacing for different rock mass levels. "Forâ…¡rock mass level, the reasonable minimum spacing is 0.5B (B is the breadth of a single tunnel); forâ…¢level, it is 0.8B; forâ…£level, it is 1.0B; for V level it is 1.3B. After the supporting, forâ…¡rock mass level, the reasonable minimum spacing is 0.3B (B is the breadth of a single tunnel); forâ…¢level, it is 0.5B; forâ…£level, it is 0.6B; for V level it is 0.7B. The minimum spacing value is largely less than the spacing value which is prescribed by "the criterion of highway tunnel". By model test and in-site test, this paper also discussed the influence to rock mass stability by rock mass level, spacing. It draws the important conclusions that rock mass level and spacing is the critical factors which influence the stability of surrounding rocks.This paper studied the functions of primary lining and secondary lining by numerical modeling which were in different rock mass levels and the educed spacing above, and stated that they could make the tunnels stable.The small clear distance tunnel construction in expressway make the expressway design more conform to the natural style and achieve the structure artistic, linear smooth, safe and reliable, saves the investment, convenience construction and create more social efficiency and economic efficiency.