Parallel tunnels with super-small-interval means that the interval of two parallel tunnels is between 1/2–1/3 tunnel span. Its stress condition of surround rock is more disadvantageous compare with the tunnels normal and small-interval tunnel during the tunnels with super-small-interval is excavated, and the excavated tunnel and the rock pillar between two tunnels are easy to be destroyed, so special design and construction requirement are asked. But its cost is only half of multi-arch tunnel, excavation method is simpler and construction period is shorter, moreover, it can reduce the width of highway in mountain area and the cost of higher class road greatly. The precise exploration before design and numerical simulation during design must be done to make sure the main danger which is easy to be destroyed, the reinforcement methods and the construction order. Taking advantage of self-supporting ability of surround rock fully is the design idea, so the rock pillar between two tunnels is the most important reinforcement place. Controlled blasting is another key. The ZhaoBaoShan tunnel locating in Zhenhai, Ningbo, Zhejiang, is a typical instance, which is a good book to show the design idea and construction technology.
Small-interval tunnel is one of the important content in Specifications of Tunnel Design for highway and means that the interval of two parallel tunnels are less than 1B (in I class surrounding rock) or 2.5B (in IV class surrounding rock) and 4B (in VI class surrounding rock), and B is the tunnel span. Because when the interval of tunnels less than the above value, the rock mass between two tunnels will be destroyed during construction, which cause that surrounding rock become unstable. And some reinforcement measures which are different from normal interval tunnel must be considered.
Due to the limit of terrain and linking to bridge, tunnels have to reduce the interval. Another successful method is multi-arch tunnel which makes two parallel tunnel one tunnel of double arch and supporting by mid-partition or pillar. But the processes of multiarch tunnel is more complicate and spend more than 30%–50% cost of the Small-interval-tunnel. If the interval of tunnel less than 1/3–1/2B, it should be called super-small-interval tunnel. Taking the tunnel span of 13–14m for example, its interval should be between 4–6m which is the best interval of supersmall- interval tunnels to reinforcement the rock mass between two tunnels, and the cost and difficulty of construction is only a little higher than the normal interval tunnels. Designing and constructing supersmall- interval tunnels probably are best choice for highway tunnel with 4–6 lines in mountainous area, which will reduce the width of highway and the amount of stonework of blasting. For the parallel tunnel in I–IV class surrounding rock, the design of super-smallinterval tunnels is not only a method have to be aim at any special condition, but also a regular choice of tunnel design, especially a better choice in the design of highway in mountain area.