Practical Guide to 2-Arch Tunnel Design: Analyzing Collapse Cases

1. 2-Arch Tunnel and Central Wall

The 2-Arch tunnel is constructed in the form of a tunnel, consisting of a central tunnel and widened side tunnels. The construction sequence involves the excavation of the central tunnel of the Pilot tunnel, followed by the construction of the central wall, and then the excavation of the left and right widened tunnels. Particularly, during the excavation of the widened tunnels, the central wall, constructed after the central tunnel, plays the most crucial role as the load is concentrated on it. The shape of the central wall in the 2-Arch tunnel differs between road tunnels and railway tunnels, so it must be considered when planning the tunnel cross-section. Initially, the road tunnel was planned with a cross-section similar to that of the railway tunnel, but currently, the cross-sections are completely different, allowing for clear differentiation.

The significant difference between the central walls of the two tunnels lies in the fact that the central wall of a road tunnel is a continuous structure, resembling consecutive walls. In contrast, the central wall of a railway tunnel takes on a different role, resembling columns with regular intervals in its structure. A brief analogy can be drawn by imagining the central columns in a subway station platform; in a railway tunnel, a similar form of central column is applied, allowing visibility to the opposite side when standing on the platform. However, in a road tunnel, with its continuous wall-like structure, visibility beyond emergency escape passages is restricted. Consequently, structural calculations for the central columns in a railway tunnel are rationalized in the form of girder structures. On the other hand, for a road tunnel, performing conventional concrete lining analysis is deemed appropriate due to its continuous wall-like structure.

2. 2-Arch Tunnel Collapse Case and Current Issues

Until now, there have been almost no collapse cases reported in the construction cases of 2-Arch tunnels. However, a collapse incident occurred a few years ago, making it the only known collapse case in 2-Arch tunnel projects. This incident took place in the 2-Arch tunnel (Gurye Tunnel) of the "Boksu~Daejeon Regional Road Widening Section."

During construction, two collapses occurred, and it is reported that excessive loads concentrated on the central wall due to aggressive construction progress and inadequate ground investigation led to the collapses. The excessive load was attributed to the presence of an undiscovered fault crush zone during the design phase (refer to the accident cause investigation report by the Korean Tunnel Engineering Society). Gurye Tunnel, a short tunnel of 300m, experienced a collapse during the excavation of the main widened tunnel after completing the central tunnel excavation and central wall construction. Despite conducting compaction grouting at the time of the initial collapse, the upper strata eventually collapsed. The ground conditions in Gurye Tunnel were unfavorable even during the design phase.

After the collapse incident, the tunnel was reported to have been changed from a tunneling method to a cut-and-cover method through an investigation commissioned by the Korean Tunnel Engineering Society and related lawsuits. I had the opportunity to visit the site and review relevant documents while participating as interpretation support in the accident cause investigation commissioned by the Korean Tunnel Engineering Society. Gurye Tunnel was situated in a section with extremely poor ground conditions, with the presence of fault crush zones and extensive weathered formations in the surrounding area. The direct cause was identified as the fault crush zone not detected through ground investigation during the design phase.

Below are a few compiled photos related to the collapse site.

A photo related to the collapse of Gurye Tunne
View of tunnel depression	fault fracture zone and Weathering zone in the tunnel face collapse
Lower freezing at the end of the tunnel	Occurrence status of icicles in influent water

Therefore, in this article, we will talk about the calculation of the central wall structure of the 2-Arch tunnel.

3. Calculation of Central Wall in 2-Arch Tunnel

In the design of 2-Arch tunnels, the tunnel cross-section is divided into a central tunnel and the main widened tunnel. The central tunnel can further be differentiated into a central wall. The central wall plays a structurally crucial role in supporting loads during the excavation of the main widened tunnel after the central tunnel excavation, making it significant. From my experience in conducting numerous 2-Arch tunnel designs, I believe it is the most challenging and critical aspect of 2-Arch tunnel design. In the case of Gurye Tunnel mentioned earlier, the occurrence of cracks in the central wall was detected before the collapse. Reflecting on such instances, one might consider that more careful consideration in the design of the central wall could potentially prevent tunnel collapses.

In the structural calculation of the central wall, the assessment of lateral pressure, especially the calculation of loosening loads, is deemed most crucial. In the early designs of 2-Arch tunnels, there were quite a few instances where the loosening load calculation method for 1-Arch tunnels was applied. However, this method is considered as overlooking the importance of the central wall in the characteristics of 2-Arch tunnels. The central wall exhibits completely different structural behavior, emphasizing the need to avoid performing structural calculations in the same manner as traditional methods.

Applying reinforcement throughout the entire length of the central wall in the 2-Arch tunnel is considered advantageous for ensuring structural stability during excavation. Therefore, it is reasonable to calculate the appropriate loosening load in the central wall and perform structural calculations. The commonly used methods to date include utilizing Matsuda's proposed formula or employing numerical analysis that takes into account ground conditions to calculate the loosening load acting on the central wall.

However, the calculation of loosening load using Matsuda's proposed formula may result in an excessive estimation of the loosening load, and therefore, an appropriate load reduction factor should be applied. Once this load is determined, it is then used in structural analysis to calculate the sectional forces. An alternative method involves using numerical analysis based on ground conditions, which differs from the previous approach in that it calculates the sectional forces acting on the central wall in terms of the applied load concept. This implies a difference from typical structural calculations that use the load multiplied by load coefficients. It is not definitive to claim that one method is correct over the other. Therefore, it is considered reasonable to utilize both representative methods to calculate the loosening load on the central wall in 2-Arch tunnels.

4. Closing Remarks

There was once a story about the exclusion of 2-Arch tunnels from road construction projects. However, the advantages of 2-Arch tunnels are undoubtedly present. Moreover, certain segments in route planning can only be achieved with 2-Arch tunnels, making their application inevitable. Despite the complexity of construction with the excavation of the central tunnel and the construction of the central wall, 2-Arch tunnels have evolved into versatile tunnel forms, including variable cross-section 2-Arch tunnels and asymmetric 2-Arch tunnels. Additionally, various forms such as 3-Arch tunnels and 4-Arch tunnels can be applied.

While a brief discussion was provided about 2-Arch tunnels, especially regarding the central wall, there will be an opportunity to talk about the sectional development forms of 2-Arch tunnels. In conclusion, here are various forms of 2-Arch tunnels that can be applied.

Reference

A Study on the Calculation of Subwall Behavior Load in the 2-Arch Tunnel (Korea Ground Engineering Association's Paper Collection, 2007.11, Oh Gyu-cheol et al.)

#2-Arch Tunnel

#Central Wall Design

#Tunnel Collapse

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Tunnel2023

Structural Engineer More than 15 years

Civil engineer specializing in tunnel design

Focused on analysis

Expert in quantity drafting

 

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