Introduction

Tunnels are fundamental structures for efficient road solutions and are widely used in regions with steep topography in order to avoid exacerbated expenditure on soil movement due to cutting and landfilling, and on construction materials in cases where the alternative is to build bridges and viaducts.

Another typical use of this structure is to optimize the flow of vehicles safely, for example by promoting level traffic between intersecting roads. In the industrial sphere, it is possible to ensure that small and medium-sized vehicles do not intersect at level with off-road vehicles, increasing the safety and comfort of the people and workers who travel there, as well as increasing the average speed of the road.

There are several categories of tunnels, but the emphasis is on corrugated tunnels, due to the various qualities associated with their construction method: ease and speed of execution, in line with the low cost of construction and the possibility of catalog sizing, making the design stage faster. Within this category, there are destructive (Figure 1-a) and non-destructive (Figure 1-b) construction methods, which must be chosen depending on the reality of the project.

Figure 1: Examples of corrugated tunnels. SOURCE: Armco Staco®.

Problem

Kot helped one of its customers after identifying that the operating condition of a tunnel was incompatible with the catalog requirements:

- The height of the soil over the tunnel (embankment) is greater than the maximum recommended by the manufacturer;
- The weight of the vehicle traveling over the tunnel is 20x greater than that used in the design;
- The soil used in the construction of the embankment did not have the minimum desirable qualities, as well as being highly heterogeneous;

In addition, the tunnel showed signs that it was resisting in borderline conditions. Several bolts were broken and several others were in the process of advanced corrosion.

Given that the traffic inside the tunnel is mostly made up of small and medium-sized cars, and that the traffic above the tunnel is made up of off-road vehicles, the possibility of this structure collapsing puts the lives of users at risk, as well as affecting the connection between neighboring towns and interrupting the client's business.

Solution

Kot carried out a detailed structural and geotechnical analysis using the finite element method (FEM)in order to verify the tunnel's safety.

This type of structure is usually analytically designed from catalogs, with well-defined vehicles and embankment heights. The change in calculation assumptions makes it necessary to build an elaborate computer model, taking into account soil-structure interaction for the mathematical modeling of the problem.

Since the asset was missing bolts and had a number of pathological manifestations, these had to be taken into account in the analysis in order to issue an opinion on the tunnel's current resistance(fitness for service). Damage due to broken bolts and a reduction in the strength of the plates due to loss of thickness were taken into account.

The asset's stress history was simulated, from its construction to the current situation, in order to take into account the impact of deflections on the stresses developed in the corrugated sheets.

The figure below shows the bending moment diagram during the asset construction stage.

Figure 2: Diagram of bending moments during the tunnel earthing process. SOURCE: Kot Collection.

The effects of the mobile load in the tunnel were simulated using a 3D model, in order to check the influence of the spreading of the loads in the ground on the structure, considering the possibility of superimposed stresses inside the massif. Figures 3 and 4 show, respectively, the increase in axial forces and bending moments due to the presence of the mobile load on the tunnel.

Figure 3: Distribution of normal stresses along the tunnel during vehicle traffic. SOURCE: Kot Collection.

Figure 4: Distribution of bending moments along the tunnel during vehicle traffic. SOURCE: Kot Collection.

From the stresses, it was possible to assess the condition during the structural analysis of the tunnel. In addition, the stresses developed in the backfill soil caused by vehicle traffic in the tunnel were checked, as shown in Figure 5.

Figure 5: Stresses in the ground caused by the flow of vehicles through the tunnel. SOURCE: Kot Collection.

Outcome

After the detailed structural and geotechnical analysis carried out by Kot, it was observed that the tunnel was not in compliance with the applicable regulatory criteria. The analysis led to a borderline situation, indicating the possibility of the structure collapsing.

Figure 6: Distribution of bending moments along the tunnel during vehicle traffic. SOURCE: Kot Collection.

Figure 7: Stresses in the ground caused by the flow of vehicles through the tunnel. SOURCE: Kot Collection.

The client was instructed to change the operating conditions of the asset in order to reduce the risk involved in operations. One of the measures consisted of lowering the embankment over the tunnel so that the calculated stresses were compatible with its current strength. This measure guarantees the safety of all those who pass through, without jeopardizing the operation of the production line.

Kot has a team of professionals qualified in structural integrity and analysis, ready to develop the best engineering solutions for your business. Contact our team for more information!

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