Túneis metálicos corrugados são soluções viárias de rápida execução e baixo custo, frequentemente dimensionados por catálogos padronizados. No entanto, quando as condições operacionais reais, como o tráfego de veículos fora de estrada de grande porte e alturas de aterro acima do limite, superam as premissas de projeto, a estrutura entra em condição limítrofe de colapso. Por meio da Análise por Elementos Finitos (MEF) e da avaliação de Interação Solo-Estrutura, é possível diagnosticar a capacidade resistente residual do ativo (fitness for service), contabilizando patologias como corrosão e ruptura de parafusos para propor intervenções geotécnicas que garantam a segurança sem paralisar as operações.

Entenda como a simulação geotécnica evitou o colapso de um túnel corrugado em operação.

 

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.

In addition, another typical use of this structure is to optimize traffic flow while ensuring safety, for example, by managing traffic at intersections. In an industrial setting, it is possible to ensure that small and medium-sized vehicles do not cross paths at grade with off-road vehicles, thereby increasing the safety and comfort of people and workers using the facility, as well as raising the average speed of traffic.

In general, there are several categories of tunnels, but corrugated tunnels stand out due to the various advantages associated with their construction method: ease and speed of execution, combined with low construction costs and the ability to use catalog-based sizing, which speeds up the design phase. For example, within this category, there are destructive (Figure 1-a) and non-destructive (Figure 1-b) construction methods, which should be chosen depending on the specific conditions of the project.

 

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

 

Problem

In this regard, Kot assisted one of its clients after determining that the operating conditions of a tunnel were inconsistent with the specifications in the catalog:

- 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.

Since traffic inside the tunnel consists mainly of small and medium-sized cars, and traffic over the tunnel consists of off-road vehicles, the possibility of this structure collapsing puts the lives of users traveling there at risk, in addition to disrupting the connection between neighboring cities and interrupting the client’s business operations.

 

Solution

Accordingly, Kot conducted a detailed structural and geotechnical analysis using the finite element method (FEM)to verify the tunnel’s safety.

In general, this type of structure is typically designed using catalog data through an analytical approach, with clearly defined vehicle loads and embankment heights. Consequently, any change in the design assumptions requires the development of a sophisticated computational model that accounts for soil-structure interaction in the mathematical modeling of the problem.

Since the structure was missing bolts and exhibited a number of defects, these factors had to be taken into account in the analyses to issue an opinion on the tunnel’s current structural integrity (fitness for service). Damage caused by broken bolts and the reduction in the strength of the plates due to loss of thickness were factored in.

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.

For example, the figure below shows the bending moment diagram during the asset's construction phase.

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

In addition, the effects of the moving load on the tunnel were simulated using a 3D model to assess the influence of soil displacement caused by the load on the structure, taking into account the possibility of stress superposition within the rock mass. Figures 3 and 4 show, respectively, the increase in axial forces and bending moments due to the presence of the live 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.

Thus, based on the applied loads, it was possible to assess the structural condition of the tunnel during the structural analysis. Furthermore, the stresses developed in the backfill soil due to vehicular traffic over the tunnel were determined, as shown in Figure 5.

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

Outcome

In conclusion, following the detailed structural and geotechnical analysis conducted by Kot, it was found that the tunnel did not comply with the applicable regulatory criteria. Consequently, the analyses indicated a critical condition, suggesting a potential risk of structural collapse.

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.

Consequently, the client was advised to modify the asset’s operating conditions in order to reduce the risk associated with its operations. One of the measures involved lowering the embankment above the tunnel so that the calculated stresses would be compatible with its current strength. This measure ensures the safety of everyone who passes through the area, without disrupting production line operations.

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FAQ

1. Quais foram as principais divergências operacionais que colocaram a estrutura do túnel em risco?

(Formato: Tópicos Estruturados)

O dimensionamento original por catálogo considerava premissas simplificadas de tráfego e altura de solo. A investigação de campo identificou múltiplos fatores críticos concomitantes:

• Sobrecarga de tráfego severa: Passagem de veículos fora de estrada com peso até 20 vezes superior ao considerado no projeto original;

• Aterro incompatível: Altura de solo sobrestante superior ao máximo recomendado pelo fabricante, combinada ao uso de solo heterogêneo e com baixa capacidade mecânica;

• Degradação do ativo: Presença de diversos parafusos de fixação rompidos e avanço de processos corrosivos com perda de espessura nas chapas metálicas.

2. Como a simulação por Elementos Finitos (MEF) considera a Interação Solo-Estrutura na avaliação do túnel?

(Formato: Texto Corrido)

A análise computacional por MEF recria o comportamento conjunto entre a calha metálica flexível e o maciço de solo que a envolve, reconstituindo numericamente todo o histórico do ativo, desde as deflexões ocorridas na etapa de aterramento até a aplicação das cargas móveis atuais. Por meio de um modelo tridimensional, a simulação avalia o espraiamento das pressões no solo e a sobreposição de tensões gerada pelo tráfego de veículos pesados, permitindo determinar com exatidão como os momentos fletores e esforços normais se distribuem ao longo das chapas corrugadas.

3. Como a avaliação de adequação ao uso (Fitness for Service) contabilizou as patologias existentes no túnel?

(Formato: Tópicos Estruturados)

Para refletir com precisão a resistência real da estrutura danificada, as manifestações patológicas foram inseridas diretamente no modelo matemático:

• Perda de Seção Transversal: Desconto da espessura de chapa perdida por corrosão avançada no cálculo da rigidez e resistência da seção;

• Redistribuição de Esforços: Remoção dos parafusos rompidos na modelagem para mapear a concentração de tensões nos elementos de fixação remanescentes;

• Geometria Deformada: Incorporação do histórico de deflexões acumuladas desde a construção para avaliar o estado atual de tensões combinadas.

4. Qual foi a solução de engenharia adotada para evitar o colapso do túnel sem interromper as operações?

(Formato: Texto Corrido)

Como o diagnóstico indicou que a estrutura trabalhava em nível iminente de colapso, a engenharia recomendou o rebaixamento controlado da altura do aterro sobre o túnel. Essa adequação geotécnica reduziu o peso morto da camada de solo sobrestante a patamares compatíveis com a capacidade resistente atual das chapas corroídas, aliviando os momentos fletores solicitantes e restabelecendo as margens normativas de segurança para o tráfego superior e inferior sem exigir a paralisação da linha de produção do cliente.