Foundation design above a collapsed hydraulic tunnel
For the construction of a parking facility with an approximate surface area of 10,500 m², intended for passenger vehicles, buses, and motorhomes, a foundation design was made at a site characterized by highly complex geotechnical and structural conditions.
The planned parking facility is located on an embankment extending above an existing hydraulic tunnel approximately 230 m in length. During site inspections and assessment of the existing conditions, it was determined that a significant portion of the tunnel structure had collapsed, resulting in surface deformations and compromising the stability of the surrounding ground.
At the location of the most severe damage, a complete failure of the tunnel’s concrete structure was identified, allowing embankment material to penetrate into the tunnel interior. Further review of photographic and video documentation revealed that the remaining sections of the hydraulic structure were also in very poor condition. Extensive concrete deterioration, reinforcement corrosion, localized spalling of the concrete cover, and pronounced longitudinal cracks in the tunnel roof slab were observed, all indicating a substantial reduction in the load-bearing capacity and overall structural stability of the tunnel.
The instability of the underground structure was also reflected at the ground surface. Within the future parking area, a large cone-shaped depression had formed as a result of settlement and collapse of the embankment above the tunnel. The depression measured approximately 16 m in diameter and 8 m in depth. Standing water with a stable water level was observed at its base, further highlighting the complexity of the hydrogeological conditions at the site.
Due to the combination of a severely damaged underground structure, an unstable embankment, and the presence of groundwater, the foundation design required a comprehensive assessment of the existing conditions and the development of a technical solution capable of ensuring the long-term stability and safe use of the future parking facility.
Geotechnical investigation and testing
The previously prepared geotechnical report was used as the basis for the development of this design solution. In addition, we carried out additional geotechnical investigation and testing, which included:
- Geological and geotechnical prospection of the wider area
- Exploratory rotary drilling
- Engineering geological mapping and drill core logging
- Dynamic Probing Super Heavy (DPSH) testing
- Laboratory testing
- Groundwater level monitoring
- Processing and interpretation of data from previous investigations, including correlation with newly acquired data
Based on the results of the conducted investigations, as well as the reinterpretation of previous investigations in the wider area, a comprehensive geotechnical report was prepared. This report served as the geotechnical basis for the development of the technical design solution.
Description of Alternative Design Solutions
This civil engineering project, developed at the conceptual design level, includes three alternative solutions aimed at ensuring the stability of the existing embankment intended for the future parking facility.
OPTION 1 – DEMOLITION OF THE HYDRAULIC STRUCTURE
– Based on the review of the available design documentation, supporting studies, and field investigations, it was concluded that the reinforced concrete tunnel structure is damaged, deteriorated, and no longer safe or stable for continued use.
– This option involves the demolition of the tunnel structure by means of controlled blasting.
– The implementation of this solution requires the preparation of comprehensive design documentation, including, among other things, a demolition design and a blasting plan.
– The demolition concept is based on the controlled and gradual blasting of the existing tunnel.
– The objective is to allow the embankment material to fill the existing tunnel void. During this process, surface craters are expected to form progressively as blasting advances, resulting in the gradual collapse of the embankment above the tunnel.
– Following the complete controlled collapse and stabilization of the embankment, the surface would be prepared for its intended use as a parking area through additional filling, material replacement, and other necessary earthworks.
OPTION 2 – CONSERVATION BY PERMANENT TUNNEL INFILL
– This option involves retaining the existing structure and filling the tunnel with a flowable cementitious material of controlled strength, following principles commonly applied in the closure and abandonment of mining tunnels.
– The implementation of this solution requires the preparation of comprehensive design documentation, including the methodology for placement of the fill material.
– The objective is to permanently isolate the tunnel from load transfer and preserve it in a stable condition.
– Upon completion of the tunnel infill and stabilization of the embankment, the surface would be prepared for its intended use as a parking area through additional filling, material replacement, and related works.
OPTION 3 – BRIDGING OVER THE EXISTING TUNNEL
Following construction of the bridging system, the area would be developed and finished for its intended use as a parking facility.
– This option involves retaining the existing tunnel structure and constructing a bridging system capable of spanning the tunnel, including the area susceptible to future crater formation resulting from potential additional collapses.
– The purpose of the bridging structure is to transfer loads safely across the affected zone while minimizing dependence on the condition of the existing tunnel and surrounding embankment material.
Conclusion
Considering the scale and scope of the project, as well as the specific foundation conditions, several alternative solutions were developed, including a cost assessment for each option. The analyses were based on soil parameters obtained from previous and additional geotechnical investigations and testing.
The analyses performed allow for a comparison of the technical and economic aspects of the proposed solutions, providing the investor with a reliable basis for selecting the optimal foundation solution about safety, constructability, and economic efficiency.
