A Case Study
Unknown foundations, minimal ground cover, and high risk: this case study by Dr. Lakshmana Rao Mantri, General Manager (Engineering & Design), AFCONS Infrastructure, and Vice President, TAI, shows how a risk-led approach, supported by advanced investigations and targeted mitigation, enabled safe tunnelling beneath a 50-year-old bridge.
Abstract
Tunnelling under existing bridge structures poses significant engineering challenges, especially when detailed foundation data are unavailable and the ground cover above the tunnel is minimal. This paper presents a case study of an underground rail corridor tunnel passing beneath a 50+ year-old bridge supported on pile foundations of unknown depth. To evaluate potential impacts of tunnelling on the existing foundation and structure, a comprehensive site investigation was undertaken using parallel seismic testing (PST), ground penetrating radar (GPR) surveys, and probe drilling from inside the tunnel. The integrated approach enabled accurate determination of pile depths, assessment of pile-tunnel spatial interaction, and identification of risks related to load transfer and settlement. Based on the findings, mitigation measures including pressure grouting around piles to enhance frictional resistance, addition of controlled overburden on the nala bed level to increase soil cover, and the installation of continuous structural and geotechnical monitoring systems were developed to ensure protection of the bridge during tunnelling operations. This study demonstrates the utility of combining geophysical and direct probing methods for reliable subsurface characterization and highlights practical strategies for mitigating structural risks in complex tunnelling projects below existing infrastructure.
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