Learn more about the factors that should be considered in ensuring the safety of an underground excavation project.
Underground excavations of tunnels are inherently risky, as they are carried out in a medium that is variable and uncertain by nature. It is important for construction owners to assess the probability of failure in any excavation, allowing them to understand the risk that they bear.
When calculating the excavation stability, it is important to remember that the variability of geotechnical parameters directly affects the probability of failure. For this reason, ground investigations should not only assess mean values of parameters but also the dispersions and coefficients of correlation. The lower the standard deviation of the soil properties, the lower its uncertainties and the greater the project reliability. This is why it is necessary to develop a design based on reliability analyses that complements the conventional deterministic approach which relies solely on global safety factors.
Although the costs of an efficient geotechnical investigation are extremely low in comparison with the total cost of the work, the common practice of minimising costs can lead to greater uncertainty. Back-analyses are often used to overcome such deficiencies, such as in the case of a probabilistic back-analysis of a length oftunnel on the Metro Line 2 in Santiago, Chile conducted by Bureau de Projetos e Consultoria, a company under the TÜV SÜD Group. The soil properties were back-analysed based on the observed tunnel performance and then wereadopted for the design of the Metro Line 3, in a departure from the values used for the Line 2 design.
TÜV SÜD Bureau also successfully served as consultant on a reliability analysis for the University of Chile Station on the Santiago Metro Line 3, a tunnel excavated just beneath an existing track tunnel of Metro Line 1. The lengths of the tunnel roof forepoling and the soil nailing in the excavation face were extended to reduce the probability of failure for the tunnel excavation to an acceptable level.
With this methodology, TÜV SÜD Bureau has found an attractive alternative to qualitative estimates for quantifying risks involved in underground excavations. TÜV SÜD Bureau is currently developing a software to better quantify the reliability of underground works by means of numerical limit analysis via the finite elements method.
CECÍLIO Jr., M.O. ; QUEIROZ, P.I.B. ; NEGRO Jr., A. “Instability risk assessment of soil tunnel excavation based on ground properties variability”. In: World Tunnel Congress - WTC. Geneva, 2013.
CECÍLIO Jr., M.O. ; VALENZUELA P., L. ; NEGRO Jr., A. “Probabilistic back-analysis of earth pressure coefficient at rest and deformability characteristics for fine soils in Line 2 of Santiago Metro”. In: International Symposium on Geotechnical Aspects of Underground Construction in Soft Ground. Seoul: ISSMGE, 2014.
CECÍLIO Jr., M.O. ; VALENZUELA P., L. ; NEGRO Jr., A. “Reliability analysis for the excavation of Universidad de Chile Station, Line 3 of Santiago Metro”. In: World Tunnel Congress - WTC. Iguassu Falls, 2014.