Influence of shotcrete composition on load‐level estimation in NATM‐tunnel shells: Micromechanics‐based sensitivity analyses

Displacement measurement‐based estimations of loads and utilization degrees in shotcrete tunnel shells as part of the New Austrian Tunneling Method (NATM), have become standard tools in tunnel practice; their quality, however, may crucially depend on the knowledge of the actual shotcrete composition after spraying. To shed light on this issue, we here determine, based on experimentally validated micromechanical representations of shotcrete, the hydration degree‐dependent elastic, creep, and strength properties of different shotcretes, characterized by water cement ratios (w/c) between 0.4 and 0.6, aggregate cement ratios (a/c) between 3.5 and 5, and Young's modulus of aggregates (E_agg) between 40 and 80 GPa. These properties are fed into a structural shell model of the Sieberg tunnel, and this model is subjected to displacement fields approximated from daily displacement measurements at five selected points along the shell's inner surface. Resulting stresses and forces in the tunnel shell allow for analyzing the influence of shotcrete composition on load‐level estimation in NATM tunnel shells: The magnitudes of circumferential and longitudinal normal forces increase significantly with decreasing w/c, while a/c and E_agg have the inverse and relatively minor effect. The utilization degree is virtually insensitive to changes in w/c(especially at early ages), and only slightly decreases with decreasing a/c and E_agg. The location of maximum loading is unaffected by the shotcrete composition underlying the analysis. Conclusively, location and magnitude of maximum utilization degrees are very robust estimates (not affected by limited knowledge on the shotcrete composition), whereas realistic estimation of stresses and forces does require more accurate consideration of shotcrete composition. Copyright © 2011 John Wiley & Sons, Ltd.