Abstract：Many practical measures indicate that the settlement distribution of ground surface and vault，plastic zone in surrounding rock masses，and stresses in the middle wall are remarkably asymmetrical，i.e. skewness distribution. The position of maximum subsidence always lies toward the initial excavation side and the ground settlement curve is steeper on the initial excavation side while gentler in the subsequent side. A few empirical formulae such as the one based on the Weibull distribution by fitting the ground settlement curve have been proposed to estimate the skewness. However，the mechanisms of the skewness settlement have not been reasonably analyzed. The issue through a detailed case study based on monitoring data，numerical simulation and back analysis for an urban shallow large-span double-arch tunnel，which is constructed using CD/CRD methods，is considered. Furthermore，specific countermeasures were proposed，i.e. grouting to ground loss，stopping-water- heavy-curtain to limit range of skewness，non-isoparametric supporting to regulate time-space effect. To guarantee the safety of the underground pipelines and ground buildings，risk threshold values and control criteria are put forward based on an integrated approach of time-space effect and structural integrity level.
