Revisiting factor of safety used for manhole uplift induced by soil liquefaction

The concept of safety factor has long been proposed to evaluate the uplift triggering of underground structure induced by soil liquefaction, and it is well known that it should be unity when the underground structure raises up. However, a close-unity value has been rarely obtained in experiments, especially for vulnerable cases. This is due to the difficulties of an accurate measurement during testing; thus, the sidewall friction below groundwater table is always ignored for simplicity. In this study, a close-unity safety factor during the seismic shaking was acquired for a variety of manhole uplift cases, being stable in ground or suffering significant uplift. More importantly, a complete time history showing the safety factor degradation was acquired for each case. It was found that the safety factor before earthquake shaking (termed as ‘prior safety factor’ subsequently) shows good correlation with the final manhole uplift displacement based on the presented centrifuge testing data both in this study and in the literature. This prior safety factor shows good potential to predict final uplift displacement for conventional manholes.