Influence of excavation rate and tension crack on the stability of an unsupported vertical cut in unsaturated soil

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University of New Brunswick


Unsupported excavation is an important activity in geotechnical engineering practice since many projects ranging from mining to infrastructure developments are initiated from unsupported excavation. Unsupported excavation should be carried out with the utmost caution since the failure of unsupported cuts can result in not only property losses but also fatalities. This research focuses on the stability of unsupported vertical cuts in unsaturated soils. For this, a series of numerical analyses is conducted in an unsaturated glacial till considering three factors: i) excavation rate, ii) depth and location of tension crack, and iii) rainfall infiltration into a tension crack. The results showed that the influence of excavation rate is not significant if an unsupported vertical cut is made to a safe height (i.e. critical height divided by factor of safety (1.2 in this study)). Tension crack is one of the major factors that can lead to the failure in unsupported vertical cuts; however, the factor of safety did not drop below unity if the location and depth of tension crack is limited within 20% and 30% of the safe height from the cut wall and the ground surface, respectively. Rainfall infiltration into a tension crack decreased the factor of safety with time and then eventually led to the failure in unsupported vertical cuts for most cases. However, extremely long duration of rainfall was required if the initial factor of safety with a tension crack is close to 1.2. The proposed approaches are then applied to a deep unsupported vertical cut (9.75 m) made into a clay for its validation, which successfully estimated the critical location and depth of a tension crack.