Influence of rainfall infiltration on the stand-up time of unsupported vertical trenches

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University of New Brunswick
Trenches are typically excavated in soils that are in a state of unsaturated condition. Stability of trenches in unsaturated soils, therefore, primarily depends on matric suction distribution with depth, which has a great influence on the shear strength of unsaturated soils. The location of the groundwater table, types of soils, and local climate conditions such as rainfall and evaporation are major contributing factors to the matric suction distribution, and thus the stability of unsupported trenches. Trench failures are labelled as the cause of many work-related injuries and deaths in the construction industry. Hence, the design of unsupported trenches should be done with utmost caution. Critical height and stand-up time are two critical factors that govern the design of unsupported trenches. The focus of this research is directed towards investigating the influence of rainfall infiltration on the stand-up time of unsupported vertical trenches excavated in cohesive (Indian Head till) and cohesionless (Unimin 7030 sand) soils. For this investigation, a series of numerical analyses were carried out to estimate stand-up time of unsupported vertical trenches considering possible practical scenarios such as various rainfall intensities and groundwater tables, impermeable membranes on the ground surface, and tension cracks. The commercial geotechnical modelling software, GeoStudio (2016, GeoSlope Ltd. Inc.) was used in this study to simulate excavation, rainfall infiltration, redistribution of porewater pressure due to escavation and slope stability analysis. The results show that rainfall infiltration reduces the stand-up time of unsupported vertical trenches. The use of impermeable membrane is found not to be effective especially in sand for shallow ground water tables.