Critical Heights of Sloped Unsupported Trenches in Unsaturated Sand

Workers are often required to enter unsupported trenches during the construction process, which may present serious risks. Trench failures can result in death or damage to adjacent properties, therefore trenches should be excavated with extreme precaution. Excavation work is often done in unsaturated soils, where the critical height (i.e. maximum depth that can be excavated without failure) of unsupported trenches can be more reliably estimated by considering the influence of matric suction. In this study, coupled stress/pore-water pressure analyses are conducted to investigate the critical height of sloped unsupported trenches considering the influence of pore-water pressure redistribution caused by excavating. Four different wall slopes (1.5V:1H, 2V:1H, 3V:1H, and 90°) and a vertical trench with the top 0.3 m sloped 1:1 were considered in the analyses with multiple depths of the ground water table in a sand. For comparison, the critical heights were also estimated using the limit equilibrium method for the same excavation scenarios used in the coupled analyses.

• Won Taek Oh
• Adin Richard

• Critical height
• matric suction
• unsaturated soil
• unsupported trench.

[1] National Institute of Occupational Safety & Health (NIOSH), “Trenching and Excavation - Workplace Safety and Health Topic,” US Department of Health and Human Services. 2007.
[2] Alberta, “Occupational Health & Safety Code - Explanation Guide,” Occupational Health & Safety Division. 2009.
[3] Manitoba, “Guide for Excavation Work,” Workplace Safety & Health Division. 2011.
[4] D. E. Pufahl, D. G. Fredlund, and H. Rahardjo, H.,“Lateral earth pressures in expansive clay soils,” Can. Geotech. J., vol. 20, no. 2, pp. 228–241. 1983.
[5] V. Whenham, M. D. Vos, C. Legrand, R. Charlier, J. Maertens, and J. –C. Verbrugge, “Influence of soil suction on trench stability,” In Experimental Unsaturated Soil Mechanics, Springer Berlin Heidelberg, pp. 495–501. 2007.
[6] P. De Vita, A. C. Angrisani, and E. Di Clemente, “Engineering geological properties of the Phlegraean pozzolan soil (Campania region, Italy) and effect of the suction on the stability of cut slopes,” Italian J. of Eng. Geol. and Environ., vol. 2, pp. 5–22. 2008.
[7] Ontario, “Trenching Safety,” Infrastructure Health & Safety Association. 2017.
[8] F. Mohamed, “A semi-empirical approach for the interpretation of the bearing capacity of unsaturated soils,” Master’s Thesis, University of Ottawa. 2006.
[9] D. G. Fredlund, and A. Xing, “Equations for the soil-water characteristic curve,” Can. Geotech. J., vol. 31, no. 3, pp. 521–532. 1994.
[10] D. G. Fredlund, A. Xing, and S. Huang, “Predicting the permeability function for unsaturated soils using the soil-water characteristic curve,” Can. Geotech. J., vol. 31, no. 4, pp. 533–546. 1994.
[11] D. G. Fredlund, and J. Krahn, “Comparison of slope stability methods of analysis,” Can. Geotech. J., vol. 14, no 3, pp. 429–439. 1997.
[12] N. R. Morgenstern, and V. E. Price, “The Analysis of the stability of General Slip Surfaces,” Géotechnique, vol. 15, no. 1, pp. 79–93. 1965.
[13] S. K. Vanapalli, D. G. Fredlund, D. E. Pufahl, and A. W. Clifton, “Model for the prediction of shear strength with respect to soil suction,” Can. Geotech. J., vol. 33, pp. 379–392. 1996.
[14] GeoSlope, “Stability modelling with GeoStudio”. 2017.