Abstract:
ABSTRACT - DFI - 2024 S3 - Aurora, CO
Embankment Stability Analyses using Lesser of Drained and Undrained Strengths on a Slice-by-Slice Basis
Garry H. Gregory, Ph.D., P. E., S. E., BC.GE1
Limit equilibrium (LE) slope stability analyses for “End of Construction†and “Long-Term†conditions for embankments and their foundations in fine grained soils are typically performed using undrained soil strengths and drained soil strengths, respectively, in two separate analyses. The two separate analyses are intended to determine the most critical condition for the embankment, based on the lower factor of safety from the separate analyses. This approach assumes that the soils at the end of construction and for a short time thereafter have not had time to drain and for pore pressures to come to equilibrium and are therefore analyzed with undrained strengths and total stresses. The assumption is also that a significant time following construction is required for the soil to come to full equilibrium and these conditions are analyzed with drained strengths and effective stresses. However, especially for larger embankments or longer construction times, conditions along a slip surface may consist of a combination of undrained and drained conditions and this combination may produce a stability factor of safety that is lower than either of the separate undrained or drained analyses. In this presentation, the author proposes a method of LE analysis where the undrained and drained strengths are compared on a slice-by-slice basis and the lower of the undrained or drained strength is assigned to each slice in a single analysis. This is illustrated in the presentation with an example embankment slope reinforced with piles in a staged construction sequence. The single Drained-Undrained analysis produces a significantly lower factor of safety than either of the separate drained and undrained cases.
1Principal Consultant, Gregory Geotechnical, Stillwater, OK, USA ggregory@gregeo.com