Load-Factor Stability Analysis of Embankments on Saturated Soil Deposits

Abstract

A continuum-based finite-element methodology is established for quantifying the stability of earthen embankments built on saturated soil deposits. Within the methodology the soil is treated as a fluid-solid porous medium, in which the soil skeleton's constitutive behavior is modeled using a smooth elastoplastic cap model that features continuous coupling between deviatoric and volumetric plasticity. In the stability analysis procedure, self-weight of the embankment soils is monotonically increased at rates characteristic of the em- bankment construction time, until instability mechanisms develop. The transient effects of excess pore pressures and their impact on soil strength are explicitly modeled, allowing for computation of embankment safety factors against instability as a function of construction rate. Details on the proposed method are presented and discussed, including (1) how the construction rate of an embankment can be modeled; (2) how load-based safety factors can differ from resistance-based safety factors; and (3) solved example problems corresponding to a case history of an embankment failure. INTRODUCTION AND MOTIVATION The objective of this work is to develop methods for sta- bility analysis of earthen embankments constructed on satu- rated soil deposits. Using classical methods and assumptions, stability analysis of such systems typically proceeds using Mohr-Coulomb soil models and various slice-type methods (Nash 1987) and by assuming that the saturated soil has a response behavior that is either fully undrained (short-term response) or fully drained (long-term response). Because the computed factors of safety against instability associated with the fully drained and fully undrained soil assumptions are gen- erally not close in value, with undrained stabilities being sig- nificantly less than drained stabilities, the classical methods can leave considerable uncertainty directly attributable to time- dependent pore-pressure diffusion effects. Methods of slope stability analysis are thus needed that take into account the rate at which the embankment is constructed together with the spatially/temporally evolving pore-pressure field it generates in the underlying soil deposit. Such stability information is often needed by engineers planning safe yet timely rates of construction for embankment systems. When analyzing geotechnical systems, both load and resis- tance factors, analogous to those used in structural engineer- ing, can be used to quantify the stability of the system. Load factors of safety against instability are simply the ratio of the load magnitude that first generates instability of the system to the magnitude of the expected load, while the strength or re- sistance of the system is held constant. Resistance factors of safety against instability are the ratio of actual system strength (or resistance) to reduced system strength at which system in- stability first occurs, while holding the loading on the system fixed. Although classical slope stability methods (such as slice methods) typically produce resistance-type safety factors, both factors are valid, although not necessarily the same. In Swan and Seo (1999), where continuum/FEM models for computing both resistance and load factors of safety against instability were presented for soil slopes, it was shown that neither method is consistently more or less conservative than the other. In this work, load-based stability analysis is investigated for sand embankments constructed on soft, saturated clay soil deposits. Load-based stability analysis techniques are poten- tially attractive, because they quite naturally permit stability analysis of embankments as a function of construction time. Geotechnical analysts who utilize load-based safety factors and load-based stability analysis should be well aware of the differences between such methods and the more traditional re-