Effects of Woody Vegetation on Seepage-Induced Deformation and Related Limit State Analysis of Levees

Abstract

Work in this paper investigates the effects of woody vegetation on soil hydraulic conductivity and the related probability of exceeding deformation-based performance limit states. A summary of results obtained from laboratory and field hydraulic conductivity tests for four levee sites is presented. Limit states, as defined based on the framework of critical state soil mechanics, are developed and simple probability analyses are used to quantify the probability of exceedance under hydraulic loading. A case study of Elkhorn Levee near Sacramento, California, is presented to demonstrate the applicability of the limit states concept. The field and laboratory hydraulic conductivity data obtained from the four levee test sites show no clear trend to support the notion that woody vegetation leads to either high or low values of hydraulic conductivity; site-specific testing is needed to discern such an effect. Even though the levee case used in this study (Elkhorn) was in a marginal condition of stability, based on limit equilibrium analysis, the probability of exceeding LS III after 10 days of a sustained high water level was 2%. This probability, however, increased over time to 37 and 72% after 20 and 30 days of sustained water loading, respectively, which signifies the importance of considering the transient nature of hydraulic loading in making condition assessments after a storm event. The results of modeling a 0.75-m top root layer illustrate the impact of woody vegetation on time duration to exceed a given performance LS level and the importance of accounting for seepage-deformation responses in assessing the vulnerability of levees.