Implications and Analysis of Seepage on Overall Stability and Design of Levee Foundation Treatments

This paper provides an overview of the theories and design methods used in evaluation of seepage through a levee foundation and the subsequent design of foundation treatments to control seepage. The paper also looks at the impact of seepage on overall stability. The discussions focus in particular on work done for the existing West Springfield Flood Damage Reduction System (WSFDRS), in West Springfield, Massachusetts. The work includes seepage and stability analyses used in the evaluation of the WSFDRS performance for flood stages ranging from the FEMA base flood (1%-annual-chance) up to the USACE original full design flood event (greater than a 0.2%-annual-chance event). The WSFDR system was analyzed using two-dimensional seepage and slope stability analyses. Portions of the system required rehabilitation of foundation treatments, which involved complete rehabilitation of the original (circa 1940 – 1950) toe drain and relief well systems. Due to the size and extent of the systems, seepage analysis incorporated both two- and three-dimensional analytical seepage models. Construction work was performed in Phases, such that design confirmation and adjustments could be proactively made based on new subsurface data and observations of the system performance obtained during construction. Complete review of the foundation treatments is provided, including review of the filter design. Results of aquifer pump test were used to help establish permeability values and to calibrate computer models. Important origins of current USACE guidelines for the design of relief well systems relate to work done in the 1930s and 1940s. While this previous work remains sound, the understanding of levee underseepage and non-linear degradation of older levee systems has evolved and remains an active area of study. In addition, the analysis and design of relief wells has significantly improved with our ability to: perform subsurface explorations; install instrumentation; conduct partial and full scale pumping tests; and complete finite element/difference analysis. What has not changed is the need for the designer to understand variations in the geologic and depositional history of the site, and to use appropriate engineering judgment. The limitations of the theories and methods used in the design and the investigation of foundation treatments are discussed. 37 pp, 18 references.