Levee analysis is becoming an increasingly important topic as the infrastructure ages. Many sections are in need of re-engineering improvements as consideration is given to their replacement. Concepts utilized in their original design are being re-examined. Many of these structures were designed 20 to 50 years ago, and the tools utilized in such analysis have changed significantly. Our understanding of the processes involved has also improved with time.
Levee structures are typically three-dimensional in character. Early designs largely overlooked the potential influences of three-dimensional effects at the intersections of levees. In particular, there are seepage and flow issues at a levee intersection which are inherently 3D. The behavior of groundwater at such intersections may also be influenced by culverts, which may be opened during storm events. Thus, rapid drawdown effects may be created during the natural operation of such structures. Multiple questions may arise, for example from a 45-degree intersection or a 'T'-type intersection:
- What is the effect of 'funneled' flow? Funneled flow can occur in the concave downstream side of a levee intersection. The extent of funneling of flow and its potential impact on both the seepage and slope stability aspects of the problem can be numerically modeled.
- How are gradients affected in a 3D flow regime? Can 3D regimes lead to piping failures, which may not be evident in a 2D analysis?
- How is the factor of safety affected for a rapid drawdown analysis when analyzing the concave or convex side of a levee intersection?
- How do operational scenarios such as opening a culvert affect the 3D analysis?
- Are there flooding scenarios for which the 3D analysis will give different factors of safety than that of a 2D analysis?
The above questions are typical of those that can be addressed with a three-dimensional analysis. These issues can be studied using a combined seepage and slope stability analysis. Previously, these questions may not have been possible to readily address. However, software that is currently available from SoilVision Systems Ltd. makes it easy to address these questions. Recent improvements made to SVSlope allow the import of pore-water pressures from a vertical slice through a SVFlux 3D seepage analysis. Therefore, multiple levee canal filling and rapid draw-down scenarios can be modeled in SVFlux in a 3D manner. The results from the seepage analysis can then be imported to the SVSlope limit equilibrium slope stability package, and the resulting factor of safety (FOS) determined.
Two example models were set up in SVFlux 3D to illustrate this process. The first model is comprised of earth levees meeting at a 90-degree angle. Each levee is comprised of fill material and a clay core. In this scenario the earth dam is subjected to a rising water table, which rises from the bottom to near the top of the outer part of the levee. It is possible with this model to examine the scenario and track the resulting gradients and the 'funneling' effect of flow on the downstream concave side of the levee intersection.
A second 'T' scenario was created to examine the effects of changing the canal water level on multiple sides of a complex structure. In the 'T' scenario (see Figure 2 above), the water elevation along the top portion of the levee stays constant. The water level on the one side of the T (Area 'A') is then allowed to drain through a culvert that is opened between zones 'A' and 'B'. The culvert then results in a rapid drawdown scenario on one side of the T-intersection and therefore a resulting change in the pore-water pressure scenarios. The animation of this modeling exercise may be seen in Figure 4, below. It can be seen that the location of the water table can be predicted as a function of time. Gradients can also be calculated at any point in time.
Both of these scenarios show that it is possible to export geometry and pore-water pressure data on any particular vertical 'slice' through the model and use the information for an analysis in SVSlope. Thus, the factor of safety of the slopes can be readily computed.
The primary benefits of this type of analysis are:
- A more accurate representation of the processes in 3D, and
- A means of combining the seepage and slope stability analysis.
Overall, this methodology results in an increased accuracy in modeling the specific processes involved in the operation of the structure:
- An engineer can optimize their design such that construction costs can be reduced.
- Problematic operational scenarios can be better identified, thus allowing the engineer to alter the design to account for them.