IV. ANALYSIS: IMPLEMENTING SEEPAGE ANALYSES
IV-21
WHERE AND WHEN TO USE METHODS OF ANALYSIS (Continued)
In most cases it is logical to start with the simplest and least expensive method and then
proceed to more complex and costly methods as the complexity of the problem becomes
apparent or the need for accuracy is evident. In seepage analysis, pinpoint accuracy is seldom
obtained and, consequently, most remedial measures are conservatively designed. For
example, if minor shallow seepage is emerging along the toe of a dam where relief wells are
providing relief for deep-seated pressure, then the design of a shallow toe drain based on
observed flows and a generous pipe size is likely adequate, and a finite element computer
model is not needed.
If time is of the essence (in other words, the problem appears to be headed for a catastrophic
conclusion), then a quick review of available information and an experienced judgment
"analysis" is in order.
As a final consideration, no analysis is better than the adequacy and quality of the necessary
input on boundaries and material properties. If information is limited, then a few simplified
flownet sketches will serve just as well as a numerical computer model based on vague
assumptions. In addition, the cost of a conservative remedial design is often less than the cost
of detailed explorations and analyses.
CONDUCTING ANALYSES
The analysis and resolution of seepage problems generally rely heavily on the judgment of
qualified geotechnical engineers and engineering geologists. Experience and knowledge of
geologic and material factors, design principles, and the principles of fluid flow through porous
media are more critical in most cases than the method of analysis. Consequently, it is desirable
that qualified people at least guide and review the work.
APPLICATION OF RESULTS
The objectives of seepage analysis are to determine whether or not an observed or perceived
problem is serious and represents an unacceptable risk and, if so, to develop effective remedial
action at a reasonable cost. The current philosophy of dam safety generally embraces the idea
that seepage should be controlled by redundant defenses to ensure safety.
The next unit provides information on remedial treatments, which must be designed in a
rational manner. The rational manner may vary from a very simple application of Darcy's law to
a complex numerical computer model. Selecting the best approach is judgmental, and both site
specific and problem specific. Generally, as the hazard imposed by the problem or the cost of
remedial action increases, more sophisticated investigation and design analyses may be
warranted. However, in most cases obtaining adequate, accurate data from existing sources or
additional field investigation is more important than the sophistication of the analysis.