Seepage Modeling with SEEP/W - GeoStudio 2007 version 7.22

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Chapter 2: Numerical Modeling SEEP/W 1.00E-04 Conductivity 1.00E-05 1.00E-06 1.00E-07 1.00E-08 1.00E-09 Intense Rainfall Coarse Fine Low to Modest Rainfall 1.00E-10 1 10 100 1000 Suction Figure 2-13 Conductivities under low and intense infiltration This is a critical lesson in modeling and the use of numerical models in particular. The key advantage of modeling, and in particular the use of computer modeling tools, is the capability it has to enhance engineering judgment, not the ability to enhance our predictive capabilities. While it is true that sophisticated computer tools greatly elevated our predictive capabilities relative to hand calculations, graphical techniques, and closed-form analytical solutions, still, prediction is not the most important advantage these modern tools provide. Numerical modeling is primarily about ‘process’ - not about prediction. “The attraction of ... modeling is that it combines the subtlety of human judgment with the power of the digital computer.” Anderson and Woessner (1992). 2.5 How to model Numerical modeling involves more than just acquiring a software product. Running and using the software is an essential ingredient, but it is a small part of numerical modeling. This section talks about important concepts in numerical modeling and highlights important components in good modeling practice. Make a guess Generally, careful planning is involved when undertaking a site characterization or making measurements of observed behavior. The same careful planning is required for modeling. It is inappropriate to acquire a software product, input some parameters, obtain some results, and then decide what to do with the results or struggle to decide what the results mean. This approach usually leads to an unhappy experience and is often a meaningless exercise. Good modeling practice starts with some planning. If at all possible, you should form a mental picture of what you think the results will look like. Stated another way, we should make a rough guess at the solution before starting to use the software. Figure 2-14 shows a very quick hand sketch of a flow net. It is very rough, but it gives us an idea of what the solution should look like. Page 14
SEEP/W Chapter 2: Numerical Modeling From the rough sketch of a flow net, we can also get an estimate of the flow quantity. The amount of flow can be approximated by the ratio of flow channels to equipotential drops multiplied by the conductivity and the total head drop. For the sketch in Figure 2-14 the number of flow channels is 3, the number of equipotential drops is 9 and the total head drop is 5 m. Assume a hydraulic conductivity of K = 0.1 m/day. A rough estimate of the flow quantity then is (5 x 0.1 x 3)/ 9, which is between 0.1 and 0.2 m 3 / day. The SEEP/W computed flow is 0.1427 m 3 /day and the equipotential lines are as shown in Figure 2-15. Figure 2-14 Hand sketch of flow net for cutoff below dam 1.4272e-001 Figure 2-15 SEEP/W results compared to hand sketch estimate The rough flow net together with the estimated flow quantity can now be used to judge the SEEP/W results. If there is no resemblance between what is expected and what is computed with SEEP/W then either the preliminary mental picture of the situation was not right or something has been inappropriately specified in the numerical model. Perhaps the boundary conditions are not correct or the material properties specified are different than intended. The difference ultimately needs to be resolved in order for you to have any confidence in your modeling. If you had never made a preliminary guess at the solution then it would be very difficult to judge the validity the numerical modeling results. Another extremely important part of modeling is to clearly define at the outset, the primary question to be answered by the modeling process. Is the main question the pore-water pressure distribution or is the quantity of flow. If your main objective is to determine the pressure distribution, there is no need to spend a lot of time on establishing the hydraulic conductivity – any reasonable estimate of conductivity is adequate. If on the other hand your main objective is to estimate flow quantities, then a greater effort is needed in determining the conductivity. Page 15
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Chapter 6: Analysis Types SEEP/W ON
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Chapter 8: Numerical Issues SEEP/W
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Chapter 9: Flow nets, seepage force
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Chapter 11: Modeling Tips and Trick
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Chapter 12: Illustrative Examples S
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Chapter 13: Theory SEEP/W k x k y Q
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Chapter 13: Theory SEEP/W Table 13-
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Chapter 13: Theory SEEP/W [ C] = wh
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Chapter 13: Theory SEEP/W { Q} = q
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Chapter 13: Theory SEEP/W Figure 13
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References SEEP/W Figure 14-1 Globa
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References SEEP/W In equation form,
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References SEEP/W Table 14-4 Interp
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References SEEP/W Burland J.B. 1996
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References SEEP/W Swanson, D. 1991.
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SEEP/W Index Adaptive time stepping
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SEEP/W Seepage faces ..............
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Seepage Modeling with SEEP/W - GeoStudio 2007 version 7.22

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