Seepage Modeling with SEEP/W - GeoStudio 2007 version 7.22

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Chapter 2: Numerical Modeling SEEP/W Fine Coarse Material to be protected OR Coarse Fine Material to be protected Figure 2-9 Two possible earth cover configurations 1.00E-04 Conductivity 1.00E-05 1.00E-06 1.00E-07 1.00E-08 Coarse Fine 1.00E-09 1.00E-10 1 10 100 1000 Suction Figure 2-10 Hydraulic conductivity functions After conducting various analyses and trial runs with varying rates of surface infiltration, it becomes evident that the behavior of the cover system is dependent on the infiltration rate. At low infiltration rates, the effect of placing the fine material over the coarse material results in infiltration being drained laterally through the fine layer, as shown in Figure 2-11. This accomplishes the design objective of the cover. If the precipitation rate becomes fairly intensive, then the infiltration drops through the fine material and drains laterally within the lower coarse material as shown in Figure 2-12. The design of fine soil over coarse soil may work, but only in arid environments. The occasional cloud burst may result in significant water infiltrating into the underlying coarse material, which may result in increased seepage into the waste. This may be a tolerable situation for short periods of time. If most of the time precipitation is modest, the infiltration will be drained laterally through the upper fine layer into a collection system. So, for an arid site the best solution is to place the fine soil on top of the coarse soil. This is contrary to what one might expect at first. The first reaction may be that something is wrong with the software, but it may be that our understanding of the process and our general thinking is flawed. A closer examination of the conductivity functions provides a logical explanation. The software is correct and provides the correct response given the input parameters. Consider the functions in Figure 2-13. When the infiltration rate is large, the negative water pressures or suctions will be small. As a result, the Page 12
SEEP/W Chapter 2: Numerical Modeling conductivity of the coarse material is higher than the finer material. If the infiltration rates become small, the suctions will increase (water pressure becomes more negative) and the unsaturated conductivity of the finer material becomes higher than the coarse material. Consequently, under low infiltration rates it is easier for the water to flow through the fine, upper layer soil than through the lower more coarse soil. Fine Coarse Low to modest rainfall rates Figure 2-11 Flow diversion under low infiltration Fine Coarse Intense rainfall rates Figure 2-12 Flow diversion under high infiltration This type of analysis is a good example where the ability to utilize a numerical model greatly assists our understanding of the physical process. The key is to think in terms of unsaturated conductivity as opposed to saturated conductivities. Numerical modeling can be crucial in leading us to the discovery and understanding of real physical processes. In the end the model either has to conform to our mental image and understanding or our understanding has to be adjusted. Page 13
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Chapter 6: Analysis Types SEEP/W ON
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Chapter 6: Analysis Types SEEP/W Fi
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Chapter 7: Functions in GeoStudio S
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Chapter 8: Numerical Issues SEEP/W
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15 Chapter 9: Flow nets, seepage fo
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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 L = a des
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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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Chapter 13: Theory SEEP/W Page 184
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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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