Seepage through a dam core with varying Ksat values

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Milo Atkins
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1 Seepage through a dam core with varying Ksat values 1 Introduction The objective of this illustration is look at steady state flow through a dam core for different values of Ksat in the core. In particular, the objectives of this illustration are to: Investigate the seepage through a dam core when the saturated conductivity of the core is 2x, x, and x less than the surround fill Show how this comparison can be carried out using four different analyses in a single project file Show how dynamic sketch text can be added to the project so that it changes automatically when the analysis changes 2 Feature highlights GeoStudio feature highlights include: Multiple analyses in one project file Dynamic sketch text auto updating 3 Geometry and boundary conditions The problem geometry and boundary conditions are illustrated below, in Figure 1. The dam is a simple cross section with similar fill material on the up- and downstream sides, as well as a lower permeability core in the center. There is a blanket drain beneath the dam on the downstream side. The upstream boundary nodes are designated as head boundaries with total head equal to the water level in the reservoir ( feet). The nodes along the contact between the dam and the drain are designated as potential seepage review nodes since the seepage exit point is unknown. With this review boundary applied, the solver will determine the appropriate nodes that have flow or no-flow. Default boundary conditions (no-flow) are assumed for all other boundaries. The mesh illustrated in this figure shows a nicely formed pattern of unstructured quadrilateral and triangular elements. Notice how along the boundaries and the contacts between soils, there are evenly shaped quadrilateral elements. This is ideal for the contact between different soil types where there is a potential for higher seepage gradients to form. Notice the text in the image. There is a main title and then below that, the phrase, Core x reduction in Ksat. The words Core x are dynamic and will change as the analysis changes. This way, when the view is changed to the model with x reduction in core Ksat, the text will update and be current with the view. The dynamic text is added using a field value as shown in Figure 2. You can see that the field value refers to the material description for region 2. SEEP/W Example File: Dam with core.doc (pdf) (gsz) Page 1 of

6 Seepage through an earth dam core with a toe blanket drain core x reduction in Ksat 4 3 1 - - -1 6 7 8 9 1 1 1 1 1 Figure 1 Seepage problem definition Figure 2 Dynamic sketch text field example 4

2 6 Seepage through an earth dam core with a toe blanket drain core x reduction in Ksat Figure 1 Seepage problem definition Figure 2 Dynamic sketch text field example 4 Material properties The dam fill material is assumed to be a silty sand with a Ksat of 1e- ft/s. The core is assumed to be a silty clay material with conductivity Ksat values that vary, as shown in Table 1. Table 1 Core conductivity values used in sensitivity study Run # Fill Ksat (ft/s) Reduced By Core Ksat (ft/s) 1 1 x - 1 x x - 2x x x - x 1 x x - x 1 x -7 SEEP/W Example File: Dam with core.doc (pdf) (gsz) Page 2 of

The actual functions for these soils are illustrated in Figure 3.

Figure 3 Hydraulic conductivity functions for dam soils The following figure shows the list of material models set up in the

You can see that the fill material is assigned to regions 1 and 3, and that the various core materials are applied to region 2.

3 The actual functions for these soils are illustrated in Figure 3. These functions were estimated within GeoStudio based on sample water content functions for silty sand and silty clay. Figure 3 Hydraulic conductivity functions for dam soils The following figure shows the list of material models set up in the project file, and where they are used within the project. You can see that the fill material is assigned to regions 1 and 3, and that the various core materials are applied to region 2. There are four analyses in the file, so region 2 can be assigned a different soil model in each one. Figure 4 Materials developed for model and assigned usage Figure Four sibling analyses in the project SEEP/W Example File: Dam with core.doc (pdf) (gsz) Page 3 of

4 Comparison of results Figure 6 to Figure 8 show the change in phreatic surface line position and leakage through the core that result from reducing the hydraulic conductivity of the core relative to the surrounding fill. It is clear that reducing the core conductivity by only two orders of magnitude, reduces leakage through the core by almost two orders of magnitude, as shown when comparing the figures above with Figure 9 below. In addition, the pore pressures on the downstream face are greatly reduced as most of the head loss occurs in the core material instead of on the downstream fill material. This may have implications when it comes to increased stability of the downstream dam face. core 2x reduction in Ksat e- ft³/sec Figure 6 2x reduction in Ksat of core core x reduction in Ksat e- ft³/sec Figure 7 x reduction in Ksat of core SEEP/W Example File: Dam with core.doc (pdf) (gsz) Page 4 of

5 3.873e-6 ft³/sec GEO-SLOPE International Ltd, Calgary, Alberta, Canada core x reduction in Ksat Figure 8 x reduction of Ksat in core Fill, no core reduction in Ksat ft³/sec Figure 9 Flow through dam with NO core SEEP/W Example File: Dam with core.doc (pdf) (gsz) Page of

Pore-Water Pressure Definition for a Levee Stability Analysis

Pore-Water Pressure Definition for a Levee Stability Analysis GEO-SLOPE International Ltd. www.geo-slope.com 1200, 700-6th Ave SW, Calgary, AB, Canada T2P 0T8 Main: +1 403 269 2002 Fax: +1 888 463 2239