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What does the "blue line" represent in both SEEP/W and SLOPE/W?

When people contact us and ask what the blue line represents in SEEP/W, we usually respond that it's the zero pressure contour (P = 0). How is it different than what we call the "piezometric" line in SLOPE/W, which is also blue?

Recently a support question of this nature was sent to us from Navead Jensen, an engineer with the USBR (United States Bureau of Reclamation) in Denver, CO. Navead's reason for requesting clarification was inspired because of two key concerns:

1. It was recognized that potential pitfalls exist when manually drawing the blue line in SLOPE/W as it may not always be the most accurate means of describing the actual pore water pressure for particular analyses.
   
   
2. The use of conflicting approaches amongst engineers to calibrate SEEP/W models using piezometer data including:

• Matching the location of the blue line to the measured height of the water in field piezometers, or 
• Comparing the pore-water pressure measured by a field piezometer to the computed pore-water pressures for a node existing at the same location in the model.

In general, there appears to be a misunderstanding of what is actually being measured by a piezometer and how to interpret the results to help calibrate a numerical model. The purpose of this article is to describe what the blue line represents in both SEEP/W and SLOPE/W, so they can be interpreted and applied with confidence.

SEEP/W
The blue line that we show in SEEP/W CONTOUR is always the zero pressure contour. In many cases the line is also representative of the general location of the water table. It's important to note that the actual water level measured in field piezometers will not necessarily match the location of the blue line as it is presented in SEEP/W. Therefore it is not appropriate to calibrate a numerical model by adjusting material properties in order to match the measured water elevation in a field piezometer to the computed location of the blue line in SEEP/W.

Take the example shown in Figure 1, where there is considerable head loss across the system and the elevation of the zero pressure contour is significantly higher than the location of the free water in two piezometers.

Figure 1: Schematic of information obtained from field piezometers superimposed on SEEP/W results

Note that the flow is not horizontal through this 2D flow system, so the equipotential contours (total head contours) are not vertical. Water flows in response to total head gradients, but piezometers measure pore-water pressures. More specifically, the water level that is measured in each piezometer reflects the pore-water pressure that exists at the bottom of the piezometer. When the measured pore-water pressure is converted to an equivalent pressure head and added to the elevation head then the total head that exists at the base of the piezometer is known. Since in this flow system the equipotential lines are not vertical, the blue line will not coincide with the measured height in the piezometer. In Figure 1 it is apparent that the height of the water in the piezometer corresponds to the height of the blue line where the corresponding total head or equipotential line intersects the zero pressure contour.

Given this information, an appropriate way to calibrate the numerical model would be to construct the finite element mesh in such a way that a specific node exists at the x,y coordinate of the screened portion of the piezometer. Material properties and boundary conditions can then be input and a solution obtained. The computed pore-water pressures reported at the node representing the piezometer should correlate with that measured by the field piezometer. If the two values do not correlate well, then the material properties and boundary conditions can be reasonably adjusted until the model is appropriately calibrated. It is not appropriate to define a pressure boundary condition at the location of the internal piezometer node as the program will inject or remove water at this node to ensure that the boundary condition is enforced. When using piezometer data, try to match numerical solutions to your field data, but don't force the solution.

SLOPE/W
In SLOPE/W we use the term "piezometric line" to describe the blue line that is drawn to determine the pore-water pressure conditions at the base of each slice. We use the term piezometric line because each soil layer can be assigned its own line, allowing for the definition of artesian pore-water pressure conditions for confined flow systems. With this option, SLOPE/W simply computes the vertical distance from the slice base mid-point up to the assigned piezometric line and multiplies the distance times the unit weight of water to get the pore-water pressure at the slice base. The pore-water pressures are then used to calculate the shear strength at the base of each slice.

The blue line in SLOPE/W is an approximate way to establish the pore-water pressures. For a horizontal flow system under hydrostatic conditions, the simple calculation of using the vertical distance and multiplying it by the unit weight of water is fine. However, if there is a steeply sloping profile as shown in Figure 2, using the vertical distance can result in computing a slightly higher pore-water pressure than necessary. If you want to be more accurate, you can apply a phreatic surface correction as shown by the equation in Figure 2, which will reduce the pore-water pressures at the base of each slice by taking into account the steepness of the sloping piezometric profile.

Figure 2: Phreatic surface correction

Note that when the piezometric line is horizontal (A = 0), the phreatic correction factor (cos²A) is 1 and Hc is equal to Hw. The good news is that if you do not apply the correction, you are erring on the conservative side, as the pore-water pressures computed using the non-corrected distance (H_w) will be higher than they should be, resulting in a lower factor of safety than when the phreatic correction is applied.

If you have highly irregular pore-water pressures that cannot be appropriately described by using one or more piezometric lines, then you should consider using SEEP/W generated pore-water pressures within your limit equilibrium stability analysis. GeoStudio's power lies in the integration between its products. Watch a video showing SEEP/W and SLOPE/W working together.

Q: What is the automatic License Update feature?

A: GeoStudio 2004 will automatically check for and download any new license files that you are licensed to use. Typically, no action will be required on your part. You will simply get an informational message on the Start Page notifying you that a license file has been downloaded.

Alternatively you may see a link on your Start Page that says "A new or updated license is ready for download"; click on the link and follow the instructions to replace your existing license files with the updated ones. If you have paid for a renewal, or if you are using a perpetual license (which is renewed for free automatically) your replacement license files will be available for download.

If you are using network licenses, GeoStudio 2004 will not notify you of new license files, because the files must be installed on the server, not the workstation. Your IT administrator should check the web site approximately one month before your licenses expire to get the replacement files.

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