Flow Net in soil mechanics Properties, Construction, Application

For graduate students, this equation will be briefly introduced with emphasis on the mass conservation and the physical meaning of each variable. Run flownet –help to see all possible command line argument how to draw a flow net options. You can download the paper by clicking the button above. The angle of intersection between each flow line and an equipotential line must be 90o which means they should be orthogonal to each other.

The head loss between each consecutive equipotential line is constant. The quantity of flow through each flow channel is constant. Flow lines must intersect equipotential lines at right angles. This assumption leads to the following two-dimensional relationship in velocity gradients. • The stream lines are so spaced that rate of flow Q is same between each successive pair of lines. • The streamlines ψ and equipotential lines Φ are mutually perpendicular to each other.

How to Construct a Flow Net for Seepage Analysis: Laplace’s Equation and Methods

Big blocks mean there is a low gradient, and therefore low discharge . Other sides of the squares are set equal to the widths as determined above. Irregularities are smoothed out, and the next flow line DF is drawn joining these bases.

The glass tubes (Fig.1) act as wells that are open at their bottom, as the simplest form of a piezometer. The well on the upgradient side shows a water level that is lower than the reservoir level on the left side of the tank, which helps students visualize the head in the sand at that point where it is measured. A small amount of liquid dye added to the glass tube can make this water level easier to see. The well on the downgradient side shows a water level that is higher than the reservoir level on the right side of the tank (Fig.1), which helps students understand an upward gradient. In some field situations similar to this, a flowing artesian well can be obtained with a sand-point well near a stream where the hydraulic gradient is upward. Since flow lines are normal to equipotential lines, there can be no flow across flow lines.

Principles of Geotechnical Engineering

Flow nets are drawn based on the boundary conditions only. They are independent of the permeability of soil and the head causing flow. Flow lines represent the path of flow along which the water will seep through the soil. Equipotential lines are formed by connecting the points of equal total head.

In using the sand tank and flow net, students learn that graphical methods are among the most accurate and useful tools in groundwater hydrology, even though more sophisticated numerical models are available. After completing part of a flow net it is usually possible to tell whether or not the final diagram will be correct. This line must now be redrawn in its corrected position and the procedure repeated again, amending the first flow line if necessary, until a satisfactory net is obtained. Buried surfaces (e.g. the base of the dam, sheet piling), which are flow lines as water cannot penetrate into such surfaces.

‘Draw flow net for the single row of sheet piles driven into permeability layer as shown in Figure 8.18, given the following:

Mays presented a 1-week module on stochastic groundwater modeling. Gómez-Hernández employed spreadsheets to teach numerical groundwater flow modeling. All the approaches above are aimed at emphasizing fundamental knowledge and stimulating student engagement. 8.3, the upstream bed level GDA represents 100% potential line and the downstream bed level CFJ, 0% potential line.

The hydrostatic pressure on the phreatic line itself is atmospheric. 3 can be solved if the boundary conditions at the inlet and exit are known. If the flow net is correct the following conditions will apply.

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2.9b, and the sketching procedure for the flow net is illustrated in Figs c, d, e and f of Fig. For the dye tracer, dry tablets of water-soluble dye such as fluorescein is recommended, which can be obtained commercially. The tablets can be crushed gently with a mortar and pestle, and tweezers can be used to insert into the sand a very small piece of crushed dye about 2 mm diameter or less. If too large a piece is used, it can take a long time for it to be flushed from the sand, making it difficult to get the sand-tank model ready for other uses or later classes. In most cases it is better for an experienced instructor to insert the dye into the sand, rather than taking the risk of allowing students to do this—the dye is extremely concentrated and can stain clothing or skin quickly.

In drawing flow nets for the sand-tank model, potentiometric contours must meet impermeable boundaries at right angles, such as along the glass or the Styrofoam dam, because flow is parallel to an impermeable boundary. On the upstream side of the sand tank, the interface of the sand and water forms the upgradient potentiometric contour and is a constant-head boundary with a head of 1.4 ft, which is the water level in the upstream reservoir. Similarly, the downstream interface of sand and open water is a constant-head boundary at 0.7 ft. (In this case, where a flow net is applied to a cross section, b is the width of the sand tank.) Equation can then be rearranged to solve for hydraulic conductivity. Can be derived from Darcy’s law, as a demonstration for students or as an assignment.

Rate of Seepage loss (Q)

Equal quantity of seepage occurs in each flow channel. The graphical method is most extensively used method among the methods stated above. The graphical method of flow net construction involves sketching by trial and error. The hydraulic boundary conditions are examined and keeping in mind the properties of flow net initial sketching is done and by trial and error the flow net is improved to make it acceptable for practical application. The method is typically applied to these types of groundwater flow problems, but it can be applied to any problem described by the Laplace equation, such as the flow of electric current through the earth.

The appearance of the entire flow net should be watched and not that of a part of it. Small details can be adjusted after the entire flow net has been roughly drawn. Project the equipotentials and repeat the procedure until the flow net is completed. The authors gratefully acknowledge the financial support by National Science Foundation .