GEOTECHNICAL ENGINEERING-I Construction of Flownet in Homogeneous Earthen Dam by MUKESH KUMAR,NITA by Verma-mukki

The equation is used in the construction of flow net. Phreatic line is a seepage line as the line within a dam section below which there are positive hydrostatic pressures in the dam. The hydrostatic pressure on the phreatic line itself is atmospheric.

As mentioned earlier the main application of flow net is that it is employed in estimating
quantity of seepage. If H is the net hydraulic head of flow (i. the difference in total head between
the first and last equipotential), the quantity of seepage due to flow may be estimated by drawing
the flow net, which is shown in Figure 15. With reference to Figures 15 & 17, the following terms
may be defined in order to estimate the quantity of seepage through the earth dam model. It has been noticed from experiments on homogeneous earth dam models that the line of
seepage assumes more or less the shape of a parabola. Also, assuming that hydraulic gradient i is
equal to the slope of the free surface and is constant with depth (Dupit’s theory), the resulting
solution of the phreatic surface is parabola. In some sections a little divergence from a regular
parabola is required particularly at the surfaces of entry and discharge of the line of seepage.

t hrough t he eart h dam m odel

 The pressure drop from one side of the embankment to the other,
 The seepage flow rate in each flow “channel”,
 The total seepage flow rate, and
 The pore pressure ratio, ru, for the embankment. A Flow net is a graphical representation of flow of water
through a soil mass. It is a curvilinear net formed by the combination of flow
lines and equipotential lines. Properties and application of flow net are
explained in this article.

• Phreatic line is a seepage line as the line within a dam section below which there are positive hydrostatic pressures in the dam.
• The average pore pressure ratio, ru, for the whole embankment of the earth dam model have
  been calculated in accordance to statistical procedure and algorithms / equations that were carried
  out.
• Starting from the upstream end, divide the first flow channel into approximate squares.

Some of the squares may, however, be quite irregular. Flow nets were originally used for determining how much water could flow under a dam. In
case that there is a substantial amount of water flow under the body of a dam, it can create a lot of
pressure on the alluvium / sediments. Over time the groundwater can erode the sediments, and the
dam can collapse, causing a disastrous flood. To solve this problem analytically it is difficult, but
flow nets can be used to give a graphical answer (Sachpazis et al, 2005). As such, this experimental research also assesses the validity of the ratio value obtained.

Experimental Conceptualisation of the

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. While sketching the flow line, care should be taken to make flow fields as approximate squares throughout. The flow line and equipotential lines should be orthogonal and form approximate squares. Where HL is the total hydraulic head causing flow, and is equal to the difference of the upstream and the downstream heads.

The
properties of the regular parabola which are essential to obtain phreatic line are depicted in Figure
6. Mathematically, the process of making out a flownet consists of contouring the two
harmonic or analytic functions of potential and flow line function. These functions both satisfy
the Laplace equation and the contour lines represent lines of constant head, i.

Methods used for Drawing the Flow Nets:

Construction of a flow net is often used for solving groundwater flow problems where the geometry makes analytical solutions impractical. The method is often used in civil engineering, hydrogeology or soil mechanics as a first check for problems of flow under hydraulic structures like dams or sheet pile walls. As such, a grid obtained by drawing a series of equipotential lines is called a flow net. The flow net is an important tool in analysing two-dimensional irrotational flow problems. Flow net technique is a graphical representation method. A flow net represents the graphical solution of the equations of the steady / continuous flow of
groundwater.

An equivalent amount of flow is passing through each streamtube (defined by two adjacent blue lines in diagram), therefore narrow streamtubes are located where there is more flow. Draw a trial flow line https://accounting-services.net/chapter-5-flow-nets/ ABC adjacent to boundary line. The line must be at right angles to the upstream and downstream beds. The size of the square in a flow channel should change gradually from the upstream to the downstream.

 A photograph of the experiment set up of the earth dam model was taken as shown in
Figure 2, where the seepage flow lines through the earth dam model and the boundary
conditions are also shown. These seepage flow lines were used as a rough guide for the
flow net construction. 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.

With F as the centre and QH as the radius, draw an arc to cut vertical line through Q in point P. Now join all the points G, S, P, B to get parabola. The phreatic line must start from B and not from C.

Laplace’s equation governs the flow of an incompressible fluid, through an incompressible
homogeneous soil medium. Continuity equation for
steady state and Darcy’s equations and for the case of isotropic soil, the permeability coefficient is
independent of direction (Craig, 2004). Irregular points (also called singularities) in the flow field occur when streamlines have kinks in them (the derivative doesn’t exist at a point).