Flow Nets and Two-Dimensional Flow

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Questions and Answers

In two-dimensional flow scenarios, the uplift pressure beneath a hydraulic structure remains unaffected by alterations in the anisotropic properties of the subsurface soil.

False (B)

The graphical solution of Laplace's equation for two-dimensional flow is uniquely represented by a velocity hodograph, irrespective of the boundary conditions.

False (B)

In a flownet, the intersection of flow lines and equipotential lines at acute angles signifies a mathematically valid solution for isotropic soil conditions, indicative of non-Darcian flow.

False (B)

For anisotropic soils where $k_h$ significantly exceeds $k_v$, the accurate representation of a flownet necessitates shortening the vertical distance of the problem by a factor of $\sqrt{k_h/k_v}$ to account for directional permeability differences.

<p>False (B)</p>
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In a confined aquifer subjected to two-dimensional flow, the hydraulic gradient is solely a function of the total head loss and the thickness of the aquifer, remaining invariant to the flow path's tortuosity.

<p>False (B)</p>
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In the context of seepage analysis beneath hydraulic structures, the exit gradient is invariably minimized at the downstream toe, ensuring structural stability against piping and soil erosion, regardless of soil heterogeneity.

<p>False (B)</p>
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The Laplace equation, when applied to two-dimensional flow through porous media, directly quantifies the advective transport of contaminants, thereby precluding the necessity for separate solute transport modeling.

<p>False (B)</p>
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In a flownet construction, equipotential lines always converge towards regions of higher hydraulic conductivity, irrespective of the geometric constraints imposed by impermeable boundaries.

<p>False (B)</p>
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A flow line represents the locus of points where the total mechanical energy of a fluid particle is constant, thus remaining invariant to changes in the fluid's velocity or elevation within the flow field.

<p>False (B)</p>
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The critical hydraulic gradient for the onset of piping in a soil is exclusively dictated by the soil's void ratio and specific gravity, rendering it independent of the effective stress conditions within the soil matrix.

<p>False (B)</p>
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In a typical flownet diagram, the total number of flow channels ($N_f$) and equipotential drops ($N_d$) are inversely proportional such that their product always equals a constant value determined by the hydraulic conductivity of the medium.

<p>False (B)</p>
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The presence of a low-permeability clay layer at the base of an excavation invariably mitigates the risk of heaving, irrespective of the pore water pressure distribution and the geometry of the excavation.

<p>False (B)</p>
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In a fully confined aquifer beneath a dam, the uplift pressure at any point is calculated assuming a linear equipotential drawdown between the upstream and downstream boundaries, irrespective of the flownet configuration.

<p>False (B)</p>
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The average path of a water particle flowing from upstream to downstream in a porous medium, also known as a flow line, is influenced by the largest gradient.

<p>True (A)</p>
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The solution of Laplace's equation gives two sets of lines, namely flow lines and equipotential lines, which intersect each other at right angles.

<p>True (A)</p>
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The space between two adjacent flow lines is called the flow channel.

<p>True (A)</p>
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In isotropic soil, the flow follows the path of largest gradient; therefore, the flow lines intersect the equipotential lines at a right angle.

<p>True (A)</p>
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A flow net is a numerical solution of Laplace's equation in 3-dimensional flow for a given set of boundary conditions.

<p>False (B)</p>
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In seepage analysis, flow nets are constructed to determine seepage losses, water pressure under a dam or structure, and the amount of exit gradients.

<p>True (A)</p>
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An imperious boundary is a flow line.

<p>True (A)</p>
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Flashcards

Seepage Analysis

The analysis of water flow (seepage) under structures like dams to determine seepage losses, water pressure, and exit gradients.

Flow Net

A graphical solution of Laplace's equation for 2D flow, illustrating flow patterns and pressure distribution.

Flow Lines

Lines representing the average path of a water particle flowing through soil.

Equipotential Line

Lines connecting points of equal potential or head in a flow field.

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Hydraulic Gradient

The gradient representing head loss between two adjacent equipotential lines.

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Flow Channel

The space between two flow lines.

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Field (in Flow Net)

A smaller section bounded by two flow lines and two equipotential lines.

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Flow Net Orthogonality

Flow lines and equipotential lines intersect at right angles.

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Piping

Failure caused by the removal of soil particles due to high hydraulic gradients, leading to instability.

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Uplift Pressure

Pressure exerted by water below a structure, which can reduce stability.

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Exit Gradient Location

Exit gradient is high at the corner in a dam.

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Critical Gradient

The gradient at which critical conditions could occur leading to piping.

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Energy Loss (Seepage)

The loss of energy as water flows through soil, represented by head loss.

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Heaving or Boiling

Occurs when effective stress reduces to zero, soil loses strength and behaves like a liquid.

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Kh # Ky

It illustrates the relationship between hydraulic conductivity.

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Study Notes

  • Two dimensional flow involves analyzing:
    • Uplift pressure below structures
    • Heaving or boiling in cofferdams and piping in earth dams
    • Heaving of clay layers at the bottom of excavations

Energy Losses in Flow Under a Dam

  • Examines energy losses as water flows through soil under a dam
  • Illustrates datum lines and hydraulic head at various points

Flow Lines and Equipotential Lines

  • A flow line represents the average path of a water particle from upstream to downstream (tailwater)
  • An equipotential line is a contour line connecting points of equal potential or head
  • The gradient is the head difference between adjacent equipotential lines divided by the distance between them
  • In isotropic soil, flow follows the path of the largest gradient, causing flow lines to intersect equipotential lines at right angles
  • The flow condition is 2-dimensional

Flow Nets

  • Flow nets are used in seepage analysis to determine seepage losses, water pressure under dams/structures, and exit gradients

  • Determined through the construction of flow nets

  • A flow net is a graphical solution of Laplace's equation in 2D flow for a given set of boundary conditions

  • Laplace's equation represents energy loss through a resistive medium

  • The solution of Laplace's equation yields flow lines and equipotential lines

  • Flow and equipotential lines intersect at right angles

  • The space between two adjacent flow lines is called a flow channel

  • The space between two adjacent flow lines and two successive equipotential lines is called a field

Properties of a Flow Net

  • Flow lines and equipotential lines intersect at right angles; they are mutually orthogonal
  • A circle can be drawn in each field, touching all four sides
  • For homogenous soil, transitions in the shapes of curves will be smooth, elliptical, or parabolic
  • The same potential drop occurs between successive equipotential lines
  • An impervious boundary acts as a flow line

Determination of Seepage

  • The gradient (i) is calculated as Δh/Δl, which simplifies to hL/Nd (total head loss divided by number of potential drops)

  • Flow rate (Δq) through the channel is k(Δh/Δl)A, which is k(hL /Nd)(a/b)

  • Since a ~ b, total seepage (q) is q = k * hL * (Nf/Nd)

  • Nf represents the total number of flow channels

  • Nd refers to the total number of equipotential drops

  • The rules mentioned are applicable to isotropic soils

  • In nature soils are anisotropic, accounted for by direction differences

  • Directional differences are taken into account by transforming the scale of the flow net:

    • If kh >> ky, shorten the horizontal distance of the problem by √(kh/kv)
    • q = (Nf/Nd) * √(kh*kv) * hL

Uplift Pressure Below a Structure

  • Uplift pressure acts upwards on the base of a concrete weir due to water seeping underneath

Exit Gradient

  • ie = dh/le
  • dh = H/Nd, le = average length of the smallest exit field adjacent to the structure
  • Critical conditions (piping) occur when the gradient reaches unity
  • Piping occurs at the corner of a dam since le is the least at this point, resulting in the largest exit gradient

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