MEKANIKA TANAH 1PERTEMUAN IV

FLOWNET & SEEPAGE

Oleh :Arwan Apriyono

PROGRAM STUDI TEKNIK SIPIL JURUSAN TEKNIK FAKULTAS SAINS DAN TEKNIK UNSOED

TAHUN 2011

Sec

Flownet is A network of selected streamlines and

equipotential lines to evaluate seepage in water construction.

Introduction

Sec

Stream line is simply the path of a water molecule.

From upstream to downstream, total head steadily decreases along the

stream line.

Introduction

Sec

Equipotential line is simply a contour of constant total head.

Introduction

SecTheory

1. Streamlines Y and Equip. lines are .2. Streamlines Y are parallel to no flow

boundaries.3. Grids are curvilinear squares, where

diagonals cross at right angles.4. Each stream tube carries the same

flow.

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Flow Net Theory

Flow Net in Isotropic Soil

Portion of a flow net is shown below

F

Y

Stream tube

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Flow Net in Isotropic Soil

The equation for flow nets originates from Darcy’s Law.

Flow Net solution is equivalent to solving the governing equations of flow for a uniform isotropic aquifer with well-defined boundary conditions.

Flow Net in Isotropic Soil Flow through a channel between

equipotential lines 1 and 2 per unit width is:

∆q = K(dm x 1)(∆h1/dl)

dm

Dh1

dl

F1

F3

Dq

F2

Dh2

Dq

n

m

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Flow Net in Isotropic Soil

Flow through equipotential lines 2 and 3 is:

∆q = K(dm x 1)(∆h2/dl)

The flow net has square grids, so the head drop

is the same in each potential drop: ∆h1 = ∆h2

If there are nd such drops, then:

∆h = (H/n)

where H is the total head loss between the first

and last equipotential lines.

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Flow Net in Isotropic Soil

Substitution yields:

∆q = K(dm x dl)(H/n)

This equation is for one flow channel. If there are m such channels in the net, then total flow per unit width is:

q = (m/n)K(dm/dl)H

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Flow Net in Isotropic Soil

Since the flow net is drawn with squares, then dm dl, and:

q = (m/n)KH [L2T-1]

where:q = rate of flow or seepage per unit widthm= number of flow channelsn= number of equipotential dropsh = total head loss in flow systemK = coefficient of permeability

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Drawing Method:

1. Draw to a convenient scale the cross sections of the structure, water elevations, and aquifer profiles.

2. Establish boundary conditions and draw one or two flow lines Y and equipotential lines F near the boundaries.

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Method:3. Sketch intermediate flow lines and

equipotential lines by smooth curves adhering to right-angle intersections and square grids. Where flow direction is a straight line, flow lines are an equal distance apart and parallel.

4. Continue sketching until a problem develops. Each problem will indicate changes to be made in the entire net. Successive trials will result in a reasonably consistent flow net.

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Method:

5. In most cases, 5 to 10 flow lines are usually sufficient. Depending on the no. of flow lines selected, the number of equipotential lines will automatically be fixed by geometry and grid layout.

6. Equivalent to solving the governing equations of GW flow in 2-dimensions.