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midas GTS Seepage Analysis

Presenter: Abid Ali, Geotechnical Engineer

June 19, 2012

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INDEX

1.Introduction

2. Types of Analysis

3. Functions in Seepage Analysis

4. Boundary Conditions

5. Application Areas

6. Results

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Introduction

• Groundwater flow occurs due to head difference or change in flux

(inflow/outflow).

• Seepage analysis is used to model groundwater flow.

• Two types of seepage analysis possible in GTS

Steady state – Time independent solution.

Transient state – Time dependent solution. (used to model effect of

changing BC).

• Flow governed by Darcy’s Law (both for saturated and unsaturated flow).

• Only pore pressure DOF considered in seepage analysis.

• Both 2D & 3D seepage analysis can be performed in midas GTS.

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Types of Analysis • Steady State Analysis – Used to model groundwater flow under equilibrium

conditions. (BC do not change with time).

• Transient State Analysis – Used to observe the immediate changes in flow due

to a change in hydraulic BC.

Function needed to define the change in permeability /water content with

pore pressure head in transient analysis.

Function can be defined to relate the variation in Degree of Saturation /

Water content with head and permeability.

User must specify the time steps for analysis.

• Semi-coupled analysis – Seepage Analysis coupled with stress analysis.

Seepage Analysis is performed first and stress analysis subsequently.

Used for determining stability of ground structures (e.g. Dams, sheet pile)

Results obtained include both stresses in soil and pore pressure.

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Functions in Seepage Analysis

• In GTS the variation of permeability and water content can be considered

through the Unsaturated Property Function.

Gardener coefficients and Frontal function - variation of permeability with

pressure head.

Van Genuchten’s function - variation of water content with pore pressure.

• Anisotropic permeability can be considered by defining permeability

coefficients along co-ordinate axes

• Time varying functions

(see right) can be

defined to model the changes

in BC (head/flux) with time.

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Functions in Seepage Analysis

Fig. on the

right shows

variation of

permeability

with pressure

head and

variation of

water

content with

pore pressure

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Functions in Seepage Analysis

Define the

nonlinear

equations of

permeability

and

volumetric

water

content

separately or

at the same

time.

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Boundary Conditions

• Boundary conditions describing pore pressure or flow conditions at a

boundary in seepage analysis termed Hydraulic Boundary Conditions.

• Hydraulic Boundary conditions in GTS

Nodal head – Used to define total or pressure head at nodes.

Seepage face – Used to review the seepage conditions at face/edge.

Flux (Nodal/Surface) – Used to model flow rate at nodes/surfaces.

Ground Water Table – The position of GWT can be explicitly defined

for the whole model or for individual mesh sets.

No flow – when no flow conditions assigned at a boundary.

• Time varying function can be used to consider head/flow variation with

time.

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Application Areas

GTS can be used to model the following types of problems

• Confined Flow - The flow does not involve phreatic surface e.g. around

sheet pile wall.

Contour lines of groundwater head for confined flow

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Application Areas

• Unconfined Flow – The flow involves phreatic surface e.g. in Dams

Unconfined flow in Dam (vector and contour plot for velocity).

• Infiltration – Can be modeled by specifying the flow rate at nodes or element

edges.

Phreatic Line

Infiltration at top of slope (left), with contour and vector plots for velocity.

Phreatic line

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Application Areas

• Sources and Sinks – Can be modeled by using nodal flux BC. e.g. Pumping /

Injection.

• Reservoir full /Sudden drawdown in dams.

Drain

Initial water level

Final phreatic line Initial phreatic line

Changes in water table at different time steps for sudden drawdown.

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Results

• Results obtained include – Flow rate, total & pore pressure head, pore pressu

re, velocity, hydraulic gradients, hydraulic conductivity and water content.

• Sign convention in seepage analysis is + for compressive pore water

pressure and vice-versa in stress analysis.

• Total head contours give the position of equipotential lines.

• Flow lines can be generated for any node at any stage.

Fig. shows the equi-potential lines (represented by the contour bands for total head) with flow lines (white).

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Results

• Floq quantity for any nod

e and for any step can a

lso be determined.

• Phreatic line for different

stages can also be

generated using the multi-

step iso-surface function.

• Results can also be

exported in tabular form.

Fig. shows the contour plot for flow rate with flow quantity through highlighted nodes

Total head results in tabular form

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Problem

Stage 1

Stage 2

Stage 3

Soil 1

Soil 2

Soil 3

Soil 4

Total Head 22m Varying Heads

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Problem

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THANKS

For Queries: email to esupport@midasit.com