numerical analysis of the influence of initial hydraulic boundary … · 2019-10-25 · 2...

Numerical Analysis of the Influence of

Initial Hydraulic Boundary Conditions on

the Infiltration Behaviour of Unsaturated

Soil Slopes

Patrick P. Pichler

Institute of Soil Mechanics, Foundation Engineering and Computational Geotechnics

Graz University of Technology

2

▪ Introduction – Why unsaturated soil mechanics?

▪Numerical back-calculation of Lysimeter-data

▪Safety analyses on benchmark slope geometry

▪Rainfall infiltration into a real slope

Numerical analysis of the influence of initial hydraulic boundary conditions on the infiltration behaviour of unsaturated soil slopes

Content

3


Introduction – Why unsaturated soil mechanics?

Collapse in Via Luigi Settembrini, Naples (2001)La Conchita, California (2005)

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Introduction


5

Matric suction & Capillary forces


𝒔 =𝟐 𝑻𝒔 𝒄𝒐𝒔𝜶

𝑹= 𝒖𝒂 −𝒖𝒘

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The initial pore-water-pressure conditions, before a (heavy)rainfall event starts, can play a major role in the instability ofsoil slopes!

▪Numerical analyses:

Due to equilibrium reasons the solely use of the initialgroundwater-level would lead to a linearly increasing negativepore-water-pressure above the groundwater-level, which maynot be realistic!

Focus on initial conditions


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▪Given data (continuously measured):

▪Precipitation▪Evapotranspiration

▪Suction in certain depths

Lysimeter

Grass-covered Lysimeter, testfield “Wagna”, Styria

(Klammler et. al, ongoing)


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Lysimeter – Climatic hydrograph

Grass-covered Lysimeter, testfield “Wagna”, Styria

(Klammler et. al, ongoing)


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19.04.2013 19.04.2014 19.04.2015 18.04.2016 18.04.2017

Su

ctio

n [

kP

a]

Pre

cip

ita

tion

/ E

va

po

tran

spir

ati

on

[m

m]

Timeline

P [mm] ETr [mm] ET0 [mm] suction 20 cm [kPa] suction 50 cm [kPa]

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Lysimeter – SWCC & relative permeability


0,0

0,1

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1,0

0,01 0,1 1 10 100 1000

Sa

tura

tio

n [

-]

suction [kN/m²]

Layer1 0-30

Layer2 30-50

Layer3 50-80

Layer4 80-130

Layer5 >130

1,0E-15

1,0E-14

1,0E-13

1,0E-12

1,0E-11

1,0E-10

1,0E-09

1,0E-08

1,0E-07

1,0E-06

1,0E-05

1,0E-04

1,0E-03

0,01 0,1 1 10 100 1000

un

satu

ra

ted

per

mea

bil

ity

[m

/s]

suction [kN/m²]

Layer1 0-30

Layer2 30-50

Layer3 50-80

Layer4 80-130

Layer5 >130

g

p

w

wp

=

( ) ( ) ( )c

ng

g

paressatresp gSSSS

+−+= 1

ressat

rese

SS

SSS

−

−=

( ) ( )

21

111

−−=

−

− n

n

ng

ng

lg

g

eg

erel SSSk

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Lysimeter – Backcalculation results


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1 6 11 16 21 26 31 36 41 46 51 56 61

Pre

cip

ita

tion

/ E

va

po

tran

spir

ati

on

[m

m]

suct

ion

[k

Pa

]

Timeline [days]

suction 20 cm depth (kPa) suction 50 cm depth [kPa] precipitation-evapotranspiration [mm]

11



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cip

ita

tion

/ E

va

po

tran

spir

ati

on

[m

m]

suct

ion

[k

Pa

]

Timeline [days]

suction 20 cm depth (kPa) PLAXIS suction -20 cm [kPa] suction 50 cm depth [kPa] precipitation-evapotranspiration [mm]

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cip

ita

tion

/ E

va

po

tran

spir

ati

on

[m

m]

suct

ion

[k

Pa

]

Timeline [days]

suction 20 cm depth (kPa) PLAXIS suction -20 cm [kPa] suction 50 cm depth [kPa]

PLAXIS suction -50 cm [kPa] precipitation-evapotranspiration [mm]

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Goals & Questions


Important Question: When do manually & automatically

generated pwp‘s match?

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Goals & Questions


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cip

ita

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/ E

va

po

tran

spir

ati

on

[m

m]

suct

ion

[k

Pa

]

Timeline [days]

suction 20 cm depth (kPa) PLAXIS suction -20 cm [kPa] PLAXIS suction -20cm GW [kPa]

suction 50 cm depth [kPa] PLAXIS suction -50 cm [kPa] PLAXIS suction -50cm GW [kPa]

precipitation-evapotranspiration [mm]

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Lysimeter – Backcalculation (suction prediction)


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Su

ctio

n[k

Pa

]

Net

-Pre

cip

ita

tio

n[m

m]

Time [days]

P-Etr [mm]

suction 20 cm [kPa]

suction 50 cm [kPa]

PLAXIS suction 20 cm [kPa]

PLAXIS suction 50 cm [kPa]

From here on:

Only numerical prediction in 20 cm depth!

16

Benchmark slope - Geometry


Same stratigraphy as in simple

Lysimeter „box-studies“

GW table initially ALWAYS

3.5 m below ground surface

Different slope angles

▪ Geometry A → 2,00:1 ( = 26.57°)

▪ Geometry B → 2,50:1 ( = 21,80°)

▪ Geometry C → 2,25:1 ( = 23,94°)

17

Completed investigations


pwp’s

Auto / Manual

Run-off criteria

max= 1 mm / 15 mm

Saturated (reference)

permeability

kref / kref:10 / kref:100

Climatic hydrograph

192 / 120-120 / const.

Hydraulic BC’s

Closed / Head

Modified hydrographs

“cutting of rainfall peaks”Location gw-level

-3.5 / -7.0 m

18

Influence of max


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cip

ita

tion

/E

va

po

rati

on

[m

m]

Su

ctio

n [

kP

a]

Timeline [days]

192 days

kref & Influence of max

P-Etr [mm] M_192_m_psi_1 M_192_50_psi_1 A_192_20_psi_1 A_192_50_psi_1

M_192_20_psi_30 M_192_50_psi_30 A_192_20_psi_30 A_192_50_psi_30

Once suction is equal to zero,

there‘s no influence of the initially

defined suction anymore!

19

kref/100 & Influence of max


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po

rato

pn

[m

m]

Su

ctio

n [

kP

a]

Timeline [days]

192 days

kref/100 & Influence of max

P-Etr [mm] M_192_20_psi_1 M_192_50_psi_1 A_192_20_psi_1 A_192_50_psi_1

M_192_20_psi_30 M_192_50_psi_30 A_192_20_psi_30 A_192_50_psi_30

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Net

-Pre

cip

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tio

n[m

m]

Timeline [days]

P-Etr [mm]

Safety calculations on Benchmark slope


First 40 days [of 192 days dataset]

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P-Etr [mm]



Starting with day 40 [of 192 days dataset]

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kref

max = 1 mm

suction in all phases

Model A

initial 1.58

after day 39 1.17

after day 40 1.15

kref

max = 1 mm

suction in all phases

Model A

initial 1.58

after day 40 only 1.21

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Real slope geometry


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Climatic hydrograph


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Vo

lum

e [m

m/m

²

da

y]

Timeline [months]Precipitation A Precipitation B Potential evaporation APotential evaporation B Infiltration A Infiltration B

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lum

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Timeline [days]

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Results of using the monthly precipitation dataset while varying SWCCs and saturated permeabilities and either considering the antecedent on year of precipitation or not

Influence of previous rainfall events

Type

Sliding mass Transition zone Original data Additional rainfall

SWCC ksat [m/s] SWCC ksat [m/s] FoS before FoS after q [m/day]duration

[days]FoS

1-33 Coarse 1.0 e-3 Coarse 1.0 e-3 1.28 1.14 0.3 2 1.13

1-33 Coarse 1.0 e-6 Coarse 1.0 e-6 1.28 1.26 0.3 10 1.24


26

Influence of previous rainfall events

Type

Sliding mass Transition zone Original data Additional rainfall

SWCC ksat [m/s] SWCC ksat [m/s] FoS before FoS after q [m/day]duration

[days]FoS

1-33 Coarse 1.0 e-3 Coarse 1.0 e-3 1.28 1.14 0.3 2 1.13

- 1 year Coarse 1.0 e-3 Coarse 1.0 e-3 1.28 - 0.3 2 1.20

1-33 Coarse 1.0 e-6 Coarse 1.0 e-6 1.28 1.26 0.3 10 1.24

- 1 year Coarse 1.0 e-6 Coarse 1.0 e-6 1.28 - 0.3 10 1.28


Results of using the monthly precipitation dataset while varying SWCCs and saturated permeabilities and either considering the antecedent on year of precipitation or not

27

▪ With respect to the reproduction of real-life suction profiles in unsaturated soils, the back-calculation of Lysimeter-data in a column-test with PLAXIS 2D delivers very satisfactory results

▪ The application of the described procedure avoids the use of uncertain hydraulic input-parameters, which could affect the infiltration behavioursignificantly

▪ Antecedent precipitation greatly influences the pore water pressure profile in slopes and should therefore always be considered

Conclusions


Thank you for your attention!

Patrick P. Pichler

Institute of Soil Mechanics, Foundation Engineering and Computational Geotechnics

Graz University of Technology

numerical analysis of the influence of initial hydraulic boundary … · 2019-10-25 · 2...

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