assessing the performance of thermo-active geotechnical ...€¦ · material behaviour •...

Assessing the performance of thermo-active

geotechnical structures

GSHPA Technical Seminar

University of Leeds, 24th May 2018

© Imperial College London

1

David M G Taborda

Imperial College London

Energy Geotechnics

• Combines all geotechnical issues of energy provision:

i) gas hydrate sediments,

ii) unconventional hydrocarbons and hydraulic fracturing

iii) energy geostructures

iv) energy geostorage

v) high-level radioactive waste disposal

vi) CO2 storage

• At Imperial College, the focus has been on energy geostructures, energy geostorage and nuclear waste disposal

• Piles

• Retaining walls

• Tunnel linings

Energy Geotechnics

Energy geostorage

Energy structures

Nuclear waste

disposal

• BHEs

• Horizontal loops

• Open loops

• Geological disposal

• £20-£30 billion

• 10 000 years design

Which do we need to know?

What are we asking?

• Thermal performance?

• Additional structural forces?

• Additional ground movements?

• Impact on adjacent structures?

• Piles

• Retaining walls

• Tunnel linings

Energy Geotechnics

Energy structures

• Thermal properties

• Thermal loads

• Thermo-hydro-mechanical properties of soil

How do we find out?

• Specialist temperature-controlled lab equipment

• Advanced numerical methods

• Field data for validation

Energy Geotechnics

How do we find out?

• Specialist temperature-controlled lab equipment

• Advanced numerical methods

• Field data for validation

• Temperature-controlled triaxial equipment

• Temperature-controlled oedometers

• Strength & stiffness under temperature changes

• Thermal expansion

• Thermal pressurisation of pore water

dT>0

• Pile expands when heated

• Soil restrains the deformation

• Additional axial forces are generated

• The more the soil effectively restrains the pile

the larger the axial force increase

• The opposite is expected when cooling

(i.e. reduction in axial force)

Behaviour of a single thermo-active pile

Increase in temperature

Coupled phenomenaNumerical algorithms

Material behaviourInitial conditions

Numerical modelling of thermo-active piles

Material behaviour

• Mechanical response

• Hydraulic properties

• Thermal properties (thermal conductivity, specific heat capacity)

Coupled phenomena

• Consolidation (HM)

• Thermal expansion (TM)

• Advection (TH)

• Temperature-induced pore water pressure (THM)

Numerical modelling of thermo-active piles

Numerical modelling

• Transient seepage

• Hydraulic boundary conditions

• Transient heat flux (advection-diffusion)

• Thermal BC & modelling of pipes

Initial conditions

• Stress state

• Pore water pressure profile

• Temperature field

Reproduction of a field test

Lambeth College main test pile (Bourne-Webb et al., 2009)

• 23 m long pile (19 m in London Clay)

• 550 mm diameter

• Loaded mechanically to 1200 kN (FoS = 2.5)

• Pile temperatures ranged from 0 ºC to 35 ºC

• Heavily instrumented pile (including OFS & VWSG)

• 𝐸 = 40 𝐺𝑃𝑎, 𝛼𝑐𝑜𝑛𝑐𝑟𝑒𝑡𝑒 = 8.5 × 10−6𝑚/(𝑚𝐾)

• Coupled THM modelling assuming non-linear elasticity below yield, properties obtained from literature on London Clay

Reproduction of a field test

0.0

10.0

20.0

30.0

40.0

0.0 10.0 20.0 30.0 40.0 50.0 60.0

Tem

per

atu

re (º

C)

Time (days)

0.00

5.00

10.00

15.00

20.00

25.00

-100.00 -50.00 0.00 50.00 100.00 150.00 200.00

Dep

th (m

)

Axial strain (με)

OFS OFS OFS

VWSG VWSG VWSG

ICFEP ICFEP ICFEP

Loading Heating

Loading

0.0

5.0

10.0

15.0

20.0

25.0

-100.0 0.0 100.0 200.0

Dep

th (m

)

Mechanical axial strain (με)

1200 kN

Cooling

0.0

5.0

10.0

15.0

20.0

25.0

-100.0 0.0 100.0 200.0

Dep

th (m

)

Mechanical axial strain (με)

Reproduction of a field test

0.0

10.0

20.0

30.0

40.0

0.0 10.0 20.0 30.0 40.0 50.0 60.0

Tem

per

atu

re (º

C)

Time (days)

0.00

5.00

10.00

15.00

20.00

25.00

-100.00 -50.00 0.00 50.00 100.00 150.00 200.00

Dep

th (m

)

Axial strain (με)

OFS OFS OFS

VWSG VWSG VWSG

ICFEP ICFEP ICFEP

Loading Cooling Heating

Loading

1200 kN

0.0

5.0

10.0

15.0

20.0

25.0

-100.0 0.0 100.0 200.0

Dep

th (m

)

Mechanical axial strain (με)

Reproduction of a field test

0.0

10.0

20.0

30.0

40.0

0.0 10.0 20.0 30.0 40.0 50.0 60.0

Tem

per

atu

re (º

C)

Time (days)

0.00

5.00

10.00

15.00

20.00

25.00

-100.00 -50.00 0.00 50.00 100.00 150.00 200.00

Dep

th (m

)

Axial strain (με)

OFS OFS OFS

VWSG VWSG VWSG

ICFEP ICFEP ICFEP

Loading Cooling Heating

Loading

End of cooling

0.0

5.0

10.0

15.0

20.0

25.0

-100.0 0.0 100.0 200.0

Dep

th (m

)

Mechanical axial strain (με)

Reproduction of a field test

0.0

10.0

20.0

30.0

40.0

0.0 10.0 20.0 30.0 40.0 50.0 60.0

Tem

per

atu

re (º

C)

Time (days)

0.00

5.00

10.00

15.00

20.00

25.00

-100.00 -50.00 0.00 50.00 100.00 150.00 200.00

Dep

th (m

)

Axial strain (με)

OFS OFS OFS

VWSG VWSG VWSG

ICFEP ICFEP ICFEP

Loading Cooling Heating

Loading

End of heating

End of cooling

Exploratory studies

• Based on the validated numerical model

• Investigate the effect of:

• Transient phenomena

• Thermo-mechanical response of soil (linear)

• Thermo-mechanical response of soil (thermo-plastic)

• Adopted boundary conditions within pile

(temperature / heat flux / modelled pipe)

• Thermal conductivity of soil

• Permeability

• Thermo-induced pore water pressures

• Reported in Gawecka et al. (2017) (ICE Proceedings) and

in Gawecka (2017)

Exploratory studies

Reference analysis

Time (months)

Change in

tem

pera

ture

12

6

A B

C D

-15°C

+15°C

1 5

7 11

0.0

5.0

10.0

15.0

20.0

25.0

0.0 500.0 1000.0 1500.0

De

pth

(m

)Total axial stress (kN)

0.0

5.0

10.0

15.0

20.0

25.0

0.0 500.0 1000.0 1500.0

De

pth

(m

)

Total axial stress (kN)

Mechanical loading

Initial cooling (A)

End of cooling (B)

Initial heating (C)

End of heating (D)

Total axial force (kN)

0.0

5.0

10.0

15.0

20.0

25.0

0.0 500.0 1000.0 1500.0

De

pth

(m

)Total axial stress (kN)

Exploratory studies

Reference analysis

Time (months)

Change in

tem

pera

ture

12

6

A B

C D

-15°C

+15°C

1 5

7 11

0.0

5.0

10.0

15.0

20.0

25.0

0.0 500.0 1000.0 1500.0

De

pth

(m

)

Total axial stress (kN)

Mechanical loading

Initial cooling (A)

End of cooling (B)

Initial heating (C)

End of heating (D)

0.0

5.0

10.0

15.0

20.0

25.0

-400.0 -200.0 0.0 200.0 400.0

De

pth

(m

)

Thermo-induced axial stress (kN)Total axial force (kN) Thermo-induced axial force (kN)

0.0

5.0

10.0

15.0

20.0

25.0

0.0 500.0 1000.0 1500.0

De

pth

(m

)Total axial stress (kN)

Exploratory studies

Reference analysis

Time (months)

Change in

tem

pera

ture

12

6

A B

C D

-15°C

+15°C

1 5

7 11

0.0

5.0

10.0

15.0

20.0

25.0

0.0 500.0 1000.0 1500.0

De

pth

(m

)

Total axial stress (kN)

Mechanical loading

Initial cooling (A)

End of cooling (B)

Initial heating (C)

End of heating (D)

0.0

5.0

10.0

15.0

20.0

25.0

-400.0 -200.0 0.0 200.0 400.0

De

pth

(m

)

Thermo-induced axial stress (kN)Total axial force (kN) Thermo-induced axial force (kN)

Exploratory studies

Reference analysis

0.0

5.0

10.0

15.0

20.0

25.0

0.0 500.0 1000.0 1500.0

De

pth

(m

)

Total axial stress (kN)

Mechanical loading

Initial cooling (A)

End of cooling (B)

Initial heating (C)

End of heating (D)

0.0

5.0

10.0

15.0

20.0

25.0

-400.0 -200.0 0.0 200.0 400.0

De

pth

(m

)

Thermo-induced axial stress (kN)Thermo-induced axial force (kN)

Animation of pile behaviour under temperature changes

Exploratory studies

Reference analysis

Time (months)

Change in

tem

pera

ture

12

6

A B

C D

-15°C

+15°C

1 5

7 11

-400.0

-200.0

0.0

200.0

400.0

0 2 4 6 8 10 12

Max

imu

m t

he

rmo

-in

du

ced

axia

l fo

rce

(kN

)

Time (months)Maximum effect of cooling

Maximum effect of heating

Exploratory studies

Reference analysis

Time (months)

Change in

tem

pera

ture

12

6

A B

C D

-15°C

+15°C

1 5

7 11

Excess pore water pressures

Initial cooling (A) End of cooling (B) Initial heating (C) End of heating (D)

+30 kPa

-30 kPa

0 kPa

Exploratory studies

Effect of transient phenomena

• Analysis without pwp or temperature degrees of freedom

• No transient seepage or heat flux

-600.0

-400.0

-200.0

0.0

200.0

400.0

0 2 4 6 8 10 12

Max

imu

m t

he

rmo

-in

du

ced

axia

l fo

rce

(kN

)

Time (months)

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

0 2 4 6 8 10 12

Ve

rtic

al p

ile h

ead

d

isp

lace

me

nt

(mm

)

Time (months)

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

0 2 4 6 8 10 12

Ver

tica

l pile

hea

d

dis

pla

cem

ent

(mm

)

Time (months)

Ful THM analysis

Non-transient analysis

Further reduction in forces can be observed

when enhancing transient effects

(e.g. higher thermal conductivity)

Full THM analysis

Non-transient analysis

• Thermal response test reported by

Loveridge et al. (2014)

• 300 mm diameter pile with a length of 26.8 m

• Single U-loop installed with spacing between

pipes of 135 mm

• Pile entirely within London Clay

• Pipe discretised using linear elements:

• If elements are 1 𝑚 long: 𝑃𝑒 = 1.33 × 106

• To satisfy 𝑃𝑒 ≤ 1, size should be 7.5 × 10−7𝑚

• Alternatively, use Petrov-Galerkin FE

Modelling pipe-pile-soil interaction

Inlet

London Clay

Outlet

26.8

m

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

5.0

10.0

15.0

20.0

25.0

0.0 3.0 6.0 9.0 12.0 15.0

DT

(C

°)

Time (day)

Inlet

Modelling pipe-pile-soil interaction

Inlet

London Clay

Outlet

26.8

m

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

5.0

10.0

15.0

20.0

25.0

0.0 3.0 6.0 9.0 12.0 15.0

DT

(C

°)

Time (day)

Inlet Outlet (measured) Outlet (modelled)

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

5.0

10.0

15.0

20.0

25.0

0.0 3.0 6.0 9.0 12.0 15.0

DT

(C

°)

Time (day)

Inlet

Modelling pipe-pile-soil interaction

Inlet

London Clay

Outlet

26.8

m

InletOutlet

-25.0

-20.0

-15.0

-10.0

-5.0

0.0

5.0

10.0

15.0

20.0

25.0

0.0 3.0 6.0 9.0 12.0 15.0

DT

(C

°)

Time (day)

Inlet Outlet (measured) Outlet (modelled)

Energy transferred to the soil

Energy extracted from the soil

Animation of heat propagation from pipes to the pile

0.0

5.0

10.0

15.0

20.0

0.0 5.0 10.0 15.0 20.0

DT

(C

°)

Time (day)

Inlet

Modelling pipe-pile-soil interaction

Inlet

London Clay

Outlet

26.8

m

0.0

5.0

10.0

15.0

20.0

0.0 5.0 10.0 15.0 20.0

DT

(C

°)

Time (day)

Inlet Outlet

0.0

50.0

100.0

150.0

200.0

0.0 5.0 10.0 15.0 20.0

Po

we

r (W

/m)

Time (day)

Outlet

𝑃 =𝜌𝐶𝑤 ∙ 𝑄𝑤 ∙ 𝑑𝑇

𝐿𝑝𝑖𝑙𝑒

𝑑𝑇

Temperature-dependent soil-fluid behaviour

1.00E-11

1.00E-10

1.00E-09

1.00 1.50 2.00

Perm

eab

ility

(m

/s)

Void ratio

25 deg C 50 deg C

70 deg C 90 deg C

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 20 40 60 80 100Temperature (deg C)

𝛼𝑓

×10−3

𝑚

𝑚

Soil p

erm

eability

Therm

al expansio

nof

fluid

IC Thermal Model

Thermo-plasticity

OCR = 1.4

OCR = 2.5OCR = 12.0

𝜀𝑣𝑜𝑙

𝑇 (°𝐶)

ContractionExpansion

Pontida Clay

Conclusions

• Thermo-active structures need to be assessed during design considering issues

of stability (forces), serviceability (deformations) and impact on

neighbouring structures

• These require new aspects of numerical analysis to be developed

• New components of analysis require more information from experimental

investigation (calibration) and from field testing (validation)

• There are substantial transient effects when modelling thermo-active piles

– ignoring these will lead to overestimation of pile axial forces

• Modelling the full pipe-pile-soil interaction provides new insight into the

pile response and its thermal performance

• Detailed modelling can be used to reduce uncertainties, remove excessive

conservatism and enables the assessment of more complex systems