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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
