Geotechnical Aspects of Energy Piles

Kenichi SogaBinod Amatya

Peter Borne-WebbEcho Ouyang

Tony AmisChris Davidson

Field Test @ Lambeth College, London

Made Ground

Terrace Deposits

London Clay

0+00

1+50

4+00

5+00

23+00

30+00

610 mm diam.

550 mm diam.

18+00

610 mm diam.

550 mm diam.

MT-Pile HS-Pile

c=0 o36

' =φc =65 kPa

c =180 kPa

c =190 kPa

0

5

10

15

20

25

30

Alluvial soil

Alluvial soil

Sandy gravelly

moraine

Moraine glacial till

Molasse

(Marl/sandstone)

kPaCo

5,30 ==φ

kPaCo

3,27 ==φ

kPaCo

6,23 ==φ

dia.=975mm

dia.=960mm

dia.=1170mm

dia.=1100mm

m

Heating OnlyPε

(a) Axial thermal strain

profiles

(b)Axial thermal

load profiles

No pile load, no end restraint

τ

(c) Thermal shaft

resistance profiles

0

5

10

15

20

25

30

0 100 200 300Axial Strain(10

-6)

0

5

10

15

20

25

30

-100 0 100 200 300 400Axial Strain(10

-6)

Dep

th(m

)

HS Pile:

London

T-1:Laussane

1.8oC

20.9oC

Observed

(Cooling)

=∆T

29.4oC

Observed

(Heating)

Free

thermal

strain=∆T 29.4oC

Observed

(Heating)

Observed

(Cooling)=∆T

Free

thermal

strain

=∆T

20.9oC=∆T

0oC=∆T

Free strain1.8oC=∆T

(Partial restraint

at top and

bottom)

(Strong restraint at

top and no

restraint at

bottom)

P P

ε ε

P

ε

(No restraint at top

and strong

restraint at

bottom)

Case-C Case-D Case-E

(a) Axial thermal strain profiles

(b) Axial thermal load profiles

Heating Only

0

5

10

15

20

25

30

-4000 -3000 -2000 -1000 0Axial Load (kN)

De

pth

(m)

0

5

10

15

20

25

30

-4000 -3000 -2000 -1000 0

Axial Load (kN)

29.4oC 20.9oC=∆T=∆T

HS Pile: London

(a) (b)

T-1:Laussane

y = -191.94x

R2 = 0.962

y = -103.77x

R2 = 0.9627

y = -50.436x

R2 = 0.8748

y = -87.197x

R2 = 0.9827

-7000

-6000

-5000

-4000

-3000

-2000

-1000

0

1000

2000

-5 0 5 10 15 20 25 30

Temperature change(oC)

Th

erm

al S

tre

ss

(kP

a)

London:HS Pile (17m)

T-1(2.5m)

T-1(21.5m)

T-1(Toe)

London case

190kPa/oC at middle

0 kPa/oC at two ends

Lausanne case

103 kPa/oC at middle

50kPa/oC at the top

87kPa/oC at the toe

0

5

10

15

20

25

30

-200 -100 0 100 200Shaft Resistance (kN/m

2)

0

5

10

15

20

25

30

-200 -100 0 100 200

Shaft Resistance (kN/m2)

Dep

th(m

)

HS Pile:

London

29.4oC 20.9oC=∆T=∆T

T-1:Laussane

y = -1.4957x

R2 = 0.5139

y = -0.4856x

R2 = 0.6611

y = 2.4574x

R2 = 0.9737

y = -2.1155x

R2 = 0.9897

-100

-80

-60

-40

-20

0

20

40

60

80

100

0 5 10 15 20 25 30 35

Temperature change(oC)

Sh

aft R

esis

tan

ce

(kN

/m2)

T-1 (5-12m)

T-1(13-21m)

HS Pile(Upper part)

HS Pile(lower part)

London clay : 2.1-2.5 kPa/ºC for the London clay

Lausanne : 1.5kPa/ºC for the alluvial clay deposit

Fig.10 Mechanisms for response of pile to thermo-mechanical loading ( No end

restraint case)

τP

a) Load only

P τ τP

P τ τP

d) Heating only e) Combined Load & Heating

b) Cooling only c) Combined Load & Cooling

0

4

8

12

16

20

24

-2000 -1000 0 1000

Axial Load(kN)

0

4

8

12

16

20

24

-2000 -1000 0 1000 Axial Load(kN)

Dep

th(m

)

Fig. 15(a) MT-Pile: London

(a-1) Cooling phase (a-2) Heating phase

MP

TP

TotalP

MP

TP

TotalP

)20( CTo−=∆ )10( CT

o=∆

0

5

10

15

20

25

30

-5000 -4000 -3000 -2000 -1000 0

Axial Load(kN)

De

pth

(m)

T-6(initial)

T-6(temp)

T-6(Total)

T-7(initial)

T-7(temp)

T-7(Total)

Fig. 15(b) Lausanne

TotalP

∆T = ~18oC

MP

TP

y = -313.46x

R2 = 0.9838

y = -21.078x

R2 = -0.106

y = -260.55x

R2 = 0.9192

y = -329.44x

R2 = 0.9214

y = -98.21x

R2 = 0.792y = -176.73x

R2 = 0.7208

-10000

-7500

-5000

-2500

0

2500

5000

7500

10000

-30 -20 -10 0 10 20

Temperature Change(oC)

Th

erm

al S

tress(k

Pa)

C-1-6m

C-1-12m

C-1-15m

(At 6m)

(At 12m)

(At 15m)

(At 6m)

(At 12m) (At 15m)

Middle section (max)

London - 330kPa/oC

Lausanne - 150kPa/oC

Ends

London - Small

Lausanne - 150kPa/oC (top), 80 kPa/oC (bottom)

Bad Schallerbach - 30kPa/oC (top), small (bottom)

Summary• Thermally induced pile stress and shaft resistance.

• London - a limited time, extreme temperature cycles

• Lausanne - heating pulses only, sat in amongst a foundation

system that was otherwise not used in a GSHP system.

• Bad Schallerbach - very limited data set, complex temperature

profile.

• Data of changes in axial stress and pile settlement for long-term

operational conditions needed.

• Cyclic thermal loads can deteriorate the inherent soil resistance

capability at soil-pile interface. Further work needed.