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GSHPA Thermal Pile Standard
Ground Source Heat Pump Association
Technical Seminar
Wednesday 16 November 2011Homerton College, Cambridge University
Duncan Nicholson
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Thermal Pile Standard – sub committee
Duncan Nicholson Arup (Chair)
Nic Wincott NeoEnergy
Peter Smith Cementation
Tony Amis GIL
Chris Wood Bullivant /
Kenichi Soga Cambridge University
Fleur Loveridge Southampton University
Jake Salisbury GSHPA - (Secretary)
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GSHPA -Thermal Pile Standard overview Contents List
- Sec 1 Preamble (as in the Vertical Borehole Standard)- Sec 2 Regulations and governments (as VBS)- Sec 3 Contractual setup- Sec 4 Training requirements (Link with FPS for piles)- Sec 5 Design - Sec 6 Thermal response- Sec 7 Pile materials and methods- Sec 8 Pipe Jointing (as VBS)- Sec 9 Thermal pile concrete- Sec 10 Loops installation- Sec 11 Pressure testing- Sec 12 Indoor piping /values (as VBS) – Header pipes- Sec 13 Thermal Transfer fluids (as VBS) – High loop temps – use water as Europe?- Sec 14 Design drawings- Sec 15 Monitoring and checking - Sec 16 Alterations
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Appendices- A Design – Geotechnical design issues- B Thermal response – Effect of large diameter piles- C Thermal pile concrete – Concrete thermal conductivity - E Loops Installation – Methods and scratching
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Section 3 Contractual responsibilities
Many parties results in division of responsibilities.
ICE Specification for Piling and Embedded Retaining Walls (SPERW) is the starting point- “Engineer” design- “Contractor” design- Standardise terms
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Contractual responsibilities “Engineer” design piles
Pile Construction, Trimming & Groundworks
(RIBA Work Stage J onwards)
Employer
M&E Designer
Main ContractorEngineer (Pile Designer)
GSHP Designer
GSHP Contractor
Piling Contractor
Groundworks Contractor
M&E Contractor
Concept Design, Design Development, Tender
(RIBA Work Stage A-H)
Denotes parties with responsibilities set out in SPERW (2007)
Contractual links
Possible non-contractual links
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Employer
M&E Designer Main ContractorEngineer
GSHP Designer
Concept Design, Design Development, Tender
(RIBA Work Stage A-D)
GSHP Contractor
(GSHP Designer)
Piling Contractor
(Pile Designer)
Groundworks Contractor
M&E Contractor
Technical Design, Pile Construction, Trimming & Groundworks
(RIBA Work Stage E onwards)
Denotes parties with responsibilities set out in SPERW (2007)
Contractual links
Possible non-contractual links
Contractual responsibilities “Contractor” design piles
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Section 5 Design requirements Thermal effects complicate traditional pile design
Factor of Safety (e.g. LDSA Guidance Notes)
Constraints
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Section 5 Geotechnical Pile design – heating pile
c1, 1'
c2, '
c3, '
Ground uplift?
Pile expansion/ contraction T = 30°C
Thermal Properties:- Soils - Concrete
Shaft friction effects:- Freezing?- Shear stress?- Radial stresses?- Cyclic loads?- W/C change?
Concrete Stress• Stress concentrationsnext to pipes?
Building Load – effect on pile F of S?
• Daily fluctuations• Seasonal fluctuations
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Geothermal Pile – Geotechnical Design Process1. Pile design for structural loads
- Normal F of S > 2.0 to 3.0 – (ULS Design)- Consider normally consolidated clays as –ve skin friction.
2. Agree temperature range with GSHP Designer- Interface must not freeze. - Pile/Soil interface eg +2 to +30oC.- Number of thermal piles – free head / Fixed head
3. Assess pile expansion and ground movements (Undrained)- Free head and fixed head (SLS design)
4. Assess concrete stresses – dead load and thermal- Max concrete stress < Concrete strength (qc)/4
5. Consolidation / Quasi – thermal creep effect- Check settlement
6. Check live loads and thermal cyclic effects - Treat thermal loads as cyclic live loads - 50 annual cycles
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Combined load and thermal test- Lambeth College, London (2007)Bourne-Webb et al, (2009) Geotechnique
Cementation / GIL / Cambridge Pile loading test undertaken incorporating cyclic
temperature effects Optical fibre sensor (OFS) system Conventional vibrating-wire strain gauges (VWSG),
thermistors and external load control elements
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Lambeth College - Geotechnical Assessments
Rapid (Undrained0 response –- Expansion of pile - Lambeth College
Long term (Drained) response –- Dissipation of pore pressure
Quasi Creep effect - Reduction in Preconsolidation pressure with increased temp.
Cyclic thermal loading- Annual thermal cycle
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Lambeth College Pile Test Layout and
Instrumentation
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Design load -1mm settlement
Cooling – 3mm change
Heating – 2mm change
Little heave during heating
Pile Temp and head movement
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Lambeth College Modelling using DYNA and OASYS PILE
Both external load and heating/cooling cycle applied to pile
LS-DYNA and Oasys PILE used to model behaviour
Pile Head Load (Free head)
Fequiv
Fequiv
Fequiv = therm.Econcrete
.Apile
Limiting shearτ= α.Cu
Vertical settlement(Mindlin)
Thermal expansionof piles
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Lambeth College – Pile Loads
-25
-20
-15
-10
-5
0-500 0 500 1000 1500 2000
Dep
th b
elow
GL
(m)
Axial Load (kN)
MeasuredOasys PILELS-DYNA
Cooling
-25
-20
-15
-10
-5
0-500 0 500 1000 1500 2000
Dep
th b
elow
GL
(m)
Axial Load (kN)
MeasuredOasys PILELS-DYNA3mm movementMatch pile head
Heating
Fixed pile head generated large axial load
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Lambeth College – Modelling pore pressures(In progress) LS-Dyna calculates excess pore pressures due to:-
- Undrained pile loading 1200kN- Thermal effects
Dissipation of water pressures allows consolidation
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Section 5 - GSHP Design of thermal piles
Fleur Loveridge has addressed issues
Pile Modelling Assumes - line source – piles up to 0.3m – Use standard packages - Uniform temperature source – larger piles – Use Pile Sim or Orphius- Finite element models
Number of loops in pile
Low thermal conductivity concrete – similar to soil
High thermal conductivity concrete – reduces thermal resistance
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Lab Testing – pore pressures
Difference in soil/porewater thermal expansion generates excess pore pressures on heating
Discussed in literature:- Campanella & Mitchell (1968)- Hueckel, Francois and Laloui (2009)
αs = thermal expansion of mineral solids
αw = thermal expansion of soil water
αst = physico-chemical structural volume change
mv = soil compressibility(Campanella & Mitchell, 1968)
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Thermal-creep effect on preconsolidation Heating reduces preconsolidation pressure (σp’) and stiffness
Creep ignored in OC clays NOT in NC clays.
(Boudali, Leroueil & Srinivasa Murthy, 1994)(Eriksson, 1989)
London Clay is very over-consolidated.
σp’ reducingwith higher Temp.
NC Clay Creep settlement
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Cyclic loading
Cyclic thermal load caused by heating and cooling pile - Pu = 3.6MN- Po = 1.2MN- Pc = 0.7MN
- Pc/Pu = 0.7/3.6 = 0.2 - Po/Pu = 1.2/3.6 = 0.33
Poulos Stability Diagram
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Section 6 – Response Tests for thermal piles
How Long should the test take?
Consider Loops on Centreline or round perimeter
Thermal conductivity of concrete relative to soil
Temperature at soil concrete interface
Response test – shallow depth - Part of Geotechnical Investigation- Part of pile test eg reaction pile
Combine with strain gauges mid depth – thermal stress in piles
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Section 7 Pipe Materials Plastic pipes - Bend Radius - PEX at 20oC
- 15/??mm -- ??m pile Can a 15mm PEX pipe fit in a 0.45m pile?
- 20/1.9 mm - 0.6m pile (20cm)
- 25/2.3mm - 0.75m pile. (25cm)
- 32/2.9mm - 0.90m pile (32cm)
- 40/3.7mm - 1.0m pile cage (40cm)
PE100 or PE100+ at 20oC - 15/??mm -- ??m pile
- 20/1.9 mm – 1.0m pile (40cm)
- 25/2.3mm - 1.2m pile. (50cm)
- 32/2.9mm - 2.2m pile (100cm)
- 40/3.7mm - N/A
- PEX bends to about half the radius of PE100 or PE100+.
- Colder temperatures increase min bend radius
- PEX is more expensive but does not need U bends at the top and bottom of loops.
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Section 10 Loop Installation
Loops on long cages – Long tremie pipe- Inside cage - Outside cage
Loops on short cages – Short Tremie-
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Section 10 Borehole Loops installation
Historically – Europe- Long cages - Internal pipes with looped pipes
In London - dry bored piles- Use short cages- Use borehole U-tubes
Paddington Basin – GIL and Cementation- Two pairs of U-tubes
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West End Green – Use of lantern spacers (2010)
+25.5mOD
Brickearth
River Terrace Deposits
London Clay
+28mOD
+20mOD
A A
BB
A-A
B-B
Thermal pipes
Reinforcement bars
Lantern spacers
To central GSHP
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Short or long tremie – Scratching test (2010)
Concrete = +3.45mOD
Pipe bottom +4.0mOD
Pile bottom -5.4mOD
Tremie length = 6m
Loops restrained by bar weights
+33.2mOD
+32.4mOD
Test set-up Photos from test
Bar weights prior to testing U bend after test
Lower pipe after testUpper pipe after test
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Scratch depth measurement on 32mm pipes Assessment of damage
- Par off pipe until scratch just disappears- Measure pared width (2C)- Calculate scratch depth
R R
CCT 2C – chord length (mm) - measured;
T – Depth of the scratch (mm) - calculatedR – radius of the pipe – measuredConclusions – Vertical pipes – <1mm scratches
- Splayed pipes - 1 to 2mm scratches
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CFA Piles, Cambridge (Bachy Web site)
Pile design - Motts
Pile Contractor – Bachy
Loop design / build – GIL
CFA piles (600mm dia) 150 No up to 25m depth
Loops - 4 pipes x 32mm dia
Pushed with 1 x T32
Heating - 188kW
Cooling - 117kW
Plunging used T32 bar + 4 pipes (2 loops)
Trimming Cage and header pipes
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Section 11 Pressure testing
Checks for loop leakage - During installation- Contract interfaces
Pressurise loops during installation- European contractors pressurise loops during installation - UK does not do this?
Relevance of pressure test in concrete- Pipe relaxation at high pressures- Stiff response increases test sensitivity- Pipe pressure can increase – Pile concrete heats water -
expansion
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Section 15 Monitoring and triggers
No freezing at Soil/Pile interface
Little data on relationship between circulation fluid temp and interface temperature
Adopt conservative minimum temperature from heat pump
Monitor
Use trigger values
Under discussion
Keble College - Oxford
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Central line tunnels –Exisiting.
Crossrail Stations
Thermal piles
Thermal walls
Thermal Walls –Crossrail Dean Street Box
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Crossrail - Ground temperatures at Oxford Street
12
14
16
18
20
22
0 20 40 60 80 100 120 140 160
Tem
pera
ture
(˚C
)
Distance from Nearest LU line (m)
Temperature vs Distance from nearest LU Line
T31R - 100.73 (mATD) T22R - 99.26 (mATD)
RT119 - 92.93 (mATD) BST15R - 86.09 (mATD)
RT120 - 89.72 (mATD) RT121C - 96.33 (mATD)
Ground temperature at tunnel level
Next to tunnel temperature 19oC
Temperature drops to to ~15oC at about 90m from tunnel
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Diaphragm wall Dyna Model - Temperature effect on wall
PLAN VIEW
Soil Diaphragm Wall Concrete
Rebars (pink)
Pipe
Soil extends to 100m from outer surface of concrete
0.25m
0.25m
Air / insulation
Soil Air / insulation
Pipe
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Temperature variation across diaphragm wall
Soil(8oC)2W/m2
Concrete
Air in Basement(19oC )20W/m2
Soil D-wall
With insulation (pipe at 6deg)
No insulation (pipe at 6deg)
Zero extraction
0oC
20oC
Pipe(6oC)
Rebars
Air
10oC
(6 months cooling)
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Thermal effects onwallbending moments
(Cracked sections)
(Expansion of soil not considered)
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Conclusions -Thermal pile standard advances Contents List
- Sec 1 Preamble (as VBS)- Sec 2 Regulations and governments (as VBS)- Sec 3 Contractual setup- Sec 4 Training requirements (Link with FPS for piles)- Sec 5 Design - Sec 6 Thermal response- Sec 7 Pile materials and methods- Sec 8 Pipe Jointing (as VBS)- Sec 9 Thermal pile concrete- Sec 10 Loops installation- Sec 11 Pressure testing- Sec 12 Indoor piping /values (as VBS) – Header pipes- Sec 13 Thermal Transfer fluids (as VBS) – High loop temps – use water as Europe?- Sec 14 Design drawings- Sec 15 Monitoring and checking - Sec 16 Alterations
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Thank you for your attention
Any Questions?
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Thank you!
References- Campanella, R.G. & Mitchell, J.K. (1968) Influence of
temperature variations on soil behaviour, Journal of the Soil Mechanics and Foundations Division, 94(SM3), ASCE, pp 709-734.
- Boudali, M., Leroueil, S. & Srinivasa Murthy, B.R. (1994) Viscous behaviour of natural clays, Proc. 13th Int. Conf. Soil Mechanics and Foundation Engineering, New Delhi, pp 411-416.
- Eriksson, LG, (1989) Temperature effects on consolidation properties of sulphide clays, Proc. 12th Int. Conf. Soil Mechanics and Foundation Engineering, Rio de Janeiro, Vol. 3: pp 2087-2090
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Design requirements – design charts
Design basis- Thermal pile load test- Computer model
Typical temperature range to consider- ±5 to 10ºC daily- ±20ºC seasonal
Model of varying length/diameter of piles and study effect on concrete stress, FOS.
0
Pile length, L
Concrete stress, σc
Pile diameter, D1.2
0.75
0.60
Etc.
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Further work
Ongoing research provided in Appendices to the Thermal Pile Standard- Soil and concrete thermal conductivity- Thermal response test interpretation for larger diameter piles- Change in soil behaviour / shaft friction / concrete stress with
temperature variations- Pile / soil interface zone temperature and thermal conductivity
Knowns and unknowns in producing the design guidance clearly stated
Several further revision cycles required to finalise the document with the T&SC
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Conclusions Thermal Piles are established in UK.
Thermal Pile / Heat pump systems - compete with gas boilers, biomass, CHP.
Thermal pile installation methods developing.- Need to check installation damage.
Geothermal design based on borehole loops guidance.
Geotechnical design developing.
Ownership of design responsibilities unclear.
Few designers and contractors able to tender for work.
Thermal walls – Design processes under development- Basement insulation, thermal stresses on wall moments, earth pressures.
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Design requirements – laboratory testing
Thermal conductivity - concrete- Soil, concrete and interface zone- Eurocode or ASTM methods (eg Guarded hot plate)