Review of Thermal (Energy) Piles

Transcription

1 Review of Thermal (Energy) Piles Duncan Nicholson CIC Workshop New developments in Ground Engineering Opportunities for the construction industry 6 th October hrs The Building Centre, 26 Store Street, London, WC1E 7BT

2 Contents Historical background Cost effectiveness borehole loops and geothermal piles GSHPA Thermal Pile Standard Design and responsibilities Geothermal designer, Geotechnical engineer and Piling contractor Thermal pile testing Temperature effects on the pile performance - Lambeth College Tests to improve our understanding of thermal piles Installation and interface with other site work Field performance of case histories 2

3 Historical background Thermal pile historical development Brandl Rankine lecture 2002 Early UK work 2002 to 2005 Cementation / Naegelebau projects. Courtesy: Skanska Post 2005 Cementation / Geothermal International Limited (GIL) projects. Cementation trademark term Energy Piles. Current UK generic name Thermal piles. Ground Source Heat Pump Association (GSHPA) - Thermal pile Standard (Sept 2012). 3

4 Cost Effectiveness of Borehole Loop / Thermal Pile Item Borehole Loop Thermal Pile Diameter 0.2m 0.6m Heat transfer (Brandl) 35W/m 65W/m - (with recharge) Number of loops One Two Length 100m (3500W) 27m x 2No (3500W) Boring / installation cost 35/m x 100m = 3500 Boring included + rig / crew time Thermal Grout 5/m x100m = 500 N/A Pipe 25mm ID 5/m 1 loop= x 2 x (27m x 2) = 540 Total Installation rig / crew time Header pipes 20m at 5/m = trenching 2x20m at 5/m = placement Grand Total for 3500W rig / crew time Construction work Not on critical path On Critical path 4

5 GSHPA -Thermal pile standard overview Contents List Sec 1 Preamble (as BHS) Definitions Sec 2 (as BHS) Sec 3 Sec 4 Sec 5 Sec 6 Sec 7 Sec 8 Sec 9 Sec 10 Sec 11 Sec 12 Sec 13 Sec 14 Sec 15 Regulatory & Government Agency Requirements Contractual Responsibilities Training Requirements Design Thermal Response Testing Pipe Materials and Jointing Methods Thermal Pile Concrete Loops Installation Pressure Testing Indoor Piping / Values (as BHS) Thermal Transfer Fluids (as BHS) Design Drawings Monitoring and Checking Alterations 5

6 GSHPA -Thermal pile standard overview Appendices Guidance notes A Fluid temperatures B C D E F G H Thermal soil properties Soil properties Load transfer mechanisms SLS design considerations Design charts Concrete conductivity Thermal loops in pile cover zone 6

7 Section 3 - Responsibilities Many contractual parties clear division of responsibilities ICE Specification for Piling and Embedded Retaining Walls (SPERW) is the starting point: Engineer design Contractor design 7

8 Section ICE SPERW Design Responsibility Who is responsible for the thermal loop design? Consultant (Engineer) or Contractor 8

9 Section Contractual responsibilities Engineer design Employer M&E Designer Engineer (Pile Designer) Main Contractor GSHP Designer Piling Contractor GSHP 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) Pile Construction, Trimming & Groundworks (RIBA Work Stage J onwards) Contractual links Possible non-contractual links 9

10 Section Contractual responsibilities Contractor design Employer M&E Designer Engineer Main Contractor GSHP Designer Piling Contractor (Pile Designer) GSHP Contractor (GSHP Designer) Groundworks Contractor M&E Contractor Concept Design, Design Development, Tender (RIBA Work Stage A-D) 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 10

11 Section Design process responsibilities Engineer design 11

12 Section Design process responsibilities Contractor design 12

13 Section Design requirements Thermal effects complicate traditional pile design Desk Study Thermal loads M&E Designer Constraints: Heating/ cooling requirement (thermal load profile) Ground heat storage Desk Study SI Data Thermal loads GHSP Designer Heat demand Heat transfer Heat storage Pile/soil interface temp. agreed with Pile Designer Pile/concrete thermal properties Desk study SI data Pile loads Pile Designer FoS (e.g. LDSA Guidance notes) Load requirements Settlement assessment Temperature range agreed with GSHP Designer Cyclic effect of large ΔT Shaft friction Limiting concrete stress Combined Pile load test / Response test 13

14 Section Interface with M&E Engineers M and E heat loads (Section 5.5) Heating / Cooling loads Hourly results Complex - Dynamic Simulation Models - Daily temperature fluctuations. - Weakness - Climate and Building occupancy changes. Monthly results Simple Simple model Average monthly heat loads Superimpose daily cycles - Worst winter heating and summer cooling peak periods 14

15 Section Interface with GSHP designer (Appendix A) Assessment of the hottest pile Maximum expansion and friction mobilised on pile Assessment of the coldest pile Maximum contraction of pile No freezing at pile soil interface Circulation fluid Single piles or series of piles. Section Ground must not freeze 1. Ensure heat pump inlet/outlet temperature above zero (tolerance of 2 degrees C) 2. Or if inlet/outlet temperature allowed to be sub-zero for a short time then check piles give sufficient thermal buffer Use Loop model for long term ground temperatures 15

16 Section Interface with pile designer Normal pile design considerations Building Load Additional thermal pile design considerations Ultimate LS ΔH ΔT ULS (Appendix C) Stratigraphy and soil properties Shear / radial stresses End bearing Serviceability LS Pile settlement Differential settlement Concrete stress Negative skin friction Δσ c u1 φ 1 ' c u2 φ 2 ' c u3 φ 3 ' Soil strength properties considering heating and cooling effects SLS (Appendix E) Axial and radial pile expansion / contraction / fixity Thermally induced axial stresses Cyclic effects of thermal loading Temperature at soil-pile interface including daily / seasonal variations 16

17 Section Ultimate Limit State Soils (Appendix C) Effects of heating soil Strength and stiffness reduces from reduction in preconsolidation pressure (quasi-creep effect) Consolidation regains the strength Over Consolidated soils less effect Undrained Excess pore pressures Drained Consolidation regains soil strength (increased strength when cooled) OC soils: little settlement σ p reduces with higher temperature NC soils: - large settlement (Eriksson, 1989) 17

18 Section Serviceability Limit State - stresses (Appendix E) Thermal pile expansion is similar to -ve skin friction Consider settlements Negative skin friction design Poulos, (2008) Pile head fixity increases thermal stresses Thermally induced concrete axial stresses Check concrete stress < concrete strength (q c ) / 4 Restrained Pile Δσ L Δσ = α T x ΔT x E conc Unrestrained Pile ΔL/2 L ΔL/2 ΔL = α T x ΔT x L 18

19 Combined load and thermal test Lambeth College, London (2007) Bourne-Webb et al, (2009) Geotechnique Cementation / GIL / Cambridge Pile loading test with thermal response test -cyclic temperature effects. Load cells and pile head levelling Optical fibre sensor system strains and temperature Conventional vibrating-wire strain gauges (VWSG), thermistors 19

20 20 Lambeth College - test layout

21 Test results Pile loads Head movements Temperatures 21

22 Section Pile thermal response test Shell HQ, London (2012) TRT borehole converted to mini pile. Instrumented. Strain and soil properties Pile and soil thermal conductivity VW strain gauges VW piezometer & thermocouples VW strain gauges 26m VW strain gauges VW strain gauges 22

23 Back Analysis using OASYS PILE and LS Dyna - Input parameters Made Ground / Terrace Gravel: Mohr Coulomb material, f = 36 o, drained behaviour. London Clay: undrained behaviour, strength profile: 4m to 18m bgl: cu,top = 60 kpa; cu,bot = 184 kpa; 18m to 30m bgl, cu,top = 184 kpa; cu,bot = 194 kpa a = 0.5, E/c u = 600, n=0.5, N c = 9. Pile Design: q s = a c u q b = N c c u Pile : Elastic, E=40GPa Thermal expansion coefficient of pile = 0.85x

24 OASYS - PILE model Pile Head Load (Free or fixed head) Thermal expansion of piles Vertical settlement (Mindlin) F equiv Limiting shear τ= α.cu F equiv F equiv = e therm. E concrete. A pile No allowance for soil volume change 24

25 Depth below GL (m) Depth below GL (m) Model thermal effects pile contracts / expands Lambeth College Pile Axial Loads thermal =700kN = 2.5MPa Axial Load (kn) Cooling Heating Axial Load (kn) Measured Oasys PILE LS-DYNA 3mm movement Match pile head Measured Oasys PILE LS-DYNA

26 Recent developments LS Dyna Model Lambeth College 23m 4m Top 5m of pile has diameter 610mm Remainder of pile has diameter 550mm 26

27 Temperature Change in Soil Near start of cooling Mid-way through cooling End of cooling Near start of heating End of heating 27

28 Pore Pressure Change (Undrained) After reload End of cooling End of heating Stresses in kpa, relative to initial stress state. Positive numbers mean that the pore pressure has increased (more compressive) compared to the initial stress state Model: /data3/rsturt/energy_piles/lambeth_jan2012/aw_curve_slip/lambeth_12_awcur.key 28

29 Permeability Effect End of Cooling - Pore pressure (kpa), relative to initial stress state. - Positive numbers - pore pressure has increased (more compressive) relative to initial stress state Permeability 1.0x10-9 m/s Permeability 1.0x10-10 m/s Undrained 29

30 LS-Dyna Conclusions Temperature changes in soil well modelled Relative expansion of soil skeleton / water is significant Large pore pressure changes in soil Could reduce undrained shear strength Consolidation occurs rapidly Soil regains strength ULS capacity of pile increases. Effect of soil expansion on axial load in pile Gravel layer (Lambeth College) hard to model 30

31 Section Serviceability LS - settlement (Appendix F) Pile head fixity % of thermal piles in the scheme Additional settlement Thermal effects Seasonal cyclic movement heating / cooling 31

32 Section Cyclic thermal effects (Appendix E) Thermal cyclic loading Comparison with cyclic stability diagram (Poulos) Factor of Safety =2.0 32

33 Conclusions Thermal Piles are established in UK but few designers / contractors. Thermal pile standard provides a framework Based on Vertical Borehole Standard Main text - Specification Appendices Guidance and current state of art Responsibilities - Design and Contract - linked with SPERW Design Interfaces - M&E, GSHP, Pile Designer Geotechnical design developing - soil properties & THM models 33

34 Thermal Pile Geotechnical Design Process Specify temperature ranges Pile / Soil interface - +2 o C to +30 o C (No freezing) How to monitor? limit heat pump output temp Assess pile extension and ground movements Assess concrete stresses dead load and thermal Max concrete stress < Concrete strength (q c )/4 Pile design - conventional Factors of Safety F of S > 2.0 to 3.0 allow for thermal effects Check cyclic effects similar to live loads Treat thermal loads as cyclic live loads - 50 annual cycles Check consolidation In progress 34

35 Conclusions on pile testing Lambeth College pile test - addresses geotechnical design issues Pile axial loading. Thermal concrete stress (700kN/0.28m 2 = 2.5MPa). Ultimate capacity. Design tools PILE and DYNA Back analysis does not fully model pile loading due to pile expansion. Need to consider small strain parameters and consolidation. Extrapolate Lambeth results to other London Clay projects Pile length. Temperature change. Soil Strength profile. In future Thermal response tests with mid height strain gauges for thermally induced concrete stress change. 35

36 Installation and interface with other site work Successful UK installation of plastic pipes in: Small / large diameter bored piles. Piles under bentonite or dry bore. CFA piles. Driven cast in situ piles. Diaphragm walls etc. 36

37 Bored piles - Pallant House - Chichester (2003) Pile design Arup Pile construction Cementation Closed loops - Naegelebau Bored piles -0.75m dia Cooling - 40kW of 75kW Heating - 60kW of 90kW Pressure testing Protection of tubes Tubes on full length cage 37

38 Pallent House connection Trimming pile head Header pipes before blinding Connecting tubes Pre blinding Plant room - Note pressure gauges 38

39 Short Cages and Borehole Loops (2007) Cementation / GIL Market dry bored piles in London Clay. Use short cages. Use borehole U-tubes. Paddington Basin GIL and Cementation Two pairs of U- tubes. 39

40 West End Green Use of lantern spacers (2010) To central GSHP +28mOD Brickearth +25.5mOD A A A-A River Terrace Deposits +20mOD London Clay Thermal pipes Reinforcement bars Lantern spacers B-B B B 40

41 Short or long tremie Scratching test (2010) Test set-up Photos from test +33.2mOD +32.4mOD Tremie length = 6m Bar weights prior to testing U bend after test Loops restrained by bar weights Pipe bottom +4.0mOD Concrete = +3.45mOD Pile bottom -5.4mOD Upper pipe after test Lower pipe after test 41

42 Scratch depth measurement on 32mm pipes Assessment of damage Par off pipe until scratch just disappears Measure pared width (2C) Calculate scratch depth C R T C R 2C chord length (mm) - measured; T Depth of the scratch (mm) - calculated R radius of the pipe measured Conclusions Vertical pipes <1mm scratches - Splayed pipes - 1 to 2mm scratches 42

43 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 Plunging used T32 bar + 4 pipes (2 loops) Heating - 188kW Cooling - 117kW Trimming Cage and header pipes 43

44 Large piles - One New Change London (2008) Planning Stage M and E Hoare Lea Wells and Geothermal Piles - Arup Detailed Stage M and E Hoare Lea Geotechnical pile design - Arup Pile construction Cementation Geothermal design (189 No) - GIL Chalk Wells (2No) Heating / Cooling - 1.7MW 44

45 45 One New Change Basement Full length heave reinf. Piles to 2.4m dia 7 loops

46 Crossrail - Thermal Piles pipes outside cages Plastic pipes outside cages. Cover = 75mm +32mmOD =107mm Full length cages - long tremie 46

47 Ground Source Heat Pump Association (GSHPA) provides:- Information to Clients Standards for Industry Guides/codes on borehole loops extend to thermal piles Training / competency standards Competency standards - GSHP designers / installers Seminars Influence Government energy policy Further details from 47

48 Conclusions Thermal Piles are established in UK. Thermal Pile / Heat pump systems - compete with gas boilers, biomass. Thermal pile standard available from GSHPA. Need to check installation damage and develop Thermal Pile Specification Ownership of design responsibilities unclear. Geothermal design - based on borehole loops guidance. Geotechnical design methods described. Few designers and contractors able to tender for work. Thermal walls Design processes under development. 48

49 49 Questions