The Effects of Heating and Cooling Energy Piles Under Working load at Lambeth College, UK
THE EFFECTS OF HEATING AND COOLING ENERGY PILES UNDER WORKING LOAD AT LAMBETH COLLEGE, UK Tony Amis, Cementation Skanska Ltd, Rickmansworth, United Kingdom Peter Bourne-Webb, Cementation Skanska Ltd, Rickmansworth, United Kingdom Chris Davidson, Geothermal International Ltd, Coventry, United Kingdom Binod Amatya, Cambridge University, Cambridge, United Kingdom Kenichi Soga, Cambridge University, Cambridge, United Kingdom There is very limited information relating to the potential impact of temperature cycles, outside the variation that occurs naturally, on the behaviour of foundation piles. Such thermal loading occurs when piles are used as heat exchange elements within a Ground Sourced Heat Pump system, in addition to carrying building loads to competent strata. An investigation has been undertaken where a conventional load test has been extended to include a period in which the load is maintained and temperature cycles applied. Instrumentation was installed in the test pile, an adjacent borehole, two of the anchor piles and the heat sink pile in order to be able to develop an understanding of the temperature variation and associated strain changes within and between the various elements. Conventional electronic strain and temperature gauges have been used in conjunction with optical fibre sensors which also measure strain and temperature changes. The results have confirmed the validity of the assumption made in the thermo-dynamic design of the system, the compatibility of the differing measurement systems and differing installation methods for the optical fibres, and initial assessment suggests that the load-settlement response of the pile is not adversely affected by the temperature cycles applied. Introduction Over the last six years there has been a The five-storey centre will incorporate specialist significant increase in the use of Energy Piles as facilities for Performing Arts and Creative part of Ground Sourced Heat Pump systems Studies, with state-of-the-art information and being used to heat and cool the buildings which communications technology provided throughout they support. Incorporating pipe loops in the the building. piles allows the geothermal energy to be transferred via heat pumps for use in theC:Documents and Settingsbc958205DesktopMy DocumentsLambeth Test pileAmis et al - Lambeth College DFI Paper - final (2).doc building as required. This method of heating and cooling provides an excellent means of reducing CO2 emissions and can greatly assist in meeting renewable energy targets now required in the United Kingdom as part of building regulation. Where piled foundations are required, it seems obvious to utilise the piles with their good thermal conductivity properties to capture the latent geothermal energy in the ground, for use in the heating and cooling of the building. Energy Piles provide not only a renewable energy source but also a cost effective engineering solution when compared to other geothermal Figure 1. Architects impression of new 6th methods. Form centre, Clapham, London, UK The project within which the test described in The building will utilize all 143 foundation piles this paper was undertaken, involved delivering a as Energy Piles, working in tandem with a new Sixth Form centre at the existing Clapham displacement air handling system, which Centre, on the south side of Clapham Common, requires lower running duties than traditional South London, Fig. 1. systems.
As part of the subcontract enquiry, a preliminary For the trial, an 8 kW heat pump was connectedtest pile was required, which under normal into a circuit which included the test and heatcircumstances involves loading an expendable sink piles. A data logger attached to the heatpile to failure in order to verify the design pump enabled continuous monitoring of directassumptions and allow the use of reduced inflow and outflow temperatures from the heatfactors-of-safety. The opportunity to understand pump during the test.geothermal technology better led to a proposalfor a geothermal test pile being developed by Test pile arrangementCementation Skanska Ltd (CSL), in partnershipwith Geothermal International Ltd (GIL) and the The pile load test was set-up with equipmentGeotechnical and Environmental Research typical of that associated with undertaking aGroup (GERG) at Cambridge University. static load test, i.e. load cell, transducers for monitoring pile movement (4 positions on theThe purpose of the trial was to improve pile head and loading frame), Fig. 2. In addition,knowledge and understanding of the effects of provision had to be made for pipe work and heatheating and cooling on piles in terms of stress, pump installation, and data-logging of thestrain, pile head movement and heat migration instrumentation.whilst under working load. In addition to the testand anchor piles, a 30 m heat sink pile totransfer heat to and from the test pile, and aborehole positioned 0.5 m from the main test pilewere constructed. The trial was conducted overa 7 week period.This paper describes the test set-up used, someof the results seen during the test and some ofthe trends seen in the results.Ground conditions and pile designAll piles were installed from a piling platformlevel of about +25.4 m above Ordinance Datumthrough 4.0 m to 4.5 m of Made Ground andRiver Terrace Deposits, before penetrating theLondon Clay Formation.Using site investigation data and loads providedby Faber Maunsell, the pile design wasdeveloped based on overall Factor of Safety of2.5 on shaft and end bearing, LDSA GuidanceNote 1 (2000). For a typical working load of1200 kN, and utilising a 600 mm nominaldiameter rotary bored pile (610 mm OD casing;550 mm diameter tool size), a pile length of 23 mwas required.Geothermal design Figure 2. Arrangement of pile test elementsThe geothermal design was developed from pile Conventional instrumentation was installedlengths required to carry the specified building within the main energy test pile in order toloads and site investigation information, to monitor temperature and strain changes duringproduce a peak heating load of 268 kW and the test. Instrumentation in the test pile includedpeak cooling load of 428 kW. Heating and vibrating wire strain gauges (VWSG) for straincooling loads were specified by Faber Maunsell measurement and thermistors for temperature. Ain order to meet the Planning Authority’s thermistor array was installed within the adjacentrequirement for 10% renewable energy source. borehole. Each set of gauges was located at or near either changes of strata, changes in pile section or at intervals on the shaft within the
London Clay, as indicated in Fig. 3 and Table 1. Table 1. Instrumentation summaryAll electronic instrumentation was connected to Test element Monitoring/control instrumentsa data logger that enabled remote and Test pile - external Four displacement transducerscontinuous monitoring from CSL Main Office. (LVDT) for pile head displacement & 1 LVDT to loading frame. Load cell.Two different types of Optical Fibre Sensor Test pile - internal VWSG arrays of 3 gauges at six(OFS) cabling; Reinforced OFS cable (Fujikura) levels over 23 m length of pile.and loose tube internal/external grade fibre OFS cables 2 loops for strain and 2 loops for temperature measurementUniversal Unitube OFS cable were trialled to Borehole Thermistor array at six levels over 23measure strain and temperature separately, mFig. 4. The test pile, two anchor piles, the heat OFS temperature cablesink pile and the borehole were all fitted with Anchor piles Two diagonally opposed piles 23 mloops of either or both types of cable, Fig. 3 and deep with OFS strain andTable 1. temperature cable Heat sink pile OFS cables 2 loops for strain and 2 loops for temperature measurement over 30 m length of pile Figure 4. Optical Fibre Sensors used in test Reinforced OFS cable sensors were also installed to measure circumferential strain changes in the test pile, at a depth of 9 m and 14 m below ground level.Figure 3. Soil profile and instrumentation OFS sensors provide a continuous profile ofdetails for test pile and borehole strain and have been used alongside conventional discrete sensors such as VWSG, in a number of projects where the behaviour of
piles has been examined, Bennett et al (2006). A in this estimate and it also appears that theBrillouin Optical Time Domain Reflectometry loading arrangement at the head of the pile has(BOTDR) analyser was maintained on site a significant impact on the observed response,throughout the test in order to provide Bourne-Webb et al (2009).continuous monitoring of the OFS cables. The initial pile settlement at working loadFindings from the geothermal test pile (1200 kN) prior to switching on the heat pump was about 2.4 mm. Over the cooling cycle pileObservations at the pile head head movement increased to 4.8 mm and whenDetails of the load applied at the pile head and the heat pump was switched to heat the pile, pileassociated settlement observed throughout the head movement recovered to 2.8 mm. Dailycourse of the test are summarised in Table 2 cycles of cooling and heating showed a range ofand shown in Fig. 5. pile head movement between 2.8 mm in heating mode and 4.4 mm in the cooling mode.Table 2. Pile head movement Observations from VWSG & thermistors Load Displ. Load case Comment (kN) (mm) Logging of all conventional pile instrumentation DVL = SWL 1200 1.7 Displacement at end commenced immediately after concreting of first cycle operations were completed. Temperature and 0 0.5 Recovery at zero load strain changes due to the hydration process, DVL+½SWL 1800 3.2 Displacement at end during the “Initial curing” period prior to of second cycle load/thermal testing are shown in Fig. 6. Data 0 1.0 Recovery at zero load from about 9.5 m bgl is shown; the general form SWL 1200 2.4 of the data from other levels is broadly similar End cooling 1200 4.8 however and has been left off to improve clarity. End heating 1200 2.8 0 3.5 Unload during cooling Interestingly, prior to commencing the initial load cycle so total value is test, temperatures in all instruments seemed to influenced by thermal effects; recovery be equilibrating to a level somewhat higher (18C movement due to load to 21C) than expected in and around London change is consistent (12C to 15C). It is believed that this may be due with original loading to heat radiating from the London Underground Max test 3600 10.3 railway tunnels which run approximately 20 m Change ~7 mm load west of the test site. Prior to the thermal trial, a conventional working pile load test was undertaken between 14th and 15th of June, with a maximum test load of 1.5x working load (1800 kN). The performance of the test pile in terms of the load and pile head displacement is summarised in Table 2. On June 18th, the pile was reloaded to working load (1200 kN) and the heat pump switched on to provide maximum cooling in the test pile. This involved circulating fluid at approximately –6C through the trial pile for a period of 4 weeks, theFigure 5. Load control and pile head pile cooled by 14C to 16C over the first 7 daysmovement during test and reached a state of near equilibrium after 14 days, Fig. 6. After 4 weeks of cooling, theObserved changes in pile head movement would average pile temperature remained aboveappear to be consistent though smaller in freezing. The lowest temperature of 0.3C wasmagnitude than those estimated from coefficient recorded near the toe of the pile.of thermal expansion values alone. However, therestraint provided by the soil is not accounted for
Figure 6. Strain and temperature variation9.5 m bgl during Energy Pile test On July 31st, the heat pump was switched to a cooling phase for a period of 24 hours, beforeThe borehole located 0.5 m away from the test switching back to a heating phase for 24 hourspile begins to show the effects of cooling after a and then cooling until 4pm, August 5th when thetime lag of approximately 24 hours. The heat pump was switched off. During the lastreduction in temperature during the cooling and cooling stage, the pile was tested to 3x itssubsequent phases develops at a much slower working load.rate than that observed in the test pile. Themaximum temperature change recorded in the In the recovery period after the heat pump wasinitial cooling phase was about 9C compared to switched off, temperatures in the pile andabout 20C in the pile. borehole recovered and appeared to be stabilising towards a value similar to that at theOn July 19th, the heat pump was switched to a start of the test.heating phase which circulated fluid at about40C through loops in the geothermal trial pile. Monitoring was terminated when the test areaThis was interrupted by a power outage during had to be handed back to the main contractor tothe weekend of 20th & 21st July, see Fig. 6. allow the building works to progress.
Observations from OFS Cable due to heating or cooling and the effect of temperature in the optical fibre glass. In all theComparison of OFS installation technique following results, the effect of thermal impact in the OFS is to be considered and necessaryAs part of the development of the use of OFS temperature compensation is yet to be done.systems, a comparison of two methods ofinstallation has been made: glued attachment(OFS fixed to bar at ~15 cm intervals) andclipped attachment (OFS strung betweenattachment points at each end of cage).This comparison is made in Fig. 7 from twostages within the test; at 1800 kN static loadwithout any thermal load and at 1200 kN load,during the initial cooling phase. It is apparentthat there is no significant difference in resultsobtained, due to the differing installation methodused. Clipped attachment at top and bottom isas effective as glued attachment and isrecommended as the preferred method ofinstallation as it is quicker, more straightforwardto implement and hence economical than theglued method. Figure 8. Test pile strain profiles In addition to axial strain, measurement of circumferential strain was made possible by looping the OFS around the reinforcement cage. Circumferential strain at a depth of 9 m, during mechanical loading and combined thermal loading are shown in Fig. 9.Figure 7. Comparative strain profiles forglued (dashed line) versus clipped (solid line)OFS installationResponse of Main Test PileStrain profiles over the depth of the main test Figure 9. Circumferential strain variationpile during loading, as well as during heating and from OFS in test pilecooling phases were generated from thereinforced OFS and are shown in Fig. 8. Temperature over the depth of the pile was measured using ‘Universal Unitube’ OFS andThe values of strain shown are due to the temperature profiles along the test pile arecombined effect of applied load, thermal loading
shown in Fig. 10. Below about 3 m depth,temperature changes are effectively uniform.The net change in temperature during the initialcooling stage was a reduction of ~22C andduring heating, an increase of ~16C with respectto the ambient temperature recorded at the startof the test. Figure 11. Temperature profiles from individual OFS cables across pile sectionFigure 10. Test pile temperature profilesAn indication of the variation in temperatureacross the pile section can be obtained from theprofiles shown in Fig. 11 which plotstemperature data from the four sets of cableplaced at different quadrants within the pile. It isclear that there was some variation oftemperature within the pile cross-section. Itwould appear that this variation is due to the Figure 12. Impact of mechanical loading onproximity of inlet and or outlet geothermal loops strain readings in OFSto the OFS; the VWSGs which were locatedfurther from the pipes showed a lesser Response of Heat Sink Pilecooling/heating effect. Strain profiles over the depth of the heat sink pileAs the OFS cables for temperature during heating and cooling phases are shown inmeasurement were embedded in concrete, it Fig. 13, and temperature profiles, Fig. 14. As forwas essential to make sure that there was no the test pile, compensation of thermal impact oneffect of mechanical loading. Strain data OFS readings has yet to be incorporated inrecorded during the initial load test when these figures.temperature did not vary significantly while themechanical strain does, provides assurance that The information from the heat sink pile is usefulthe ‘Universal Unitube’ results are not influenced as it is not carrying any mechanical load andby mechanical strain effects, Fig. 12. therefore provides a picture of the effect of temperature changes only, on the behaviour of a pile.
In the strain profiles, it appears that there may piles, e.g. the temperature in the heat sink pile isbe some residual strain developing as more uniform over its length than the test pile –comparison of the profiles at different stages at this may be due to there being four pipe loops innotionally the same temperature shows the heat sink pile and only two in the test piledifferences – this is the subject of deeper providing less chance for surface radiation toexamination as to whether the effect is real and impact in the manner seen in the test pile.whether it will impact on the pile behaviour. Spatial temperature variation Temperature variation derived from the OFS, in each of the instrumented elements (test pile, heat sink pile, two anchor piles and borehole) at about mid-depth of the pile is shown in Fig. 15. The variation is plotted against duration of the test and shows the change in temperature with respect to ambient conditions at the start of the test (actual ambient temperature about 19C prior to thermal load being applied). Temperature variation in the borehole was measured using thermistors and temperature OFS. While the borehole was only 0.5 m from the test pile, temperature changes were less than half those measured in the test pile; reducing by ~10C with respect to ambient, at the end of the cooling and recovering to near ambient at the end of the heating phase. Temperature propagation from the test pile toFigure 13. Heat sink pile strain profiles the anchor piles was investigated using temperature OFS installed in two of the anchor piles. The anchor piles are located about 2.15 m from the test pile and some cooling and heating was detected; about a 4C reduction during the cooling stage and a return to near ambient conditions during heating. Figure 15. Temperature variation during testFigure 14. Heat sink pile temperature profiles in all elements at about 10 m bglThere are some interesting differences in thetemperature profiles for the test and heat sink
There is also some evidence that the thermal detail of the testing procedure is not coveredwave front radiating from the heat sink pile also here but relies on the assertion that the rate ofimpacted on the temperature data. change of temperature in a test loop (with the natural logarithm of time), under heating orThermo-dynamic behaviour cooling load is proportional to the conductivity averaged across the medium.One of the most frequently used models ingeothermal system design is based on a To enable the current test data to be analysed aderivation of Maxwell’s Relations, more “clean” section of data, exhibiting a steady rise incommonly used in electromagnetic science and temperature over a prolonged period wasengineering, and often referred to as a “Line chosen, Fig. 16. The variation of average waterSource Model”. In this treatment, the geothermal temperature in the test loop against the natural“collector” pipe is likened to an infinitely long two logarithm of time is shown; the gradient of thedimensional source or sink of heat energy, Bose latter portion of this curve yields a conductivity(1988). vale of 1.52 W/m/K +/-0.06 W/m/K. This corresponds closely to previously measuredDesign software based on these principles is values for conductivity in the London area ofused to calculate the amount of pipe required to about 1.5 W/m/K and largely validates theallow adequate heat energy transfer to maintain design approach described above.the system within preset temperature limits overthe modelling period, typically 100 years.Refinements are then added to allow forborehole depth, relative pipe placement,borehole diameter, and backfill material (grout)and conductivity. Energy Pile design cantherefore be understood as an extension of theconventional bored closed loop geothermalmodel with the key parameter comparisonsoutlined in Table 3.Table 3. Comparison of key parameters forborehole and Energy Pile systems Parameter Borehole Energy Pile Depth range, m 60 – 120 12 – 45 Diameter range, mm 125 – 150 >450 Figure 16. Analysis of flow & return fluid Backfill conductivity, W/m/K 1.5 – 2.5 0.8 – 1.5 temperatures Ground conductivity, W/m/K 0.4 – 3.5 Spacing, m Regular Irregular Concluding Remarks 3.5 – 5.5 2.5 – 4.5 Loops per hole 1–2 1–7 The test undertaken at Lambeth College, London has provided some new and interestingMost ground loop design software packages insights into the impact of temperature cycles onallow limited adjustment of parameters such as the behaviour of piles under compressiveborehole depth and diameter and relative loading, and in relation to the propagation ofseparation. By extending this ability within the thermal energy into the ground surrounding suchdesign software, Energy Pile designs can easily a pile.be catered for. The test has allowed the validityof the revised model to be confirmed by Overall instrumentation reliability was excellentmeasuring the conductivity of the test and heat with all conventional gauges remaining fullysink piles, and comparing this to the values used functional throughout the test period. The OFSby the ground loop design software and in more cables have proved to be an effective tool forconventional geothermal system design. establishing strain (longitudinal and circumferential) and temperature profiles in pilesOne of the most respected methods for subjected to mechanical and thermal loading.measuring ground conductivity for geothermalsystems was developed during the 1990s at It should be noted that outputs from the heatOklahoma State University, Austin (1998). The pump were set to maximum cooling, -6C and
maximum heating, +55C during each cycle. The line source model approach to Energy PileThese temperatures are outside the normal design has been verified as a valid extension ofoperational range of heat pumps (more typically conventional geothermal system design.-1C to +38C) and heat pumps would normallyoperate at maximum outputs for much shorter Referencesperiods of time than was the case in this test. 1. AUSTIN, W. A. 1995. Development of anA 4 week extreme cooling phase (-6C output in situ system for measuring ground thermalfrom heat pump) of the test pile increased pile properties. Oklahoma State University,settlement by about 2 mm. Although circulating Stillwater, Oklahomatemperatures were below freezing, the trial wasunable to cool the pile below freezing, although 2. BENNETT, P.J., KLAR, A., VORSTER,locally the temperature may have been below T.E.B., CHOY, C.K., MOHAMMAD, H., SOGA,freezing, in particular close to the pipe loops. K., MAIR, R.J., TESTER, P. and FERNIE, R. 2006. Distributed optical fibre strain sensing inThe extreme heating phase (+40C output from piles. Proc. Intl. Conf. on Reuse of Foundationsthe heat pump) of the test resulted in recovery of for Urban Sites, BRE, Watford, HIS BRE Press,the pile head movement to approximately the p. 71-78value at the start of the thermal test. Thebehaviour seen during the heating phase is 3. BOURNE-WEBB, P.J., AMATYA, B., SOGA,thought to have been affected by the thermal K., AMIS, T., DAVIDSON, C. & PAYNE, P. 2009.mass that was built-up in the surrounding soil Energy Pile test at Lambeth College, London:during the cooling phase. The response seen Geotechnical and thermodynamic aspects of pileduring the heating phase may not be response to heat cycles. Submitted forrepresentative of an operational GSHP system publication in Geotechnique symposium in printwhere cooling and heating cycles will be of much – Thermal characteristics of the ground.shorter duration. 4. BOSE, J.E. 1988. Closed-loop/ground-sourceBoth heating and cooling phases affected heat pump systems. Intl. Ground Source Heatground temperature within the borehole and the Pump Association, Stillwater, Oklahoma, USA.anchor pile which were located about 0.5 m and2.15 m from the test pile respectively. 5. LDSA Guidance Note 1, 2000. GuidanceTemperature changes had reduced by about notes for the design of bored straight shafted50% and 80% relative to those seen at the test piles in London Clay. London District Surveyorspile, at the borehole and anchor pile Association (LDSA) Publications, 10 pagesrespectively. AcknowledgementsIt appears that there is a complex interaction Cementation Foundations Skanska Limitedbetween the restraint offered by the surrounding funded the field test and many otherssoil and the load/restraint conditions applicable contributed in kind to the success of the test.at the pile head which impacts on the observed The authors thank Lambeth College, Osborneresponse of the pile when temperature changes Construction Ltd and Faber Maunsell withoutare imposed on the system. This aspect of whose agreement and co-operation thebehaviour will be the focus of future assessment investigation reported here would not have beenof the data from this test and future studies. possible. Furthermore, acknowledgement goesHowever, at this early stage it would seem that to the numerous CFSL staff who providedthere has been no detrimental effect on the load- assistance in the preparation of the test pile; Drsettlement behaviour of the test pile as a Taro Uchimura of the University of Tokyo, Mrconsequence of the temperature cycles applied Andy Leung and Hisham Mohamad researchduring the test. Future investigations will include students at GERC who assisted with the OFSthe monitoring of Energy Piles within an installation.operational GSHP system, as well as furtherextended load testing such as that described inthis paper.