Integrating Geothermal Loops into the Diaphragm walls of the Knightsbridge Palace Hotel Project
INTEGRATING GEOTHERMAL LOOPS INTO THE DIAPHRAGM WALLS OF THE KNIGHTSBRIDGE PALACE HOTEL PROJECTTony Amis, Geothermal International Limited, Coventry, United KingdomChristopher A. W. Robinson, Cementation Skanska Ltd, Doncaster, South Yorkshire, United KingdomSamuel Wong, WSP Cantor Seinuk, London, United Kingdom The Knightsbridge Palace Hotel development located in Knightsbridge, London required construction of a six level basement to house a double height ball room, dining areas, swimming pool, offices and plant rooms. The site formerly housed The Normandie Hotel and is sandwiched between Knightsbridge, Raphael Street, Knightsbridge Green and the adjacent 199 Knightsbridge development. Construction commenced in September 2009 and is anticipated to be completed in late 2011. These significant basement construction works required construction of a diaphragm wall, rotary plunge column bearing piles and rotary tension piles. The diaphragm wall and bearing piles also served a secondary role through the incorporation of geothermal loops to facilitate heating or cooling of the structure. The diaphragm walls and rotary piles were designed and constructed by Cementation Skanska Limited, with WSP undertaking the role of Structural Engineer for the scheme and Geothermal International Limited designing, supplying and installing the geothermal elements of the scheme. This paper will provide an overview of the scheme, describe the reasons for including incorporation of the geothermal system into the diaphragm wall, summarise how the technical challenges of incorporating the geothermal system into the wall construction were overcome, and describe the potential effects the geothermal system may have on the wall.INTRODUCTIONThe Knightsbridge Palace Hotel (KPH) project is a water tightness criterion, the structural basementscheme to construct a ten storey hotel and walls were constructed adopting diaphragm walls.apartment complex with a six level basement. The 800mm wide diaphragm wall panels werebasement is proposed to house a double height constructed up to 36m below platform level.ball room, dining facilities, swimming pool, plantrooms as well as housing a ground source heatpump (GSHP) system.The site is located on the site of the formerNormandie Hotel on the south side ofKnightsbridge (see Fig. 1 below). The footprint ofthe site is only 1100m² so space constraintsduring construction, together with the proximity ofthe site to neighbouring structures and the generalhigh profile locality made working on this siteparticularly challenging.The six storey basement excavation will extend toover 24m below pavement level adopting topdown construction techniques to minimiseretaining wall deflection and also assist to reducethe overall construction programme. Due to thedepth of excavation and the specified grade 1 Fig. 1 Site location plan
Construction of the diaphragm wall commenced inSeptember 2009 with the geotechnical elementsof the scheme being completed in March 2010.Construction remains underway and is anticipatedto be completed during late 2011. This paper willdescribe the ground conditions, diaphragm wallconstruction, geothermal requirements of theproject and the implications these had on thedesign and detailing of the diaphragm wall, andthe potential effect the geothermal system mayhave on the basement walls.GROUND CONDITIONSThe site investigation undertaken at the siteprincipally comprised cable percussion boreholes.The site geology follows the general sequence:-EGL – 9.00m OD Made ground9.00 – 4.50m OD Firm silty clay4.50 – -2.00m OD River Terrace Gravels-2.00 – EOH London Clay(EGL = existing ground level, approx. 12m OD;EOH = End of hole)Equilibrium groundwater level at the site fordesign was taken as 2.00m OD. The basementwas to be designed for a long term groundwaterlevel of 11.00m OD (i.e. a flood level 1m belowaverage existing ground level).The site investigation generally gave the followingmaterial descriptions:- Fig. 2 Design borehole profileMade ground:Firm to stiff grey brown clay and silty clay with subrounded and sub angular gravel and occasionalconcrete and brick.Silty Clay (Brick earth Clay):Firm to stiff grey and orange mottled silty CLAY,sandy in parts.River Terrace Gravels:Medium dense to dense grey brown fine andmedium SAND with some sub-angular to sub-rounded gravel.London Clay:Stiff to hard grey fissured CLAY with traces ofdark grey pyritic silt.The design SPT „N‟ values and undrained shearstrength profile adopted for design are presentedin Figures 3 and 4 below. Fig. 3 SPT „N‟ vs. Level
damaged during the cutting down operation, installation of reinforcement cage, installation of connecting manifold pipes, supply and return feed to the loop.Fig. 4 Undrained Shear Strength „N‟ vs. LevelTHE STRUCTURAL ENGINEER‟SPERSPECTIVE.Top Down ConstructionWith 6 levels of basement and 10 levels ofsuperstructure (Fig 5) in a relatively small site,diaphragm wall and plunge column piles togetherwith top down construction method was adoptedto meet the tight construction programme. Top Fig 5 3D section showing superstructure anddown construction method also minimises the basement through ball room area.ground movement to the surrounding propertieswhich was a major consideration on this project. Plunge column pilesDiaphragm walls Plunge column piles involve large diameter pilesThe 800mm thick diaphragm walls are installed in incorporating steel column sections. Installation oflarge panels with geothermal loops attached to the these steel sections required specialist plungeoutside face of the cage near the external surface. frames to control the position and verticality of theThe external cover was increased to maintain steel section when being plunged into the75mm of cover to the loops. The geothermal loop concrete before it sets. Where geothermal loopsdiameter is relatively small and thus has no are also incorporated in the piles, the loops aresignificant effect on the bending and shear installed onto the cage as it is being lowered intocapacity of the pile wall. Future penetrations position; the loops have to be secured to ensurethrough D-wall for incoming and outgoing services the pipes are not snagged during the installationrequire careful consideration to avoid damaging of the plunge columns. The free ends of the loopthe loop. have to be protected until being exposed for connection when the lowest raft is constructed.Capping beams The effective area of the concrete pile is only marginally reduced by the introduction ofThe top of the Diaphragm wall requires trimming geothermal loops and can be ignored in theand cutting down to receive the reinforced capping capacity calculation.beam. Care is required to ensure the loops are not
Fig. 6 Diaphragm Wall Panel LayoutDIAPHRAGM RETAINING WALL Diaphragm wall panels were constructed using aDiaphragm wall panels were designed for retained crane mounted hydraulic grab with bentoniteheights of up to 24.35m. Panel joints incorporated support fluid to maintain panel trench stability,water bars to a depth of 27m. particularly within the Made Ground and River Terrace Gravel deposits. Fig. 7 below shows the hydraulic grab in operation. The grab is mountedThe contract specification required the diaphragm on a rotator which enables the grab to be easilywall panels to provide a basement environment of rotated through 180° to maintain panel verticality.Grade 1 to BS8102: 1990 for the basement walls.Given the depths of excavations involved and the The diaphragm wall grab and crane incorporateshigh water pressure behind the retaining wall, on-board instrumentation which provides both anachieving tight construction tolerances for the instantaneous graphical output of grab and paneldiaphragm wall panels was vital. Verticality verticality and also records this data (together withtolerances of better than 1:200 were routinely other relevant information) to form part of the asachieved. Diaphragm wall panels were built record information for each diaphragm wallconstructed using C32/40 concrete. panel.A drained cavity liner wall was required to achieve In addition to supporting the basement excavationthe final Grade 3 environment required for the and excluding groundwater, the diaphragm wallscompleted basement. also carried significant vertical superstructure loads. The magnitude of vertical load meant that certain elevations required the diaphragm wall39 No. 800mm wide diaphragm wall panels were panels to be constructed to deeper toe levels thanconstructed to form the structural basement walls, was required from consideration of stability alone.as shown on Fig 6 below. The overall length ofdiaphragm basement wall was approximately 155linear meters.
inclusion of additional length of geothermal pipework (or loops). The initial concept for the GSHP pipework to be incorporated within the diaphragm wall panels took the form of slinky pipes. These, as their name suggests, are loops of pipework formed into horizontal loops (see Fig.8 below) orientated in the vertical plane. This configuration is a variation of more typical slinky arrangement often found in GSHP applications which have the form of a curtate cycloid.Fig. 7 Hydraulic Grab for D-wall ConstructionGROUND SOURCE HEAT REQUIREMENTS OFTHE SCHEME Fig. 8 Schematic “slinky” pipe arrangementFrom an early stage the KPH scheme included arequirement to incorporate ground source heating When the scheme progressed beyond the concept stage, consideration of this form of GSHP/ cooling elements within the various geotechnical pipework by the various parties raised significantstructures being constructed to support the concerns in terms of system resilience as a resultsuperstructure and retain the ground / exclude of the potential complexity of the pipeworkgroundwater. Whilst GSHP systems have beenincorporated into a limited number of diaphragm geometry and the additional connection details which would be required along with creatingwalls on the continent, this was a first in the UK. significant programme implications compared toOne elevation of the basement wall was not potentially simpler alternatives. Both Cementationrequired to include any GSHP capability since the Skanska Limited (CSL) and Geothermalimmediately adjacent property (199 Knightsbridge) International (GI) have a considerable track recordhas a three level basement car park which wouldsignificantly reduce the efficiency of a GSHP installing GSHP pipework into piles of various types (e.g. large diameter rotary piles [constructedsystem along this elevation. both dry and under bentonite support fluid], CFA piles, driven cast in-situ piles etc.). Taking theThe additional depth of diaphragm panel required experience from these previous schemes CSLto support vertical superstructure loads gaveadditional opportunity to enhance the available and GI working closely together developed acapacity of the proposed GSHP system through solution which maximised the ground sourced heating and cooling potential of the diaphragm
wall (with geometry etc, determined by the Conductivity Test Boxretaining or bearing capacity functions) whilstminimising the impact on reinforcement quantitiesand potentially deleterious effects on constructionquality.Unlike Energy Piles®, which will be surrounded bysoil on all faces; Energy Walls™ will have oneface permanently partially exposed as thebasement; at Knightsbridge Palace Hotel thisequated to a basement depth of 20m out of a totalwall depth of approximately 25m. The activegeothermal loop length installed was 24m, loopsbeing installed 1 m above the founding level of thediaphragm wall to avoid any possible effect on the Fig. 10 Stage One of the Conductivity Testload bearing capability of the wall. The results of this conductivity test supported the conservative values used in the ground loop design. The second stage of the study is due to be undertaken shortly, after excavation of the basement is completed later this year as illustrated in Fig 11. Conductivity Test BoxFig.9 Schematic of Energy Wall™It is important that geothermal loops within eachpanel are installed as close as can be practicallyachieved to the side of the diaphragm wall panelthat will remain unexcavated as illustrated in Fig 8.Assessing the effects (of one face being exposed Fig. 11 Stage Two Of The Conductivity Testin this way) on the conductivity values needed tobe taken into account within the ground loop The second stage of conductivity testing will thendesign. A review of available geothermal literature enable GI to compare and assess any reduction inrevealed that there were no papers dealing with the conductivity values, and ultimately assess thethis effect and thus it was imperative that GI levels of reduction arising from the basementdesign a scheme using some very conservative excavation that needs to be taken into accountconductivity and resistivity values for the loops when designing ground loops within futureinstalled within the diaphragm wall. From an early basements.stage it was GI‟s intention to undertake a two partstudy into the effects of geothermal loops installed GI and CSL ultimately developed a hybrid groundwithin basement walls. The first part of the study, loop solution that was the first of its kind in the UK,completed in May 2010, was to undertake a Geothermal loops were installed within both 100conductivity test prior to excavation of the linear meters of Energy Wall™ and 49No. Energybasement as outlined in Fig 10. Piles® that will ultimately deliver 150kW of peak heating and 150kW of peak cooling to the hotel as illustrated in Fig 12.
detail of the GSHP pipework through the near face (NF) of the diaphragm wall. The slinky pipes were not to be connected vertically between cage sections, rather the pipework being brought horizontally to a box out at the NF of the reinforcement cage near the top of each cage section such that the pipework tails could be exposed and connected (headered in) at a convenient time following basement excavation. This would have required the vertical position of the pipework box outs to have been vertically co- ordinated with the position of the basement slabs.Fig. 12 Schematic of Energy Wall & Pile Various construction details associated with theScheme original concept gave CSL & GI significant concerns. The prefabricated mesh arrangementIMPLICATIONS ON DIAPHRAGM WALL and horizontal pipework terminating in fairly largeREINFORCEMENT DESIGN AND DETAILING. box outs all lead to additional congestion within the diaphragm wall reinforcement cage whichOne of the most significant considerations for CSL through structural requirements was already fairlywas to ensure that the integration of the GSHP congested. An example of the potential effectssystem into the diaphragm wall, in whatever form, significant cage congestion can have onwould not have any adverse impact on the construction quality is illustrated in Fig. 13 below.construction process and quality of the completed The detailing of cages with a relatively smalldiaphragm wall panels. One of the principle aperture size can lead to “pillowing” of thefactors affecting diaphragm wall panels is to concrete as flow between bars is restricted, ratherensure that the construction process is as than flow being uninhibited and flowing to thecontinuous as possible, particularly following extremities of the panel excavation.panel excavation and the subsequent bentonitecleaning process. The diaphragm walls werereinforced with pre-fabricated cages in threesections which required splicing together duringcage construction. The cage splice zones werelocated at suitable locations to avoid significantbar congestion in the areas of peak bendingmoment. The heaviest reinforcement consisted ofpaired B40 bars at 175mm centres, so attemptingto form splices with paired bars of this size wouldhave caused considerable additional difficulty insplicing the cages in a timely manner during panelconstruction. An additional constraint on splicelocation was the relatively tight access fromKnightsbridge into the site. The access Fig. 13 Concrete quality defects due torestrictions lead to a maximum practicable cage reinforcement cage congestionlength of 15m. Complete cages were typicallyformed from two 15m sections and a 5m section The solution favoured by both CSL and GI was an(including the splice lengths). adaptation of techniques developed for Energy Piles™ constructed under bentonite support fluid.The original slinky pipework concept would have In essence the geothermal loops are site fixed torequired GSHP pipework to be prefabricated onto the outside of the far face reinforcement duringsteel mesh and site fixed to the far face (FF) of the cage placement. The geothermal loops arediaphragm wall. The FF is that face of the fabricated at GI‟s facility in Coventry under factorydiaphragm wall panel against the retained soil with controlled conditions. The loops are thenthe near face (NF) being on the internal basement pressure tested to assure their quality at this stageside of the panel. The GSHP pipework could not of the process. The loops are then coiled readybe pre-fabricated onto the reinforcement cage as for dispatch to site. On site the coiled loops arethese are transported on their back (i.e. far face then placed onto a drum arrangement (as showndown) which would have resulted in potentially in Figures 15, 16, and 17 below) to enable thesignificant damage to the pipework which may not loops to be fed out and fixed onto thehave been easily evident unloading / installation reinforcement cage as it is lowered into theof the cages. A further additional potential prepared panel. Generally two loops weredifficulty with this solution was the connection installed in each panel (the exception being the
Fig. 14 Typical reinforcement and geothermal panel construction is undertaken with an absoluteloop configuration minimum of delay to maintain a high quality finished product.corner panels and those adjacent to 199Knightsbridge). Each loop comprises a flow and Once the reinforcement cage was installed to thereturn line, there therefore being a total of 4 No. correct level, the loops were then pressurised topipes fixed to the reinforcement cages. test their integrity and ensure that no damage had occurred during the installation process as shown in Figure 18. The pressure was maintained duringTo accommodate the loops the FF cover was concreting of the panel and held until the followingincreased and the longitudinal reinforcement day. The level of pressure testing adopted givesarrangement altered such that there was no net the best guarantee of a future systemincrease in the degree of cage congestion from performance.that detailed for cages not required toaccommodate the geothermal loops. Wherediaphragm wall panels were deepened to carryvertical loads an additional length of cage wasinstalled to take advantage of the extrageothermal capacity afforded by the geometry.This additional length of cage was detailed to be alight as possible whilst maintaining sufficientrobustness for handling and placing operation.The typical cage reinforcement and geothermalloop arrangement is shown in cross section inFig. 14.The basic method of loop installation adopted had Fig 15 Feeding Geothermal Loop ontobeen used on previous piling contracts and had Reinforcement Cagebeen refined to ensure that the panel constructioncycle took no longer than if the geothermal loopswere not installed. This is critical to ensure that
Fig 16 Geothermal Loop and Reinforcement Fig 18 Pressure Testing Geothermal LoopsCage Installation Once Reinforcement Cage Installed PROGRAMME IMPLICATIONS OF INSTALLING LOOPS INTO DIAPHRAGM WALL. The installation of a diaphragm wall cage within a completed panel is a slow and careful process even without geothermal loops. The 3 cage sections for each panel meant that considerable time was required to splice cages together. Thus, as long as loops were in position on loop reelers ahead of cage installation, the time required to attach loops to the far face of the cage adequately matched the speed of cage insertion. GI were on hand to assist and undertake flow test and pressure test during preparation for concreting works. Thus geothermal loop work remained a non-critical activity and no additional time was required for this element of works. ANTICIPATED EFFECTS OF PLACING GEOTHERMAL LOOPS IN DIAPHRAGM WALLS AND PILES For the last 10-15 years geothermal loops have been installed within foundation piles and diaphragm walls in Europe with no adverse effectsFig 17 Geothermal Loop and Reinforcement being reported. The UK unfortunately has laggedCage Installation behind and only in recent years is catching on to the benefits associated with this simple technology.
Brandl 2006 – reported on several projects across concrete during a winter‟s day. Work currentlyAustria and concluded that shaft resistance, base continues on site to link up loops in diaphragmpressure and bearing resistance of soil are not wall panels and piles in a similar vein, with goodaffected by heat absorption and that temperature coordination with the ground workerinduced settlement or heave is negligible The secondary usage of the structural element as thermal mass enhances the sustainabilityLaloui 2006 – Identified that the heating-cooling credential of the development.process of the building foundations inducessignificant modifications in the soil-structureleading to additional stresses in the piles,decrease of the lateral friction and the possibilityof a gap between the pile and the soilBourne Webb et al 2009 – concluded thattemperature change in piles leads to increasesand decreases in shaft resistance and axial load.Working stresses in pile should be kept low, andmaintain high factor of safety on shaft to withstandheating and cooling loadsTemperatures within geothermal loops will rangegradually between -1°C and 30°C over a 12 monthperiod as the season changes from winter heatingdominant, to summertime cooling dominant. Overa single 24 hour period the ground looptemperature is unlikely to change by anything Fig. 19 Architects‟ impression of completedgreater than 8°C, thus the likely effects compared developmentto the thermal effects imposed on an externalfaçade in spring time, when temperatures can ACKNOWLEDGEMENTSrange from below freezing in the morning to a highmidday temperature can be considered to be The authors wish to thank Squire and Partners,minimal. the Architect along with all the other members of the professional team and especially the ClientCONCLUSIONS Prime Developments Limited for their kind permission to publish this paper.The requirements for the Knightsbridge PalaceHotel development have led to a UK construction REFERENCESfirst with the successful construction of EnergyWalls™ for basement construction (i.e. Bourne Webb, PJ et al (2009) Geotechnique 59incorporation of geothermal loops within No3 237-248 Energy pile test at Lambeth College,diaphragm wall panels). The wide range of London: geotechnical & thermodynamic aspectsexpertise and techniques employed by CSL, GIand WSP combined with the close relationships of pile response to heat cyclesdeveloped with the Client‟s team have resulted inthe construction of a first class project. Good early Laloui, L., Nuth, M. & Vulliet, L. (2006).coordination between all parties enabled the Experimental and numerical investigations of thesuccessful installation of geothermal loops within behaviour of a heat exchanger pile. Int. J. Numer.diaphragm walls and piles with no additional time Anal. Methods Geomech. 30, No. 8, 763–781being needed to be added to the constructionprogramme. Brandl,H (2006) Geotechnique 56 No 2, 81-122 Energy Foundations and other thermo-activeLoop layout within reinforcement cages in both ground structures.Energy Pile™ and Energy Walls® requires carefulcoordinationCareful consideration needs to be made for futurepenetration requirement for incoming andoutgoing servicesOnce operational, daily loop temperaturefluctuations will be considerably less than exposed