INTEGRATING GEOTHERMAL LOOPS INTO THE DIAPHRAGM WALLS OF THE

KNIGHTSBRIDGE PALACE HOTEL PROJECT

Tony Amis, Geothermal International Limited, Coventry, United Kingdom Christopher A. W. Robinson, Cementation Skanska Ltd, Doncaster, South Yorkshire, United Kingdom Samuel 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.

INTRODUCTION The Knightsbridge Palace Hotel (KPH) project is a scheme to construct a ten storey hotel and apartment complex with a six level basement. The basement is proposed to house a double height ball room, dining facilities, swimming pool, plant rooms as well as housing a ground source heat pump (GSHP) system. The site is located on the site of the former Normandie Hotel on the south side of Knightsbridge (see Fig. 1 below). The footprint of the site is only 1100m² so space constraints during construction, together with the proximity of the site to neighbouring structures and the general high profile locality made working on this site particularly challenging. The six storey basement excavation will extend to over 24m below pavement level adopting top down construction techniques to minimise retaining wall deflection and also assist to reduce the overall construction programme. Due to the depth of excavation and the specified grade 1

water tightness criterion, the structural basement walls were constructed adopting diaphragm walls. 800mm wide diaphragm wall panels were constructed up to 36m below platform level.

Fig. 1 Site location plan

Construction of the diaphragm wall commenced in September 2009 with the geotechnical elements of the scheme being completed in March 2010. Construction remains underway and is anticipated to be completed during late 2011. This paper will describe the ground conditions, diaphragm wall construction, geothermal requirements of the project and the implications these had on the design and detailing of the diaphragm wall, and the potential effect the geothermal system may have on the basement walls. GROUND CONDITIONS The site investigation undertaken at the site principally comprised cable percussion boreholes. The site geology follows the general sequence:- EGL – 9.00m OD Made ground 9.00 – 4.50m OD Firm silty clay 4.50 – -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 for design was taken as 2.00m OD. The basement was to be designed for a long term groundwater level of 11.00m OD (i.e. a flood level 1m below average existing ground level). The site investigation generally gave the following material descriptions:- Made ground: Firm to stiff grey brown clay and silty clay with sub rounded and sub angular gravel and occasional concrete 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 and medium SAND with some sub-angular to sub-rounded gravel. London Clay: Stiff to hard grey fissured CLAY with traces of dark grey pyritic silt. The design SPT „N‟ values and undrained shear strength profile adopted for design are presented in Figures 3 and 4 below.

Fig. 2 Design borehole profile

Fig. 3 SPT „N‟ vs. Level

Fig. 4 Undrained Shear Strength „N‟ vs. Level

THE STRUCTURAL ENGINEER‟S PERSPECTIVE. Top Down Construction With 6 levels of basement and 10 levels of superstructure (Fig 5) in a relatively small site, diaphragm wall and plunge column piles together with top down construction method was adopted to meet the tight construction programme. Top down construction method also minimises the ground movement to the surrounding properties which was a major consideration on this project. Diaphragm walls The 800mm thick diaphragm walls are installed in large panels with geothermal loops attached to the outside face of the cage near the external surface. The external cover was increased to maintain 75mm of cover to the loops. The geothermal loop diameter is relatively small and thus has no significant effect on the bending and shear capacity of the pile wall. Future penetrations through D-wall for incoming and outgoing services require careful consideration to avoid damaging the loop.

Capping beams

The top of the Diaphragm wall requires trimming and cutting down to receive the reinforced capping beam. Care is required to ensure the loops are not

damaged during the cutting down operation, installation of reinforcement cage, installation of connecting manifold pipes, supply and return feed to the loop.

Fig 5 3D section showing superstructure and basement through ball room area. Plunge column piles Plunge column piles involve large diameter piles incorporating steel column sections. Installation of these steel sections required specialist plunge frames to control the position and verticality of the steel section when being plunged into the concrete before it sets. Where geothermal loops are also incorporated in the piles, the loops are installed onto the cage as it is being lowered into position; the loops have to be secured to ensure the pipes are not snagged during the installation of the plunge columns. The free ends of the loop have to be protected until being exposed for connection when the lowest raft is constructed. The effective area of the concrete pile is only marginally reduced by the introduction of geothermal loops and can be ignored in the capacity calculation.

Fig. 6 Diaphragm Wall Panel Layout DIAPHRAGM RETAINING WALL Diaphragm wall panels were designed for retained heights of up to 24.35m. Panel joints incorporated water bars to a depth of 27m. The contract specification required the diaphragm wall panels to provide a basement environment of Grade 1 to BS8102: 1990 for the basement walls. Given the depths of excavations involved and the high water pressure behind the retaining wall, achieving tight construction tolerances for the diaphragm wall panels was vital. Verticality tolerances of better than 1:200 were routinely achieved. Diaphragm wall panels were constructed using C32/40 concrete. A drained cavity liner wall was required to achieve the final Grade 3 environment required for the completed basement. 39 No. 800mm wide diaphragm wall panels were constructed to form the structural basement walls, as shown on Fig 6 below. The overall length of diaphragm basement wall was approximately 155 linear meters.

Diaphragm wall panels were constructed using a crane mounted hydraulic grab with bentonite support fluid to maintain panel trench stability, particularly within the Made Ground and River Terrace Gravel deposits. Fig. 7 below shows the hydraulic grab in operation. The grab is mounted on a rotator which enables the grab to be easily rotated through 180° to maintain panel verticality. The diaphragm wall grab and crane incorporates on-board instrumentation which provides both an instantaneous graphical output of grab and panel verticality and also records this data (together with other relevant information) to form part of the as built record information for each diaphragm wall panel. In addition to supporting the basement excavation and excluding groundwater, the diaphragm walls also carried significant vertical superstructure loads. The magnitude of vertical load meant that certain elevations required the diaphragm wall panels to be constructed to deeper toe levels than was required from consideration of stability alone.

Fig. 7 Hydraulic Grab for D-wall Construction GROUND SOURCE HEAT REQUIREMENTS OF THE SCHEME From an early stage the KPH scheme included a requirement to incorporate ground source heating / cooling elements within the various geotechnical structures being constructed to support the superstructure and retain the ground / exclude groundwater. Whilst GSHP systems have been incorporated into a limited number of diaphragm walls on the continent, this was a first in the UK. One elevation of the basement wall was not required to include any GSHP capability since the immediately adjacent property (199 Knightsbridge) has a three level basement car park which would significantly reduce the efficiency of a GSHP system along this elevation. The additional depth of diaphragm panel required to support vertical superstructure loads gave additional opportunity to enhance the available capacity of the proposed GSHP system through

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. 8 Schematic “slinky” pipe arrangement When the scheme progressed beyond the concept stage, consideration of this form of GSHP pipework by the various parties raised significant concerns in terms of system resilience as a result of the potential complexity of the pipework geometry and the additional connection details which would be required along with creating significant programme implications compared to potentially simpler alternatives. Both Cementation Skanska Limited (CSL) and Geothermal International (GI) have a considerable track record installing GSHP pipework into piles of various types (e.g. large diameter rotary piles [constructed both dry and under bentonite support fluid], CFA piles, driven cast in-situ piles etc.). Taking the experience from these previous schemes CSL and GI working closely together developed a solution which maximised the ground sourced heating and cooling potential of the diaphragm

wall (with geometry etc, determined by the retaining or bearing capacity functions) whilst minimising the impact on reinforcement quantities and potentially deleterious effects on construction quality. Unlike Energy Piles®, which will be surrounded by

soil on all faces; Energy Walls™ will have one

face permanently partially exposed as the

basement; at Knightsbridge Palace Hotel this

equated to a basement depth of 20m out of a total

wall depth of approximately 25m. The active

geothermal loop length installed was 24m, loops

being installed 1 m above the founding level of the

diaphragm wall to avoid any possible effect on the

load bearing capability of the wall.

Fig.9 Schematic of Energy Wall™

It is important that geothermal loops within each

panel are installed as close as can be practically

achieved to the side of the diaphragm wall panel

that will remain unexcavated as illustrated in Fig 8.

Assessing the effects (of one face being exposed

in this way) on the conductivity values needed to

be taken into account within the ground loop

design. A review of available geothermal literature

revealed that there were no papers dealing with

this effect and thus it was imperative that GI

design a scheme using some very conservative

conductivity and resistivity values for the loops

installed within the diaphragm wall. From an early

stage it was GI‟s intention to undertake a two part

study into the effects of geothermal loops installed

within basement walls. The first part of the study,

completed in May 2010, was to undertake a

conductivity test prior to excavation of the

basement as outlined in Fig 10.

Fig. 10 Stage One of the Conductivity Test

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.

Fig. 11 Stage Two Of The Conductivity Test

The second stage of conductivity testing will then

enable GI to compare and assess any reduction in

the conductivity values, and ultimately assess the

levels of reduction arising from the basement

excavation that needs to be taken into account

when designing ground loops within future

basements.

GI and CSL ultimately developed a hybrid ground

loop solution that was the first of its kind in the UK,

Geothermal loops were installed within both 100

linear meters of Energy Wall™ and 49No. Energy

Piles® that will ultimately deliver 150kW of peak

heating and 150kW of peak cooling to the hotel as

illustrated in Fig 12.

Conductivity

Test Box

Conductivity

Test Box

Fig. 12 Schematic of Energy Wall & Pile Scheme IMPLICATIONS ON DIAPHRAGM WALL REINFORCEMENT DESIGN AND DETAILING. One of the most significant considerations for CSL was to ensure that the integration of the GSHP system into the diaphragm wall, in whatever form, would not have any adverse impact on the construction process and quality of the completed diaphragm wall panels. One of the principle factors affecting diaphragm wall panels is to ensure that the construction process is as continuous as possible, particularly following panel excavation and the subsequent bentonite cleaning process. The diaphragm walls were reinforced with pre-fabricated cages in three sections which required splicing together during cage construction. The cage splice zones were located at suitable locations to avoid significant bar congestion in the areas of peak bending moment. The heaviest reinforcement consisted of paired B40 bars at 175mm centres, so attempting to form splices with paired bars of this size would have caused considerable additional difficulty in splicing the cages in a timely manner during panel construction. An additional constraint on splice location was the relatively tight access from Knightsbridge into the site. The access restrictions lead to a maximum practicable cage length of 15m. Complete cages were typically formed from two 15m sections and a 5m section (including the splice lengths). The original slinky pipework concept would have required GSHP pipework to be prefabricated onto steel mesh and site fixed to the far face (FF) of the diaphragm wall. The FF is that face of the diaphragm wall panel against the retained soil with the near face (NF) being on the internal basement side of the panel. The GSHP pipework could not be pre-fabricated onto the reinforcement cage as these are transported on their back (i.e. far face down) which would have resulted in potentially significant damage to the pipework which may not have been easily evident unloading / installation of the cages. A further additional potential difficulty with this solution was the connection

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. Various construction details associated with the original concept gave CSL & GI significant concerns. The prefabricated mesh arrangement and horizontal pipework terminating in fairly large box outs all lead to additional congestion within the diaphragm wall reinforcement cage which through structural requirements was already fairly congested. An example of the potential effects significant cage congestion can have on construction quality is illustrated in Fig. 13 below. The detailing of cages with a relatively small aperture size can lead to “pillowing” of the concrete as flow between bars is restricted, rather than flow being uninhibited and flowing to the extremities of the panel excavation.

Fig. 13 Concrete quality defects due to reinforcement cage congestion The solution favoured by both CSL and GI was an adaptation of techniques developed for Energy Piles™ constructed under bentonite support fluid. In essence the geothermal loops are site fixed to the outside of the far face reinforcement during cage placement. The geothermal loops are fabricated at GI‟s facility in Coventry under factory controlled conditions. The loops are then pressure tested to assure their quality at this stage of the process. The loops are then coiled ready for dispatch to site. On site the coiled loops are then placed onto a drum arrangement (as shown in Figures 15, 16, and 17 below) to enable the loops to be fed out and fixed onto the reinforcement cage as it is lowered into the prepared panel. Generally two loops were installed in each panel (the exception being the

Fig. 14 Typical reinforcement and geothermal loop configuration corner panels and those adjacent to 199 Knightsbridge). Each loop comprises a flow and return line, there therefore being a total of 4 No. pipes fixed to the reinforcement cages. To accommodate the loops the FF cover was increased and the longitudinal reinforcement arrangement altered such that there was no net increase in the degree of cage congestion from that detailed for cages not required to accommodate the geothermal loops. Where diaphragm wall panels were deepened to carry vertical loads an additional length of cage was installed to take advantage of the extra geothermal capacity afforded by the geometry. This additional length of cage was detailed to be a light as possible whilst maintaining sufficient robustness for handling and placing operation. The typical cage reinforcement and geothermal loop arrangement is shown in cross section in Fig. 14. The basic method of loop installation adopted had been used on previous piling contracts and had been refined to ensure that the panel construction cycle took no longer than if the geothermal loops were not installed. This is critical to ensure that

panel construction is undertaken with an absolute minimum of delay to maintain a high quality finished product. Once the reinforcement cage was installed to the correct level, the loops were then pressurised to test their integrity and ensure that no damage had occurred during the installation process as shown in Figure 18. The pressure was maintained during concreting of the panel and held until the following day. The level of pressure testing adopted gives the best guarantee of a future system performance.

Fig 15 Feeding Geothermal Loop onto Reinforcement Cage

Fig 16 Geothermal Loop and Reinforcement Cage Installation

Fig 17 Geothermal Loop and Reinforcement Cage Installation

Fig 18 Pressure Testing Geothermal Loops 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 effects being reported. The UK unfortunately has lagged behind and only in recent years is catching on to the benefits associated with this simple technology.

Brandl 2006 – reported on several projects across Austria and concluded that shaft resistance, base pressure and bearing resistance of soil are not affected by heat absorption and that temperature induced settlement or heave is negligible Laloui 2006 – Identified that the heating-cooling process of the building foundations induces significant modifications in the soil-structure leading to additional stresses in the piles, decrease of the lateral friction and the possibility of a gap between the pile and the soil Bourne Webb et al 2009 – concluded that temperature change in piles leads to increases and decreases in shaft resistance and axial load. Working stresses in pile should be kept low, and maintain high factor of safety on shaft to withstand heating and cooling loads Temperatures within geothermal loops will range gradually between -1°C and 30°C over a 12 month period as the season changes from winter heating dominant, to summertime cooling dominant. Over a single 24 hour period the ground loop temperature is unlikely to change by anything greater than 8°C, thus the likely effects compared to the thermal effects imposed on an external façade in spring time, when temperatures can range from below freezing in the morning to a high midday temperature can be considered to be minimal. CONCLUSIONS The requirements for the Knightsbridge Palace Hotel development have led to a UK construction first with the successful construction of Energy Walls™ for basement construction (i.e. incorporation of geothermal loops within diaphragm wall panels). The wide range of expertise and techniques employed by CSL, GI and WSP combined with the close relationships developed with the Client‟s team have resulted in the construction of a first class project. Good early coordination between all parties enabled the successful installation of geothermal loops within diaphragm walls and piles with no additional time being needed to be added to the construction programme. Loop layout within reinforcement cages in both

Energy Pile™ and Energy Walls® requires careful

coordination

Careful consideration needs to be made for future

penetration requirement for incoming and

outgoing services

Once operational, daily loop temperature fluctuations will be considerably less than exposed

concrete during a winter‟s day. Work currently continues on site to link up loops in diaphragm wall panels and piles in a similar vein, with good coordination with the ground worker The secondary usage of the structural element as

thermal mass enhances the sustainability

credential of the development.

Fig. 19 Architects‟ impression of completed development ACKNOWLEDGEMENTS

The authors wish to thank Squire and Partners, the Architect along with all the other members of

the professional team and especially the Client Prime Developments Limited for their kind permission to publish this paper.

REFERENCES Bourne Webb, PJ et al (2009) Geotechnique 59

No3 237-248 Energy pile test at Lambeth College,

London: geotechnical & thermodynamic aspects

of pile response to heat cycles

Laloui, L., Nuth, M. & Vulliet, L. (2006). Experimental and numerical investigations of the behaviour of a heat exchanger pile. Int. J. Numer. Anal. Methods Geomech. 30, No. 8, 763–781 Brandl,H (2006) Geotechnique 56 No 2, 81-122 Energy Foundations and other thermo-active ground structures.