Paper Presented to GE NCE Basement & Structures Conference October 2010

1. 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 water tightness criterion, the structural basement
scheme 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 were
basement is proposed to house a double height constructed up to 36m below platform level.
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 Fig. 1 Site location plan

2. 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:- Fig. 2 Design borehole profile
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. 3 SPT „N‟ vs. Level

3. 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. 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 Fig 5 3D section showing superstructure and
down construction method also minimises the basement through ball room area.
ground movement to the surrounding properties
which was a major consideration on this project.
Plunge column piles
Diaphragm walls
Plunge column piles involve large diameter piles
The 800mm thick diaphragm walls are installed in incorporating steel column sections. Installation of
large panels with geothermal loops attached to the these steel sections required specialist plunge
outside face of the cage near the external surface. frames to control the position and verticality of the
The external cover was increased to maintain steel section when being plunged into the
75mm of cover to the loops. The geothermal loop concrete before it sets. Where geothermal loops
diameter is relatively small and thus has no are also incorporated in the piles, the loops are
significant effect on the bending and shear installed onto the cage as it is being lowered into
capacity of the pile wall. Future penetrations position; the loops have to be secured to ensure
through D-wall for incoming and outgoing services the pipes are not snagged during the installation
require careful consideration to avoid damaging of the plunge columns. The free ends of the loop
the 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 of
The top of the Diaphragm wall requires trimming geothermal loops and can be ignored in the
and cutting down to receive the reinforced capping capacity calculation.
beam. Care is required to ensure the loops are not

4. Fig. 6 Diaphragm Wall Panel Layout
DIAPHRAGM RETAINING WALL
Diaphragm wall panels were constructed using a
Diaphragm wall panels were designed for retained crane mounted hydraulic grab with bentonite
heights 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 mounted
The contract specification required the diaphragm on a rotator which enables the grab to be easily
wall 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 incorporates
high water pressure behind the retaining wall, on-board instrumentation which provides both an
achieving tight construction tolerances for the instantaneous graphical output of grab and panel
diaphragm wall panels was vital. Verticality verticality and also records this data (together with
tolerances of better than 1:200 were routinely other relevant information) to form part of the as
achieved. Diaphragm wall panels were built record information for each diaphragm wall
constructed using C32/40 concrete. panel.
A drained cavity liner wall was required to achieve In addition to supporting the basement excavation
the final Grade 3 environment required for the and excluding groundwater, the diaphragm walls
completed basement. also carried significant vertical superstructure
loads. The magnitude of vertical load meant that
certain elevations required the diaphragm wall
39 No. 800mm wide diaphragm wall panels were panels to be constructed to deeper toe levels than
constructed to form the structural basement walls, was required from consideration of stability alone.
as shown on Fig 6 below. The overall length of
diaphragm basement wall was approximately 155
linear meters.

5. 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 Construction
GROUND SOURCE HEAT REQUIREMENTS OF
THE SCHEME
Fig. 8 Schematic “slinky” pipe arrangement
From an early stage the KPH scheme included a
requirement 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 significant
structures being constructed to support the
concerns in terms of system resilience as a result
superstructure and retain the ground / exclude
of the potential complexity of the pipework
groundwater. Whilst GSHP systems have been
incorporated into a limited number of diaphragm geometry and the additional connection details
which would be required along with creating
walls on the continent, this was a first in the UK.
significant programme implications compared to
One elevation of the basement wall was not
potentially simpler alternatives. Both Cementation
required to include any GSHP capability since the
Skanska Limited (CSL) and Geothermal
immediately adjacent property (199 Knightsbridge)
International (GI) have a considerable track record
has a three level basement car park which would
significantly reduce the efficiency of a GSHP installing GSHP pipework into piles of various
types (e.g. large diameter rotary piles [constructed
system along this elevation.
both dry and under bentonite support fluid], CFA
piles, driven cast in-situ piles etc.). Taking the
The additional depth of diaphragm panel required
experience from these previous schemes CSL
to support vertical superstructure loads gave
additional opportunity to enhance the available and GI working closely together developed a
capacity of the proposed GSHP system through solution which maximised the ground sourced
heating and cooling potential of the diaphragm

6. wall (with geometry etc, determined by the Conductivity
Test Box
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 Fig. 10 Stage One of the Conductivity Test
load 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 Box
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 Fig. 11 Stage Two Of The Conductivity Test
in this way) on the conductivity values needed to
be taken into account within the ground loop The second stage of conductivity testing will then
design. A review of available geothermal literature enable GI to compare and assess any reduction in
revealed that there were no papers dealing with the conductivity values, and ultimately assess the
this effect and thus it was imperative that GI levels of reduction arising from the basement
design a scheme using some very conservative excavation that needs to be taken into account
conductivity and resistivity values for the loops when designing ground loops within future
installed within the diaphragm wall. From an early basements.
stage it was GI‟s intention to undertake a two part
study into the effects of geothermal loops installed GI and CSL ultimately developed a hybrid ground
within 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 100
conductivity test prior to excavation of the linear meters of Energy Wall™ and 49No. Energy
basement 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.

7. 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 the
Scheme original concept gave CSL & GI significant
concerns. The prefabricated mesh arrangement
IMPLICATIONS ON DIAPHRAGM WALL and horizontal pipework terminating in fairly large
REINFORCEMENT DESIGN AND DETAILING. box outs all lead to additional congestion within
the diaphragm wall reinforcement cage which
One of the most significant considerations for CSL through structural requirements was already fairly
was to ensure that the integration of the GSHP congested. An example of the potential effects
system into the diaphragm wall, in whatever form, significant cage congestion can have on
would 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 small
diaphragm wall panels. One of the principle aperture size can lead to “pillowing” of the
factors affecting diaphragm wall panels is to concrete as flow between bars is restricted, rather
ensure that the construction process is as than flow being uninhibited and flowing to the
continuous as possible, particularly following extremities of the panel excavation.
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 Fig. 13 Concrete quality defects due to
restrictions lead to a maximum practicable cage reinforcement cage congestion
length of 15m. Complete cages were typically
formed 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 to
required GSHP pipework to be prefabricated onto the outside of the far face reinforcement during
steel mesh and site fixed to the far face (FF) of the cage placement. The geothermal loops are
diaphragm wall. The FF is that face of the fabricated at GI‟s facility in Coventry under factory
diaphragm wall panel against the retained soil with controlled conditions. The loops are then
the near face (NF) being on the internal basement pressure tested to assure their quality at this stage
side of the panel. The GSHP pipework could not of the process. The loops are then coiled ready
be pre-fabricated onto the reinforcement cage as for dispatch to site. On site the coiled loops are
these are transported on their back (i.e. far face then placed onto a drum arrangement (as shown
down) which would have resulted in potentially in Figures 15, 16, and 17 below) to enable the
significant damage to the pipework which may not loops to be fed out and fixed onto the
have been easily evident unloading / installation reinforcement cage as it is lowered into the
of the cages. A further additional potential prepared panel. Generally two loops were
difficulty with this solution was the connection installed in each panel (the exception being the

8. Fig. 14 Typical reinforcement and geothermal panel construction is undertaken with an absolute
loop configuration minimum of delay to maintain a high quality
finished product.
corner panels and those adjacent to 199
Knightsbridge). Each loop comprises a flow and Once the reinforcement cage was installed to the
return line, there therefore being a total of 4 No. correct level, the loops were then pressurised to
pipes 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 during
To accommodate the loops the FF cover was concreting of the panel and held until the following
increased and the longitudinal reinforcement day. The level of pressure testing adopted gives
arrangement altered such that there was no net the best guarantee of a future system
increase in the degree of cage congestion from performance.
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 Fig 15 Feeding Geothermal Loop onto
been used on previous piling contracts and had Reinforcement Cage
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

9. Fig 16 Geothermal Loop and Reinforcement Fig 18 Pressure Testing Geothermal Loops
Cage 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 effects
Fig 17 Geothermal Loop and Reinforcement being reported. The UK unfortunately has lagged
Cage Installation behind and only in recent years is catching on to
the benefits associated with this simple
technology.

10. Brandl 2006 – reported on several projects across concrete during a winter‟s day. Work currently
Austria and concluded that shaft resistance, base continues on site to link up loops in diaphragm
pressure and bearing resistance of soil are not wall panels and piles in a similar vein, with good
affected by heat absorption and that temperature coordination with the ground worker
induced settlement or heave is negligible The secondary usage of the structural element as
thermal mass enhances the sustainability
Laloui 2006 – Identified that the heating-cooling credential of the development.
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 Fig. 19 Architects‟ impression of completed
greater than 8°C, thus the likely effects compared development
to the thermal effects imposed on an external
façade in spring time, when temperatures can ACKNOWLEDGEMENTS
range from below freezing in the morning to a high
midday 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 Client
CONCLUSIONS Prime Developments Limited for their kind
permission to publish this paper.
The requirements for the Knightsbridge Palace
Hotel development have led to a UK construction REFERENCES
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expertise and techniques employed by CSL, GI
and WSP combined with the close relationships of pile response to heat cycles
developed with the Client‟s team have resulted in
the 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 the
successful 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, 763–781
being needed to be added to the construction
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Loop layout within reinforcement cages in both ground structures.
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