PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering

Stanford University, Stanford, California, February 11-13, 2013

SGP-TR-198

DEVELOPMENTS IN THERMAL PILE AND THERMAL TUNNEL LININGS FOR CITY

SCALE GSHP SYSTEMS

Nicholson D. P., Chen Q., Pillai A., Chendorain M.

Ove Arup and Partners Limited

13 Fitzroy Street

London, W1T 4BQ, England

duncan.nicholson@arup.com

ABSTRACT

The city scale development of horizontal trench

closed loops and vertical borehole loops for ground

source heat pump (GSHP) systems is limited by the

lack of ground surface space. This paper discusses

the use of thermal piles below city buildings.

Consideration is given to the development of

standards that maintain compatibility between the

M&E, geothermal, and structural designs. The

monitoring requirements are discussed.

Cities also use tunnels to provide transportation

and utility networks. Some of these tunnels must be

cooled to meet operational requirements. This paper

discusses the use of system-wide thermal tunnels

with closed loops embedded in the tunnel segments.

Water is circulated through the loops to cool the

tunnels. The geothermal design of the loops and the

impact on the structural performance is considered.

The heat extracted from the tunnel is then used with

GSHP systems to heat adjacent buildings via a

district heating system. When planning a thermal

tunnel system the future use of the heat for buildings

must be investigated.

1 INTRODUCTION

The limited availability of open land in cities reduces

the use of horizontal trench closed loops for ground

source heat pump (GSHP) systems. Vertical

borehole closed loops systems use space more

efficiently, but still require open areas next to

buildings. City developments are often high rise

buildings with piled foundations and basement car

parks. Thermal pile closed loop systems provide

another type of ground heat exchanger for GSHP

systems. The pile depths are generally controlled by

building structural loads rather than the heat storage

requirements and therefore for tall buildings thermal

piles are best used in combination with other heating

and cooling systems. The design, installation and

operation of thermal pile systems are more onerous

than vertical borehole loops because of the

temperature effect on the piles structural

performance. This paper considers the recent

develops in thermal pile design and installation

standards to ensure compatible structural and

geothermal designs.

In cities, tunnels provide access for rail, road and

utilities. They can also be used as ground heat

exchangers for GSHP systems. These tunnels can be

considered as cold and hot tunnels. The cold tunnels

access heat from the surrounding ground and the air

inside the tunnel is similar to the ground surface

temperature. The hot tunnels access heat from the

surrounding ground and also equipment inside the

tunnels, such as trains and HV cables. These hot

tunnels also require air ventilation for cooling to keep

the tunnels within the operational temperature range.

This paper considers the design of thermal energy

segments (TES) for thermal tunnels. These segments

are installed system wide and therefore studies are

required for future connections to surface and district

heating systems to supply heat to adjacent buildings.

2 THERMAL PILES

This section discusses the background to the UK‟s

development of a range of thermal pile installation

methods. The contractual arrangements for designing

and constructing thermal piles are discussed. Full

scale pile tests have been undertaken and this has

lead to the development of design methods.

2.1 Background

The thermal (energy) pile development in the United

Kingdom is summarised in Table 1. Early thermal

pile designs and construction work relied on Brandl

(2006) and Austrian contractors to assist UK piling

contractors. However, from about 2005 the design

and construction work was largely undertaken by

Cementation Skanska Ltd. and Geothermal

International Limited. Figure 1 shows the UK‟s

increased use of thermal piles between 2005 and

2010.

Table 1: Milestones for thermal pile development

in UK

Date Milestone Reference

2002 Brandl - Rankine lecture

(2002)

Brandl (2006)

2002 -

2005

Early projects – Keble

College

Suckling

(2004)

2002 -

2005

PII - Ground storage of

heat energy

Arup (2005)

2005

onward

Cementation / GIL

projects

Amis et al

(2009)

2007 Lambeth College pile

test, (2007)

Bourne-Webb

et al (2009)

2010 NHBC Piling for Houses

Guide

NHBC (2010)

2012 GSHPA Thermal Pile

Standard

GSHPA

(2012)

Figure 1: Thermal piles installed in UK (2005 –

2010

In the UK and USA, Cementation Skanska Ltd have

the UK Trademark No. 0606293.9 and the US

Trademark No 77704419 on the term “energy pile”.

Therefore the UK industry has adopted the term

“thermal pile” to avoid conflict with trademarks.

2.2 Installation

The early bored, cast in-situ thermal piles used full

length cages with plastic pipes attached inside the

reinforcement cages. The small pipes were bent

around tight radii at the top and bottom of the cages.

Since about 2005, the borehole U-connectors have

been used at the top and bottom of the cages to

maintain pipe continuity. Since 2010 the pile cover

has been increased and the pipes are being mounted

on the outside of the cages. This simplifies the

thermal loop installation on long cages which have to

be spliced together.

The use of U-connectors allowed bundled pipes to

extend below short cages in dry bored piles. Since

about 2010, lantern spacers have been used to place

the pipes around the perimeter of the piles, see Figure

2. Consideration is also given to the impact of

freefalling aggregate into the pile.

Figure 2: Short cage with borehole thermal loop:

a) Bundled, b) Lantern spacers to

place pipes round pile perimeter, c)

Borehole connecter showing minor impact

damage from falling concrete

In continuous flight auger (CFA) piles, double U

tubes have been pushed into the fluid concrete using

a central 32mm bar to depths of up to 25m.

2.3 Contractual Responsibilities

In the UK the ICE (2007) Specification for Piling and

Embedded Retaining Walls (SPERW) is often used

for pile design. This considers both Engineer and

Contractor designed piles. The GSHPA (2012) have

produced a Thermal Pile Standard which extends the

SPERW to incorporate the roles and interfaces for the

M&E and Geothermal Designers, see Figure 3. The

construction interfaces during construction for Piling,

Geothermal, Ground-works and M&E Fit-out

Contractors are also shown. There is a need for a

Main Contractor coordination role. The roles and

responsibilities of the different organisations during

the design and construction process are shown on

Figure 4 for Engineer designed piles.

Figure 3: Roles and responsibilities for engineer

designed piles

Figure 4: Design and construction process for

engineer design piles

2.4 Thermal Pile Testing

As part of the site investigation, the conventional

borehole thermal response test (TRT) has been

extended to assess pile thermal effects. The borehole

is about 300mm diameter and backfilled with

concrete to form a mini pile. Extensometers, strain

gauges and piezometers have been installed to assess

the thermal expansion, concrete stresses, and pore

pressures induced in and around this mini pile. These

can then be back analysed and scaled up to the

contract piles. The test also provides data on the

thermal conductivity of the soils.

As part of the contract work, full scale thermal pile

testing has been undertaken on well instrumented

bored piles. The main example in the UK is Lambeth

College, described by Bourne Webb et al (2009).

The static loading, cooling and heating cycles are

reproduced in Figure 5. This information has been

used to develop the thermal pile design processes.

Figure 5: Lambeth College pile test

2.5 Thermal Pile Design

The tasks for the thermal pile design team are

summarized in Figure 6.

Figure 6: Tasks undertaken by designers

The M&E Designer is responsible for defining the

annual heating and cooling profile to the heat pump.

This must consider variations in summer heat wave

and winter cold periods.

The Geothermal Designer for the ground source

system is responsible for assessing the number of

thermal piles that are needed to keep the temperature

above freezing. The GSHPA (2012) Standard

requires that the pile soil interface does not drop

below freezing point. It recommends a minimum

circulation fluid temperature of 2°C. This

temperature may be reduced where detailed modeling

is carried out on transient effects and pipe cover /

spacing. The maximum fluid temperature will

depend on the stresses induced in the pile but may be

up to 40°C. The temperatures should be agreed with

the pile designer. Conventional borehole loop design

packages such as Earth Energy Designer (2012) or

Pilesim (Pahud, 2007) have been developed to

include pile effects and assess the variation of fluid

temperatures with time.

The Pile Designer is responsible for assessing the

thermal effects on the pile. Where the pile is fully

restrained, additional compression stresses will be

mobilized at the head of the pile. Where the pile is

unrestrained by the surrounding soil there will be

movement of the building, see Figure 7. In practice,

the pile behavior will be between these extremes.

Arup have developed a thermo-hydro-mechanical

finite element soil model using the LS DYNA code to

assess the heating of the pile and the surrounding

ground. The changes in stresses and the load bearing

capacity of the soil are assessed. The changes to the

water pressure in clay soils with time are also

considered. This model has been calibrated against

the Lambeth College pile test. In addition, the

OASYS PILE program has been modified to

incorporate the expansion of the pile and restraint

from the surrounding soil (Bailie, 2012). Some

results for the end of cooling and heating stages of

the Lambeth College pile test are shown in Figure 8.

This work has been undertaken with Cambridge

University.

Figure 7: Restrained and unrestrained pile model

Figure 8: Lambeth College back analysis – Measured

and predicted pile axial loads

The seasonal expansion and contraction of the

thermal pile has also been considered, see Figure 9.

This is compared with the vertical cyclic loading of

piles and the effect on factor of safety, Poulos (1989).

The accumulative pile head settlement has also been

considered based on Matlock and Foo (1980).

Details are given in GSHPA (2012).

Figure 9: Seasonal cyclic expansion of pile leads to

increased movements

2.6 City Scale

On a city scale, the existing London underground

system has experienced a long term increase in the

temperature within tunnels and in the surrounding

ground, Botelle et al (2010). The Crossrail project

has introduced thermal piles and walls systems into

the station boxes. These will be available for heating

the overlying site developments above the stations

and will help to cool the ground and the adjacent

underground.

In the UK at present, only open GSHP systems,

which use advection, are regulated to achieve a

balanced annual heating and cooling load. The

schemes must be approved by the Environmental

Agency to ensure that existing abstractors are not

affected. Closed systems rely on conduction and are

not regulated.

3 THERMAL TUNNEL LININGS

This section discusses the background to the

development of thermal tunnels using thermal energy

segments (TES) in the UK. Additional background

information is given in the paper by Franzius and

Pralle (2011).

3.1 Concept

The concept for the thermal tunnels is shown in

Figures 10 and 11. The TES use plastic pipes

embedded within either the steel cage or fibre

reinforced segments. The plastic pile is PEXa grade

to enable tight bed radii to be formed and ensure 100

year durability at 15 bar pressures. The pipes also

enable permanent mechanical connections to be

formed in the segment box-outs, see Figure 11. The

box-out also allows for the pipe extension /

compression if joint rotation occurs.

Figure 10: The thermal tunnel concept

Figure 11: Thermal energy segments and box-out

connections

The ring to ring connections have to allow for the

segment rotation to allow the tunnel to be steered.

This flexibility is provided by using plastic pipes

mounted on the segment surface. Intermittent access

needs to provide for the header pipe connection, see

Figure 11.

The header pipe connections occur at about every

fifth segment ring. A reverse return system is used to

balance the head loss around all the ring circuits, see

Figure 12. The header pipes transfer the flow back to

the surface connection points at between 250m and

400m centres. Larger spacing of connection points

should be avoided to control hydraulic head losses.

The surface connection points can be provided at

shafts, ends of the stations, cross passages or possibly

from adjacent boreholes, see Figure 10. Where

possible, a plant room is provided at ground level to

incorporate circulation pumps and de-airing systems.

Heat exchangers can be provided if the tunnel system

is to be hydraulically separate from the building

circulation system.

Figure 12: Header piles with reverse return and

surface connections

3.2 Design Considerations

The efficiency of the thermal tunnel is influenced by

the air temperature inside the tunnel as well as the

surrounding ground temperature. Short-run tunnel air

temperatures are often controlled by the surface air

temperature and are called “cold” tunnels, see Figure

13. Longer metro and cable tunnels develop high

internal air temperatures and are called “hot” tunnels,

see Figure 13. For example, the motors in a London

Crossrail trains will emit about 1MW of heat and one

train passes at 2.5 minute intervals during peak times.

Air conditioning adds an extra 0.1MW. At peak

times this is about 22W/m2 and on a weekly average

this is 13W/m2. Braking adds to the heat generation.

The ground permeability and hydraulic gradient can

also affect the thermal tunnel efficiency.

Figure 13: Cold and hot tunnels

The thermal tunnel design is based on modeling the

heat transfer from the tunnel air and the surrounding

ground to the water filled pipes in the segments. The

LS DYNA numerical model used to assess this

transfer is shown in Figure 14. The model has been

calibrated against periods with TES heat extraction

rates of zero, 10W/m2 and 30W/m2 and the effects of

the circulation water are modeled, see Figure 15. The

model includes the heat load from the trains and the

temperatures in the surrounding ground.

Figure 14: TES model showing heat flow from tunnel

to pipes and soil

Figure 15: Temperature variation in the pipe – for

zero, 10 and 30w/m2 continuous

extraction rates

The stress in the concrete segments and the

surrounding ground was also studied in LS DYNA

using a Thermo-Hydro-Mechanical (THM) soil

model, see Figure 16. Some results are shown in

Figure 17. These results highlighted that the hoops

stress increased by about 7% due the heating and

expanding the lining and the ground. compared with

the „no heating‟ case. The circulating water also

induced thermal gradients and stresses across the

segments. This lead to a further 2% increase in hoop

stress. Local tensile stresses were induced next to the

pipes due to the cooling effects.

Figure 16: FE model used for stress analysis of

tunnel segments with pipes and

surrounding soil

Figure 17: Comparison of maximum principal stress

distribution in segments during summer

A tunnel heating and ventilation program was used to

calculate the combined effect of trains running

through the tunnels. It included allowances for heat

given off at station stops, braking heat during entry to

the stations and acceleration heat when exiting

stations, see Figure 18. The heat extraction from the

TES was also included and some results are shown in

Figure 19. These show that in summer the tunnel air

temperature is reduced by about 5°C with the flow

and return water circulation water temperatures of 21

and 26°C in central London. This leads to a heat

transfer of about 10W/m². These water circulation

temperatures are high enough to be used with a heat

pump for domestic hot water.

Figure 18: Tunnel ventilation model extended to

include TES heat extraction

Figure 19: Predicted tunnel air and pipe

temperatures along Crossrail eastbound

tunnel during summer peak hours, with

and without TES installed throughout the

tunnel

The effect of train fire on the TES system was also

assessed. The majority of the segment pipe work is

embedded at sufficient depth to avoid fire effects, see

Figure 20. The pipes near the surface and at box-outs

were not possible to protect. Instead the short-run

tunnel length where a fire occurs would be isolated

from the system and not reinstated. The smoke and

fumes given off was assessed and considered to have

little effect. This is because ventilation systems

should have sufficient capacity to extract the smoke.

Figure 20: Fire modelling – Segments and pipes and

header pipes

3.3 Buildings connections and city scale

development

The thermal tunnel system requires an early

investment when the tunnels are being built to

provide pipe work for heating adjacent buildings.

This requires a city scale investment in long term

heat supply. For Crossrail, a preliminary study was

carried out for the buildings in a corridor extending

100m beyond the tunnels. These buildings were

assessed using the London heat map and aerial

photos. The buildings were divided into 3 tiers:-

Tier 1: 34 hotels, large residential, hospitals

Tier 2: 4 schools, colleges, libraries, museums

Tier 3: 327 offices, leisure centres, retailers

An example of part of the Crossrail route map with

building heat mapping is shown on Figure 21.

Figure 21: Comparison of Crossrail header pipe

access points and building heat map study

Based on header pipe access points connecting to

500m of twin tunnels, the heat output would be about

200 to 600kW for 10 to 30 W/m2 of tunnel surface

heat extraction. Examples of header pipe access

points are also shown on Figure 21. On a city scale,

an Energy Supply Company (ESCo) could be used to

market the distribution of this heat through local

district heating networks.

5 CONCLUSIONS

The main conclusions associated with thermal piles

are:

The development of thermal piles in the UK is

reviewed. The GSHPA Thermal Pile Standard

incorporates recent experience and provides

basis for their future use.

Thermal pile installation and testing has

progressed from long to short reinforcement

cages and installation into CFA piles.

The contractual roles and responsibilities for the

pile and geothermal designers are becoming

clearer and are linked to the existing piling

contracts. The interfaces during construction are

linked to handover testing.

The structural design methods are being

developed to assess concrete stresses and pile

settlements due to temperature changes.

At a city scale, thermal piles are used where

there is insufficient space to install vertical

GSHP borehole loops. They can support metro

systems where cooling the tunnel is becoming

increasingly important.

The conclusions associated with thermal tunnel

linings are:

The thermal tunnel concept and key components

are explained.

The design is linked to hot tunnels where heat is

extracted from the tunnel air and transferred to

the surface for domestic hot water production

and heating.

The DYNA modeling of the tunnel heat transfer

and segment concrete stresses is explained. In

addition the conventional tunnel heating and

ventilation system model is extended to include

the heat extracted by the circulating fluid. The

impact on cooling the tube is considered.

Fire protection is discussed but found not to be

appropriate for the surface pipe work.

At a city scale, investment is needed in the

tunnel pipe work. Early studies are required to

assess the way the buildings use tunnel heat and

to make the business case.

ACKNOWLEDGEMENTS

The work by the GSHPA thermal pile subcommittee

is acknowledged in the standard development. The

research work carried out by Cambridge and

Southampton universities has provided a framework

for instrumentation, monitoring and design.

The thermal tunnel work required a multi discipline

approach from within Arup / Atkins Crossrail team.

The continuing support to the project by Crossrail is

recognized, together with Mott MacDonald‟s input

on the H&V modeling and Rehau‟s work on the

pipework design.

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