Pile Design and Construction Practice, Fifth edition

474 Miscellaneous piling problems
with a 5 m deep probe at each pile tip to locate cavities. The pile hole was extended by underreaming
where the probes located cavities, the tube re-driven as necessary to sound rock and
filled with concrete. The overburden sand, up to 20 m deep, was treated by vibroflotation to
improve the relative density to 85% to 90% in order to reduce liquefaction potential.
9.8 Energy piles
Ground temperatures in much of Europe are reasonably constant at 10�C to 15�C (and in the
tropics as high as 20�C to 25�C), below a depth of 10 m. This near-surface geothermal energy
potential is being exploited to provide a consistent low level, but cost-effective and environmentally
friendly, source of heating for buildings, using the thermal properties of the building
foundations. Concrete has a high thermal storage capacity and good thermal conductivity and
heat from the ground taken up by the pile, diaphragm wall or other foundation can be transferred
from the concrete to a heat exchanger coil buried within the concrete and moved by a
simple heat pump to heat the building. Conversely, in suitable soils the heat from the building
can be transferred to the concrete and ground for cooling during the summer. Brandl (9.42)
describes the heat transfer mechanisms in the ground and between the absorber fluid in the
exchanger pipework and the structural concrete and provides recommendations for the design
and operation of geothermal piles and other ‘earth-contact’ concrete elements.
The geothermal properties of the ground (thermal conductivity and capacity) and
groundwater flow and direction have to be determined for the complex heat exchange
calculations, but the pile diameter and length should be designed to resist the applied structural
loads and not increased to suit the geothermal requirements. The primary circuit within
the pile comprises absorber pipes of high density polyethylene plastic, 25 mm diameter and
2–3 mm wall thickness, formed into several closed-end coils or loops and fixed evenly
around the inside of a rigid, welded reinforcement cage for the full depth. Typically loops of
eight vertical runs would be provided in a 600 mm diameter pile. The geothermal effectiveness
of piles less than 300 mm diameter is much reduced due to lower surface area and the
limited number of loops which can be fitted; the economically minimum depth of an energy
pile is about 6 m. Each loop is filled with the heat transfer fluid, water with antifreeze or
saline solution, and fitted with a locking valve and manometer at the top of the pile cage.
This may necessitate off-site fabrication. The piling method must produce a stable hole for
the careful insertion of the cage and absorber pipework. Bored piles, with or without drilling
fluid support, or a cased or withdrawable tube method, are acceptable for most schemes.
Before concreting, the absorber pipes are pressurized to around 8 bar for an integrity test
and to prevent collapse due to the head of fluid concrete. The pressure has to be maintained
until the concrete has hardened and then re-applied before the primary circuit is finally
enclosed. Concreting should be by tremie pipe placed to the base of the pile to avoid
damaging the pipework. Plunging the absorber pipes, either as separate tubes or attached to
the reinforcement cage, into fluid concrete in a CFA pile is not currently recommended.
The primary circuits in each pile are connected via header pipes to manifold blocks which in
turn are connected usually through a heat pump to the secondary circuit embedded in the floors
and walls of the building. Using a heat pump with a coefficient of performance of 4 (the ratio
of the energy downstream of the heat pump to the energy input of the pump), the ground temperature
can be raised from 10�C–15�C to between 25�C and 35�C at the building. Depending
on soil properties and installation depth of the absorbers, Brandl notes that 1 kW heating needs
between 20 m 2 of saturated soil and 50 m 2 of dry sand in contact with the pile surface.