Environmental Engineering Reference
In-Depth Information
the groundwater against the borehole with the filter cake caused by excess
pressure in the borehole, opposed to the surrounding soil. For jetting purposes,
water is normally used, sometimes with certain added substances (e.g. ben-
tonite; see also DVGW-Guidelines W 115 and W 116); this supports the jetting
in adopting the described characteristics and fulfilling its tasks better.
−
The borehole hammer method has been largely accepted. Air driving the bore
hammer and transporting the drillings above ground at a rate of ascent of 15 to
20 m/s is used for jetting. Depending on the borehole hammer, air pressures of
over 10 bar and air volumes of over 10 m3/min are required /9-4/. If necessary,
a foaming agent can be added to the air to enable an improved transport of
drillings and to avoid detritus in the borehole.
After sinking the ground probe into the borehole, it has to be filled up again in or-
der to guarantee a good heat transfer between soil and probe. The filling can be a
bentonite-cement suspension.
There have been attempts to also apply the method of direct evaporation for
ground probes instead of using a heat carrier circuit. Around 1990, several of
these systems were built in Austria and the USA. Various problems occurred e.g.
with the return of the compressor oil and the large amount of filling, with then still
largely utilised working media that were damaging for the ozone layer. These
problems stopped this development. Recently, direct evaporation has been dis-
cussed again. Nowadays, ammonia is used as a working medium. A pilot plant
was built in Coswig near Dresden, Germany.
A more promising new development in the field of ground probes is their de-
sign as a heat pipe and the use of CO
2
. Thus using water-damaging antifreeze
compound can be avoided. Energy in the circulation pump is saved due to the heat
pipe functionality and the disadvantages of direct evaporation are avoided by
separating the heat pipe and the refrigerant circuit. Ground-coupled heat pumps
with such heat pipes are already run successfully in Upper Austria.
Components with earth contact (energy piles, slot-die walls).
A further variation
of vertical ground-coupled heat exchanger are the heat transfer piles, so-called
"energy piles" /9-8/, /9-9/. They are foundation piles, used for difficult subsoil
conditions for laying the foundation of buildings. These piles are equipped with
heat transfer tubes and allow the installation of ground-coupled heat exchanger at
low additional costs in locations where foundation poles have to be used anyway.
Energy piles can basically be combined with all well-known geotechnical
structure-on-pile foundation methods. So far, cast-in-situ piles (Bore piles) and
ready-made piles (ramming piles) from reinforced concrete with a full cross-
section and hollow piles as well as steel piles have been used. Every pile type has
specific advantages and disadvantages. Cast-in-situ concrete piles are very flexi-
ble, but from a technical and an economic point of view should only be used start-
ing at a minimum diameter of approximately 600 mm. Their production is quite
cost-intensive and requires a lot of care. Ramming piles are easy to produce in a
factory; however, during the ramming process adequate protection for the tube
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