Geothermal energy (absorber piles) for heating and cooling buildings

		1. Cast-in-situ piles	4. Piled retaining walls
		2. Precast piles	5. Diaphragm walls
		3. Hollow piles	6. Foundation slabs

Bored piles:

Bored piles are normally used for larger diameters of 60 cm to 150 cm. These are also used for large depths. The bore can be produced with or without bentonite suspension or with a casing tube. Once the reinforcing cage has been installed the concrete is filled from below and at the same time the bentonite suspension (if used) is drawn off or the casing tube withdrawn.

Augered piles:

Augered piles are a special form of bored pile. Once the bore has been sunk by the continuous flight auger, the concrete is pumped into the bore through the hollow stem as the auger is removed. The reinforcing cage is pressed or vibrated into position immediately afterwards.

Mixed-in-place piles:

In the case of mixed-in-place piles a continuous flight auger is first used to bore down to the required depth. During the boring and auger extraction operation a cement suspension is added through the hollow stem (central tube) of the auger. This is mixed with the surrounding soil and thus forms the pile. Diameters are between 40 cm and 90 cm and depths are up to 22 m. Steel sections or rigid reinforcing cages are pressed into piles which are to take structural loads.

Cast-in-situ driven piles:

With this type of pile a casing tube is top-driven or vibrated into the ground, the reinforcing cage installed and the casing tube extracted at the same time as the concrete is filled and compressed. This type of pile is suitable for smaller diameters up to 60 cm and can be used for greater pile lengths.

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Steel-reinforced concrete driven piles:

Steel-reinforced concrete driven piles are precast in square cross sections of 30/30 cm to 40/40 cm and lengths of up to 14 m. Once transported to the construction site, the piles are driven. For handling reasons, these piles can only be fabricated and driven to a maximum length of 14 metres. In many cases, however, foundation depths are considerably greater with higher energy potential present in the lower ground layers. In order to be able to extract and utilize this energy, a pile joint was developed through which the inflow and outflow lines can be laid.

Available range of precast absorber piles:

Pile cross section

Pile length

Load bearing capacitiy

30/30 cm

up to 13 metres

up to 700 kN

35/35 cm

up to 13 metres

up to 1000 kN

40/40 cm

up to 14 metres

up to 1400 kN

30/30 cm (tapered)

up to 8 metres

depends on ground conditions

40/40 cm (tapered)

up to 13 metres

depends on ground conditions

(also available without enercret technology)

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Steel-reinforced concrete hollow piles:

Steel-reinforced concrete hollow piles are precast in the factory as hollow piles and come in round cross sections of 30 cm to 60 cm. Once transported to the construction site, the piles are driven. The piping used for heat transfer is installed before the cavities are filled with concrete.

Micro-piles:

Small-diameter micro-piles in steel or cast iron tubing are driven into the ground and then filled with concrete, or cement mortar is compressed to form a bond with the surrounding soil. The heat exchangers are installed before the piles are filled with concrete.

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Piled retaining walls are constructed with pile diameters of between 30 cm and 150 cm. There are three types of piled retaining wall:

Kingpost pile wall:

The number of piles required by the structural design are set at suitable intervals. The spaces between the piles are either filled with mixed-in-place piles prior to the excavation or with shotcrete during excavation of the construction pit.

Contiguous pile wall:

Here the piles are set directly next to one another.

Secant pile wall:

The non-reinforced primary piles are bored at suitable intervals and filled with concrete. The reinforced secondary piles are bored between them before the concrete of the primary piles sets so that they can intersect the primary piles on both sides. The amount of intersection is 10% to 20% of the pile diameter. Poorer quality concrete is used for primary piles.

Mixed-in-place walls:

Mixed-in-place piling is also used to stabilise construction pits. Depending on structural engineering requirements, steel sections or rigid reinforcing cages are installed at suitable distances apart. In the case of contiguous and secant bored pile walls, bore templates are manufactured before the boring operations are begun in order to ensure exact spacing. These are used to guide the casing tubes. Otherwise the usual pile boring methods apply.

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Depending on ground conditions and depth, diaphragm walls are constructed using either a grab excavator or milling machine. Depths of over 100 m are possible. Wall thicknesses are usually between 40 cm and 120 cm. With milling machines, wall thicknesses of up to 300 cm are possible. There are three methods of wall construction:

Two-stage method:

A non-hardening suspension (usually bentonite) is usually added during excavation to stabilise the walls of the excavated diaphragm segment. Once the diaphragm segment is completed the reinforcing cage is installed and the concrete is filled from below. The displaced bentonite suspension is drawn off for reuse.

Single-stage method:

This method is mainly used for pile diaphragm walls which are exposed to low structural loads. The slow-hardening cement-based suspension is not drawn off and thus becomes part of the wall. With this type of diaphragm wall no reinforcing cage is installed.

Combination methods:

Sealing elements such as precast concrete elements of the steel sheet pile sections are installed in the wall which has been constructed using the single-stage method.

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General:

The vast majority of buildings are constructed on foundation slabs, strip footings, column foundations or combinations of these foundation types. A subbase of lean concrete or fine gravel is almost always provided beneath these foundations. It is in this layer that the absorber lines can be laid, thus enabling the use of geothermal energy.

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