The large borehole annulus should be tightly and permanently backfilled to protect the pipes, to ensure good heat transfer from the rock to the pipes and finally to the circulating water-glycol fluid („brine“) and to stabilize the borehole wall.

For reasons of groundwater protection, the incursion of surface water should be prevented and a seal should be made between two different groundwater levels (Note: This case does not apply to Bavaria, where only drilling in the 1st groundwater level are permitted).

Thermal conductivity describes the material property of a material. The thermal conductivity has the unit Watt per Kelvin and meter (W/m*K) and describes how well thermal energy is transferred through a material.

With high thermal conductivities, thermal energy is transferred very well. This results in good system efficiency in geothermal systems and ensures good heat transfer in heating mode and rapid dissipation into the surrounding soil in cooling mode.

The freeze-thaw resistance change is important for the grouting material with regard to system operation. Possible frost actions result from the circulation of supercooled (temperatures below 0°C) water-glycol mixtures („brine“) and not due to the frost actions from the surface of the terrain (Note: The geothermal heat pipes have placed the frost-resistant below the natural frost incursion limit at approx. 1-1.5 m below ground level ).
The Freeze-thaw resistance change in building materials, such as Fischer GeoSolid® products, are not damaged by the temperatures occurring in the geothermal probe pipes by freezing excess water or ground water in the pores. This property is important to have a durable and stable backfill material around the geothermal probe in the borehole.

The borehole resistance is determined as a measured value in the so-called Thermal Response Test (TRT), which is carried out as a test on a geothermal probe at the planned plant location.

The borehole resistance consists of a value that is influenced by the probe geometry, the thermal properties of the grouting material and the influence of disturbances (possibly existing groundwater). Thermally improved grouting materials are favourable because it reduce the resistance of the borehole.

A high-quality grouting material prevents the leaching of harmful substances and their entry into the groundwater or the soil through good environmental compatibility.

Due to its good long-term stability (against frost effects from the geothermal probe or cementitious materials in the groundwater), it ensures a good connection of the probe pipes to the surrounding soil. The incursion of surface water is prevented, thus ensuring the protection of groundwater. The use of thermally improved grouting materials increases system efficiency by reducing the resistance between the borehole wall and the probe pipes. This ensures a better heat transfer.

Damage to geothermal boreholes has recently been caused by unprofessional work during the construction of geothermal probes or due to difficult geological and hydrogeological conditions.

If the grouting material is used professionally, unfavourable geological conditions can be compensated under certain circumstances (e.g. safe closure of a borehole in which pressurised groundwater rises to the surface).

There are, however, geological conditions in which the construction of geothermal probes should generally be avoided. If problems occur during the construction phase, a balance must be achieved so as to check whether these can be controlled with professional technology or whether drilling must be stopped. The decision on this is made by the approval authority.