Site characterization and testing

As with any underground engineering works such as foundations, excavations, drainage or tunneling, information on geology, strata, structure, groundwater, geochemistry, underground infrastructure and potential contamination is essential when considering Borehole Heat Exchanger systems (BHE).

Starting of with a desktop approach, the inventory of relevant information can lead to the conclusion to conduct site testing, such as drilling a test borehole, water quality testing or carrying out a geothermal response test (GRT).

Insight in site characteristics is not only useful in design but is also essential to project planning and quantifying risks when implementing and operating BHE systems.

Minimal site information requirement

- Administrative information

- Regional and local geo(hydro)logical setting

- Geological strata, structure and texture

- Groundwater table, flow & direction, geochemistry

- Underground structures & cables

- Environmental site aspects

- Site specific or project related Health & Safety aspects

To backfill a borehole

After drilling, the loops are installed in the boreholes and to ensure long term (> 50 years) structural integrity, thermal conductivity and environmental security, the borehole needs to be securely backfilled from the bottom to the top.

Routinely boreholes are backfilled with a pumped bentonite/cement grout slurry. The clay based grout certainly does not benefit optimal thermal performance, but often does not even provide the assumed structural stability and environmental protection. This is especially the case where permeability of the strata (gravels, fissured formations etc.) in combination with a high groundwater table leads to dilution of the slurry and loss of the essential grout properties.

A similar loss of properties can occur when the geochemistry (salt water or sulfates) of the groundwater on site prohibits the swelling of the clays.

When using grouts, boreholes are pumped from bottom to top until the quality of the exiting material is comparable to that of the grout being pumped into the borehole. In situations with a high groundwater table this ensures large quantities of water and diluted grout to be contained.

Other site specific ways of backfilling boreholes are the use of coarse sand or gravels, clay plugs or pellets. Essential is optimal thermal performance, long term borehole stability and EA approval for the applied materials and methodology under the site specific environmental conditions.