Grouting Practices for Horizontally-Bored (Directionally-Bored, HDD) Loopfields

Saturday, May 15, 2010

Horizontally-bored loopfield installations are becoming a more widely recognized viable alternative to the vertically-bored loopfield configuration.  However, designers must be aware of the different aspects and limitations associated with HDD (horizontal directional drills) loop installation to properly design such a system.  Special attention must be paid to the installation depth, soil properties, pipe placement methods, and grouting practices during the HDD loop installation process. 

One major point of debate related to HDD loop installation is whether it is necessary to completely grout the holes after pipe placement.  Arguments against grouting HDD bores are:

  • Hole collapse around the pipe
  • The presence of heavy drilling mud left in the boreholes

Our response to each of these arguments is:

  • There is no way to guarantee that the native soil will collapse around the pipe uniformly and with the density needed to promote adequate heat transfer through system
  • Drilling mud and/or drill cuttings are not recognized by IGSHPA or the National Ground Water Association to be adequate grouting materials   

Grouting is the only way to ensure consistent thermal contact between the pipe and the earth to promote heat transfer.  In fact, in Section 7.7 of IGSPHA’s Ground Source Heat Pump Residential and Light Commercial Design and Installation Guide(2009), it is recommended that all horizontally-bored holes be grouted from end to end to ensure contact between the earth and GHEX piping and to protect the integrity of our environmental groundwater supply. 

Our experience with HDD loopfield installations further reinforces this concept.  We have dealt with numerous installations in neighboring states.  The majority of the problematic HDD installations we have encountered have been where the loop contractor did not grout the holes, but left them to bore collapse and thickened drilling mud for performance.  It must be stressed that the ultimate goal during construction is to build a heat exchanger.  Proper performance of that heat exchanger is critical for the performance of the system as a whole. 

In the interest of putting our best foot forward as an industry, best practice principles suggest that we should completely grout all boreholes, vertical and horizontal, and leave nothing to chance.