Okay to (partially) fill borehole with cuttings?

Discussion in 'Vertical and Horizontal Loops' started by chrispitude, Mar 2, 2011.

  1. chrispitude

    chrispitude New Member

    Hi folks,

    I'm a homeowner getting a 4T ClimateMaster 27 installed in the next month or so. The well driller came out to the property yesterday to have a look at the lay of the yard. I asked about the grouting process, and he told me that he usually backfills most of the borehole with cuttings, then fills just the upper portion with grout. I asked if the cuttings would reduce the thermal transfer, and he said that this method is cheaper, plus his book says that soil actually has superior conductivity versus grout. I asked if I could look this up on the Internet somewhere, but he told me that he's old-school and does not own a computer with Internet access. I asked how high up the cuttings are used before he switches to grout, and he told me it depends on how the bedrock and casings play out.

    I hate to be the homeowner that second-guesses the contractor, but... should I be? Should I be asking for a fully grouted borehole? Are cuttings superior to grout for thermal conductivity?

    I haven't talked to the HVAC contractor yet. I wanted to educate myself before going into that conversation.

    - Chris
  2. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Lots of thought

    Firstly, there may be code requirements that supersede any engineering preferences. Meaning, it may just be code to fully grout in your area.

    But, the only time that I would be convinced to not grout would be if the pipe is fully submerged in water. And even then, I may have some thoughts about frosting causing pipe movement. One now has to have a good knowledge of aquifer(s) depths and vertical hydrology.

    Most of the time, geothermal drilling is not really a delicate procedure in comparison to say geotechnical or contaminated sites type drilling. In the latter two cases, we pay particular attention to soil statigraphy, moisture content and so forth. In geothermal, we are looking at production and just getting a hole in. So, we often don't have all the information one would need in order to justify partial grouting or not; thus, another default to full grout.

    Yes, the original soils may have better thermalconductivity/thermodiffusivity over grout, but that is in their native state. Air gaps, bridging, and so forth become issues when returning native materials to a borehole. So another reason to fully grout.

    If you are drilling in dry bedrock and trying to return that material back to the borehole, you are dealing with a poor material and I would differ back to fully grouting.

    I'm sure the driller would like to just use cuttings, but point him towards grouting. If you get resistance, then someone will have to sign off on the performance of the loop. That may be enough to change some minds.
  3. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    what he said plus 1
  4. Amazingly the majority of PA with the exception of here in Centre County has no water well or geothermal regulations at all. Now I believe in filling the borehole with material that is at least as thermally conductive and with similar permeability to what was taken out. This way you don't do ever any harm. The exception is if you ever cross aquifers it should be grouted and also the top 20-30' should always be grouted to stop surface water from entering the shallow water table.

    It's possible that cuttings can be used below the water table within a single aquifer and they can perform fine in that situation if placed appropriately so they don't bridge. Thermal grout, however, will perform just as well in 95% of the cases and is never a bad idea. However, many drillers in PA don't own a grout pump so they won't grout boreholes or often they don't own a pump capable of pumping thermal grout which is fine as long as the boreholes are sized accordingly (you may have to add up to10% to the length). The best you can often do in these cases is to get them to hydrate bentonite pellets in the top 20-30'.

  5. Texas Cooler

    Texas Cooler New Member

    How can you be certain?

    Every reference from IGSHPA requires grout to be used starting at the bottom and grouting up. The grout must also be mixed per the recommendations. It is not sufficient to just mix a slurry of grout and water and pump it down the hole. When your driller puts the cuttings back into the hole, how can he be certain that they pack all around the pipe without voids? You may have water in your well which is great but I've always found trying to circumvent established procedure usually ends up with unknown performance. (sometimes better, sometimes worse)

    Properly mixed grout is hard to pump without the proper equipment. Proper equipment is not inexpensive. Improper installation is less expensive up front and costs the homeowner more in utility costs long term.

    Your driller can order the book "Grouting for Vertical GHP Systems" directly from IGSHPA for $30. It is not available in a downloadable format so the internet excuse won't work.
  6. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    My experiance has been that the local powers that be have been sloooow to address any issue concerning ground loops. They are only marginally concerned with the aquifer and dis interested in loop performance. That is where IGSHPA comes in. The installation of grout ensures a predictable outcome in relation to performance based on original data, the side benefit is that it protects the aqifer beyond the local concern. It is a win win. As more drillers migrate to looping to feed their families there is no exc use that is plausible to support not following industry standards and making a capital investment in training and eqipment, period.
  7. GCI

    GCI Member

    Well said sir...
  8. SoundGT

    SoundGT New Member

    Cuttings are not a good idea and rareely get to bottom before bridging the hole. The borehole needs to be grouted for good thermal conductivity and to seal the borehole against cross contamination.

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