The Benefits and Detriments of Using Flowable Fill as a Grout Material in Geothermal Boreholes

     Flowable Fill is a construction material that is mixed in a concrete batch-plant and delivered in ready-mix concrete trucks to a construction site where it is commonly used to fill in voids in soil and bedrock to achieve an adequate bearing strength to support a building.  It is a mixture of predominately sand, Portland cement, and water.  A proprietary liquefying agent is added to make the mixture flow readily with a low viscosity.  In geothermal boreholes that encounter open bedrock fissures and solution openings that would drain away standard or thermally-enhanced bentonite grouts and cement grout, flowable fill is used as a bridging agent to fill the open voids and the geothermal borehole.  Due to its sand and cement content, it has a high thermal conductivity.  Because it is readily available from most concrete batch plants, requires no equipment on site to emplace it in the boreholes, will bridge and fill large bedrock solution openings, and has a high thermal conductivity, it is a popular replacement for cement and bentonite grouts in problem boreholes.  Flowable fill is poured into the top of a geothermal borehole directly from the delivery chute of a ready-mix concrete truck without the use of a tremie pipe because it is not easily pumpable.

  However, because of its reduced cement content compared to cement grouts, flowable fill does not meet the impermeability requirements to protect groundwater quality.  This presentation will show the use of flowable fill at both commercial and residential loop field sites, and will present recommendations for groundwater quality protection when flowable fill is used as a bridging agent in geothermal boreholes.