In order to model low resistivity backfill material, such as Bentonite, surrounding grounding conductors, there are two approaches you can consider:

1. For short conductors, such as ground rods, or for small grounding systems in which voltage drop along the grid conductors can be neglected (this is the case for any grounding system modeled with AutoGrid Pro or the MALT module of other SES software packages), you can consider increasing the conductor diameter in your model such that it occupies the entire backfill volume. Thus, a 3/8” diameter ground rod in a 12” diameter hole would be modeled as a much larger conductor, with a 12” diameter. This approximation is quite reasonable if the resistivity of the backfill material is one or more orders of magnitude less than the resistivity of the surrounding soil (or rock!).
2. With the MALZ and HIFREQ modules, you can represent the backfill as a thick coating on the conductors. Specify a coating thickness equal to the actual thickness of the backfill outside the conductor and specify a coating resistivity equal to that of the backfill material.