TN 6 Importance of Backfill in Underground References

The backfill surrounding a bagged underground reference electrode is a mix of gypsum and bentonite. The primary purpose of backfill is to retain water which ensures that a low contact resistance between the electrode and the surrounding earth is maintained.  Additionally, backfill usually prevents the inner core of the electrode which contains a saturated salt gel from drying out.  However, during severely dry conditions, the electrode may still dry out.  The backfill will eventually rewet with local groundwater, and the electrode should re-activate.  However, local ground water will have many other chemicals dissolved in it that can affect the accuracy of the electrode.  If this situation is suspected, the electrode should be calibrated against a reference electrode of known accuracy to determine whether replacement is necessary.

When installing underground reference electrodes in areas known to have extreme seasonal dry periods, a good practice is to place additional gypsum-bentonite backfill around the reference bag.  This backfill is commonly known as driller’s mud.  This extra backfill will hold additional water around the reference electrode and extend the time before it dries out.

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