GEOTECHNICAL INSTRUMENTATION
GEOTECHNICAL INSTRUMENTATION
Cement-Bentonite Grout Backfill for
Borehole Instruments
P. Erik Mikkelsen
Solid Bentonite Seals
The backfill for a borehole instrument is often an item that receives a disproportionate lack of attention. The behavior of the backfill, the material that is in the most intimate contact with both the formation and the instrument, is critical for obtaining correct measurements. In many situations, instrument observations may just reflect unstable backfill, lack of backfill or backfill that is too stiff or too soft. Sand, gravel and various bentonite products have proven to be both too difficult to place and often entirely inappropriate. Experience has shown the author that cement-bentonite grout is the most universally applicable material for successfully backfilling a borehole instrument.
Single-component bentonite grouts have been used in related industries a long time, and have been adopted for borehole instrumentation with mixed success. Their uses are more involved and, as explained below, should be avoided. The use of fly ash as a substitute for cement promises to be a good way for reducing grout stiffness when required.
Current Use of Bentonite
Materials and Technology
Although this article advocates the use of cement-bentonite grout it is appropriate to review current bentonite backfill and sealing products to illustrate why their use should be limited. To the uninitiated, there is a confusing array of various pellets, chips, granulated and powd er-forms of sodium bentonite commercially available in North America and elsewhere. Calcium type montmorillonite and opalite are also used.
The proper use of these products is a mature and complex technology employed by the environmental, water-well and petroleum industries. Basic research has been done and their properties are known. For example, Baroid
Industrial Products (1994), a manufacGeotechnical News,
December 2002
turer of about two-dozen
Cement-Bentonite Grout Backfill for
Borehole Instruments
P. Erik Mikkelsen
Solid Bentonite Seals
The backfill for a borehole instrument is often an item that receives a disproportionate lack of attention. The behavior of the backfill, the material that is in the most intimate contact with both the formation and the instrument, is critical for obtaining correct measurements. In many situations, instrument observations may just reflect unstable backfill, lack of backfill or backfill that is too stiff or too soft. Sand, gravel and various bentonite products have proven to be both too difficult to place and often entirely inappropriate. Experience has shown the author that cement-bentonite grout is the most universally applicable material for successfully backfilling a borehole instrument.
Single-component bentonite grouts have been used in related industries a long time, and have been adopted for borehole instrumentation with mixed success. Their uses are more involved and, as explained below, should be avoided. The use of fly ash as a substitute for cement promises to be a good way for reducing grout stiffness when required.
Current Use of Bentonite
Materials and Technology
Although this article advocates the use of cement-bentonite grout it is appropriate to review current bentonite backfill and sealing products to illustrate why their use should be limited. To the uninitiated, there is a confusing array of various pellets, chips, granulated and powd er-forms of sodium bentonite commercially available in North America and elsewhere. Calcium type montmorillonite and opalite are also used.
The proper use of these products is a mature and complex technology employed by the environmental, water-well and petroleum industries. Basic research has been done and their properties are known. For example, Baroid
Industrial Products (1994), a manufacGeotechnical News,
December 2002
turer of about two-dozen
References: Bariod Industrial Products (1994), “Drilling and Boring Fluids Workshop”, Houston, TX. Dunnicliff, John, (1988, 1993), “Geotechnical Instrumentation for Marsland, A. (1973), “Discussion, Principles of Measurement”, in Field Instrumentation in Geotechnical Engineering, British Geotechnical Papp, J.E. (1996), “Sodium Bentonite as a Borehole Sealant”, Chapter 12, in Vaughan, P. R. (1969), “A Note on Sealing Piezometers in Boreholes”, Vaughan, P. R. (1973), “Discussion, Principles of Measurement”, in P. Erik Mikkelsen, Consulting Engineer, Geometron, 16483 SE 57th Place,