Grout for borehole instruments
The backfill for a borehole instrument is an item that does not receive the attention it deserves. The behaviour 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.
This topic often causes much discussion (and disagreement) and is also often missing from instrumentation specifications and/or left to the installer to determine or, at worst, left to the instrument manufacturer to specify. The responsibility for grout specification should rest entirely with Instrumentation Design Engineer, nobody else.
The grout mix should ideally be selected to be compatible with the in-situ field instruments installed in the borehole and also be engineered to match closely the geotechnical properties of the parent soils.
A stable grout can be made using cement with various proportions of bentonite. Grout stability is important during both the liquid and set conditions.
The liquid grout fluidity should be as viscous as possible to avoid segregation, yet fluid enough to be easily pumpable and fill voids and over-break in the borehole.
The responsibility for grout specification should rest entirely with Instrumentation Design Engineer, nobody else.
Strength and Deformation
The general rule for grouting any kind of instrument in a borehole is to try and mimic strength and deformation characteristics of the surrounding soil rather than permeability.
While it is feasible to match strengths, it is not really feasible to match the deformation modulus and it is a more sensible approach to minimize the area of the grouted annulus with an approximate strength grout so that the column will only contribute a weak force.
Figure 1 shows strength data collected by (Mikkelson 2002) and demonstrates the decrease in strength with increasing water-cement ratio.
Work by Marsland (1973) showed that the water-cement ratio controls the strength of the set grout and Marsland’s rule-of-thumb is to make the 7-day strength of the grout to match one quarter that of the surrounding soil.
Typical mixes
Grout strength decreases with water-cement ratio and controlling this ratio is the most important factor for grouting and it is therefore recommended that the water and cement is mixed first. Water and cement ratios greater than 0.7 – 1.0 by weight will segregate without the addition of Bentonite to provide a thick but “pumpable” grout mix.
The tables below provide guidelines for typical mixes that may be adopted for varying soil types but is only intended as a guideline. We have also developed an easy-to-use Grout Calculator for working out the quantity of cement, Bentonite and water required according to soil type, bore diameter, depth of bore and number of boreholes for borehole instruments such as Inclinometers and Extensometers.
| HARD SOILS | MEDIUM SOILS | SOFT SOILS | ||||
| Materials | Unit | Weight Ratio | Unit | Weight Ratio | Unit | Weight Ratio |
| Cement (OPC) | 50kg | 1 | 50kg | 1 | 50kg | 1 |
| Bentonite | 15kg | 0.3 | 15kg | 0.3 | 20kg | 0.4 |
| Water | 125 lit | 2.5 | 225 lit | 4.5 | 325 lit | 6.5 |
Instrumentation installations often go through several different soil types from hard to soft and whilst stage grouting to match the individual soil types would be the best technical solution it is not really practicable or considered worth doing. In most cases the measurements required are in the axial direction of a borehole and where reasonably large quantities of grout are used, and for extensometers for example, it is best to have a softer rather than stiffer grout.
Mixing
The key issue is controlling the water-cement ratio and this is accomplished by mixing the cement with the water first and then adding the Bentonite.
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