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We have developed the following Excel spreadsheet as an easy-to-use 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.

Disclaimer: The following spread sheet is provided in good faith as an aid for data reduction but it is the sole responsibility of the user to check that the calculations are suitable for their own use.

Introduction

The backfill for a borehole instrument often causes much discussion and disagreement and is often missed from instrumentation specifications and/or left to the installer to determine.

The grout specification should be provided by the Site/Design engineer and designed to mimic the surrounding soil conditions. However as installations may have a combination of soil types it is recommended to err on the softer side of the spectrum especially where extensometers are concerned.

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.

Permeability

Permeability of grout is mainly an issue that is limited to piezometer installations.

It is general practice to grout the borehole above a Bentonite seal placed above the piezometer response zone and it should have permeability no greater than one or two orders of magnitude above the clay to get representative reading.

Recently piezometer installations are following the “Fully Grouted Method” as described by (Vaughan (1969, Mikkelsen & Green 2003, Contreras, Grosser, Ver Strate, Dunnicliff 2008) and it is recommended referring to grout mixes as outlined in these papers (see links below).

The permeability for piezometers in the fully grouted method is an issue in clay and the grout should have permeability no greater than one or possibly two orders of magnitude above the clay to get representative readings.

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.

  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.