Grout selection – permeability requirements
The minimum permeability that is commonly encountered in natural soils is on the order of 10-9 cm/s and the cement-bentonite grout mix used in the fully-grouted method is required to have at most a permeability of 10-6 cm/s for these low permeability soils.
The appropriate permeability of the cement-bentonite grout is therefore crucial for the success of the fully-grouted method.
Below is a timeline with information on the research and development for grout mixes for the Fully Grouted Method which began gaining significant traction within the Geotechnical community in 2012 and has been used successfully since then.
2003 Mikkelsen and Green Piezometers in fully grouted boreholes
Research identified that the appropriate permeability of the cement-bentonite grout is crucial for the success of the fully-grouted method and is based on the fact that the pressure gradients in the radial direction from the borehole wall to the piezometer tip are normally one to several orders of magnitude greater than those in the vertical direction within the borehole. As a result, the radial gradients control the piezometer response and this is correct as long as flow in the vertical direction does not develop due to higher permeability of the cement-bentonite grout than the ground.
2008 – Contreras et al GIN June
Carried out computer modelling and laboratory testing of six different grout mixes and found the following conclusions:-
- The permeability of the grout can be up to three orders of magnitude higher than the permeability of the surrounding soil without inducing a significant error.
- Laboratory test results show that the permeability of the cement-bentonite grout mixes is a function of the water to cement ratio. As the water to cement ratio (void ratio) decreases, the permeability decreases.
Permeability of the grout can be up to three orders of magnitude higher than the permeability of the surrounding soil without inducing significant error in the measured pore water pressure
2012 – Contreras et al Update of Fully Grouted Method GIN June
Field work was carried out to verify the computer modelling by comparing results from Open Standpipe Piezometers and nearby fully-grouted Vibrating Wire Piezometers similar ground conditions. It concluded that the permeability of the grout can be up to three orders of magnitude higher than the permeability of the surrounding soil without inducing significant error in the measured pore water pressure. It also confirmed that the hydrodynamic time lag /response time of the piezometer is very short.
2016 – Presentation by Joel Swenson at iGSM2016
From further laboratory and field testing grout mixes recommendations for various soil types was concluded as follows:-
| Soil type | K soil cm/sec | K grout cm/sec | Mix design C:W:B | Q grout psi |
| Sand | 10-4 | 7.2×10-6 | 1:6.55:1.0 | 15 |
| Clayey sand | 10-5 | 3.1 x 10-6 | 1:4:0.67 | 35 |
| Clay | 10-7 | 2 x 10-6 | 1:2.5:0.4 | 100 |
| Clay | 10-8 | 1 x 10-7 | 1:2:0.36 |
2020 Measurement of pore water pressure: Piezometers ISO 18674-4
The grout mix shall be designed before installation and the important variables to consider are the permeability, viscosity and strength of the grout.
The grout shall act as a filter so as not to adversely affect the horizontal flow of water to the piezometer and as a seal so as not to allow vertical flow along the borehole. This can be achieved if permeability of the grout is no more than 1000 times the permeability of the ground (this includes grouts that are less permeable than the ground).
When the grout is less permeable than the ground, response time is increased and permeability can decrease up to a factor of 1000 in the months after installation.
The viscosity of the grout should allow it to be pumped and avoid excessive penetration of the grout into the surrounding ground and in soils with a high permeability the viscosity of the grout is more important than the permeability.
The cement and the water should be mixed first as the water / cement ratio determines the permeability (as well as the strength and stiffness of the mix). Bentonite shall be used to control viscosity and to avoid excessive bleed.
High air entry porous filters shall not be used with the fully grouted method unless there is a means of removing air from the piezometer
When using the fully grouted method, care should be taken not to overstress the piezometer, as grout is heavier than water.
Typical grout mixes (ratios in weight) and order of magnitude of hydraulic conductivity and uniaxial compressive strength
| Cement | Water | Bentonite | K grout m/sec @28 days | qu grout kPa |
| 1 | 6 | 1 | 7.2×10-6 | 100 |
| 1 | 4 | 0.7 | 3.1 x 10-6 | 200 |
| 1 | 2.5 | 0.4 | 2 x 10-6 | 700 |
| 1 | 2 | 0.36 | 1 x 10-7 | 1500 |
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.
Mix Design rules
In order to keep field procedures simple the emphasis should be on controlling the water-cement ratio. This is accomplished by mixing the cement with the water first. When water and cement are mixed first, the water-cement ratio stays fixed and the strength/modulus of the set grout is more predictable. If bentonite slurry is mixed first, the water-cement ratio cannot be controlled because the addition of cement must stop when the slurry thickens to a consistency that is still pumpable.
There are many types of bentonite and the viscosity of the grout will depend on the quantity and type of bentonite used
High air entry porous filters shall not be used with the fully grouted method unless there is a means of removing air from the piezometer.
When using the fully grouted method, care should be taken not to overstress the piezometer, as grout is heavier than water.
Typical grout mixes (ratios in weight) and order of magnitude of hydraulic conductivity and uniaxial compressive strength
| Cement | Water | Bentonite | K grout m/sec @28 days | qu grout kPa |
| 1 | 6 | 1 | 7.2×10-6 | 100 |
| 1 | 4 | 0.7 | 3.1 x 10-6 | 200 |
| 1 | 2.5 | 0.4 | 2 x 10-6 | 700 |
| 1 | 2 | 0.36 | 1 x 10-7 | 1500 |
Mix Design rules
In order to keep field procedures simple the emphasis should be on controlling the water-cement ratio. This is accomplished by mixing the cement with the water first. When water and cement are mixed first, the water-cement ratio stays fixed and the strength/modulus of the set grout is more predictable. If bentonite slurry is mixed first, the water-cement ratio cannot be controlled because the addition of cement must stop when the slurry thickens to a consistency that is still pumpable.
RESEARCH CONTINUES INTO GROUT SPECIFICATION FOR THE FULLY GROUTED METHOD
Other references available
References Contreras, I.A., Grosser, A.T., and Ver Strate, R.H. 2007. “Basic Strength and Deformation Properties of Cement-Bentonite Grout Mixes for Instrumentation Installation.” Proceedings of the 55th Annual Geotechnical Engineering Conference University of Minnesota. pp. 121-126.
Contreras, I.A., Grosser, A.T., and VerStrate, R.H. 2007. “The Use of the Fully-grouted Method for Piezometer Installation.” Proceedings of the Seventh International Symposium on Field Measurements in Geomechanics. FMGM, 2007. Boston, MA. ASCE Geotechnical Special Publication 175.
Contreras, I.A. and Solseng, P.B. 2006. “Slope Instabilities in Lake Agassiz Clays.” Proceedings of the 54th Annual Geotechnical Engineering Conference. University of Minnesota. pp. 79-93. Dunnicliff, J. 1993. “Geotechnical Instrumentation for Measuring Field Performance.” J. Wiley, New York, 577 pp. Hvorslev, M.J. 1951. “Time Lag and Soil Permeability in Groundwater Observations.” Bulletin No. 36, U.S. Waterways Experiment Station, Vicksburg, MI.
McKenna, Gordon T. 1995. “Grouted-in Installation of Piezometers in Boreholes.” Canadian Geotechnical, Journal 32, pp. 355-363.
Mikkelsen, P.E. and Green, E.G. 2003. “Piezometers in Fully Grouted Boreholes.” International Symposium on Geomechanics. Oslo, Norway. September 2003.
Mikkelsen, P. Erik. 2002. “Cement-Bentonite Grout Backfill for Borehole Instruments.” Geotechnical News. December 2002.
Vaughan, P.R. 1969. “A Note on Sealing Piezometers in Boreholes.” Geotechnique, Vol. 19, No. 3, pp. 405-413.
Contreras, I.A., Grosser, A.T., Ver-Strate, R.H., 2012, “Update of the fully-grouted method for piezometer installation”. Geotechnical News, June.
