MOORE & TABER                     

 

 

Soil Grouting and Ground Improvement Specialists

 

         

Permeation grouting is a term used to describe a ground treatment method in which grout is injected into a porous medium without disturbing its original structure.  In geotechnical engineering, this usually refers to the process of filling the pores and joints in a soil and/or rock deposit to change its geotechnical properties.  Almost any grout material may used for permeation grouting, but there are distinct limits on the grout mix used for specific types of soil or rock.  Applications are for enhanced foundation bearing value, improvement of excavation character in running sands and reduction of liquefaction potential.

Particulate grouts are typically water-based slurries of cement, fly ash, lime or other finely ground solids that undergo a hardening process with time.  These materials may be used to fill pores and joints in soil and rock, provided the grout particles are small enough to be carried through the pore or joint openings.  A good rule of thumb is that the effective particle diameter in the grout suspension should be less than the dimension of the pore or joint aperture divided by 5. 

Slurry grout mixes used for permeation grouting are designed primarily to promote passage of the grout particles into the porous medium.  The grain size of the slurry is matched to the pore aperture and steps are taken to assure the grout particles are properly dispersed in the grout.  Both high speed mixing and wetting agents are used to break up clumps and aggregations of grout particles that would cause the grout to have a lager apparent grain size than the actual grain size of the slurry.  Water content is adjusted in the mix design to control the mean free path between the slurry particles rather than simply providing enough water to allow complete hydration.   

Two types of slurries are used.  The so called stable slurries exhibit less than 10% bleed (separation of water from the slurry) at final set.  Unstable slurries bleed from 10 to 90% of the water prior to setting.  

When solids and water are mixed, the solid particles begin to settle out and water is displaced upward (bleed).  The forces acting in the suspension to reduce the settling of the particles are random impacts of water molecules against the particle, viscosity of the water, interparticle attraction and friction.  Since the interparticle attraction is inversely proportional to the square of the mean free path length between particles and the other forces are inversely proportional to the cube of the particle diameter, either reducing the particle size or increasing concentration reduces the bleed.  As a general rule, for Portland Cement grout 0.66 : 1 is the water to cement ratio (by weight) which is the borderline between stable and unstable grout. 

Stable slurries are too thick to be used for permeation grouting of all but the most coarse-grained soils or extremely fractured rock.  Unstable slurries having water to cement ratios from 0.66 : 1 to 3 : 1 (by weight) may be used to permeation grout granular soils with effective grain size down to coarse sand or fractured rock with joint widths as low as 0.01 inch.  However, the bleeding of these grouts causes channels and open pathways to remain through the grout. 

To eliminate the effect of bleed on Portland cement grout, additives are used to hold the cement grains in suspension at water to cement ratios that would otherwise be quite unstable.  The most common additive is a water suspension of bentonite.  Even small amounts of bentonite increase the interparticle forces dramatically and hold the cement particles in suspension.  Typically, cement/bentonite grout used for permeation grouting has water to cement ratio varying between 1 : 1 and 2.65 : 1 and exhibits zero bleed. 

In many cases, it is necessary to grout soil and rock formations having an effective pore aperture smaller than the allowable aperture for Portland cement grout.  Type III cement or microfine cement grouts are used to grout these finer materials.  The grain size of Type III cement is about 20 microns v. 50 microns for Type II, while the grain size of the microfine cement grouts is between 4 to 8 microns.

The image on this page shows a sample of permeation grouted sand from a project that required steep-walled footing excavations in running sands.  The proposed excavation area was permeation grouted with a microfine cement slurry prior to excavation of footing trenches, resulting in a significant reduction in project cost through the elimination of shoring.  Unconfined compressive strength tests performed on samples trimmed from the field yielded 28-day strengths on the order of 800 psi.

Chemical grouts are liquid resins or water solutions of grout constituents such as polyurethane, sodium silicate or prepolymers.  In some instances, the chemical grout is an emulsion of water and a liquid resin.  These materials my be injected through the pore or joint aperture depending on the viscosity of the solution instead of the particle size.