 Rock Grouting Overview
Grout Mix Formulations Grouting Equipment Grouting Procedure Selection of Grout Type The choice of grout type is a function primarily of the aperture of the rock joints and cost. For grouting projects that involve filling large fissures, the use of stable grouts formulated with locally available ordinary Portland cement is recommended. Based on many years of experience, the aperture of joints that can be grouted with ordinary Portland cement is (Houlsby, 1990): Estimating the aperture of joints is not a trivial task, especially in the planning process where the grouting site may not be accessible. Snow, (1968) proposed the following equation for estimating the hydraulic conductivity of a rock mass with two sets of planar fractures. 
Knowing the joint spacing, and the hydraulic conductivity estimated from Lugeon tests, the average joint aperture can be calculated. The groutability of fine cracks is related to the width of the crack and the grain size of the grout material, expressed as a groutability ratio for rock in the following formula (Weaver, 1991): 
For groutability ratios greater than 5, grouting is considered consistently possible. For groutability ratios less than 2, grouting is not considered possible. The D95 and practical grouting range for various grouts are provided in Table 1. Table 1 Joint Apertures Range for Various Cement Grouts Cement Product | D95 of grout | Practical Joint Aperture Range | Ordinary Portland Cement | 80-100 microns | >400 microns | High Early Strength Cement | 40-60 microns | >200 microns | Microfine Cement | 10-12 microns | >50 microns |
For reasons of low cost, ordinary Portland Cement is always preferred on grouting projects, provided that the fissure widths are greater than 400 microns. For a nominal extra cost, high early strength Portland cement will provide enhanced penetration for joint apertures down to 200 microns, due to its lower d95 size. For grouting rock joints down to 50 micron aperture, microfine cement is recommended. Microfine cement is normally a proprietary blend of finely ground blast furnace slag and Portland cement. Where high sulphate groundwater is a potential problem, sulphate resisting Type 20 or Type 50 Portland cements should be used in combination with fly ash. Water testing results (described below) can be used as a guideline to selecting the most suitable grouting material. Table 2 indicates grouting materials for various rock mass hydraulic conductivities. For most rock grouting applications, a target permeability range of 1-2 Lugeons is achievable. Table 2 Suitable Grout vs Permeability Lugeon Value | Permeability | Grouting Material | 4 | 5.2 x 10-7 m/s | Microfine cement grout | 8 | 1.0 x 10-6 m/s | Microfine cement grout | 15 | 2.0 x 10-6 m/s | Microfine cement grout | 30 | 3.9 x 10-6 m/s | Cement grout with stabilizing additives | 60 | 7.8 x 10-6 m/s | Cement grout with stabilizing additives | 120 | 1.6 x 10-5 m/s | Cement grout with thixotropic additives |
Grout Mix Additives Performance enhancing grout additives enable stable, balanced cementitious grouts to be formulated for a wide variety of field conditions. Stable grouts are grouts that exhibit generally less than 5% bleed in 24 hours. Balanced grouts resist premature blockages due to pressure filtration and thereby achieve improved penetration at lower grouting pressures. Common grout additives include: Bentonite - used generally up to 2% by weight of cement to improve stability under pressure and reducing shrinkage and bleeding of the grout. Superplasticizer- normally a napthalene sulphonate admixture used as a dispersing agent to reduce the viscosity of the grout. Typical addition rates are 0.5-1% by weight of cement. Fly Ash - used as a pozzolan to replace up to 25% of the cement where cheaper fly ash is available. Fly ash can also be used with Type 20 cement to produce a grout better resistant to aggressive groundwater. Silica Fume - used up to 10% by weight of cement to produce a stronger, less permeable grout with enhanced stability and resistance to pressure filtration. Accelerators - normally used during cold weather grouting operations or wherever faster setting times are required. Sodium Silicate - a flash setting additive used under high inflow/high pressure conditions. Thixotropic Modifier- used in flowing water conditions to produce a cohesive, water repellent grout that resists washout. For most common grouting applications it is necessary to use only bentonite and superplasticizer. | 
