Grouts are broadly classified into two main categories based on their physical composition:

• Particulate (Suspension) Grouts: These are mixtures of solid particles suspended in a fluid (usually water). Common examples include Portland cement, microfine cement, and clay (bentonite) grouts. Their penetrability is strictly limited by the size of the particles relative to the size of the soil or rock voids. They exhibit Bingham plastic behavior, requiring a minimum shear stress (yield stress) to initiate flow.
• Chemical (Solution) Grouts: These are true solutions free of suspended particles. Examples include sodium silicate, acrylamide, and polyurethane resins. Their penetrability is constrained only by their viscosity, allowing them to permeate much finer soils (like silts and fine sands) than particulate grouts. They typically behave as Newtonian fluids.

The groutability ratio relates the pore size of the target soil (represented by $D_{15}$) to the particle size of the grout suspension (represented by $D_{85}$).

• Soil Grain Size ($D_{15}$): The diameter at which 15% of the soil mass is finer. This represents the effective size of the interconnected void spaces within the soil matrix.
• Grout Particle Size ($D_{85}$): The diameter at which 85% of the grout particles are finer. This represents the larger particles in the suspension that are most likely to bridge across pores and cause clogging.

• Permeation Grouting: The fluid flows into existing voids without displacing the soil structure. Low pressure is used to avoid fracturing.
• Compaction Grouting: Injection of a stiff, low-mobility mortar under high pressure. It forms an expanding bulb that forcefully displaces and densifies surrounding loose soils without permeating voids.
• Fracture Grouting (Hydrofracture): A mobile fluid is injected at a pressure high enough to intentionally exceed the soil's tensile strength, creating a network of grout-filled lenses that actively heave the overlying ground.
• Compensation Grouting: A highly specialized application of fracture grouting, primarily used during soft-ground tunneling. As the Tunnel Boring Machine (TBM) advances, it inevitably causes some ground loss and settlement above. Compensation grouting precisely injects grout into the soil between the tunnel and overlying sensitive structures simultaneously with the excavation, perfectly compensating for the volume loss and preventing any surface settlement.
• Jet Grouting: High-velocity jets erode the soil matrix while injecting a cementitious slurry to form a "Soilcrete" column, bypassing inherent groutability limitations.

• The Grouting Plant: Consists of high-shear colloidal mixers (to ensure thorough wetting of cement particles), agitator tanks (to keep suspensions from settling), and positive displacement pumps capable of delivering precise volumes at required pressures.
• Real-Time Monitoring: Modern systems utilize automated computer logging to continuously record flow rate, total volume, and injection pressure at the header. This allows engineers to instantly detect issues like premature refusal, hydraulic fracturing (a sudden pressure drop), or grout traveling outside the target zone.
• The Lugeon Test: Specifically for rock grouting, a pre-treatment water pressure test (Lugeon test) is performed to quantify the formation's permeability (hydraulic conductivity of fissures). One Lugeon unit ($1\text{ L/min/m}$ at $10\text{ bars}$) corresponds to approximately $1.3 \times 10^{-5}\text{ cm/s}$. It dictates the starting grout mix and pressure, and post-treatment Lugeon tests verify the seal.