Pregrouting

Pregrouting is a geotechnical ground-treatment process in which grout is injected into soil or rock ahead of a primary construction operation to improve conditions for subsequent works. The goal is to reduce permeability, limit water inflow, stabilize fractures, and create a more uniform ground mass to receive excavation, tunneling, or lining.

Common grout materials used in pregrouting include cementitious grout (cement-based mixtures), chemical grout (such as polyurethane or vinyl ester blends), and microfine cements. The choice depends on ground conditions, the desired penetration, and environmental considerations. Grout is delivered through boreholes equipped with packers, and injection is conducted under controlled pressure and flow rates. Grouting patterns may aim to form a curtain around a excavation zone, fill fractures selectively, or saturate a specified volume.

The process typically involves site characterization, drilling and installing injection points, designing the grout mix, and executing staged injections with monitoring of pressure, volume, and ground response. Verification methods include borehole surveys, permeability tests, and observation of water inflows or ground movements. Quality control is essential to avoid over-pressurization, excessive ground disturbance, or incomplete saturation.

Applications are common in tunneling (including TBM and NATM methods), underground stations, dam foundations, and other excavations where control of water ingress and ground stability is critical. Pregrouting is generally regarded as a preparatory measure that supports subsequent ground support and lining works; it is not a substitute for proper design, monitoring, and reinforcement.