7.3 Grouting principles and methods

7.3.1 General The introduction of grout in a host medium is achieved either with or without displacement of the ground. Figure 2 illustrates the various injection methods associated with these two principles.

Grouting principles and methods
Figure 2 — Grouting principles and methods The term consolidation grouting is sometimes used to emphasize an improvement in the strength or deformation characteristics of a soil imparted by the permeation treatment. The aim of consolidation grouting in soils and rock is to improve the ground so that it does not undergo any inadmissible deformation. As this term does not refer to a grouting principle or injection method, it will be avoided in the following. The term compensation grouting is used when the objective of grouting is to concurrently compensate for ground loss caused by tunnelling or excavation works.

7.3.2 Grouting without ground displacement (non-displacement grouting) Permeation (impregnation) grouting Permeation grouting aims at filling the accessible interstices between grains in permeable soils by a grout without destruction of the integrity of the ground. It reduces the permeability of the host material and usually increases the strength and density. In order to avoid displacement, permeation grouting shall be carried out at carefully controlled pressures and flow rates. Fissure and contact grouting Fissure grouting aims at filling open fissures, fractures or joints in a rock mass with grouts without creating new or opening existing fractures, in order to reduce the permeability and/or increase the strength of the grouted mass. The frequency, orientation, extent, width, roughness and infilling of the discontinuities shall be known in order to formulate the design. Bulk filling Bulk filling is used for the filling of large natural or man made openings. The term is generally applied to the placement of large volumes of grout under gravity or at low pressures. If a large volume of cement is placed in an enclosed space, the effect of high local temperatures and induced stresses shall be taken into account. Bulk filling may be followed by a phase of grouting under pressure to fill the remaining voids.

7.3.3 Grouting with ground displacement (Displacement grouting) General considerations Displacement grouting refers to the injection of grout under pressure with the deliberate intent of spatially displacing the host medium. The term includes injection methods such as compaction grouting, and hydraulic fracturing (claquage). The method is used to increase the density of a plastically deformable material, and the volume of the treated mass where the plastic deformation limit is reached. Controlled displacement grouting can be employed to strengthen the ground under existing structures. Hydraulic fracturing Grouting by hydraulic fracturing is used to:

  • reinforce or stabilize the ground (soil or rock);
  • produce controlled uplift of structures;
  • achieve watertightness by creating compartments. It is difficult to control the propagation of an hydraulic fracture plane. Hence, the grouting objective should usually be achieved by an incremental series of injections, spread over a period of time. Compaction grouting Compaction grouting refers to the intrusion of a comparatively stiff (viscous) particulate grout into the ground to induce displacement and deformation. The grout is usually extruded from open-ended injection tubes. The grout consistency is such that the grout remains as a homogeneous mass and neither permeation nor hydraulic fracturing of the host medium occurs. Compaction grouting is most often used to compact and densify loose ground and to raise and support structures which have settled. The final grid of grout holes is generally defined during the grouting process, in accordance with the results of control tests performed in the centre of the primary grid. The slow dissipation of pore pressure in low permeable ground shall be taken into account for the control measures.

EN 12715:2000 Execution of special geotechnical work – Grouting