Put simply, grouting aims to fill the cavities in soil or rock. In its initial stages, the grouting material is liquid (so it can be pumped and become dispersed) but it has the capacity to set in a timely manner. This increases the cohesion and shear strength of the ground while decreasing its permeability.

Successful design of grout chemistry requires adequate investigation to determine the characterisation of ground, groundwater and the identification of fractured rock, weathered rock, granular soils and natural cavities.

Most grouting material used in underground projects can be generally classified into either cement or chemical grout. Within each grout class, there are primary grout subtypes. Portland cement grout (particulate sizes on average of 15 microns), microfine cement (6 – 10 microns), and ultra-fine cement (average particulate sizes of 3 – 5 microns) are among the subtypes of cementitious grouts.

Examples of the subtypes of chemical grouts include:

  • Sodium silicate – this is a two-component grout which has a low viscosity in most cases, but it will often expunge water after gelling by a process called syneresis. With relatively short gel times ranging from a few minutes to a few hours (depending on mix design), sodium silicates are commonly used as a temporary solution for water control and structural support.
  • Acrylics – they are free of suspended solids and have extremely low viscosity (similar to water). The acrylic family consists of acrylamide and acrylates. Each requires a base resin to be mixed with a catalyst to create a gel matrix within a soil (or rock). Acrylamide typically changes from a liquid to a solid in a controllable gel time ranging from three seconds to ten hours.

Grouting Methodologies

The most common form of grouting is referred to as permeation grouting, which simply means filling open voids and involves filling any cracks, joints or voids in rock, concrete, soil and other porous materials. All other grouting methodologies include some measure of permeation grouting, but they also have other characteristics of their own.

Examples include:

  • Compaction grouting – used to increase the density of the soil and usually used to rehabilitate settlement of sensitive structures. A low-mobility grout is injected under pressure through cased boreholes to form bulbs. This compacts the surrounding soil, increasing its density.
  • Fracture grouting – this involves a low-viscosity cement grout injected under high pressure. The grout will split open the ground using hydrofracturing and creates lenses that would penetrate void spaces or fractures. Soils are also displaced using this process. Fracture grouting is commonly used for re-levelling structures and stabilising overlying structures during tunnelling.
  • Jet grouting – creates in-situ columns of grouted soil using a very high-pressure grout injection by pumping high velocity jets of grout. The jets erode and mix the soil as the drill string is being rotated and withdrawn.

To plan for a grouting operation, it is critical to determine and monitor grout flow, volume and pressure along the grouting lines. The difference between assumed target values and actual field measurements can help the grouting specialist detect a range of issues and adjust the operation as needed. For instance, if the volume (m3) of injected grout rises above the design target volumes while line pressure remains constant, it could be a sign of grout leakage or channelling into adjacent areas. On the other hand, if designed injection volumes are not nearly reached while the flow is low to none and line pressure shows higher than normal values, it could be the result of a line blockage or the consequence of less permeable ground than initially anticipated.

The grout volume to be injected depends on ground porosity, the geometry of the treated zone, grout-hole spacing and total depth to be treated. A grout’s ability to penetrate a rock fissure largely depends on particulate size, whereas its ability to permeate a soil is also dependent upon surface tension within the grout. The groutability of soil with particulate grouting has been evaluated based on the N value (Mitchell and Katti 1981). N is defined as N = (D15) Soil / (D65) grout. Grouting is considered feasible if N > 24 and not feasible if N < 11.

As with all tunneling projects, the type of ground and geology will be the basis and guide for designing and executing the right type of grout and injection procedures.