What is Compaction Grouting?

Compaction grouting is a ground improvement method which is used to increase the density and stability of the soil. By stabilising the ground, it enhances the safety and longevity of structures built on top. This method of grouting is usually used to rehabilitate the settlement of sensitive structures. [1]

It is carried out by injecting stiff mortar-like grout under pressure through cased boreholes to form bulbs, which push the surrounding soil to the side and increase its density. This grouting method is used for treating a wide variety of loose soils, with relatively good drainage, and is performed at depths ranging from 2m, right down to several tens of meters. [2]

The key parameters for compaction grouting are the grid pattern, the injection pressure and the grout take – these determine the performance of the grout take.

• The grid pattern is devised such that each drill hole nominally treats a given area in the plan. The grid can be square or triangular and generally makes the distinction between primary and secondary (and sometimes tertiary) holes. The grid is determined by the type of treatment required (localized or en masse)
  and the radius of influence (Ri).
• The radius of influence is the distance from the centre of the drill hole to the furthest point at which there is a change in void ratio as a result of the treatment.
• The injection pressure is dependent on the specific site conditions: the presence of buildings, civil engineering structures, open sites, treatment depths, etc.
• Pressure is prescribed by depth in stages at 1 bar (100KkPa) per meter of depth measured from the bottom of the stage.
• Most compaction grouting is performed using grid patterns of 4 to 9m² with grout take varying from 2 to 6%. In the particular case of sinkholes, grout takes are highly variable and have been known to be as high as 14%.

The benefits of compaction grouting include the following:

1. Cost-effective than other methods
2. Maintenance-free solution for void filling
3. Prevents collapse of granular soils and settlement under adjacent foundations
4. Controls groundwater movement and protects utilities from damage
5. Enhances soil strength and rigidity, and reduces ground movement
6. Provides predictable degree of movement, critical for structural integrity
7. Suitable for limited working spaces, offering designable strength and permeability