Grouting into a soil or around a structure makes it possible to render soil impermeable and/or fully consolidated by filling its voids with a pumpable product (grout) that subsequently sets hard.

Grout can be used to fill voids in soil, cracks in rock and voids caused by dissolution (referred to as crack, impregnation or cavity filling grouting), and/or penetrate by displacing the soil by means of pressure or hydrofracturing (this is referred to as compaction grouting or solid grouting and hydrofracture grouting.). Displacement grouting can be used to limit structural deformation as a result of excavation (galleries and tunnels, large-scale urban excavations, etc.); this is referred to as compensation grouting (of structural settlement).

Grouting is always injected by drilling, often using devices to control the flow of grout into the soil (injection tubes, obturator bags, “tubes à manchettes”, etc.)

Applications

Underground excavation has always provided the broadest scope of applications for impermeability and consolidation grouting. The grouting techniques used vary widely depending on the desired end result and soil type.

Tunnel construction work is very often accompanied by a range of different grouting types. They can be used – for example – to reduce the permeability of a working face (progress treatment), treat water ingress by grouting cracks in the rock, reduce surface settlement as a result of tunnel construction (compensation grouting) or to fill voids in the outer surfaces of tunnel segment assembly.

Grouting also has a role to play in soil improvement. Solid grouting is used to consolidate granular soils by mortar injection to reduce the void index.

Grouting is also widely used to create hydraulic barriers, for a slab construction, for example, a grouted cut-off at the foot of a diaphragm wall and an impermeable base level to limit pumping flows when excavating at extreme depths.

It is also used very frequently in dam construction and maintenance to consolidate the dam core or slopes, or for cut-off walls, for example.

Grouting can also be used to protect structures. Examples of this application include safety work to old mine shafts and the galleries of redundant underground quarries, and filling karstic and gypsum dissolution voids. It is also frequently used to consolidate old masonry walls.

Lastly, grouting techniques are essential tools for the securing of anchor bolts and micropiles.

What are the advantages of this technique?

Implementation

What are the different types of grouting and how are they used?

The scope of applications for grouting is extremely wide. So for the purposes of example, we can make a distinction between:

• rocky terrain where grouting can be done on an ‘open hole’ basis, because the walls of the drilling are stable
• soft ground where the drilling will be lined by a casing or “tube à manchettes” to contain a grouting obturator.

The “tube à manchettes” is a tube with a smooth internal surface and with perforations every 30 to 40cm. These perforations are covered by rubber sleeves, called «manchettes», which act as non return valves. The tube is sealed into the grout hole with a weak bentonite-cement slurry (the sleeve grout) to prevent the subsequent grout from travelling along the annular space.

When grouting into rock, the universally accepted method is called “split hole grouting”, which begins with primary drillings spaced at fairly wide intervals, followed by intermediate drillings (secondary, tertiary or more) to create an increasingly tight-knit matrix of drillings.

Grout is injected into alluvial soils using “tube à manchettes”. This is usually a two-phase process: the first involving injection of a bentonite/cement grout to fill the coarse grain size, and the second using a more penetrative grout (liquid or ultra-fine suspension grout). The main criterion is the quantity of grout needed to provide optimum filling of soil porosity.

The grout injected can be:

• liquid: the penetrative power of these grouts is essentially a function of their viscosity and the way that viscosity changes over time
• suspensions: in addition to viscosity, these grouts also have rigidity or cohesion, which limits their scope of action. The granularity of the slurry also limits the size of the voids or pores into which the grout can penetrate. Suspension stability is also an important parameter for this form of grouting
• mortars: these high-rigidity grouts are used to fill cavities and voids, or in displacement grouting (solid or compensation grouting)

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