Grouting in civil engineering
Grouting in civil engineering refers to the injection of pumpable materials into a soil or rock formation under pressure to change the physical characteristics of a natural formation. It is one of the ways in which ground water can be controlled during civil engineering works. Grouting is suitable where soil permeability would create a heavy demand on pumping or where ground conditions mean it may be economically inefficient to bore wells.
Different materials may be used for grouting depending upon the soil type, the size of rock fissures and so on. However, the basic process is the same: the soil or rock is injected with fluids which set and reduce or act as a sealant on the material’s permeability.
Grouting is relatively costly and so wastage must be controlled. This can be achieved by the use of additives which assist with the gelling properties of the grout and limit their spread through the ground. Care must also be exercised in relation to existing services or underground structures such as sewers, since the pressure of grouting may result in fine cracks that may be present being penetrated.
 Injection methods
Typically, grouting is carried out is by driving pipes or boring holes into the ground, and then pumping the grout solution at high pressure through inserted tubes.
The extent of grouting required for a particular area is determined through investigation of ground conditions and the calculation of a drilling pattern. This considers the size, spacing and depth of the holes required. The type of grout and the particular ground conditions will influence the spacing of the holes. Generally, they range from 600 mm centres for sand to 10 m centres for rock.
Site conditions will influence the tools used for the boring process, but pneumatic tools, diamond drills or wash-boring are the most common. Alluvial soils are prone to collapse and so holes are usually cased.
The pressure of the grout injection is dependent on soil conditions, and in-situ testing may be carried out before the correct pressure is determined. Pressures usually range from 1 N/mm2 for sands to 7 N/mm2 for rock.
There are several different types of material used for grouting:
Holes are bored in a radius around the area to be excavated before being injected with a thin grout, the viscosity of which is then increased by reducing the water-cement ratio. If required, secondary holes are bored between the primary holes to ensure the complete grouting of the area.
Bentonite is produced from clay which has thixotropic properties, meaning it forms a highly water-resistant gel which, when mixed with additives, can create a permanent barrier to water flow. This is used where soil particles are too small for cement grouting, most commonly to combat seepage in alluvial soils beneath the foundations of dams or other water-bound structures.
Chemical grouting is used in soils of medium to coarse grading. Materials such as sodium silicate and calcium chloride are mixed together in liquid form and solidified into a gel. There are two main processes:
- ‘Two-shot’ process: Pipes are driven into the ground. One chemical is injected followed by another meaning that the reaction, and soil strengthening, is rapid.
- ‘One-shot’ process: This involves chemical mixing prior to injection, with the hardening being delayed by the composition. This allows for wider borehole spacing.
As opposed to chemical grouts, resin grouts have a very low viscosity which are able to penetrate fine sands. The type of resin used depends on the chemical content of the local water table and may result in different times for setting. Common types include:
- Tannin-based grouts.
Bitumen emulsion can serve as a suitable grouting material that can be injected into fine sands as an impermeable barrier to water. Soil strength will not be increased, but cut-off walls beneath dams and other water-bound structures can be formed effectively.
 Related articles on Designing Buildings Wiki
- Bridge construction.
- Cracking and building movement.
- Driven piles.
- Geothermal pile foundations.
- Pile foundations.
- Prestressed concrete.
- Reinforced concrete.
- Retaining walls.
- River engineering.
- Temporary works.
- Screw piles.
- Water engineering.
 External references
- ‘Introduction to Civil Engineering Construction’ (3rd ed.), HOLMES, R. (1995)
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