Last edited 31 Aug 2020

Grouting in civil engineering



[edit] Introduction

Grouting in civil engineering refers to the injection of pumpable materials into a soil or rock formation to change its physical characteristics. It is one of the ways 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. Grout may also be used in the formation of pile foundations, ground anchors, under-reaming, underpinning, in road construction, dam construction, and other applications.

Different materials may be used for grouting depending upon factors such as the soil or rock type and the area to be grouted. However, the basic process is the same: the soil or rock is injected with fluid grout which sets and reduces or acts as a sealant on the material’s permeability.

Grouting is relatively costly and so wastage must be controlled. This is achieved by the use of additives which improve the gelling properties of the grout and limit its spread through the ground.

[edit] Injection methods

Typically, grouting is carried out 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 by investigation of ground conditions and 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.

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 grout injection pressure 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.

[edit] Grouting materials

There are several different types of material used for grouting:

[edit] Cement grouting

Cement (or cementitious grout) is used for high-permeability ground. Neat cement and water or a mixture of sand (4 parts) to cement (1 part) is the usual composition.

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.

[edit] Bentonite grouting

Bentonite is produced from clay which has thixotropic properties and 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. For more information, see Bentonite.

[edit] Chemical grouting

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.

Chemical grouting has the advantages of allowing economical spacing of bore holes, greater penetration of the grout, and more flexibility in terms of the time of grouting.

[edit] Resin grouting

As opposed to chemical grouts, resin grouts have a very low viscosity and 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.
  • Phenol-formaldehyde.
  • Resorcinolformaldehyde.

[edit] Bituminous grouting

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.

[edit] Related articles on Designing Buildings Wiki

[edit] External references


Designing Buildings Anywhere

Get the Firefox add-on to access 20,000 definitions direct from any website

Find out more Accept cookies and
don't show me this again