Grouting: Things a Civil Engineering must know about grouting

Articles > Grouting: Things a Civil Engineering must know about grouting

Introduction:

Grouting is a form of ground improvement technique, which includes injection of a mixture of water,cement and sand, injected with high pressure in the ground. Grouting can sometimes also be treated as reinforcement. The modern form of grouting was first introduced in mining works for arresting seepage and strengthening. Grouting is majorly used in case foundation purposes for strengthening, controlling settlement, and increasing the bearing capacity of the soil, etc.

Importance of Grouting:

Sealing pockets or leaks of permeable or unstable soil or rock prior to the excavation of tunnel heading
Sealing the base of structure founded on a pervious dam
Fixing ground anchors for sheet pile walls, retaining walls, stabilising rock cuttings, tunnels, etc.
Repairing a ground underneath a foundation or cracks on building masonry or pavement
Filling the voids between the lining and rock face in tunnel works
Forming grout curtains in layers of permeable strata below dam
Fixing the tendons in prestressed post tensioned concrete
Producing mass concrete structure and piles
Sealing the gap between the surface of a concrete foundation and the base plate of a foundation
Used for seepage control in rock under dams
Used for vibration control in case of machine foundation
Used for soil solidification and stabilization

Grout Characteristics:

In order to obtain the desired effect a grout should not set quickly because this may affect the pumping, but should set and harden after the completion of the pumping. The viscosity and subsequent rate of hardening are regulated during pumping by addition of additives, because the viscosity gradually changes with time to time when the grout transforms from liquid to solid state. The following principles are to be followed while selecting a grout:

Grout must be able to penetrate the voids of the mass to be injected
Grout should be able to resist any chemical attack when in place
Grout should have sufficient shear strength to withstand the gradient imposed during injection and on flowing groundwater.

Grouting Plant and Equipment:

Similar types of grouting plants are used for both solution and suspension type of grout, they only differ in storage and mixing. A grouting plant includes a mixer, an agitator, a pump, and piping connected to the grout holes. The items required for a grouting plant with their functions are:

Measuring tank: Used to control the volume of the grout injected
Mixer: Used to mix the grout ingredients
Agitator: To keep the solid particles into suspension until pumped
Pump: To draw grout from the agitator and deliver to the pumping line
Control fittings: To control the injection rate and pressure so that the hole can be regularly blended with water and thin grout.

Fig: Grouting Plant and Equipment

Courtesy: Wikipedia

When mixing the grout certain precautions are to be followed which were given by Houlsby, they are as follows:

Water is placed first in the mixer
The mixer is run at maximum speed before adding the cement
Grout is mixed in batches
Ingredients have to be measured by volume
Enough water should be provided to cover the rotor while it is functioning
Mixer should not be allowed to run for a longer duration between batches
Mixer should be cleaned thoroughly after each work.

Grouting Materials:

Grouting materials are grouped into two different types as follows:

Suspension Grouts: When the grout particles are in suspension in a liquid medium, the particles are generally small in size. These are multi phase systems capable of forming subsystems after being subjected to natural sieving processes, with properly checked chemical properties.
Solution Grouts: When the grout consists of a liquid mixture of two or more substances. These are intimate one phase systems that balances the chemical until completion of the relevant actions. Solution grout can be injected into the ground by either
- One-Shot System: Here all chemicals are injected together after pre mixing and the setting time is controlled by varying the catalyst concentration according to the grout concentration, water composition and temperature.
- Two-Shot System: Also known as the Joasten process, here one chemical is injected followed by the injection of the second chemical, which reacts with the first chemical to form a gel that subsequently hardens. This system is slower than the one shot system and requires greater pressure for injection, and also closer grout spaces are required.

Groutability:

Groutability is the ease with which the grout can be injected into the ground. In order to obtain satisfactory performance, the grain distribution of the grout particles must be known because it shows the relationship between the suspended particles and the void dimensions. Factors which decide the effect of rate of injection of grout are:

The viscosity of the grout
Permeability of the soil
Shear strength of the soil

Groutability Ratio:

When grout particularly Suspension grout is injected into the soil, the relationship between grout particles and the void ratio of the soil is very important. So a ratio is considered which has to be greater than 20 for the grout to be injected properly, this ratio is known as the Groutability ratio which was defined by Kravetz.

The groutability ratio is the ratio of the D₁₅ particles of the formation and the D₈₅ particles of the grout. D₁₅ is the particle size at which 15% of the soil is finer of the formation being grouted, and D₈₅ is the particle size at which 85% of the soil is finer of the grout being injected.

GR=D₁₅/D₈₅

Compaction and Displacement Grouting:

Compaction Grouting: In this form of grouting a thick viscous paste of low mobility is injected into the soil, upon reaching the ground it forms a bulb shape and compresses the surrounding soil.

Displacement Grouting: In this form of grouting, both low and high viscous paste can be injected into the soil. Here the grout displaces the surrounding soil,thus compacting the soil. It is further subdivided into

Displacement Compaction Grouting
Displacement Soil Fracture Grouting.

Conclusion:

Grouting though is not a new concept in the field of civil engineering, but with time its advancement and uses are enhancing. Grouting is used in repair works where there are cracks in concrete walls or below the foundation, hence with proper use grouting, a structure can be saved from collapsing as well as it also helps for strengthening the soil and enhances the characteristics of the soil upon which the foundation has to stand. Therefore, with the help of grouting many challenging situations can be solved in case of civil related works.

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