Ground improvement and stabilization techniques-Ground grouting
Grouting is the process of injecting grout materials into the ground. The purpose of ground grouting
- reduces the permeability of the ground.
- Improve the strength and stabilize the soil.
The grouting can be performed from the ground surface or from an adjacent shaft in the case of excavation for a tunnel or from within the excavation itself. Grouting can be used to stabilize an existing structure foundation that likely to be impacted by the excavation.
For tunnel grouting. The holes for injecting grouting are drilled at an acute angle to the axis of the tunnel to form overlapping cones of treated ground. Also, it can be drilled from the face of the tunnel. For TBM method the holes can be drilled forward from the rear of the machine to avoid disturbing the cutter wheel. Also, grouting can be performed toward the face of the tunnel and radially using the shield for shield TBM. Figure 1 shows different forms of ground grouting for tunnels excavation.
Figure 1 Examples of grouting tunnels during construction, a) from within a tunnel, b) using an adjacent tunnel (after Woodward 2005), c) from the ground surface, d) from an adjacent shaft or e) as protection to adjacent structures
Figure 2 shows the tube-a-manchette (TAM) or sleeved tube. The TAM will perform a successive injection of grout at a specified location. The pipe perforated at a specific location. The perforation is closed by a plastic sleeve. The plastic sleeve will open to allow the grout to flow outside. The grout is passed to the injection point by a movable separate internal tube.
Types of grouting technique:
- PERMEATION GROUTING (CHEMICAL GROUTING): This technique fills the voids in the soil with either chemical or cement binders with the intention of not disturbing the fabric of the ground. The range of particle sizes over which it can be applied is from sands (0.06 mm) to coarse gravels (60 mm).
- JET GROUTING: these techniques use the high-pressure jets to break the soil and replace it with a mixture of excavated soil and cement. Jet grouting can be performed in pre-drilled holes or by self-drilling. the jet grouting used for clays ( 0.002 mm) to fine gravels (10 mm). Once the jet reached the desired depth, it will rotate and pour the pressurized fluid while moving upwards. The output of the process is a column, as shown in figure 3. there are three jetting systems. The first system involves jetting grouting liquid only. The second system involves jetting grouting liquid with air. In third system grout injected from one hole and from another hole air and water pumped. The selecting of the system type will depend on the soil type. For the sand single system is suitable. Other systems will be ideal for finer grains.
- COMPACTION GROUTING: In this technique, a low slump grout (25mm-100mm) is injected to the ground. The injection of the stiff grout will form an expanding bulb. The expansion will densify and compact the soil around grout and improve the soil properties. The method is carried out by either drilling or driving small diameter casings (89–114 mm typically) to the required depth, withdrawing the rods or knocking off the drive point and then pumping the grout to the bottom of the hole. The range of applicable soils for this method is similar to permeation grouting ranging from sands (0.06 mm) to medium gravels (30 mm).
- Figure 5
- Stability: grouts should remain stable during the mixing and injection processes and not separate prematurely in the case of suspension grouts, or set prematurely if it is a liquid grout.
- Particle size: particle size will determine the lower limit of the grain size that can be penetrated by suspension grout.
- Viscosity: viscosity will determine the ability of the grout to penetrate the ground. Other flowing properties and gelling time will determine the maximum injection radius.
- Strength when set or gel strength