Jet Grouting
Jet grouting involves the erosion of the soil by cement grout, jets of water, and/or compressed air, and the mixture of the grout with the soil to form grouted columns or walls. The grout pipe with jets may be self-drilled or inserted into a predrilled hole. The jetting fluids are pumped at high pressure while the pipe is withdrawn with rotation for columns or without rotation for walls.
Jet grouting’s ability to construct soilcrete in limited spaces and around subsurface obstacles such as utilities, provides unique design flexibility. In any situation requiring control of groundwater or excavation of unstable soil (water-bearing or otherwise) jet grouting is often a preferred solution.
The procedure for jet grouting operation typically includes the following steps:
1. Position the drilling machine at a desired location for injection.
2. Drill into the ground to the desired depth. The tolerance of the drilling location should be less than 50 mm.
3. Insert the jet grouting pipe to the predrilled grout hole. Some machine installs the jet grouting pipe directly with an auger at the tip.
4. Once the pipe reaches the desired depth, start the jet grouting process by withdrawing the pipe and injecting grout until reaching a desired elevation. The operation should follow the specifications for injection pressure, injection rate, injection time, pipe withdrawal rate, and pipe rotation rate.
5. Clean up the injection pipe and other associated tools.
6. Move to the next jet grouting location.
During Jet grouting procedure, high pressure flow of cement grout erodes soil body and makes a soil-cement combined column that can improve mechanical properties of soil.
Water jet and drilling equipment
Selection of Jet System
The single jet system injects neat cement grout through a small nozzle at high pressure, which is mixed with in-situ soil. This method produces the most homogeneous soil–cement columns or walls with the highest strength and the least amount of grout spoil return. It is the simplest system among the three systems and has more choices of qualified specialty contractors.
The double jet system injects neat cement grout at a lower pressure, which is aided by a cone of compressed air. The air reduces the friction loss and allows the grout to travel a farther distance to produce a greater column diameter. However, the presence of the air reduces the strength of the column and produces more spoil return than the single jet.
The triple system injects water at high pressure, which is aided by a cone of compressed air. This process produces an air lifting effect, which erodes the soil. The grout is injected at a lower pressure through a separate nozzle below the water and air nozzles to fill the void created by the air lifting process.
Design principles
Jet grouting columns are used to increase bearing capacity, control settlement, mitigate liquefaction potential and reduce soil permeability. In order to reach to aforementioned goals jet grouting columns can be designed like piles. For instance toe and skin friction resistance of jet grouting columns should be considered in design procedure and their capacity will be determined based on their geotechnical and structural capacity.
Structural capacity is determined using unconfined compression strength of soilcrete samples. Unconfined compresion strength of soilcrete can be obtained from 28-day samples taken from sacrificial columns.
Application:
Jet grouting is a widely used method of soil improvement and can be acceptable in various soil conditions. Jet grouting has been used for the following applications:
- Densification of granular soils
- Raising settled structures
- Settlement control
- Underpinning of existing foundations
- Excavation support
- Protection of existing structures during tunneling
- Liquefaction mitigation
- Water control
Application of jet grouting method comparing to other methods
Quality control and assurance:
The general procedure for quality control and assurance are similar to that for deep mixing. Quality control for grouting may include the following steps:
- Evaluate the grouting record including the grout volume, injection pressure, rate of injection, time of injection.
- Inspect the integrity and uniformity of the grouted column or wall.
- Verify the dimensions of the grouted column or wall.
- Evaluate the strength of the grouted column.
Quality assurance includes the following field testing:
- Core samples of grouted columns or walls to verify the strength and stiffness of the stabilized soil
- SPT and CPT
- Single column and composite foundation loading tests
- Cross-hole geophysical testing
Advantages and Limitations
Jet grouting has the following advantages as compared with alternate technologies:
- No need for removal and replacement
- Effective for underpinning and protecting existing structures
- Easy to access and operate within constrained space
The limitations associated with grouting are:
- Quantity of grout is hard to estimate.
- Effectiveness of some applications cannot be predicted.
- Area of improvement is sometimes uncertain.
- Grouting may cause ground movement and distresses to existing structures.
- Certain chemical grouts may contain toxicity and have adverse impact to groundwater and underground environment.
- Specialty contractors are required for the operation.
