Jet Grouting

Jet Grouting

History

Jet grouting has established itself as a leading technique in soil improvement within the construction industry. Known for offering effective solutions to a diverse range of geotechnical challenges, jet grouting has a rich history dating back to 1962 when Cementation Co., Ltd. implemented the first cut-off wall in Pakistan.

Initially developed in Japan, the technique gained momentum as specialists explored the use of high-speed jets to cut through rock and similar materials, ultimately applying these principles to soil improvement. The process involves injecting fluid binders into pre-drilled boreholes, eroding and mixing the soil in situ to create cemented soil bodies. The first patented version, Chemical Churning Pile (CCP), utilized chemical binders, which were later replaced by water-cement grouts.

In the early 1980s, jet grouting was introduced to the United States. Despite initial legal uncertainties, it quickly gained traction due to its practical and cost-effective solutions for various geotechnical issues, including excavation support, groundwater barriers, slope stabilization, and underpinning existing foundations.

Today, jet grouting is utilized worldwide, including South American countries like Brazil, and is recognized as an essential technique in modern soil improvement practices.

 

Jet Grouting Systems

Jet grouting stands out as a vital technique in soil improvement, highly valued for its ability to address various geotechnical challenges. Three primary systems define this method: Single fluid, Double fluid, and Triple fluid, each distinguished by the number and type of fluids injected into the subsoil.

 

Single Fluid System

The single fluid jet grouting system injects a water-cement (W-C) grout into the ground through one or more nozzles. This technique not only remoulds the soil but also cements it simultaneously using the same fluid. By injecting neat cement grout at high pressure through a small nozzle, it mixes effectively with the in-situ soil. This method yields highly homogeneous soil-cement columns or walls, known for their strength and minimal grout spoil return. It remains the simplest and most efficient among jet grouting systems, making it the preferred choice for projects requiring robust and uniform soil stabilization.

 

 Double Fluid System

The double fluid jet grouting system combines soil disaggregation and cementation through a water-cement grout jet, enhanced by a coaxial air jet. This air jet reduces energy losses and allows the grout to travel farther, forming larger columns. A variation of this system uses water jets in the upper nozzles and W-C grout in the lower ones, which separates the eroding and cementing actions. While this version resembles the triple fluid system, the air-assisted approach can reduce column strength and increase spoil return compared to the single jet system.

 

Triple Fluid System

The triple fluid jet grouting system clearly distinguishes between soil remoulding and cementation. A high-velocity water jet, delivered through an upper nozzle, disaggregates the soil and is shrouded by a coaxial air jet to minimize energy loss. The W-C grout is then injected through a separate lower nozzle at a lower velocity, specifically for cementing the remoulded soil. In some cases, both water and grout are injected at high speeds, subjecting the soil to dual erosion stages, which can expand the treatment radius. This system effectively erodes the soil and fills the voids with grout, providing durable soil stabilization.

 

Grout & Spoil Features

The grout mix used in jet grouting typically comprises water and cement, with weight ratios (W/C) ranging from 0.6 to 1.3. Bentonite, a commonly used additive, is often included in the mix as a suspension to reduce bleeding, especially when the W/C ratio is high, and high strength is not required.

There are two primary methods to incorporate Bentonite into the grout:

1. Static Mixer Method: Bentonite is added by inserting a static mixer after the agitator, just before the high-pressure pump, to minimize the risk of gel setting within the circuit.
2. Direct Injection Method: Sodium silicate is injected through a separate conduit, mixing with the cement directly in the ground.

 

During the jet grouting process, a portion of the injected grout and eroded soil rises to the surface through the annular gap formed between the pipe string and the borehole wall, creating spoil. A moderate amount of spoil is crucial for effective treatment, as the upward flow prevents hole clogging. However, minimizing spoil is essential for cost-effectiveness and addressing the challenges of disposal or recycling under increasingly stringent environmental regulations.

 

Key Equipment in Jet Grouting

The essential tools for jet grouting utilities include the manufacturing system of the W-C grout, grout and water pumps, air compressors, drilling rigs, string rods, monitors, and hydraulic circuits (high-pressure hoses and connecting junctions).

 

Pumping System

The Core Element in jet grouting technology is the pumping system. This system requires extremely high-pressure pumps, which can reach pressures up to 50 MPa or more for pumping grout in single and double fluid systems and water in the triple fluid system. In the triple fluid system, grout pressure typically does not exceed 10 MPa.

 

Air Compressor

An air compressor is crucial in double and triple fluid systems, providing pressure ranges between 1.2 and 2.5 MPa with airflow rates around 200–300 l/s

 

 Drilling and Grouting Rigs

Drilling and grouting rigs have different characteristics for outdoor and underground works:

• Equipped with hollow rods with diameters ranging from 60 to 140 mm.
• Special rods for rotary-percussion drilling to pass through very stiff soil layers, rock blocks, or masonry.

 

Column Properties: Diameter of Columns

 

The diameter of the columns is influenced by jet hydrodynamic properties and the soil's resistance to erosion. Thus, it's crucial to select the appropriate technique, considering the jet grouting system and treatment parameters based on the soil and required diameter.

 

Mean Diameter of Columns

The mean diameter of the columns is a critical parameter. It can be affected by factors such as the type of soil and depth. As depth increases, the soil's shear strength typically increases due to the overburden effective stress. This increase in soil strength can reduce the diameter of the columns, which is significant in soils with high friction angles. In soils with lower friction angles, the diameter reduction with depth is usually negligible.

 

It's suggested to treat the diameter as a stochastic variable with a probability distribution. This approach helps to account for the natural variability in soil conditions.

To achieve the desired mean diameter of columns, one should:

• Select an appropriate injection technique (single, double, or triple fluid) based on the specific soil.
• Set proper treatment parameters, including the number and diameter of nozzles, injection pressure and/or flow rate, lifting speed of the monitor, and grout mix composition.

• This detailed consideration ensures that the desired column diameter is achieved, tailored to the specific conditions of the soil being treated.

 

Jet Grouting Applications

 

Jet grouting is a widely used method for soil improvement, suitable for various soil conditions. It has been applied in numerous scenarios, including:

 

• Densification of Granular Soils: Enhancing the density and strength of loose sandy soils.
• Raising Settled Structures: Elevating structures that have settled unevenly.
• Settlement Control: Managing and reducing soil settlement issues.
• Underpinning of Existing Foundations**: Providing support and stability to existing foundations.
• Excavation Support: Stabilizing the soil surrounding excavation sites.
• Protection of Existing Structures during Tunneling**: Safeguarding buildings and infrastructure near tunneling operations.
• Liquefaction Mitigation: Preventing soil liquefaction in earthquake-prone areas.
• Water Barriers: Creating impermeable barriers to control water flow.
• Hydraulic Cutoffs: Preventing the flow of water in specific areas.
• Tunnel Canopies and Vertical Shafts: Supporting and stabilizing tunnel roofs and vertical shafts.

 

 

Jet Grouting Controls

 

Jet grouting involves a series of Control methods, Field trials, Property assessments, and Load tests to ensure efficiency and safety.

 

Control Methods

The various control methods serve several main purposes:

• Ensuring Material Quality: Ensuring that the basic materials adopted for jet grouting possess adequate characteristics.
• Checking Construction Procedures: Checking that the construction procedure is correctly carried out and that the equipment is working properly.
• Quantifying Dimensions and Properties: Quantifying the dimensions and properties of the jet-grouted elements.
• Verifying Performance: Verifying the performance of jet-grouted structures.
• Monitoring Environmental Effects: Monitoring the jet grouting effects on the surrounding environment and structures.

 

Field Trials

Field trials aim to:

• Select the Best Jet Grouting System: Choose the most appropriate jet grouting system and treatment parameters.
• Assess Treatment Results: Evaluate the treatment results with respect to the project requirements.
• Check Environmental and Structural Effects: Monitor the effects of treatments on the surrounding environment and structures.
• Refine Control Procedures: Improve the control procedures to be implemented during construction.

 

Treatment Control

• Materials Qualification: Ensuring the materials used in the jet grouting process meet the necessary standards.
• Grout Preparation: Proper preparation of the grout mixture.
• Drilling and Grouting: Ensuring precise drilling and grouting procedures.
• Spoil Return: Managing the return and disposal of spoil material.

 

Properties of the Jet-Grouted Structure

The properties of the jet-grouted structure include:

• Control of Geometrical and Mechanical Characteristics: Ensuring the geometric and mechanical properties of the jet grouting elements.
• Control of Performance: Assessing the performance of jet-grouted elements and structures.
• Diameter of Columns: Determining the diameter based on soil resistance and jet hydrodynamic properties.
• Continuity and Homogeneity: Ensuring continuity and homogeneity of jet-grouted elements.
• Physical and Mechanical Properties: Monitoring the physical and mechanical properties of the jet-grouted material.

 

Load Tests

Axial load tests can be performed with the same procedures recommended for foundation piles. Specific requirements for jet-grouted columns usually include the regularisation of the column head (which must be plane and horizontal) and the insertion of a steel reinforcement into the upper part of the column to prevent local buckling. Due to their dimensions and irregular shape, jet-grouted columns are often similar to large diameter piles, capable of transferring high loads to the surrounding soil.