September 7, 2022
Mechanism Analysis of High-Pressure Jet Grouting Pile
High-Pressure Jet Flow Structure
Single-tube high-pressure rotary jet grouting uses high-pressure jet cement slurry flow to cut soil, and its jet structure can be explained by the mode of a continuous high-speed water jet in the air. Assuming that the jet flow is not mixed with the surrounding air, the jet boundary is at atmospheric pressure, the friction force is ignored, the nozzle is subjected to external force, and the flow rate at the nozzle outlet is uniform, the high-pressure jet structure can be divided into three regions.
The initial region: includes the jet core and the migration section that maintains the nozzle outlet pressure. The axial dynamic pressure of the jet core is constant and the velocity is uniform, which is infiltrated by the boundary airflow, and this part becomes smaller and even disappears. There is a transition section at the end of the jet core, which is called the migration section. The diffusion width of the jet core in this section increases slightly, and the axial dynamic pressure decreases.
Main area: the axial dynamic pressure suddenly decreases, the jet velocity further decreases, its diffusion rate is constant, and the diffusion width is proportional to the square of the distance. When high pressure is sprayed in the soil, the jet flow and the soil are mixed in the region.
End zone: the kinetic energy of the jet stream is depleted, the jet stream becomes an intermittent flow, and the end is atomized, mixed with air, and finally dissipated in the atmosphere.
The effective length of high-pressure jet grouting reinforcement is the sum of the length of the initial area and the main area. If the effective length is longer, the distance of the soil is larger, and the diameter of the pile is larger. Different media, different effective range. For example, in the air resistance is small, and the range is far, in the water jet diffusion is fast, and dynamic pressure drop near the effective range; the jet resistance in the soil is greater and the effective range is closer.
In the air, water is sprayed at a pressure of 20 MPa.Within 8 cm from the nozzle, the pressure can be kept constant, and the pressure gradually decreases beyond 8 cm. The water is sprayed within 3 cm from the nozzle, which can maintain 20 MPa, and the pressure drops sharply when it exceeds 3 cm. In order to improve this situation, if an annular gap is set around the nozzle and the airflow is sprayed simultaneously with the jet water, the damage ability and action distance of the jet will increase. When the air velocity is close to the speed of sound, the airflow will not spread rapidly and remain around the jet water to form a jet state, two substances like a dense jet out, until the speed is reduced.
The double pipe and triple pipe jetting method is developed based on the above theory. The use of gas, water, and slurry coaxial injection can obtain a greater injection effect and larger pile diameter. In addition, the single pipe is based on the cement slurry as the energy-carrying medium of the jet, its consistency and viscous force are larger, and the diameter of the formed jet grouting pile is smaller, while the triple pipe is based on the water as the energy-carrying medium of the jet, and the resistance of the water flow in the pipeline is smaller than that of the mud. Under the same pressure, the water is used as the jet medium, and the destructive force of the soil is larger, so the diameter of the formed jet grouting pile is larger.