9.3.2 Jet grouting method
The jet grouting method implants a grouting pipe，with a jet，into the soil to a certain depth with the help of a boring machine and emits high pressure grout or water（about 20 MPa），pounding and cutting soils at the same time.When the pulsing current，at high pressure and speed，exceeds the strength of soils，the soil particles will be separated from the soil body.Some fine soil particles will flow with water or grout out of the ground while other soil particles will mix with the grout under the influence of pounding，centrifugal force and gravity.The mixed soil particles will be rearranged into a grout⁃soil mixture.When the grout is congealed，it forms a solid body.Figure 9.2 is a diagram of the sequence of jet grouting.
Some commonly used methods of jet grouting are the single tube method，the double tube method，and the triple tube method，as shown in Figure 9.3.The single tube method is to implant the boring rod with a special jet fixed on the side of the rod bottom with the help of the boring machine and to emit pressurized grout（about 20 MPa）into soil to cut and destroy soil bodies.With the rotation and lift of the rod，the grout and the cut soils will be churned and mixed.After a certain period，the whole of the grout and soil will congeal and form a column.The method is often called the CCP（chemical churning pile）method.
As shown in Figure 9.3（b），the configuration of the double tube is a coaxial double jet.The inner jet emits grouting materials at 20 MPa while the outer one pumps compressed air at 0.7 MPa.Under the double effect of jetting grout and the surrounding air pressure，the capability to destroy and cut soil is highly enhanced.With the rotational jetting and lift of the double jet，grouting materials and cut soils are churned and mixed and form a column⁃like solid body together.The diameter of the column formed with the double tube method is larger than that formed with the single tube method.The method is often called JSG（Jumbo Special Grouting）method，or JSP（Jumbo Special Pile）method.
Figure 9.2 Procedure of the jet grouting method
Figure 9.3 Types of the jet grouting method
The triple tube is a jet which emits water，air，and grouting materials simultaneously.As shown in Figure 9.3（c），on the side of the bottom of the grouting pipe is attached a coaxial double jet，the inner one of which emits water at about 20 MPa while the outer one emits compressed air at about 0.7 MPa.With the double contribution of high pressurized water and the surrounding air，the power of cutting and destroying soil bodies is greatly enhanced.Grouting materialsof 2⁃5 MPa are emitted from another jet right below the double jet.With the rotation and lift of the jet，the grouting materials and the cut soils will be churned and mixed and form a column⁃like solid body.The diameter of the column formed with the triple tube method is larger than that formed with the double tube method.The method is often called the CJG（Column Jet Grout）method.
The diameter of jet grouting piles depends on soil properties and jet pressure.The average piles diameter can be estimated according to Table 9.3.
Table 9.3 Diameter of jet grouting piles
The commonly used grouting materials for jet grouting are lime（calcium oxide or calcium hydroxide）and cement.Sometimes fine aggregate or fly ash is added.Besides，depending on the functions of the machine and the soil properties，fluid materials（such as cement mortar or cement grout）or dry materials（such as cement or lime powders）are employed.