Through the coordinated hole-forming of high-pressure jet and spiral composite drilling tools, during the rotary drilling process, the high-pressure jet grouting mixing function is used to complete the cement-soil outer pile while forming the hole. After drilling to the design depth, concrete is pressure-grouted. While lifting the drill pipe, concrete is continuously pumped, and at the same time, rotary jet grouting is applied to the side wall of the pile hole for supplementary grouting. After lifting the drill pipe above the pile top elevation, the reinforcement cage is implanted into the concrete pile body to form a rigid cement-soil integrated pile.
 
　　Equipment:

　　It mainly consists of three parts: the pile frame, the drilling system, and the rotary jet grouting system. The drilling rig shown uses a Beibiao 580 power head, driven by a single 75KW motor, a 3e120 variable-frequency grouting pump, with a power of 90Kw, a pressure of 12MPa, and a flow rate of 0-120 liters/minute.




　　Composite drilling tools:
　　The spiral soil-displacement drilling tool is equipped with side and vertical high-pressure nozzles on the outside. A grouting pipe is arranged on the outside of the drilling tool or between the outer pipe and the core pipe. The drill bit is provided with vertical nozzles and horizontal nozzles, allowing vertical and radial grouting.
　　A grouting rotary joint is installed at the lower end of the power head, and then connected to the drill pipe.
　　Drill pipe: outer diameter Φ245mm (the thickest part of the drilling tool structure is Φ360mm), 3-meter threaded drill pipe, 12-meter plain pipe; total length of the drilling tool is 16 meters;
　　Drill bit: outer diameter Φ400mm, length 1 meter, Φ500mm at the nozzle protection part of the lead head, with two nozzles: side jet and down jet.


　　3. Process Flow

　　Position the drilling rig and align with the hole → rotary jet grouting test → rotary drilling while performing high-pressure rotary jet grouting to form the cement-soil outer pile → stop drilling at the design depth → while lifting the drill pipe, pump concrete into the hole through the inner cavity using a concrete pump, and at the same time perform rotary jet grouting on the side wall of the borehole for supplementary grouting → lift the drill pipe above the pile top elevation → move the drill pipe away and place the reinforcement cage → pile completion

　　4. Features of Rotary Jet Grouting Enlarged Diameter Screw Soil-Displacement Cast-in-Place Pile

　　(1) Reliable pile quality and high cost-effectiveness
　　(2) Low torque and fast drilling speed
　　(3) High single pile bearing capacity
　　(4) Wide applicability
　　(5) Green and environmentally friendly, energy-saving and emission-reducing
　　There is no loose soil at the pile end; concrete is pressed into the hole from the center of the drill pipe, resulting in dense material and fewer occurrences of pile breakage, necking, and hole collapse; the soil around the pile hole is integrated with the pile by the rotary jet grouting action, and at the same time compacted by the spiral extrusion.
　　The high-pressure jet from the vertical nozzles at the front end of the drill bit meets the requirements of cutting soil, pre-emulsifying, cooling the drill bit, and reducing resistance, achieving low torque, low rotational current, and improved drilling speed. 　　“Four longitudinal enlargements, four transverse zones” greatly improve the single pile bearing capacity.
　　“Four enlargements” refer to enlarged top, enlarged diameter, enlarged support plate, and enlarged head.
　　“Four zones” refer to the concrete core pile zone, the high-pressure rotary jet grouting core consolidation zone, the rotary jet grouting cement-soil consolidation zone, and the high-pressure rotary jet grouting slurry hydraulic compaction and permeation zone.
　　The cement-soil outer pile formed by high-pressure rotary jet grouting and the concrete core pile are formed in one pass, so the cement-soil outer pile and the core pile are an integrated pile, which increases the equivalent diameter of the pile and greatly improves the bearing capacity.
　　(4) Wide applicability
　　Suitable for silt, mucky soil, fluid-plastic, soft-plastic or plastic cohesive soil, silty soil, sandy soil, plain fill and gravel soil layers, weathered rock, and also suitable for various soil layers with groundwater.
　　a. Pile foundation (enlarged diameter, enlarged support plate, enlarged head)
　　b. Composite foundation (enlarged top, enlarged diameter, enlarged support plate, enlarged head)
　　c. Uplift pile
　　d. Water-stop and support integrated pile 
　　(5) Green and environmentally friendly, energy-saving and emission-reducing
　　No spoil, no dewatering, low noise, no disturbance to residents, and a relatively clean construction site.
　　Low concrete consumption per unit bearing capacity, and low cost per unit bearing capacity.

     (This article is excerpted by the Foundation Engineering Network from the 2016 Cross-Strait Geotechnical Engineering/Geotechnical Technology Exchange Seminar Sub-venue Report, Reporter: Wang Qingwei)