The high-pressure jet grouting method uses a drilling rig to drill a grouting pipe with a nozzle into a predetermined position in the soil layer and then uses high-pressure equipment to make the slurry or water (air) become a 20-40MPa high-pressure jet that is ejected from the nozzle to punch, disturb, and destroy the soil. At the same time, the drill rod is gradually lifted at a certain speed to force the slurry and soil particles to be mixed. After the slurry solidifies, a cylindrical consolidation body (i.e., rotary jet pile) is formed in the soil to achieve the purpose of strengthening the foundation or stopping water and seepage.
1. It can be widely used in silt, silty soil, clay, silty clay, silt (sub-sand), sand, loess, and plain fill in artificial fill, even gravel soil, and other soil layers.
2. It can be used for foundation reinforcement of existing buildings and new buildings, and can also be used for foundation anti-seepage; it can be used as a temporary measure during construction (such as retaining soil or water on the side walls of deep foundation pits, waterproof curtains, etc.), and can also be used for foundation reinforcement and anti-seepage treatment of permanent buildings.
3. When used to treat peat soil or foundation projects with corrosive groundwater, excessive groundwater flow rate, and water gushing, it is advisable to determine its applicability through tests.
Different injection methods, can be divided into single-tube method, double-tube method, and triple-tube method.
Single-tube method: single-layer injection pipe, only cement slurry is injected.
Double pipe method: also known as slurry gas injection method, it uses double grouting pipes to simultaneously eject two medium jets of high-pressure cement slurry and air horizontally to impact and destroy the soil. Under the combined action of high-pressure slurry and the surrounding airflow, the energy for destroying the soil is significantly increased, and finally, a larger consolidated body is formed in the soil.
Triple pipe method: It is a slurry, water, and air injection method. It uses triple grouting pipes that transport water, air, and slurry respectively. A cylindrical airflow surrounds the high-pressure water flow generated by a high-pressure generating device such as a high-pressure pump. The high-pressure water jet and the airflow are coaxially injected to cut the soil to form a larger gap. The mud pump then injects cement slurry into the cut and broken foundation at a lower pressure. The nozzle rotates and lifts to mix the cement slurry with the soil, solidifies in the soil, and forms a larger consolidated body. The diameter of the consolidated body can reach 2m.
The reinforcement radius of the jet grouting method is related to many factors, including the jet pressure P, the lifting speed S, the shear strength τ of the reinforced soil, the nozzle diameter d, and the slurry consistency B. The reinforcement range is proportional to the jet pressure P and the nozzle diameter d, and inversely proportional to the lifting speed S, the shear strength τ of the soil, and the slurry consistency B. The strength of the reinforced body is related to the amount of cement added to the unit’s reinforced body and the soil quality.
The pile formation mechanism of high-pressure jet grouting includes the following five effects:
1. High-pressure jet flow cutting and destroying soil. The jet flow pressure impacts and destroys the soil in the form of pulses, causing holes in the soil and expanding cracks in the soil.
2. Mixing and stirring. During the rotation and lifting process of the drill rod, a gap is formed at the rear of the jet. Under the jet pressure, the soil particles are forced to move in the direction opposite to the direction of the nozzle (i.e., the direction with small resistance), and are mixed with the slurry to form a new structure.
3. Lifting and displacement effect (triple tube method). While the high-speed water jet cuts the soil, a part of the cut soil particles are discharged to the ground due to the introduction of compressed gas, and the gaps left after the soil particles are discharged are supplemented by cement slurry.
4. Filling, penetration, and consolidation. High-pressure cement slurry quickly fills the gaps between the opened grooves and soil particles, precipitates water for consolidation, and can also penetrate the sand layer to a certain thickness to form a consolidated body.
5. Compaction effect. During the process of cutting and crushing the soil layer, the high-pressure jet flow still has residual pressure at the edge of the crushed part and can produce a certain compaction effect on the soil layer, making the compressive strength of the edge part of the rotary jet pile body higher than that of the center part.
1. Pre-construction preparations
(1) Based on the various technical data provided in the design documents, supplementary engineering geological exploration is carried out to further understand the nature and burial conditions of the foundation soil at each construction site.
(2) Prepare sufficient cement reinforcement materials and water. The type, specification, and production time of cement shall be inspected by the laboratory to meet national standards and design requirements and have a quality certificate. It is strictly forbidden to use expired, damp, hardened, or deteriorated reinforcement materials. The general cement is No. 425 ordinary Portland cement. The water should be clean, with a moderate pH value, and the pH value should be between 5 and 10.
(3) Based on the supplementary exploration data, a necessary amount of original soil is collected from each representative stratum within the reinforcement range of the selected test site using a thin-walled soil sampler and sent to the laboratory. The soil samples obtained should be properly preserved before testing to keep the physical and chemical properties of the soil samples as unchanged as possible.
(4) Indoor mix ratio test. According to the design requirements of the spraying volume or the situation of the on-site soil samples, several mix ratios are designed and adjusted according to different water contents. The soil samples taken on-site are air-dried (dried) and crushed indoors. The powdered soil samples are passed through a 2-5 mm sieve and mixed, cured, and mechanically tested according to the designed spraying volume and water-cement ratio to determine the construction spraying volume and water-cement ratio. The general water-cement ratio can be 1.0-1.5. To improve the performance, anti-sedimentation performance, and strength of cement soil, calcium lignin sulfonate, gypsum, triethanolamine, sodium chloride, calcium chloride, sodium sulfate, clay, alkali, and other admixtures can be appropriately added. If the water content of the soil sample changes before the test, it should be adjusted to the natural water content.
(5) Pile test. According to the construction spraying volume and water-cement ratio determined by the indoor test, the cement slurry is prepared and several test piles are set at the test site. According to the test pile results, the spraying volume of the reinforcement material is adjusted, and the construction process parameters such as the lifting speed of the mixing pile mixer, the rotation speed of the mixing shaft, the injection pressure, and the stop slurry surface are determined.
(6) Bulldozers and excavators cooperate with dump trucks to remove 0.3m thick planting soil and debris on the ground surface, level the original ground according to the design requirements, and fill the road arch. According to the actual situation of the construction site, temporary drainage and water interception facilities are constructed, and waste mud pools and drainage ditches between the construction holes and the mud pools are excavated outside the construction scope.
(7) Complete the construction layout according to the design requirements, use wooden stakes to determine the pile positions, and use white lime to make obvious marks.
2. Construction process
(1) Drilling rig positioning. Move the rotary jet pile driver to the designated pile position, align the drill bit with the center of the hole position, and level the drill rig at the same time, place it stably and horizontally, and the verticality deviation of the drill rod is not more than 1% to 1.5%. After positioning, first conduct a low-pressure (0.5MPa) water jet test to check whether the nozzle is unobstructed and the pressure is normal.
(2) Prepare cement slurry. When the pile driver is moved, start mixing cement slurry according to the mix ratio determined by the design. First, add water to the bucket, then pour in cement and admixtures, start the mixer and stir for 10 to 20 minutes, then unscrew the valve at the bottom of the mixing bucket, put in the first screen (aperture 0.8mm), filter it and flow into the slurry pool, then pump it into the second filter (aperture 0.8mm) through the mud pump, filter it for the second time and flow it into the slurry bucket for use when grouting.
(3) Drilling (triple pipe method). When drilling with a geological drill, the drill bit drills a hole at the predetermined pile position to the designed elevation (the pre-drilled hole diameter is 15cm).
(4) Inserting a pipe (single pipe method, double pipe method). When using a rotary jet grouting pipe for drilling, the two processes of drilling and inserting a pipe can be combined into one. When the first stage is penetrated the soil, the jet or vibration of the jet pipe itself can be used for penetration. The process is as follows: start the drilling rig and start the high-pressure mud pump to deliver cement slurry at low pressure, so that the drill pipe vibrates along the guide frame and sinks by jetting until the pile bottom is designed. The working current should not be greater than the rated value. After the triple-tube drilling rig drills a hole, pull out the drill pipe and insert the rotary jet pipe. During the pipe insertion process, to prevent mud and sand from clogging the nozzle, a small pressure (0.5-1.0 MPa) can be used to spray water while lowering the pipe.
(5) Lift the spray pipe and stir. After the spray pipe sinks to the designed depth, stop drilling and rotate continuously. The pressure of the high-pressure mud pump is increased to the construction design value (20-40 MPa). After 30 seconds of bottom spraying, spray and rotate at the same time. At the same time, lift the drill pipe strictly according to the lifting speed determined by the design and the test pile. If the double-pipe method or triple-pipe method is used, after reaching the designed depth, connect the high-pressure water pipe and air pressure pipe, start the high-pressure clean water pump, mud pump, air compressor, and drilling rig to rotate, and use instruments to control the pressure, flow and air volume. When the preset values are reached, start lifting, continue to spray, and lift until the expected reinforcement height is reached, and stop.
(6) Treatment of the pile head. When the rotary spray pipe is lifted close to the pile top, it should be lifted slowly from 1.0m below the pile top, sprayed for a few seconds, and then slowly lifted upward by 0.5m until the pile top stops.
(7) If encountering gravel strata, in order to ensure the pile diameter, repeat the spraying and mixing: repeat the spraying and mixing according to the above steps 4 to 6 until the spraying pipe is lifted to the top surface, turn off the high-pressure mud pump (clean water pump, air compressor), stop the delivery of cement slurry (water, air), rotate and lift the rotary spray pipe out of the ground, and turn off the drilling rig.
(8) Cleaning. Pour an appropriate amount of clean water into the slurry tank, turn on the high-pressure pump, and clean the remaining cement slurry in all pipelines until they are clean. And clean the soil adhering to the grouting pipe head.
(9) Shift. Move the pile driver to construct the next pile.
(10) Grouting. After the jet grouting operation is completed, due to the water separation effect of the slurry, there is generally a certain degree of shrinkage, resulting in a concave hole on the top of the consolidation body. It is necessary to promptly fill it with cement slurry with a water-cement ratio of 1.0.
3. Recommended main construction technical parameters
(1) Single-tube method. Slurry pressure 20-40MPa, slurry specific gravity 1.30-1.49, rotary spraying speed 20rmin, lifting speed 0.2-0.25m/min, nozzle diameter 2-3mm, slurry flow rate 80-100L/min (depending on the pile diameter, the flow rate can be increased).
(2) Double-tube method. Slurry pressure 20-40MPa, compressed air pressure 0.7-0.8MPa. (3) Triple tube method. Slurry pressure 0.2-0.8MPa, slurry specific gravity 1.60-1.80, compressed air pressure 0.5-0.8MPa, high-pressure water pressure 30-50MPa.
2. Key points of quality control
(1) Before the official start of construction, the test pile should be carefully carried out to determine the reasonable construction technical parameters and slurry ratio.
(2) During the rotary jetting process, it is normal for the slurry volume to be less than 20% of the grouting volume. If it exceeds 20% or no slurry is emitted at all, the cause should be found out, and the rotary jetting parameters should be adjusted or the nozzle diameter should be changed.
(3) The rotation and lifting of the drill rod must be continuous and uninterrupted. When disassembling and extending the drill rod or continuing the rotary jetting, the drill rod should be kept at a 10-20 cm overlap length to avoid broken piles.
(4) During the rotary jetting process, if the rotary jetting is interrupted due to mechanical failure, it should be re-drilled to the designed elevation of the pile bottom and then the rotary jetting should be restarted.
(5) When making the slurry, the water-cement ratio should be strictly controlled according to the design and should not be changed at will. During the rotary jetting process, the mud should be prevented from settling and the concentration should not be reduced. Moist or expired cement should not be used. After the slurry is stirred and sent to the suction bucket, it should be filtered with a screen. The sieve hole should be smaller than 1/2 of the nozzle diameter.
(6) During the rotary spraying process, if there are isolated rocks or large boulders, the piles can be moved appropriately (according to the stress conditions, piles can be added if necessary) to avoid deformed piles or broken piles.
3. Treatment of common quality problems
(1) No slurry or small amount of slurry: The usual reason is that the particle size of the reinforced soil layer is too large and there are many pores. The following measures can be taken.
1) Increase the slurry concentration, which can be increased from 1.1 to about 1.3, and continue to spray.
2) Pour clay slurry or add fine sand or medium sand, and continue normal spraying after the pores are filled.
3) Add aggregate to the slurry.
4) Add mud balls to seal and continue normal spraying.
5) After pouring cement mortar, replace the cement slurry in the hole with clay slurry, and continue normal spraying after the pores are filled.
(2) Excessive grouting volume: This is usually related to the incompatibility of the effective spraying range and the spraying volume. The following measures can be taken.
1) Increase the spraying pressure.
2) Appropriately reduce the nozzle diameter.
3) Appropriately increase the lifting speed. Since the grouting volume contains a mixture of formation particles and slurry, there is currently no suitable method for separating and recovering the cement in the grouting. In construction, filtration, sedimentation, recovery, and concentration adjustment are often used for reuse.
(3) Cavity treatment.
1) When the jet grouting is completed, the grout is continuously or intermittently injected into the jet hole with static pressure until the mixed liquid in the hole solidifies and no longer sinks.
2) After the jet grouting is completed, a second static pressure grouting is carried out into the gap between the solidified body and its upper structure. The slurry ratio should be a non-shrinking and expansive material, such as a slurry with a cement: water: aluminum powder ratio of 9.8:6.9:0.3.
1. High-pressure mud pumps, air compressors, and high-pressure clean water pumps must be operated by designated personnel, and pressure gauges should be regularly inspected and calibrated to ensure normal operation.
2. Drilling rig operators should have proficient operating skills.
3. High-pressure equipment and pipeline systems should be checked before construction. Their pressure and flow (air volume) must meet the design requirements. The durability of the pipeline and whether the pipeline connection is reliable should be checked. There should be no debris in the pump body, grouting pipe, and nozzle. All kinds of sealing rings must be in good condition and leak-free. Otherwise, the joints will be disconnected and the hoses will rupture, which will lead to safety accidents such as slurry, high-pressure water flow, and hose throwing out. The safety pin in the safety valve must be pressure tested (pressure test to the local pressure vessel inspection institute for inspection). It must be ensured that the pin can be broken and the pressure can be relieved when the specified pressure is reached. Self-made safety pins that have not been pressure tested must not be easily installed.
4. When the jet slurry is ejected from the nozzle, it has very high energy, so the distance between the human body and the nozzle should not be less than 60cm.
5. The slurry for jet grouting is currently mainly cement slurry, but sometimes other chemical additives are added. Generally speaking, the slurry is harmless to humans and animals after hardening, but if the liquid before hardening enters the eyes, it must be fully cleaned immediately and treated in a hospital in time.
6. When hanging or placing the jetting pipeline, it is strictly forbidden to stand under the pipe. The cable must not be soaked in water and mud to prevent leakage and injury.
1. Fine-grained bulk materials such as cement and lime should be covered for storage; they must be covered during transportation and must not be scattered along the way.
2. Formulate measures to prevent dust by sprinkling water, and designate a person to be responsible for sprinkling water and reducing dust on site.
3. Take effective noise reduction measures.
4. Abandoned reinforcement materials must not be discharged at will.
===Thanks===
Bored cast-in-place piles have the advantages of low construction noise, and low vibration, pile length…
Bored cast-in-place piles have the advantages of low construction noise, and low vibration,…
The four major construction technologies are dry hole forming technology, slurry static pressure technology, casing…
Soil nail support is a new type of retaining structure developed in recent years for…
Pile Foundation Classification and Construction Principles 1. Classification According to Load-Bearing Piles ① Friction type…
Steel sheet pile cofferdams are suitable for basic projects such as sandy soil, gravel soil,…
