Research On Large Diameter Singular-Jet Grouting Method And Its Engineering Application

The processes of jet grouting method are as follows: First, the special high pressure jetting head is drilled (auto-drilling) or driven (pre-drilled) into the planned depth in the soil by the drill rig. Then, the soil is directly cut and broken or cut and broken by the injected high pressure water, and simultaneously, the low pressure cement slurry jet is jetted and mixed with the broken soil. Finally, a certain soilcrete with high strength, low compressibility, and low permeability is formed after a series of physical and chemical reactions. With these processes, the ground is reinforced and improved.According to different media transmitted into the soil, there are three types of jet grouting methods, i.e., singular-jet grouting method, dual-jet grouting method and triple-jet grouting method. In 1973, the singular-jet grouting method was invented in Japan. Since then, with the continuous development of equipment manufacturing technology and application of electronic technology, various new jet grouting methods have been developed by Japanese. The soilcrete produced by these new methods is of larger pile diameter (up to 5.0m with RJP method) and deeper reinforcing depth (up to 80m with MJS method). The successful development of new methods and equipment has largely expanded the application range of high pressure jet grouting. With remarkable advantages such as simple equipment, easy operation and easy quality control, the singular-jet grouting method has been applied in different types of ground reinforcement. However, the singular-jet grouting method has a most obvious disadvantage, that is, the produced pile diameter is relatively smaller. Based on theoretical analysis, jet grouting tool development, field test result analysis, and engineering application, this paper studied thoroughly how to increase the pile diameter of the singular-jet grouting method and expand its engineering application range.1. Component characteristic indexes of the grouted soilcrete:The grouted soilcrete mainly consists of three components: water, cement grains and soil grains. The proportion relationship between these three components determines the strength characteristics of soilcrete. Based on the basic hypotheses such as mass balance, volume balance, soilcrete homogeneity, and limited reinforced range, the proportion relationship between these three components for a certain soil, jet grouting parameters and reinforced diameter could be achieved.Theoretically, the key indexes of soilcrete formed by the singular-jet grouting method could be calculated as follows:Where: Gc—Specific gravity of cement particle; R_g—Water/cement ratio of slurry;ρ_s—soil density(kg/cm3);ω_s—Water content of the soil; q —Slurry flow rate(cm3/min);Lv—Lifting velocity of the jetting head(cm/min);D—Formed pile diameter(cm)For pre-determined jet parameters and soil physical indexes, the above mentioned characteristic indexes of the soilcrete were only related to the expected pile diameter. These indexes can be used for the laboratory strength mixture ratio experiment for different pile diameters. The measured in situ densityρmo could be used to predict the pile diameter. Also, the initial water/cement ratio could be used to predict the strength of soilcrete.2. Relationship between uniaxial compressive strength fcu and initial water/cement ratio RmoThe singular-jet grouting method used the high pressure slurry jet to erode and cut soil to produce a certain soilcrete. In addition to the original water content of soil, the injected high pressure slurry jet also increased the water content of soilcrete evidently. Therefore, the influence of water on the strength of soilcrete was very significant for the jet grouting method. It has been resulted that the relationship between uniaxial compressivestrength f_cu and initial water/cement ratio of the soilcrete could be described asWhere, a, b, and n could be determined by jetting parameters, soil properties, cement types and curing time.The uniaxial compressive strength f_cu of soilcrete of marine soft clay in Lianyungang on the 28th day could be expressed as the following formula:3. Relationship between diameter and bearing capacity of the pileWhen the grouted pile diameter increased, the initial water/cement ratio would increase, and the pile strength would decrease. However, the bearing capacity of pile would increase. When the grouted pile diameter was changed from 0.5m to 1.0m, the pile bearing capacity would increase by 80%.4. Development of new types of large diameter singular-jet grouting methodBased on analysis of key factors on grouted pile diameter, two nozzles, one installed on the surface of the jetting head, and the other controlled by the hydraulic cylinder or installed on the fixed horizontal shaft spread horizontally from the surface of jetting head with a certain distance, were installed on one jetting head in the newly invented large diameter singular-jet grouting method. After first cut by the jet from the nozzle on the surface of the jetting head, the soil was cut and broken repeatedly by the jet from the spreading nozzle. Thus the pile diameter would be greatly increased (Figure 1).Method A: The singular-jet grouting method with a telescoping hydraulic nozzle (A). This jetting tool has a single channel. After the jetting head was placed in the pre-determined depth in the soil, the down-nozzle was spread out of the jetting head by the hydraulic cylinder. By the up-nozzle and down-nozzle, the soil would be cut and broken simultaneously as the jetting head was being rotated and lifted.Method B: The singular-jet grouting method with a spreading nozzle fixed on the horizontal mixing shaft (B). This jet grouting tools was a system of double channels. Channel 1 was connected with the down-nozzle, and channel 2 was connected with the up-nozzle. When the jetting head was being drilled down, high-pressure slurry (Method B1) or low-pressure slurry (Method B2) was jetted from the lower nozzle through channel 1. The soil would be cut and broken at the first time. When the jetting head got the pre-determined depth, channel 1 and channel 2 would closed and opened separately. The soil would be cut and broken at the second time, while the jetting head was being lifted and rotated.New jet grouting tools and technological parameters (Table 1) were developed for these two methods. According to the technological parameters of relative domestic equipment, the auxiliary equipment systems of the large diameter singular-jet grouting methods A, B1, and B2 were determined separately. 5. In situ experiment and its results analysisWith these newly invented jet grouting tools and related equipment, 57 jet-grouted piles (3.0 m in length) were tested in this experiment in three test sites A(new filling sandy soil), B(old filling cohesive soil) and C(new filling saturated cohesive soil). In the experiment, excavation inspection, pile shaft core drilling tests and temperature measurement of soilcrete were performed.The main conclusions were as follows.(1) The relationship between grouting pressure, lifting velocity, nozzle diameter and pile diameter was as follows.(a) The higher was the grouting pressure, the larger was the formed pile diameter. For piles A5 and A4, when the grouting pressure was changed from 20 MPa to 30 MPa, the formed pile diameter of these two piles was enlarged from 506.8 mm to 721.3 mm.(b) The lower was the lifting velocity of the jetting head, the larger was the formed pile diameter. Three lifting velocity tests, i.e., 25cm/min, 35cm/min and 45cm/min, were performed in Site C. The corresponding diameters of formed piles were respectively 850mm, 770mm, and 750mm.(c) The bigger was the jet nozzle, the larger was the formed pile diameter. As for the test piles A9 and A12, nozzles ofφ1.8mm andφ2.2mm were respectively used in Site A, and the formed pile diameters were respectively 535.3 mm and 668.8 mm.(2) The pile diameter increased with the spreading distance of nozzle in the secondary cutting. Under these conditions(jetting pressure=30MPa; lifting velocity of jetting head=20cm/min; rotation velocity=22r.p.m), as for the secondary cutting nozzlesφ1.8mm andφ2.2mm, when the spreading distance≥180mm, the diameters of all the piles (A6, B13, B15, B18) both in Site A and Site B were larger than 800m. Under these conditions(the jetting pressure=25MPa; jetting bit lifting velocity≤35cm/min), as for the secondary cutting nozzleφ1.8, when the spreading distance≥180mm, the diameters of the piles (C10¹2, C16 and C17) were larger than 750m.(3) Pile shaft strength of all tested piles satisfied the engineering application requirements on the whole. The average values (fcu,D28)of unconfined compressive strength of the Site A(new filling sandy soil), Site B(old filling cohesive soil), and Site C(new filling saturated cohesive soil) were respectively 14.1MPa, 5.3MPa and 3.79 MPa.(4) Changing law of the formed pile soilcrete temperature is as follows. In sandy soil, the hydration process was relatively faster, and the heat releasing speed was also higher. For sandy soil in Site A, the average value of the highest temperatures was 43.56℃, and the average value of time reaching the highest temperatures was 15.81h; for cohesive soil in Site B, the average value of the highest temperatures was 33.57℃, and the average value of time reaching the highest temperatures was 22.84h.(5) The newly invented jet grouting tools and auxiliary equipment were of stable structure and dependable properties. These tools and equipment could be put into the jet grouting construction in accordance with the pre-determined experimental parameters. It has been proved that the newly invented jet grouting tools and auxiliary equipment could be applied in the practical construction.(6) To ensure the pile diameter produced by the large diameter singular-jet grouting method be larger than 750mm, the main jet grouting parameters should be the following:(a) Grouting pressure≥25MPa;(b) Nozzle diameter≥1.8mm;(c) Lifting velocity of the jetting head: method A≤20cm/min; method B≤35cm/min;(d) The spreading distance of the secondary cutting nozzle≥200mm.In the practical engineering application, the jet grouting parameters should be modified according to the field pile testing results. 6. Engineering applicationThe newly invented large diameter singular-jet grouting method B1 has been applied successfully in a large deep foundation pit support project. The foundation pit is adjacent to the sea with the depth of 16.4m and the perimeter 705m. Within the range of excavation, there were three soil layers—filled cohesive soil, mud and sand. Bored pile and prestressed anchor were adopted for the supporting structure. The main functions of the singular-jet grouting method reinforcement are soil (for mud) and groundwater (for sand) retaining between the bored piles. It has been proved that there were no mud sliding destruction or ground water leakage in this excavated deep foundation pit. Compared with the traditional dual-jet grouting method, the newly invented singular-jet grouting method (B1) could save the water sealing construction cost about 15%.