1 Introduction
In the construction of underground engineering in soft and water-rich stratum, the auxiliary construction method of pre-reinforcement in advance is generally adopted. At present, the main methods of advance pre-reinforcement include small pipe pre-grouting, small pipe pre-grouting, large pipe shed, deep hole grouting, horizontal jet grouting pile, horizontal mixing pile and horizontal freezing. , should be based on geological conditions and surrounding environment, from the aspects of safety, technical economy, applicability and mature reliability analysis.
Shenzhen Metro Grand Passenger Section is located directly below Shennan Avenue, the main road in Shenzhen, with a single-track length of 1 144.7 m and a buried depth of10 ~19 m. It is divided into two left and right single-track tunnels. Line spacing 13.2 ~ 17.2 m ... in which SK32 10 ~ SK33 15 section crosses bad geological section, which is buried at a depth of 15 ~ 17 m and covered with plain fill and plain soil. Among them, the fluid plastic cohesive soil intrudes into the tunnel section for 3 ~ 6 m. The self-stabilization ability of the soil in this area is extremely poor, and there are pockets in the soil layer, which are filled with a mixture of water and mud and sand, and will erupt without warning during the tunnel excavation, causing soil instability and danger.
Due to environmental restrictions, surface precipitation and grouting reinforcement are not allowed in this section. At first, only small pipes were used for intensive grouting in the tunnel, but the effect was not good, safety, quality and progress could not be guaranteed, and it was once in a state of suspension. 200 1,101On October 22nd, more than 50 experts and scholars in the field of underground engineering gathered in Shenzhen to join hands in "pulse consultation". Through comprehensive demonstration, it is suggested that the construction scheme of horizontal jet grouting pile combined with small pipe grouting reinforcement should be preferred. At present, the project is progressing smoothly, and the secondary lining has been completed, achieving the goal of "safety, economy, speed and high quality".
2 horizontal jet grouting pile reinforcement mechanism and characteristics
The horizontal jet grouting pile takes the high-pressure pump as the power source, and injects the prepared slurry into the soil through the drill pipe and nozzle of the horizontal drilling rig. The jet shoots through a certain range of soil with great energy, cuts the soil while the nozzle slowly rotates and advances and retreats, forcing soil particles to mix with mud. After the slurry is solidified, a horizontal cylindrical cement-soil consolidation body, namely a horizontal jet grouting pile, is formed. When jet grouting piles are engaged with each other, a closed horizontal jet grouting pile curtain body is formed in the form of concentric circles at the vault and periphery of the tunnel, which plays a role in preventing sand flow, sliding and leakage.
Horizontal jet grouting pile has the following characteristics:
(1) controllability. The slurry of horizontal jet grouting pile is limited to the range of soil damage, and the position and range of grouting can be controlled. By adjusting the grouting parameters (soil cutting pressure, grouting speed, curing material ratio and grouting quantity, etc.), the consolidated body meeting the design requirements can be obtained. ).
(2) Unity. The jets meet before the energy decays, and the cutting energy cancels each other at the collision point. When the distance between the pile center and the collision point is greater than that, the jet has no ability to cut soil and the solid addition uniformity is good.
(3) low cost and high efficiency. Due to the limited grouting range, the grouting amount is greatly reduced, and the cement consumption is only 100 ~ 150 kg/m, and the construction speed is 2 ~ 3 times higher than that of large pipe shed or deep hole grouting.
(4) It has multiple functions such as improving the strength of composite soil, preventing seepage, preventing sliding and pre-supporting.
3. Pre-reinforcement scheme of horizontal jet grouting pile in advance
Through indoor simulation test, field test, theoretical analysis and expert opinions, it is decided to adopt the advanced pre-reinforcement scheme of "single-layer occlusion horizontal jet grouting pile reinforcement around (double-layer in particularly difficult sections), supplemented by small pipe grouting and horizontal jet grouting piles at intervals on the tunnel face".
3. 1 reinforcement scope
Through the combination of SIR22000 GPR (American GSSI Company) and Luoyang shovel, the geology in front of the excavation face is predicted in advance, and the pre-reinforcement range of the surrounding horizontal jet grouting pile is determined according to the stratum boundary. In general, the reinforcement range of arch is 150, and the reinforcement range of flowing plastic cohesive soil invading the lower part of tunnel is appropriately adjusted to 180 ~ 270. The scope of tunnel heading reinforcement is the core soil of excavation face steps and the inner edge of arc pilot tunnel. See Figure 1 and Figure 2 for the longitudinal and transverse sections.
3.2 Main design parameters
The diameter of the peripheral jet grouting entity is 500 mm, and the hole depth is 15 m. Considering that there is no shed expansion, in order to reduce the workload of peripheral pile breaking, the hole is formed only within 3.0 m of the pile tail, and jet grouting is not carried out. The actual pile length is 12 m, the longitudinal lap length is1.5m. The circumferential spacing at the opening is 0.33m.. Extrapolation angle is 5 ~ 8 (depending on geology and line slope), occlusal thickness of adjacent entities is increased by >: 10 cm, a circular jet grouting pile is constructed, excavation and initial support footage are10.5 m. In order to reduce the instability of tunnel face caused by soil slip in front and aggravate structural deformation and ground settlement, horizontal jet grouting piles are also arranged on tunnel face.
4 technical points of each working procedure
4. 1 construction parameters
(1) Drilling speed of drill pipe: 0125 ~ 0135m/min;
(2) Rotation speed of drill pipe (shaft):120 ~ 70r/min; w = 1∶ 1
(3) Cement slurry mixture ratio: c: 017;
(4) Cement dosage per linear meter: 75kg；;
(5) A rotary jet drilling rig with a drill pipe length of 3 m, an outer diameter of 50 mm and an outer diameter of 90 mm, with a single hole and an aperture of 2 mm;
(6) Jet grouting pressure: 20 ~ 25mpa.
4.2 Construction preparation
Closed up and down the steps, shotcrete thickness ≮20 cm. Accurately measure the center line and elevation, build a working platform, spread bamboo plywood and sleepers on the platform, and arrange drilling machines, high-pressure pumps and other machines and tools in place. Set up temporary ditches and waste mud pools.
4.3 Mud preparation
Prepare cement slurry in strict accordance with the design mixture ratio, fully mix evenly, and the mixing time is ≮3 min. Before the slurry is poured into the slurry storage barrel from the mixer, it should be filtered with a sieve to prevent the slurry outlet from being blocked.
4.4 Drilling and Jet Grouting
Jet grouting construction shall be carried out in the order of "periphery first, then heading face", with one hole at a time around the periphery, and the hole positions alternate from bottom to top. Jumping pile formation and balanced strength on both sides can reduce the problem of low bite rate between piles caused by drill pipe deviation.
According to the designed extrapolation angle, the deflection angle and elevation angle of each pile are calculated hole by hole, and the drilling rig is accurately positioned by using three-dimensional coordinates. When drilling, drill slowly to 1 m, and then drill to the design depth at normal speed. When the slurry is sprayed from the nozzle and reaches the design pressure, the jet grouting is started, and the front end of the pile is in-situ jet grouting for ≮ 30 s. In order to ensure the diameter of the pile and the bite between piles and make up for the shortage of the fast retreat speed of jet grouting in horizontal drilling in China at present, the re-injection technology (i.e. once retreat, once advance, once retreat again and jet grouting for three times) is adopted to improve the diameter increasing effect and bite rate of the consolidated body. The front end of the pile is affected by the extrapolation angle. In order to ensure the strengthening effect, increase the front-end pressure or reduce the rotating lifting speed of the nozzle. When the jet grouting reaches the orifice 3 m, stop and immediately quit the drill pipe, and plug the orifice with cotton yarn to prevent the slurry from leaking out.
4.5 Mud Treatment Process
In the process of rotary jet grouting, a certain amount of soil particles often come out along the wall of grouting pipe with some slurry. Through the observation of mud outburst, we can know the formation situation in time, judge the overall effect of jet grouting and evaluate the rationality of jet grouting parameters. According to experience, it is normal that the slurry leakage is less than 20% ~ 30% of the grouting amount. If it exceeds 30% or does not exist at all, it is necessary to find out the reasons and take timely measures.
When the jet flow rate is constant and the pressure drops suddenly, check the leakage of each part, and if necessary, pull out the grouting pipe to check the sealing performance. In the case of no grouting or intermittent grouting, if the soil is loose, it is regarded as a normal phenomenon and can be re-sprayed appropriately. If there is a cavity near the department, continue grouting or pull out the grouting pipe, and then re-grouting until the slurry boils, or use quick-setting slurry to solidify the slurry near grouting pipe.
The main reason for the excessive grouting amount is that the grouting amount within the effective spraying range greatly exceeds the grouting amount required for rotary spraying reinforcement. Measures such as increasing the injection pressure (the amount of grouting is constant) or appropriately reducing the nozzle diameter (the injection pressure is constant) and accelerating the lifting and rotating speed can be adopted.
5 rotary jet reinforcement effect
(1) test shows that the diameter of jet grouting consolidation body is 40 ~ 70 cm (the weaker the stratum, the larger the pile diameter), and the average value meets the design requirements. The strength of the consolidated body is close to C 10 plain concrete, and the bite rate between piles is more than or equal to 80%, forming a good arch shell support, which can bear the pressure of soil piles outside the excavation line during excavation (circulating footage is 0.7 m) and ensure the excavation safety.
(2) When the soil around the consolidated body is squeezed, the void ratio of the soil decreases, the impermeability is improved, and the leakage of the initial support is obviously reduced, which provides a good external environment for the construction of composite waterproof layer and secondary lining.
(3) Ground subsidence and vault subsidence are effectively controlled. The surface subsidence is reduced by 565,438 0%, and the tunnel vault subsidence is reduced by 54%. At the place where SK3 355 crosses the gas pipeline, the surrounding double-layer jet grouting pile and tunnel face jet grouting pile are used for reinforcement. According to the construction principle of "short step, short step, fast excavation, fast support, fast closure and fast passage", temporary inverted arch and dense foot-locked conduits are added, and post-grouting is carried out in time to control gas pipes in plastic stratum.
(4) In the plastic clay soil section with a large flow, the jet grouting pile construction is 65,438+065,438+0,000 linear meters, the construction time of each pile is about 2 h, the circulation time is 3 d, and the monthly driving footage is about 30 m, which is much higher than the construction progress with large pipe shed or deep hole grouting. Using horizontal jet grouting pile increases the cost by 6 000 ~ 8 000 yuan per meter, which is lower than that of large pipe shed, deep hole grouting and horizontal freezing, and has good technical and economic advantages.
6 concluding remarks
The application and research of horizontal jet grouting pile in China started late, and most of them are still in the experimental stage. There is still a lot of work to be done in construction method, numerical model, theoretical research, mechanical selection and matching. With the development of underground engineering in China, the research of horizontal jet grouting pile will be further deepened. It is believed that in the near future, the horizontal jet grouting pile will be gradually popularized in the weak stratum of underground engineering and will show its talents.
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