1 characteristics of pipe jacking construction
Pipe jacking method, also known as trenchless pipe laying technology, has the characteristics of not excavating the surface layer and crossing the ground structures, underground pipelines, highways, railways and rivers. Compared with the excavation and laying technology, the investment and construction period will be greatly saved. At the same time, pipe jacking construction technology can reduce noise, dust, interference and damage to urban traffic conditions and environmental conditions, and it is a truly pollution-free and efficient construction technology. Pipe jacking construction method has been widely used in municipal engineering, especially in municipal water supply and drainage pipeline engineering because of the above advantages. To sum up, the pipe jacking construction technology has several advantages: the construction surface is reduced from line to point, and the occupied area is small; Ground activities are not affected by construction and have little interference to traffic; The noise and vibration are low, the construction in the city has little interference to the living environment of residents, and does not affect the use of existing pipelines and structures; Pipelines can be laid deep underground or underwater, and can safely cross railways, highways, rivers and buildings, reducing the workload of demolition along the line and reducing the project cost. Its main disadvantage is that the construction technology is difficult and requires detailed engineering geological and hydrogeological exploration data.
2 pipe jacking technology construction application analysis
2. 1 Selection of pipe jacking Generally, reinforced concrete pipes are used for pipe jacking, and steel pipes can be used if there is no anti-corrosion requirement. The specification design, reinforcement and stress check of reinforced concrete pipeline shall conform to the relevant standards and technical regulations of reinforced concrete, especially reinforced concrete pipeline.
2. 1. 1 pipe jacking pipe diameter selection is to determine the inner diameter according to the nature and needs of the project, the reinforcement and wall thickness of the concrete pipe according to the load on the pipe jacking, and then determine the outer diameter. Because the working face of pipe jacking project needs to be equipped with excavators, the inner diameter of the pipe is generally not less than 500 mm.
2. 1.2 Selection of pipe jacking length The length of pipe jacking has great influence on the controllability and economy of pipe jacking process. Using long pipes in the case of straight pipe jacking can reduce the number of pipe installation and achieve good results. However, with the increase of pipeline length, if it deviates from the original route, it will be more difficult to restore the correct route than using short pipelines. When constructing the top pressure pit, the length of the top pressure pit will also increase, and the cost of excavation, support, backfill and repair will also increase accordingly. On the other hand, it is difficult for short pipes to advance in a straight line, because short pipes are easy to squeeze into the surrounding soil layer, which leads to the snake-shaped bending of the whole pipe string and reduces the controllability of pipe jacking.
Under normal circumstances, the pipe length must be measured relative to the pipe diameter. When L/D≤ 1. 10, it is a short pipe. When the length-diameter ratio = 1. 15, it is a standard pipe; When L/D≥2. 10, it is a long tube.
2.2 Pre-preparation for pipe jacking construction
2.2. 1 The general layout of the site includes the layout of hoisting equipment, automatic control room, material room, segment storage yard, slurry mixing shed, slurry mixing material storage yard, water injection system and slag pit. The launching equipment is as follows: water frame, pipe jacking machine, front jacking iron, main jacking jack and reaction frame. It is installed in the originating working well, and the side of the working well is provided with a descending escalator for the construction personnel to get up and down.
2.2.2 Reinforcement of the soil at the inlet and outlet of the pipe jacking machine and at the rear of the pipe jacking machine In order to ensure the absolute safety of the outlet of the pipe jacking machine, it is necessary to reinforce the soil at the rear of the pipe jacking machine and the soil at the inlet and outlet area with high-pressure jet grouting piles. In order to prevent the mud loss caused by the entry and exit of the reserved hole of the pipe jacking machine and ensure that the thixotropic mud injected during the jacking process is not lost, it is necessary to install a water stop device in the hole in the working well.
2.3 pipe jacking construction technology pipe jacking construction, also known as jacking construction, refers to the construction method of jacking prefabricated box or circular structures into subgrade step by step by using jacking equipment to form a three-dimensional intersection channel or culvert. In pipe jacking construction, it is necessary to set a working well and a receiving well between some pipe sections, and then install propulsion equipment in the working well, and push the head of the pipe jacking machine on the guide rail into the soil, and the prefabricated reinforced concrete pipe will be pushed forward by the head, and the soil at the front end will be transported out through the working well, and finally the pipeline laying will be completed.
2.3. 1 Design of top tube well There are two types of top tube wells: working wells and receiving wells. There are many types of top tube wells, and reinforced concrete structures are generally adopted. The structural forms of working wells usually include single-hole wells and single-row wells. The former is round, square, rectangle, etc. The latter is mostly rectangular, and its structural mechanical properties are circular, square and rectangular from high to low. When the structure is installed, an internal support can be installed in the well to improve the stress of the structure. During the construction process, the working wells are designed by two-way jacking, which are arranged at intervals from the receiving wells, and the spacing is consistent with the designed inspection wells. After the construction is completed, inspection wells shall be drilled at the positions of working wells and receiving wells according to the design requirements.
2.3.2 pipe jacking construction technology ① wall penetration: open the wall bulkhead to push the tool pipe out of the well, and install a wall penetration water stop device. The main construction technical measures are as follows: a. Filling compacted paper-reinforced clay or low-strength cement-clay mixed soil in the wall-penetrating pipe to temporarily stop water and soil; B. In order to ensure that the soil is basically stable and has sufficient strength in a certain range outside the wall-penetrating hole, grouting reinforcement measures should be taken at the outer side of the wall-penetrating pipe before the tool pipe penetrates the wall in the working well; C. Analyze the possible problems before going through the wall and formulate corresponding treatment measures; D. After the boring plate is opened, quickly push the tool tube, and at the same time do a good job of plugging the mouth. In this project, a water-sealed flange pressure plate is adopted, and a natural high-quality rubber water-sealed plate ring with a thickness of 20mm is installed in the middle, which requires high tensile rate and wear resistance. With the help of pipe jacking, the installed rubber plate is driven to form a reverse water seal device, which should prevent the disturbed rheology caused by the long exposure time of the soil outside the wall pipe. (2) Pipe jacking outside the hole: Pipe jacking outside the hole is a very noteworthy problem in pipe jacking operation, that is, the pipe jacking machine and the first section of pipe break out of the working well to seal the door and bury it. The starting process before normal pipe jacking is a key process in pipe jacking technology, and it is also an accident-prone process. In order to prevent pipeline deviation, measures should be taken, such as zeroing the tool pipe, adding support on the shaft wall below the tool pipe, immediately rectifying deviation with the main jacking oil cylinder if deviation is found, and setting the initial angle before the tool pipe goes out of the hole to make up for the decline. (3) Drag reduction by grouting: Another important technical measure in pipe jacking construction is to inject thixotropic mud under pressure to fill the gap around the pipeline and form a mud protective sleeve, which plays a role in supporting the stratum, reducing ground settlement and reducing jacking resistance. During construction, the head and tail of the pipe jacking machine shall be grouted at first, and the grouting shall be carried out synchronously with the jacking work, and then the mud shall be tracked and supplemented at the proper position of the middle section and concrete pipeline to supplement the mud loss in the jacking process. Grouting technology is generally used in long-distance pipe jacking construction. ④ Deviation correction of pipe jacking: Deviation correction means that after the head deviates from the design axis, the direction of the end face of the head is changed by using the deviation correction jack arranged at the back to reduce the deviation and make the pipe jacking along the design axis. Jack-up rectification is rectification by adjusting four sets of rectification jacks. If the pipe is on the left, the jack will extend to the left and retract to the right, and vice versa. If there is elevation and direction deviation at the same time, the side with large deviation should be corrected first. When jacking, it should be corrected step by step at a small angle, and it should be carefully corrected frequently. When the head of the pipe jacking machine rotates, it can be corrected by adding a counterweight in the opposite direction in the pipe or providing a rotation correction torque at the intermediate station.
Pipe jacking construction technology is technically feasible, and it has more advantages than slotted buried pipe in social and economic benefits. Fundamentally changed the phenomenon of indiscriminate excavation of urban pipe network; On the other hand, from the perspective of effective environmental protection, it is imperative to strengthen the promotion of pipe jacking construction technology. It can be predicted that the future pipeline laying technology will be supported by pipe jacking technology.
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