Mechanical pipe jacking (mud-water balance pipe jacking) is used in some sections of sewage pipeline project in high-tech zone of Jiangbei sewage pipe network in Nanning, which has the characteristics of short distance, shallow depth, tight construction period and great difficulty. According to previous construction experience, it is decided to adopt eccentric broken slurry balance pipe jacking machine, combined with grouting friction reduction technology, nozzle docking technology, jacking control technology and emergency technical measures to ensure the safety, progress and quality of mechanical pipe jacking construction. Sewage pipe network high-tech zone sewage pipe project (Fengda Road, Gaoxin East 3rd Road, Keyuan East 11th Road, etc. ) Planning is located in Jiangbei District of Nanning, with a total length of11941m. According to the engineering geological survey report of sewage pipeline in the design drawings, it can be known that the soil layers crossed by pipe jacking are mainly red clay and red clay. Before construction, we observed the characteristics of the construction site, the distribution position of surrounding buildings and underground pipelines, and combined with the hydrogeological conditions within the project scope, decided to adopt mechanical pipe jacking (mud-water balance pipe jacking) to treat some sections. 1 The basic principle and technical characteristics of mechanical pipe jacking refer to the construction technology of jacking tool pipes or roadheader into the receiving pit from the working pit by using the jacking force of the main jacking oil cylinder and the relay room, and then burying pipes between the two working pits, which mainly includes three construction methods: air pressure, earth pressure and mud-water balance. Mud-water balance pipe jacking refers to a mechanical pipe jacking technology that uses external forces such as hydraulic and mechanical forces to cut soil and then transport waste soil, and uses mud pressure to offset groundwater pressure and soil pressure. The basic principle is as follows: the mud with a certain pressure is stored in the mud bin, and a waterproof mud film is formed on the surface of the excavation face to prevent the mud from infiltrating into the excavation face and ensure the overall stability of the excavation face; At the same time, groundwater pressure and earth pressure are balanced by the pressure of mud itself. Therefore, combined with this project example, the main characteristics of mechanical pipe jacking construction technology are: first, the construction area is narrow, the transportation space of the nose is small, and there are many times; Second, the pipe jacking area is covered with shallow soil, and the traffic flow is large. Because of the continuous and changing dynamic load, it is easy to cause various engineering accidents. Third, the construction depth is shallow, the construction period is tight, the difficulty is great, and the underground soil quality is poor, so civilized construction, safe construction and traffic safety must be ensured; Fourth, mechanical equipment is bulky and consumes a lot of electricity. Attention should be paid to the safety of the support and the operation of electrical equipment. 2 Selection of pipe jacking machine This project has higher requirements on construction safety, quality and time limit for a project. According to previous construction experience and successful technology, NPD-B 1000, NPD-B800 and NPD-B500 eccentric mud-breaking balanced pipe jacking machine are adopted. Their main features are as follows: (1) They are suitable for sandy soil, pebble soil, cohesive soil and soft rock. (2) Pipe jacking machine, mud-water circulation system, mud-water separation system and main jack are configured in complete sets, which can not only form a separation system with flexible operation and good sealing performance, but also save installation space; (3) Using the ground remote control operating system, direction control and construction monitoring can be carried out on the console. 2. 1 Grouting anti-wear technology The friction resistance in the process of mechanical pipe jacking is mainly composed of the normal pressure of cutting soil of the tool pipe, the friction resistance of the pipe wall and the gas-water pressure of the tool pipe. In this project, grouting anti-wear technology is used to control jacking force and reduce resistance. Its basic principle is to inject thixotropic mud into the gap between the pipe wall and the soil wall, which not only reduces the friction between the pipe wall and the soil wall to increase jacking force, but also forms a mud lubrication sleeve to prevent collapse accidents. Thixotropic mud is composed of bentonite, alkali and water. Under special requirements, additives such as lime putty, retarder and plasticizer can be properly added to enhance the consolidation of thixotropic mud, and it has good fluidity and pumpability during transportation and grouting. Matters needing attention in using thixotropic mud: First, determine the layout of grouting holes according to the diameter of the pipeline, and each section should be provided with about 3 grouting holes; Secondly, the grouting pressure must be adjusted according to the actual situation, and grouting should be carried out in turn according to the order of grouting holes; Third, problems such as joint leakage, pipeline blockage and mechanical failure occur during grouting, and jacking can only be continued after treatment. 2.2 Nozzle docking technology Nozzle docking technology refers to the construction technology of turning in the process of mud balance mechanical pipe jacking construction. In actual construction, first of all, we should pay attention to the selection of pipeline joints and the on-site inspection and acceptance of pipelines according to relevant specifications. If unqualified products are found, we will resolutely return them to provide material guarantee for the quality of nozzle docking. In the process of pipeline transportation, shelving and hoisting, special protection devices are adopted to prevent the pipe joint from being deformed by compression. Secondly, check the dimensions of pipe joints, rubber rings, gasket plates and other parts before the nozzle butt joint to confirm whether their appearance and texture are qualified. Finally, apply a thin layer of lubricating material which is not corrosive to rubber, such as silicone oil, to the joint, and apply force evenly to ensure the coaxiality with the steel collar of the upper pipe, and ensure that the rubber ring does not reverse, shift or expose; After the pipe jacking is completed, apply a smooth gradual change surface on the nozzle, and fill the gap in the pipe with elastic sealing paste. In the process of jacking the bearing and nozzle, we must always pay attention to and observe the pipeline allowance. If the supply is not timely and in place, it will easily increase the consolidation and friction of the surrounding soil, and even cause serious axis deviation and nose settlement problems. 2.3 jack-up control technology jack-up control technology refers to the technical control of axis deviation, precision, slope and direction in the process of jack-up. The technical requirements of this engineering survey are as follows: the central control accuracy reaches 1/70000 or meets the relevant technical requirements of fourth-class triangulation in engineering survey; The accuracy of elevation control meets the relevant technical requirements of national third-class leveling. The main steps of bearing and jacking control are as follows (Note: the leveling instrument and theodolite can only be used after passing the verification, and they must be verified regularly): (1) Measuring and setting out: according to the traverse points and line positions provided by the project department, determine and set out the construction center line, the center line of the working pit and the center of the receiving well, and temporary leveling points must be set for elevation control to meet the accuracy requirements; (2) Check the setting-out: check whether the measured drawings are consistent with the design drawings; If the retest deviation value meets the requirements of relevant specifications, the results and straightening results can be reported to the supervisor for approval; (3) Recheck the leveling points: the temporary leveling points should be closed every time, and if the closure difference is re-measured, it should be reported to the supervisor for approval in time; The poor closure of the second interview is unqualified (greater than 12k 1/2) and shall not be used; (4) Control the center of the pipeline: after the center retest is qualified, set up the center control pile and make obvious signs (such as bricklaying protection, etc.). ); (5) Control the elevation of pipeline: After the leveling points are re-measured, at least two temporary leveling points will be set in the working pit for use in the construction process. In addition, the measurement results should be closed and verified before use; (6) Re-survey of the center and elevation of pipe jacking: In the process of pipe jacking construction, it is necessary to re-survey the internal center line and elevation piles regularly to prevent the center of pipe jacking reaction from deviating. Finally, the direction control in the jacking process refers to operating the nose to correct the deviation and always ensuring that the laser spot is in the center of No.2 optical rake, that is, the steps and key points of direction control are as follows: First, strictly check the lofting and make original records. Before jacking, strict lofting measurement must be carried out, and the three-level measurement procedure of construction team surveyor → project manager department → supervision engineer must be adhered to to ensure the accuracy and precision standard of measurement; Second, timely adjust and regularly retest the instrument base arranged behind the working well to avoid its movement, deformation and deviation during the jacking process; Thirdly, the control of jack-up rectification must follow the principle of "diligent measurement, multi-precision adjustment and diligent rectification", and set a reasonable deviation warning line to keep the rectification angle between10 ′ ~ 20 ′; Fourth, the direction of the initial propulsion stage is mainly controlled by the main jacking oil cylinder, and the main jacking speed is controlled to ensure the quality, and the cylinder grouping and head are constantly adjusted to meet the deviation correction standard; And constantly measure the elevation rotation direction of the tool pipe, so that the pipe can be jacked up according to the design elevation and the required direction, and adjust the correction jack in time according to the measurement results, so that the head can be jacked up according to the design axis to change the direction, thus realizing the control of the jacking direction; Fifth, before jacking construction, we should make a slope plan in advance, and calculate the elevation position of each meter and each section of pipeline in advance, so as to predict the fault problems in the jacking process and formulate solutions to ensure that the design slope requirements and quality standards are met. In addition, because this project adopts laser positioning, it is necessary to ensure the accuracy of checking the center and elevation of the laser beam; Moreover, some pipe jacking sections may lead to pavement settlement due to crossing the road, so it is necessary to set settlement observation points on the center line and both sides of the auxiliary pipeline and observe them regularly so as to take relevant emergency measures in time when problems are found. 2.4 Emergency technical measures Emergency technical measures refer to the technical treatment when encountering different geological conditions or other special circumstances in the construction of slurry balance mechanical pipe jacking. There are mainly the following four situations. First, when the soil suddenly hardens, water or mud must be added to the soil bin through the mud hole on the roadheader; Secondly, when the soil suddenly becomes soft, the first three sections of pipeline and tool head must be connected to form a unified whole, so as to increase the stiffness and avoid the sudden sinking failure of the head due to the soft soil; Third, in the process of pipe jacking construction, if there is abnormal deviation or rectification failure, the construction must be stopped immediately within the allowable deviation standard, and then the reasons can be analyzed, the countermeasures can be found out, and the jacking can be continued after the fault is solved, and blind action cannot be taken; Fourthly, in the process of pipe jacking construction, once the bearing capacity of the soil at the top and bottom of the pipe is too low or the soil is loose, grouting must be carried out at the top and bottom of the pipe. Conclusion To sum up, the mechanical pipe jacking technology should first consider the specific engineering geological conditions and hydrogeological conditions, and combine the principles and characteristics of mechanical pipe jacking technology to select a reasonable type of pipe jacking machine; Secondly, in the process of jacking, grouting antifriction technology, nozzle docking technology and jacking control technology should be adopted to ensure the safety, progress and quality of mechanical pipe jacking construction. Finally, when there are different geological conditions or special circumstances, emergency technical measures can be adopted to properly handle them.

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