What are the five key points for construction with cement pipe? Today, I will give you a detailed introduction.
Track
The guideway is composed of four steel rails and a number of sleepers, with sleepers placed on the subgrade beneath the work pit at a height of 1/2 the sleeper height. The spacing between sleepers is 800-1000mm. The length of the steel rails equals the length of the work pit bottom surface minus the position occupied by the rail piles. The spacing of the steel rails should be determined based on the outer diameter of the pipe section being jacked, generally ensuring that the bottom surface of the pipe section is elevated 20mm above the top surface of the sleepers after placement. After the jack installation, the cross-section of the pipe section should align, and the steel rails should be installed straight. The front end should have a slope of 0.5-1.0%.
2. Directional Drilling Operations
Trench excavation for pipe jacking must be conducted according to the construction plan and specific environmental conditions. The length and width of the trench should be determined by the soil type, the diameter and length of the pipe sections to be jacked, the machinery and equipment, as well as the methods for pipe installation and soil removal. In addition to installing the machinery and equipment for jacking the pipe, guides, and pipe sections, the working trench should also facilitate soil removal and the operation of personnel. Generally, the front edge of the working trench opening should be at least 2 meters away from the roadside curb. Each side should have a 1-meter work area after the pipe sections are placed. There should be sufficient space for soil removal between the pipe sections and the jacks. In a waterlogged environment, a water pit and drainage facilities should be set up. The slope coefficient of the working trench wall should meet the requirements based on the soil conditions, and the bottom of the trench should be compacted.
3. Top-Notch Back Support
The strength of the back is crucial for the effectiveness of pipeline jacking construction, thus, the back must possess the capability to meet the thrust requirements. The back is composed of back piles and back beams, along with compacted earth behind the back piles. The back piles are typically replaced by steel rails, buried about 1.5 meters below the pit bottom. The earth behind the piles is filled and compacted in layers. The back piles are perpendicular to the jacking direction's axis. Steel back beams are placed on the guide tracks in front of the piles. Other configurations of the jacking back include those made of rubble masonry and those composed of precast reinforced concrete blocks.
4. Installation of jacking equipment and pipe sections
The jacking equipment consists of a high-pressure oil pump and two 200-500t jacks. The jacks are mounted between the rear beam and the pipe sections. A steel guard ring, rope, or rubber pad protects the concrete pipe ends between the pipe section's rear end and the jacks. The pipe exterior is coated with paraffin as a lubricant to reduce friction resistance during jacking. The jacks push the pipe sections into the subgrade through force transmission columns.
Excavation, jacking, surveying, and alignment correction
Upon successful trial operation of the installed equipment, excavation can proceed. The depth of excavation should be controlled based on the soil type and the thickness of the overlying soil on the pipe top. For dense soil and thick overlying soil, it is beneficial to form a relief arch, allowing for slightly more excavation. For loose soil or thin overlying soil, excavation should be less frequent and more shallow. Ensure that the diameter of the excavation does not exceed the outer diameter of the pipe section. The tools for excavation and soil transportation depend on the pipe diameter; for diameters between 880-1500mm, small carts are used; for diameters above 1500mm, double-wheeled carts can be directly used for soil removal. Vertical transportation of soil can be achieved using cranes or electric hoists.
The jacking process requires constant measurement. After each jacking step, the elevation and lateral deviation of the pipe should be measured once. Any issues should be corrected promptly. Measures to correct lateral deviation and uplifting include setting a diagonal brace at the front of the pipe, supported by the soil wall in front, and combining it with the over-excavation of soil as needed for continuous correction. At the junction of the first two sections, a steel expansion ring welded from steel plates is reinforced to serve as a measure against deviation.
Quality Requirements: The pipeline jacking should be straight, with the center of the pipe deviating from the axis no more than 3cm for excellent quality.
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