Steel pipe pile construction technology in industrial projects

(I) Site preparation: According to the on-site survey, the first thing to do after entering the site is to prepare the construction site, mainly including the cleaning and leveling of the original cultivated field surface, to make full preparations for the hoisting and stacking of steel pipe piles after they arrive and the positioning of the pile driver after they enter the site. Site leveling and cleaning are done by manual and mechanical operations.

(II) Construction plan: According to the design position of the pile and the layout of the measurement control network, draw the pile position map and measurement and layout data. According to the national measurement specifications, the center point of the pile is laid out on the ground, and a Φ6 steel bar with a length of about 30cm is driven into the ground at the center point of the pile position so that it is exposed 5 to 8cm above the ground, and then a red cloth strip is tied on it as a sample pile. At the several piles, positions planned for piling on the day, use white lime to draw a circle on the ground near the sample pile, which coincides with the sample pile and is the same size as the pile diameter, to facilitate the centering of the pile and ensure the correct position of the pile.

(III) Positioning of pile frame: Before the bottom pile is in place, the length mark in meters should be drawn on the pile body, and the length of the pile should be marked in order from bottom to top, to observe the depth of the pile into the soil and record the number of hammer blows per meter. Generally, the steel pipe pile is hoisted straight by multi-point hoisting. First, the pile head is inserted into the pile cap under the pile hammer, and then the lower end of the steel pipe pile is manually supported to position the pile tip in the white lime circle. After the bottom pile is in place, the centering and straightening process plays a key role in the quality of the pile. "Straightening" means, first, to make the pile body vertical; second, to make the center lines of the pile body, pile cap, and pile hammer coincide. According to the specification, when the bottom pile is hoisted and inserted into the ground, the verticality deviation of the pile body shall not exceed 0.5%, otherwise, the hammer shall not be started. To measure the verticality of the steel pipe pile body including the guide rod of the pile frame, two theodolites can be used to observe in an orthogonal direction 15m away from the pile driver, or two vertical lines of the hanging weight can be set in an orthogonal direction for observation and correction. Each pile driver should be equipped with a long-level ruler to measure the verticality of the pile body at any time.

(IV) Piling and inspection: Before the construction of steel pipe piles, each pile should be inspected again, that is, to check whether the concrete piles have serious quality problems. The two ends of the steel pipe piles should be cleaned, and the welding surface should be cleaned if there is paint and debris contamination. It is very difficult to correct the angle of the pile during the pile-pressing process, so it should be placed correctly when in place. When the section pipe pile is inserted into the ground, the position and direction must be kept correct. At the beginning, press lightly and check carefully. If there is any deviation, it should be corrected in time. If necessary, pull it out and press it hard. The verticality of the pile is monitored by a dedicated person using two theodolites. The theodolites should be set in a place that is not affected by the pile driving and in a direction of approximately 90° from each other. They should be leveled frequently, and the monitoring guide frame should remain vertical. Adjustments are made through the rotation, sliding, and stopping of the pile driver guide frame.

(V) Hammering piles: First of all, you should be extra careful when driving the bottom piles, and at the same time, you should take some necessary technical safety measures to prevent the surface soil from being too soft and causing the entire pile section to be submerged in the soft soil all at once, and the entire diesel hammer to fall to the ground or even embed in the surface soil, threatening personal safety and affecting the construction quality. Secondly, it should be emphasized that during the hammering process, the center lines of the pile hammer, pile cap, and pile body should coincide, and avoid eccentricity. When the pile is difficult to sink, check whether the pile frame guide rod is tilted or eccentric and whether the pile body is vertical, and each pile should be completed continuously to avoid difficulty in continuing to press down.

(VI) Pile connection: When connecting piles, pay attention to the consistency of the axis of the new pile section with the original pile section, and the dirt, oil stains, rust, etc. on the two welding surfaces should be cleaned in advance. When the pile head of the lower pile section is 1-1.2 m from the ground, the pile connection can be welded. When connecting piles, after the upper pile section is aligned, 4-6 points of symmetrical spot welding are used to fix it. The welding construction of steel pipe piles should be carefully carried out by experienced welders with professional welder certificates according to the requirements of technical regulations. The welds should be even and full. After welding, the construction can only be continued after the cooling time specified in the specification, to avoid the welds being rapidly cooled into the soil, which will make the joints cold and brittle and affect the service life. If the design requires rust prevention, the welds must also be treated with rust prevention.

(VII) Hammering: When the pile tip is driven into the designed bearing layer to a certain depth, the construction personnel should consider the issue of stopping hammering. If the hammering is stopped too early, the bearing capacity of the pile may not meet the design requirements; if the hammering is stopped too late, the pile body and pile head may be damaged. Before stopping the hammering, the construction personnel generally need to obtain various pile sinking information such as the last ten penetrations of the pile body and the last 1m of pile hammering. If it meets the pre-determined stop control conditions, the hammering can be stopped. After the pile is collected, it is required to carry out multiple re-pressing to eliminate the weakening of the ultimate bearing capacity of the pile caused by the rebound of the soil at the pile end, and each re-pressing lasts for no less than 1 minute to ensure that the bearing capacity of the pile meets the design requirements.

(VIII) Steel pipe pile foundation inspection: After the construction is completed, dynamic testing is used to detect its integrity, and static load test or high strain dynamic testing method is used to detect the bearing capacity. The number of tests is carried out by the specifications and design requirements. For small strain tests, each pile is tested, and for static load tests or large strain tests, random inspections are carried out.