Paper Keywords: PHC highway engineering soft foundation treatment test pile

Abstract: PHC pile has been used in soft foundation treatment of highway engineering for many years. Since it was applied in the field of highway engineering construction, people have always attached importance to quality inspection. Because the accuracy of test and detection work directly determines the quality of soft foundation treatment, the quality of test data and detection work must be guaranteed. In order to achieve this goal, we must understand and master the requirements of testing work. Next, I will discuss all the testing matters about PHC piles with you according to my own testing experience. ?

PHC pile (prestressed concrete pipe pile) is not uncommon as one of the main soft foundation treatment methods in today's highway engineering construction. At present, its construction control technology has matured. In recent years, the Ministry of Construction and the Ministry of Construction have constantly updated and revised the relevant specifications of PHC piles, so that their production and construction have been well controlled. However, for a technician who has just participated in the test and detection work, how to detect PHC piles on site and how to detect them afterwards after piling are all problems that must be faced in the first contact detection work. In order to let more beginners master the relevant detection items of PHC piles more quickly and comprehensively, I combined my many years' experience in test and detection, and made some induction on the detection requirements and related detection items of PHC piles from the perspective of practicality and convenience for monitoring and detection.

First, the main inspection requirements for the quality of the approach

Whether the pipe pile is commissioned prefabrication or purchased directly, the original quality should be controlled well. The entrusting unit or outsourced prefabrication yard shall have the construction qualification for processing and producing high-strength prestressed pipe piles. Before determining the manufacturer, we should go to the site with the supervisor and the owner to carefully inspect and understand the use effect of the products previously produced by the factory to ensure that it is a regular and qualified manufacturer. If you buy existing products directly, carefully demonstrate and scrutinize the products at the production site? All kinds of documents and procedures are available. The appearance of finished pile shall be free of honeycomb, exposed reinforcement, cracks, uniform color and cracks at the top of pile. If there are mandatory requirements in the specifications such as pile diameter, pipe wall thickness, center line of pile tip, flatness of top surface and bending of pile body, the relevant requirements must be met. Vibration and collision should be avoided during the lifting of pipe piles to ensure that the products transported to the site are qualified products.

After the pipe piles are transported to the construction site, the specifications, models, dimensions, appearance quality, dimensional deviation, pile piling and pile damage of all the pipe piles entering the construction site shall be thoroughly inspected, and those that do not meet the requirements shall not be used. The qualified testing unit shall conduct random witness sampling testing on the pipe piles entering the construction site, and the testing shall meet the following requirements:

(1) Before pile sinking, a cross section of each pile type produced by each manufacturer is randomly selected for destructive testing. The testing items include the tensile strength of prestressed steel bars, the number of steel bars, the diameter of steel bars (the weight per linear meter can be checked), the arrangement of steel bars, the material and thickness of end plates, the dimensional deviation, the appearance quality and the thickness of protective layer of steel bars. When the sampling results do not meet the quality requirements, the inspection should be doubled. If unqualified pile joints are found again, this batch of pipe piles are not allowed to be used and must be evacuated from the site. Engineering piles shall not be constructed without random inspection.

(2) In the process of pile sinking, each building shall conduct spot checks on the cut pile head, and detect the number of steel bars, the diameter of steel bars, the tensile strength of prestressed steel bars, the arrangement of steel bars, the size of end plates, and the thickness of protective layer of steel bars. The number of single project inspection shall not be less than 1% of the total number of pipe piles, and shall not be less than 3.

(3) Inspect the thickness of steel plate, the size of pile tip and the quality of weld seam of closed pile tip, and the inspection quantity shall be no less than 1% of the total number of piles in each building, and no less than 2 pile tips.

Before the construction of engineering pile, the vertical compressive static load test of single pile should be carried out according to relevant regulations until it is destroyed. When the high-strain dynamic test is proposed for the acceptance test of the vertical compressive bearing capacity of a single pile, the high-strain test should be carried out on the test pile first, and then the vertical static load test of the single pile is carried out and crushed to failure, and the high-strain dynamic test can be implemented in the acceptance test only after reliable dynamic and static comparison data are obtained. The number of comparison tests shall not be less than 3, and when the total number of piles is expected to be less than 50, it shall not be less than 2.

Second, the single pile static load test requirements

When the geotechnical engineering conditions are simple and the pile length is controlled by the compressive pile force, or when the geotechnical engineering conditions are simple and have similar experience, the vertical compressive static load test of single pile can be carried out by using engineering piles, but the number of tests should be doubled according to relevant regulations, and the test should meet the following requirements:

(1) The number of piles tested for a single building in each case shall not be less than 6 (when the total number of piles is less than 50, it shall not be less than 4), of which 3 (when the total number of piles is less than 50, it is 2) shall be tested before the construction of a large number of engineering piles.

(2) For multiple buildings with the same geotechnical engineering conditions in the same site, when the engineering pile conditions are the same, the number of test piles in each building should not be less than 2, and the number of test piles in each construction unit should not be less than 6. Each building has 1 pile. Before a large number of engineering piles are constructed, each construction unit should test 3 piles. When high-rise buildings and test results are scattered, the design unit should increase the number of tests as appropriate.

(3) Except for the test before construction, the rest of the tests shall be randomly sampled after the engineering pile construction is completed and the pile is cut according to the design elevation of the pile top; When the excavation of the foundation pit is deep and it is difficult to test in the pit, the design unit can also specify the pile location during the construction of the engineering pile for testing.

(4) When the total number of piles in a single building is less than 30 under a certain condition, and it is a small pile in a podium or annex, at least one pile should be subjected to static load test.

(5) When the inspection is carried out according to the above requirements, the acceptance of the bearing capacity of the single engineering pile may not be carried out under normal construction conditions.

(6) High-strain dynamic detection is not used to partially or completely replace the above-mentioned vertical compressive static load test of single pile. For all engineering piles, the inner wall of the pile hole should be illuminated by light or photographed visually in the hole one by one to observe whether there is soil seepage and water seepage in the hole, whether there is obvious damage, dislocation and deflection, and make detailed records, indicating the location where defects are found and the depth of soil seepage and water seepage.

Three, the main detection requirements of pile location, verticality and horizontal displacement

(a) `Check the elevation of the pile top of the engineering pile, and the deviation shall not exceed +20mm and -50 mm. ..

(2) During foundation pit excavation, the verticality of exposed pile heads of engineering piles or pile holes shall be detected, and the sampling number shall not be less than 5% of the total number of piles. During foundation pit excavation, if the displacement or movement of soil affects the verticality of piles, the detection times should be increased. Piles with inclination greater than 3% shall not be used: for piles with inclination of 1%~2% (including 2%) and 2%~3%, vertical compressive static load tests shall be conducted on at least two piles respectively, and the compressive bearing capacity of a single pile obtained from the tests shall be multiplied by the reduction factor as the basis for the use of this batch of piles. The maximum load of the load test should be the ultimate bearing capacity of a single pile required by the design, and the horizontal displacement of the pile top can be measured simultaneously in the test.

Four, the main requirements of pile integrity testing

Engineering piles should be tested for pile integrity. When the low strain integrity test is adopted, the number of bolts for pile foundation with design grade A should not be less than 30% of the total number of piles, and should not be less than 20. The sampling number of other pile foundations shall not be less than 20% of the total number of piles and shall not be less than 10.

The number of piles sampled under each pile cap shall not be less than 1, and all piles under single pile and two pile caps shall be tested. Uplift piles, compressive piles designed according to the control of pile strength, foundation piles of high-rise buildings with more than 25 floors, and piles with inclination greater than 1% shall be tested. When using low strain integrity testing method to test the integrity of pile body, the following requirements shall be met:

(1) The permanent structural uplift piles with cracks and defects or the piles mainly bearing horizontal force shall be designed as Class III or Class IV piles.

(2) Compressive piles with damaged concrete and inclined cracks or vertical cracks in the pile body are judged as Class III or Class IV piles.

(3) Compressive piles with slight defects should be judged as Class III piles first. When the pile type is finally judged, shallow defects should be excavated for inspection and verification, and low strain waveform should be used for discrimination and evaluation. During excavation inspection, when the crack length is less than 1/3 of the perimeter of the pile section and it is a horizontal crack, the pile with similar waveform can be judged as Class II pile.

Verb (abbreviation of verb) conclusion

In short, as a qualified test technician, in addition to understanding the requirements of various test regulations and test specifications, it is more important to pay attention to the improvement of their own work experience and professional technology, because when a group of unqualified data is misjudged due to lack of personal experience, it will often bring quality accidents to the project. Therefore, as the main role of quality evaluation, we must be prepared, be cautious, cultivate professional experience and improve testing skills. Only in this way can the quality of engineering construction be guaranteed.

References:

1. Code for Quality Acceptance of Building Foundation and Foundation Engineering (GB50202-2002[S])

2. Lu Chunhua. Construction technology of static pressure high-strength prestressed concrete pipe pile [J]. Science and technology information in 2005

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