How to test pile integrity by low strain?

- Mar 06, 2020-

How to test pile integrity by low strain?

1 Principle


The reflected wave method is derived from the stress wave theory. The basic principle is to vertically excite the top of the pile, and the elastic wave propagates down the pile body. There is a significant wave impedance interface (such as the bottom of the pile, broken piles or severe segregation) on the pile body or the change of the cross section of the pile body (such (Reduced or expanded diameter), the transmission wave will be generated. After receiving, amplifying, filtering and data processing, it can identify the reflection information of different parts of the visitor, and calculate the wave speed of the pile body, judge the integrity of the pile body and the concrete strength level.



2. Foundation pile inspection

 

2. 1 preparations before pile testing


(1) First-hand information should be obtained before entering the test: the pile formation process, pile length, pile diameter, pile formation date, concrete strength, etc. of the project.


(2) Enter the site, observe and knock on the pile head to understand its actual construction quality, such as whether the pile head is wet, mud trapped, loose, or contains mud.


(3) After the headline of the pile head has reached the design level, it should be cleaned. The pile head should be flat, intact and not damaged, and 3-4 smooth surfaces with a diameter of 8-10cm should be polished with a grinding wheel, which can be used as the excitation point and facilitate the installation of sensors The exposed steel bar should fall down to both sides, and there should be no large shaking. For large-diameter piles, several additional positions must be measured to obtain the true and complete reflection signal of the pile body.


 (4) Because the intensity of radon is closely related to its age. The test results at different ages, especially at the early stages, are quite different. These differences are manifested in the degree of discrimination of isolated nature defects, and have a great relationship. When the strength of the concrete reaches a certain value, the generated stress wave can be effectively transmitted downward along the pile body when the pile head is struck with a force bar. According to the requirements of the code, low-strain testing should be performed after the pile reaches its age, especially for long piles and piles with poor geological conditions.