How to select and install sensors in low strain pile test?
The natural frequency of a pile-soil system is determined by the mass and stiffness of the system. With a certain mass, the greater the stiffness, the higher the natural frequency of the system; the lower the stiffness, the lower the natural frequency of the system. Under the condition of constant stiffness, the greater the mass, the lower the natural frequency of the system; the lower the mass, the higher the natural frequency of the system. At present, in the reflected wave method test, the application of the speedometer and accelerometer has achieved good test results. The frequency band of the accelerometer has good high-frequency characteristics; the frequency band of the accelerometer is narrow, but its low-frequency characteristics are better. In the field test, different sensors should be selected depending on the specific project and specific occasion, in order to obtain a good curve in time. Generally, the accelerometer is used to detect short piles and small diameter piles to detect shallow defects and reduce the "blind zone" in the shallow area. In the detection of large diameter and long piles, the speedometer is used to obtain deep defects and good pile bottom reflection signals. However, in actual engineering, it is appropriate to use the two sensors together to make up for the shortcomings. A speedometer can be used for general inspection, and an accelerometer is used to detect the pile in question, and the curves are compared to make a judgment. When installing the sensor, the longitudinal axis of the sensor should be parallel to the longitudinal axis of the pile to ensure that the sensor is perpendicular to the plane of the pile top, so that the received longitudinal wave signal is not distorted. The coupling between the sensor and the pile top should use adhesives such as cooked gypsum powder, plasticine, butter, etc., so that the sensor and the pile top are tightly integrated into one to avoid vibration clutter.