Axial strain monitoring method of cast-in-place piles based on ultra-weak fiber Bragg grating

The axial strain distribution of cast-in-place piles under the static load test is a reliable basis for analyzing the compressive bearing capacity of the pile foundation. However, it is still difficult to achieve high-precision, high-sensitivity, real-time, and distributed monitoring of the pile foundation at the same time. To improve the monitoring of the stress distributions of the pile foundation, a fixed-point ultra-weak fiber Bragg grating (UWFBG) strain-sensing optical cable is designed on the basis of the large capacity characteristic of UWFBG. The strain sensitivity of this optical cable is 1.15 pm μϵ −1 within the range of 10 000 μϵ, which meets the accuracy requirements of pile health monitoring. The effectiveness of the designed UWFBG in pile foundation monitoring is verified through a static load test of the cast-in-place pile. The results show that the measured results of UWFBG and BOTDA (Brillouin optical time-domain analysis) have good consistency, and their average error is less than 7.5%. Compared with BOTDA, the UWFBG sensing system exhibits stronger anti-interference capability and faster response. The monitoring method proposed in this paper overcomes the shortcomings of previous monitoring methods in the static load test of the pile. The measured data can be used to calculate the detailed axial strain distribution of piles and analyze the distribution of axial force and side friction resistance of the pile. It not only provides a new monitoring method for static load test of cast-in-place piles, but also has great potential in monitoring large diameter pile.