New LGFBG-Based Structural Integrity Evaluation Method for Cement-Grouted Soil Nails

Abstract

This paper proposes a novel optical fiber sensor-based nondestructive method, namely, the long-gauge fiber Bragg grating (LGFBG) method, for integrity assessment of cement-grouted soil nails. This LGFBG sensor was developed by encapsulating a number of straight, bare FBG sensors inside an aluminum tube, and then mounting them on a steel bar surface for monitoring average strain distributions of soil nails subjected to tension force. Measured strain distributions of LGFBG sensors were finally used for the prediction of effective elastic modulus (EEM) distributions along the soil nails and the assessment of soil nail integrity in terms of the predicted EEM. A number of pullout tests of soil nails with different defects in the laboratory were performed to verify the feasibility of this proposed method. Pullout test results indicate that the elongation contributions of defective soil nail sections are 1.5 to 3 times the intact soil nail sections, whereas the related EEM values of the two defective soil nail sections are around 0.3 to 0.5 times that of intact soil nail sections. The statistical hypothesis Student’s t-test was used to determine the magnitude ranges of the modulus ratio leading to a rejection of the null hypothesis for a prespecified level of significance. In this study, a significance level of 0.01% is suggested for the integrity evaluation of cement-grouted soil nails.