First Author: LI Songhui
Corresponding Author: ZHANG Yan
Journal: Structural Health Monitoring

Abstract

The structural form of composite immersed steel-shell concrete immersed tube (SSIT) is highly complex. During the casting of dense concrete, voids are likely to form at the steel-concrete interface, leading to debonding defects. Such defects can adversely affect the overall structural safety and service life of the tunnel. However, nondestructively detecting debonding beneath thick steel plates at the millimeter scale remains a major technical challenge in engineering practice.

To address this, this paper proposes a novel nondestructive testing method for detecting millimeter-scale debonding at the steel-concrete interface of immersed tunnel segments by combining impact-echo imaging with the “intercept method”. First, based on near-field wave theory and the intercept algorithm, the calculation methods for debonding depth and height were derived. Second, a multi-level approach for debonding detection and defect quantification was proposed. Third, by covering the steel-concrete interface with transparent pipe segments, experimental verification of debonding detection effectiveness was carried out. Finally, the method was applied to the quality inspection of immersed tunnel construction for the ShenZhong Link Project.

Results indicate that the proposed method, when used with impact-echo imaging, can effectively identify the location and shape of debonding defects, though it cannot achieve highly accurate quantitative depth estimation. The intercept method, in contrast, provides more accurate depth evaluation, but suffers from issues such as a limited detection unit size and relatively high test cost. Experimental tests demonstrated that the positional accuracy of debonding defect detection exceeded 95%, and the debonding area determination accuracy reached 89%. When the thickness deviation was within ±2 mm, the depth detection accuracy was 87.5%.

This method provides deep insight into the mechanisms of debonding at the steel–concrete interface of immersed tunnels. Moreover, by summarizing experimental patterns, it highlights the importance of construction processes such as vibrating concrete adequately during pouring, thereby offering practical guidance for improving construction quality. The method thus provides an important technical basis for quality assurance of SSIT projects.

Keywords: Immersed tunnel; Steel-concrete interface; Nondestructive testing; Impact-echo imaging; Intercept method; Full-scale model test