Monitoring of cracks and crack growth rates is a crucial aspect of structural health monitoring for concrete infrastructure, and multiple manual and automatic monitoring techniques have emerged over the years. This study focuses on an in-depth review of concrete crack sensing using distributed fiber optic sensing (DFOS) technology. DFOS provides the option to sample distributed data points through dedicated optical fibers or cables, thereby effectively addressing the spatial limitations associated with conventional discrete point sensors such as foil strain gauges and transducers. The main findings include that (1) smart concrete crack sensing generally involves three objectives: detecting crack initiation, identifying the crack location and determining the crack width and its evolution; (2) for DFOS used for crack sensing, the three main sensing principles are to measure localized strain spikes in optical fibers or cables that span across cracks, to detect signal intensity losses caused by micro-bending of optical fibers in proximity to cracks and to measure precise local temperature variations within the crack areas; (3) strain-based crack sensing has become the predominant method due to its superior sensing performance and application versatility. This dominance is supported by extensive experimental demonstrations and successful implementations in field monitoring practices; (4) the sensitivity of optical fibers or cables to concrete cracks depends on the installation method, while quantitative crack width measurements require the precise determination of crack locations followed by a subsequent integration or exponential fitting of strain along the length at fiber-concrete interface. This study helps to advance the application of the smart DFOS for structural health monitoring and maintenance of concrete infrastructures.

This study demonstrates the design and field implementation of an innovative servo concrete bracing system in foundation pit excavation. The bracing system comprises concrete struts, revised purlins, and hydraulic jacks, and its field performance is evaluated in a deep foundation pit project in Shanghai, China. The field measurements demonstrate that the servo bracing system effectively reduces the maximum lateral displacement of the retaining wall by up to 31%. Moreover, the servo jacks modify the wall’s flexural behavior by introducing local inflection points at certain depths and driving the displacement peak upward. Furthermore, the system’s performance varies with strut configuration, and servo forces influence not only the corresponding acting strut but also the adjacent struts’ behavior, implying that the monitoring scope should be expanded when applying the servo bracing system in actual engineering. This study provides a meaningful technical reference for future servo concrete bracing system applications in foundation pit engineering.

The development of modern cities has often led to increased traffic congestion and limited usable space. One effective solution to these problems is to construct roadway tunnels, which can expand urban space and alleviate traffic congestion. However, building large underpass tunnels in urban areas, especially in soft ground, presents technical challenges owing to its extensive environmental disturbance. The pipe-umbrella box jacking method offers a promising solution to these challenges as this method does not require breaking the ground from the surface or disrupting surface traffic. This study reviews the key techniques of the pipe-umbrella box jacking method for constructing large rectangular tunnels with small burial depths in soft ground. The study focuses on three main technical issues: pipe umbrella installation, soil face excavation, and box jacking control. Furthermore, two tunnel projects in Shanghai, China, are presented as case studies to showcase the construction challenges and countermeasures during box jacking tunneling with pipe-umbrella. Practical construction experience has revealed that the installation of the pipe umbrella before box jacking serves as an effective method of ground pre-support. However, its performance can be influenced by factors such as pipe installation deviation and inter-pipe connection design. Both open-face steel grid extrusion (SGE) shields and closed-face earth pressure balanced (EPB) shields can be used for soil excavation during box jacking, with different work modes resulting in distinct ground deformation patterns. Additionally, lubrication grouting has been found to effectively reduce friction resistance and affect vertical ground deformation. The study also examines the lessons learned from case studies and proposes optimization measures. This study can serve as a valuable reference for the tunneling industry, providing insights into the pipe-umbrella box jacking method and its application in constructing large rectangular tunnels in soft ground.