## 1 Introduction  
In recent years, with the rapid development of underground space in China, a large number of foundation pit projects have emerged, and the geological and environmental conditions have become increasingly complex [1; 2; 3; 4]. When the stratum where the foundation pit is located is rich in groundwater, it may cause the pit wall leakage, basal heave, surface collapse, etc. [5; 6; 7]. It is reported that about 60% of underground engineering accidents are caused by groundwater [8]. Therefore, it is necessary to take some measures to reduce the adverse effects of groundwater. The waterproof curtain is widely used in engineering as an effective waterproof measure [9; 10], and its construction quality has a significant impact on foundation pit safety and environmental responses.  
At present, the commonly used construction technologies for waterproof curtains include the deep mixing column [11; 12], jet grouting column [13; 14], sleeve valve pipe grouting [15], freezing method [16], etc. For the above technologies, the construction depth is generally less than 30 m, so it is difficult to cut off the hydraulic connection inside and outside the pit. Moreover, the suitable strata conditions for deep cement mixing column and jet grouting column are clay, silt, and other strata with low strength. In the dense sandy soil or pebble stratum, there exist some problems such as difficult mixing and spraying, as well as poor construction quality.  
Due to the limitations of geological conditions and construction workmanship, the waterproof curtain inevitably has some quality defects, such as leakage, cavities, and cracks [17; 18]. Therefore, there is a need to detect the construction quality of the waterproof curtain before excavation. At present, the commonly used nondestructive detection technologies for waterproof curtain defects include thermal [19; 20; 21; 22; 23], sonic [24; 25; 26], electromagnetic [27; 28; 29] methods, etc. However, each detection method has its technical applicability and emphasis. As the waterproof curtain faces complex geological and environmental conditions, if only one detection method is adopted, the detection results may not reflect the real situation objectively, so it is necessary to adopt multiple detection methods for comprehensive evaluation.  
In this study, a foundation pit was constructed in the floodplain of Yangtze River, and a deep and thick pebble gravel layer exists below the base slab with high water pressure and strong permeability. In view of the fact that the conventional construction technologies for waterproof curtains such as the deep mixing column and jet grouting column are difficult to meet the engineering requirements, a new waterproof curtain construction technology that combines the trenching technology of the diaphragm wall with the TRD (Trench cutting Remixing Deep wall) technology is proposed, i.e., the trenching-and-replacing-style TRD technology, and the construction process of this new technology is introduced. After the waterproof curtain was built using this technology, the strength, integrity, uniformity, and service performance of waterproof curtain were tested and evaluated by the comprehensive methods of coring, borehole television imaging, resistivity CT, and a group well pumping test. It is expected that the research can provide useful references for similar projects.