Deep foundation pit excavations have become more extensive for the construction of underground spaces with rapid urbanization. Diaphragm walls are commonly used to support deep excavations. However, due to the complex geological conditions in karst areas, construction accidents frequently occur during the excavation of foundation pits. This study aims to investigate the performance of diaphragm walls in karst areas through field monitoring analysis. A kick‐in deformation mode of the diaphragm wall is revealed during the foundation pit excavation. Furthermore, the results show that the diaphragm walls present multiple deformation modes rather than a single mode. Additionally, this study proposes a method to calculate the lateral displacement of the diaphragm walls at different depths. It is found that the karst caves have a considerable impact on the stability of diaphragm walls, as demonstrated by their lateral displacement. The hidden karst caves reduce the bearing capacity of the bedrock, rendering it insufficient to resist the active earth pressure. As a result, the bottom of the diaphragm wall is kicked into the foundation pit, causing significant lateral displacement and posing risks during excavation. The findings of this study contribute to the design and construction of similar excavations in karst areas.

Ultra‐deep excavations in karst areas pose a substantial risk due to the presence of unknown karst caves. However, the literatures on deep excavations in karst regions remain limited and lack comprehensive discussion. This study investigates the effects of karst caves on the stability of the diaphragm walls (D‐walls) during an ultra‐deep excavation. A statistical analysis was conducted to examine the distribution of various parameters related to karst caves, including height, embedded depth, and roof thickness. It is revealed that these parameters follow a log‐normal distribution. There is a strong probability (99.8%) that the embedded depth will exceed 10 m, a high likelihood (82.6%) of the height being under 6 m, and similarly, an 82.2% chance that the roof thickness will be less than 5 m. A three‐dimensional finite element analysis was conducted to investigate the effects of the karst cave height, roof thickness, and location on the horizontal displacement of the D‐walls. The results show that the karst cave have significant effects on the horizontal displacement of the D‐wall, particularly at the bottom of the D‐wall. Specifically, when the karst cave has a height of 6 m, a roof thickness of 1 m, and is located at a distance of 1 m from the wall, the horizontal displacement at the bottom of the D‐wall can increase by 59 times compared with the condition without the presence of the karst cave. This severe increase in displacement results in a pronounced kick‐in failure of the D‐wall. The reason lies in the presence of the karst cave greatly reduces the bearing capacity of the bedrock within the foundation pit, thereby, rending it incapable of resisting the active earth pressure. The findings in this study can provide valuable references for formulating principles and treatment planes of the karst caves.

Karst cavities can cause substantial concrete loss, construction incidents, and inadequate bearing strata for bored piles. Advance drilling provides direct geological information but samples only a limited volume, while surface methods may lose resolution with depth and cross-borehole methods require multiple boreholes and additional tomographic processing. This study presents single-borehole high-density electrical resistivity profiling (SHERP) as a rapid, near-borehole screening method for karst cavities at pile locations. A high-density electrode cable and a Schlumberger-type acquisition sequence are used to generate apparent resistivity pseudo-sections that can be inspected on site without tomographic inversion. Three-dimensional finite-element forward models were used to examine the response of simplified soil–bedrock and water-filled cavity configurations. For the modeled conductivity contrast and acquisition geometry, the cavity produced a characteristic double-peaked, or ‘M-shaped’, anomaly in the pseudo-section because the conductive cavity redistributed the electric potential field as the current electrodes crossed the target depth. Field measurements from three boreholes at a karst site in Guangzhou, China, were then compared with borehole logs. Among the tested electrode spacings of 0.3, 0.5, and 1.0 m, the 0.3 m configuration provided the closest agreement with logged cavity boundaries and the highest vertical resolution under the investigated site conditions. SHERP is therefore proposed as a complementary method for screening possible off-axis cavities around pile locations and for supporting subsequent drilling and foundation design.

Shield tunnelling in upper-soft and lower-hard mixed ground is characterised by strong geological heterogeneity and coupled construction risks, including uneven cutterhead loading, face instability, cutter wear and abnormal fluctuations in tunnelling parameters. This study assessed risk of shield tunnelling under such conditions by using a coupled subjective–objective weighting and fuzzy set pair analysis model. Seven key tunnelling parameters, namely total thrust, advance rate, cutterhead rotational speed, cutterhead torque, chamber pressure, grouting pressure and shield attitude, are selected as evaluation indicators. The Analytic Hierarchy Process (AHP) and coefficient of variation method are combined to determine indicator weights, thereby integrating expert judgement with the statistical characteristics of field data. A five-element connection number is then introduced to characterise the identity–discrepancy–contrary relationships between measured indicator values and risk grade standards, while segmented triangular fuzzy numbers are used to determine discrepancy coefficients and address the fuzziness of grade boundaries. The results show that the risk grades for Rings 1–500 fluctuate considerably and are dominated by Grade II–III risks, with 11% of the evaluated segments reaching Grade IV. For Rings 501–1000, the predicted risk level is generally lower, with Grade II and Grade III accounting for 29% and 67% of the evaluated segments, respectively. The assessment results are consistent with the geological conditions and construction observations, indicating that the model can effectively quantify shield tunnelling risk and identify key high-risk parameters in mixed ground. The proposed hybrid framework provides a new theoretical reference for real-time dynamic risk assessment of shield tunnelling under geological transitional conditions.