When sinking large onshore caissons, the dynamic changes in the bearing capacity at the cutting edge and the side friction along the caisson wall are critical factors affecting sinking efficiency and the accuracy of attitude control. Traditional monitoring equipment, such as earth pressure cells, is limited by passive measurement, data lag, and susceptibility to damage, making it difficult to accurately capture the real-time evolution of foundation bearing characteristics. To address these challenges, an active device for determining foundation bearing capacity and a monitoring device for side friction were developed based on the super-large caisson foundation of the North Anchorage of the Hannan Yangtze River Bridge.

The active detection device consists of a hydraulic cylinder, a loading plate, a displacement sensor, a high-pressure oil pump, a control system, and a data acquisition system. It applies stepwise vertical loads to the soil beneath the cutting edge, using the caisson’s self-weight as the reaction force, and records the load–displacement curve (P–S curve) in real time. The ultimate bearing capacity of the foundation is determined by identifying the inflection point on the P–S curve. The side friction monitoring device is designed with compact dimensions and high sealing performance. Its core components are a sensing panel, a force transmission element, a tension–compression stress sensor, a reaction column, and a sealed housing. The device is embedded flush with the caisson wall, and the shear stress generated by soil movement is converted into an electrical signal to enable real-time measurement of side friction. Both devices were calibrated in the laboratory: the active detection device showed excellent linearity, with a calibration coefficient K close to 1.0, and the side friction sensor exhibited good repeatability, with a sensitivity loss of approximately 10% after sealing. Field monitoring was carried out during caisson sinking using four active detection units arranged at the cutting edge and four side friction units installed at different heights along the caisson wall. Numerical simulations were also performed using PLAXIS 3D with the Hardening Soil Small (HSS) model to calculate the reaction forces of the foundation at various sinking stages.

During field monitoring, the active detection device accurately determined the ultimate bearing capacity of the foundation through stepwise loading. The measured cutting edge resistance ranged from 2.0 to 2.2 MPa at greater depths, in good agreement with the numerical calculations (1.8–2.0 MPa), with a relative error of less than 10%. The data stability and detection accuracy of the active detection device were significantly superior to those of traditional earth pressure cells. The side friction monitoring device passed a water pressure sealing test at 0.7 MPa, demonstrating its adaptability to environments with high water pressure and slurry concentrations. The measured peak side friction was 77.65 kPa at the deepest measuring point and 18.6 kPa at the shallowest measuring point, consistent with the theoretically predicted trend. The relative errors between the measured and theoretical side friction values ranged from 5.6% to 9.0%, with an average of approximately 6.7%. Moreover, the measured side friction curves exhibited obvious dynamic fluctuations, reflecting the effects of soil disturbance, slurry lubrication, and air curtain operation. When the air curtain was activated at a penetration depth of roughly 44 m, the side friction decreased by approximately 28%–33%, further confirming the device’s ability to capture real-time construction effects.

The active device for detecting foundation bearing capacity and the side friction monitoring device successfully achieved active and continuous measurement of key mechanical parameters during the sinking of a large caisson. The active detection device overcomes the limitations of passive monitoring by providing reliable ultimate bearing capacity data. The side friction monitoring device effectively captures dynamic variations caused by construction activities, which theoretical models alone cannot reflect. The close agreement between the field measurements and numerical simulations validates the reliability of both devices. This study provides a novel and practical technical solution for safety control during large caisson sinking, with significant potential for engineering applications.