The accumulated plastic deformation induced by a cyclic traffic load will lead to destruction of the subgrade. Coarse-grained soil is a widely used subgrade filler. The GDS dynamic triaxial test was carried out on typical coarse-grained soil fillers to investigate the influence of different confining pressures, consolidation ratios and numbers of cyclic actions on the hysteresis curve under step cyclic loading. The results show that surrounding pressure can significantly reduce the energy lost from a soil sample under cyclic loading. Under the same stress level, increasing the consolidation ratio can effectively reduce the area enclosed by the hysteresis curve. When the stress is increased above a certain value, the strain of the response clearly changes with an increase in the number of cyclic loading. The research findings can theoretically guide the design of coarse-grained soil roadbeds in practical engineering.

The foundation piles in karst areas have different mechanical properties from those in other areas. Targeting a critical highway bridge in a karst area, this paper designs two kinds of foundation pile models: friction pile, and KCSP, based on theories on dynamic tests. Then, shaking table tests were carried out to ascertain the features of pile strain distribution of KCSP under earthquakes. During the tests, a large laminar suspended shear box was adopted to mimic the boundary effect of soil. In addition, numerical simulations were conducted to disclose the effects of karst cave on pile strain. The test results indicate that: the peak strain of KCSP increased with the peak acceleration; For both KCSP and friction pile, the peak strain decreased first and then increased along the depth; The presence of karst cave can adversely affect the seismic response of foundation pile; The taller the karst cave, the larger the peak strain of the pile; the peak strain of KCSP was larger at the two ends, and smaller in the middle. The numerical analysis shows that: the peak strain of foundation pile in karst cave increased significantly with cave height; The peak strain of the pile passing through multiple caves was similar to that of the pile passing through only one cave, under the same cave height; But the multi-cave scenario differed from the single-cave scenario in peak strain distribution. The research results provide new insights into the seismic design of pile foundation of bridges in karst areas.

Karst landforms constitute one of the most harmful geological conditions, which have an adverse effect on the deep foundation structures of bridges. During earthquakes, the existence of karst caves can cause serious seismic damage to the bridge pile foundation. In order to investigate the seismic response of rock-socketed piles under vertical loads in complex karst cave conditions, finite element numerical simulation analyses were carried out, referring to the practical major bridge structure rock-socketed pile project in China. The peak strain distributions of rock-socketed pile foundation influenced by single-karst cave factors under the combined action of vertical loads and ground motions were investigated, and the influences of complex multi-caves were further explored. The results showed that the restraint effect of the bedrock near the pile would gradually decrease with the increase of the height of the karst cave and the decrease of the height of the karst cave roof; under the condition of a beaded karst cave, the constraint of bedrock between the karst caves makes the pile present the distribution characteristics of “multi-segment and multi-broken line”; under the condition of an underlying karst cave, the existence of the underlying karst cave would decrease the restraint of the bedrock at the bottom pile and increase the peak strain of the pile to a certain extent. This paper revealed the seismic response law of the rock-socketed pile under vertical loads within various complex karst cave conditions and developed reasonable reinforcement measures aiming at dangerous locations, providing important engineering guidance and a reference for the seismic design of rock-socketed pile foundation in complex karst areas.

Excavation and dewatering are the primary factors governing diaphragm wall deformation and ground surface settlement in deep foundation pits. However, their coupled effects in soft-over-hard composite strata remain insufficiently understood. This study investigates a deep metro foundation pit in Jinan, China, and develops a three-dimensional hydro-mechanical coupled model in ABAQUS to simulate the complete staged excavation and dewatering process. The evolution of diaphragm wall lateral displacement, ground surface settlement, and pore-water pressure was systematically analyzed, and the simulation results were validated against field monitoring data. The results show that both excavation and dewatering induced significant wall deformation and surface settlement, with excavation playing the dominant role. The incremental lateral displacement of the diaphragm wall caused by excavation was approximately 2.6–3.8 times that caused by dewatering, while the corresponding ground surface settlement was 7.9–10.7 times greater. Owing to the strong restraint provided by the underlying rock stratum, the maximum lateral displacement of the diaphragm wall occurred at approximately 0.67 H_e, where H_e is the final excavation depth. The primary influence zone of ground surface settlement extended to approximately 2 H_e. In addition, dewatering altered the seepage field inside and outside the pit, leading to a continuous decrease in pore-water pressure within the pit, whereas the external pore-water pressure remained largely unchanged because of the seepage-barrier effect of the diaphragm wall. These findings provide practical guidance for the design and construction of deep foundation pits under similar geological conditions.

Deep excavations in composite “soft upper-hard lower” strata present significant deformation control challenges due to strong stiffness contrasts. This study investigates the deformation characteristics of a deep metro foundation pit in Jinan under zoned excavation conditions. A three-dimensional finite element model was developed to simulate the staged excavation process, and the spatiotemporal evolution of diaphragm wall deflection and ground settlement was analyzed, with particular focus on the influence of soft soil thickness. The results show clear spatial variation, with maximum lateral wall displacements exhibiting a typical “bulging” profile along the longer sides of the pit. The normalized maximum wall displacement (δ_hm/H_e) ranges from 0.051% to 0.090%, while the ratio of maximum ground settlement to wall displacement is 0.35–0.57, indicating lower deformation levels compared to homogeneous soft soils. As the soft soil thickness increases, the wall displacement increases and the location of maximum displacement migrates downward and outward. Under the investigated conditions, a transition toward a “kick-out” deformation mode occurs when the soft soil thickness reaches the transition range identified in the parametric analysis. These findings provide a quantitative basis for deformation prediction and support design in composite strata.