Wenbo Zhu

Large caisson foundations in sea-crossing bridges are often constructed in non-uniform bedrock with fractured weak layers, where the mechanical response during sinking, bottom sealing, and core filling is complex and not well understood. Existing studies generally assume uniform and intact bedrock and focus primarily on the service stage, leaving the entire construction process in such complex strata largely unexamined. To address this gap, this study investigates a large caisson foundation using field investigation data and three-dimensional numerical simulation with PLAXIS 3D (version: V20), taking the Xihoumen Rail-cum-Road Sea-crossing Bridge as an engineering case. Sonar-derived bedrock geometry, geological exploration data, and laboratory rock properties are incorporated, and the full construction process is simulated. After sinking, the maximum longitudinal and transverse displacements are 0.6269 mm and 0.6298 mm, respectively, both located on the right side of the middle compartment, while the maximum vertical displacement is 2.918 mm in the suspended part. Stress concentrates at the caisson–bedrock interface, with a maximum vertical stress of 3825 kPa at the cutting edge. During core filling, the vertical displacement increases to 4.65 mm, while the longitudinal and transverse displacements show a fluctuating upward trend, reaching 0.854 mm and 0.87 mm, respectively. As the weak-layer thickness decreases from 0.8 m to 0.2 m, the displacements under the most unfavorable load combination decrease. For the investigated foundation, a weak-layer thickness of 0.6 m or more causes a notable increase in deformation, while the base stress changes little. These results are numerical predictions, and the 0.6 m threshold should be regarded as a project-specific recommendation rather than a universal design criterion. The findings provide a reference for similar projects.

Large gravity anchorages founded on soft marine deposits often rely on partial cement-based ground improvement, creating a composite-foundation base in which treated area and mechanical contribution are not equivalent. This study develops a normal-load-sharing framework for evaluating base shear resistance of rigid circular foundations on partially cement-treated ground. Large-scale interface shear tests and three-dimensional finite-element analyses were used to determine constituent interface properties and examine composite-interface behaviour. The normal-load replacement ratio R_N was introduced to distinguish the actual normal load carried by treated ground from the area replacement ratio R_A. For a rigid circular foundation at R_A = 76.7%, the homogeneous solution gives geometric normal-load fractions of 0.8756 for outer-ring treatment and 0.5170 for centre treatment. Idealised elastic analyses showed a saturating increase in outer-ring R_N with stiffness ratio, represented by a one-parameter relation with β = 2.42 (R² = 0.9998). Under measured 35% and 40% CTS conditions, the simplified relation overpredicted regional load concentration, while overall horizontal-resistance estimates remained within about 7% of full finite-element results. The framework provides a first-order basis for assessing treatment layout, stiffness contrast, and base shear resistance.

The suction caisson foundation has been extensively adopted for offshore wind turbine infrastructure owing to its adaptability to deep-water environments, cost-effectiveness, and convenient construction. However, such foundations suffer from relatively low horizontal and vertical bearing capacities when embedded in soft clay deposits. To address this limitation, this study proposes a novel mobile jet-reinforcement technique and the corresponding composite suction caisson configuration. Physical model tests are conducted to investigate the soil fracturing-erosion mechanism induced by jet injection and the bearing performance of the reinforced composite foundations. Test results reveal that the soil breaking depth increases with injection pressure and injector diameter, whereas the soil breaking width increases with jet angle. Larger breaking depth and width contribute to an expanded horizontal–vertical bearing capacity failure envelope. The ultimate bearing capacity of the composite caisson increases with greater soil breaking depth, and a larger number of circumferentially arranged jet pipes enables more uniform cement–soil cladding around the caisson body. Overall, the reinforced foundations achieve a bearing capacity 3.0–5.0 times that of conventional unreinforced suction caissons. Furthermore, a time-dependent hyperbolic model for soil breaking depth prediction and a bearing capacity failure envelope method are established for the reinforced composite suction caissons. The outcomes of this study can provide a reference for the engineering design of jet-reinforced suction caisson foundations in offshore areas with soft clay.

In the marine environment, the suction caisson foundation (SCF) is often subjected to combined inclined and cyclic loading from wind and waves, which may significantly affect its ultimate bearing capacity. Under combined loading conditions, the evolution of ultimate bearing capacity is influenced by multiple factors, and the corresponding bearing capacity envelopes have become key issues that urgently need to be addressed. In this study, a series of model tests and numerical simulations were conducted considering the effects of load inclination angle, loading position, aspect ratio, soil undrained shear strength, and interface friction coefficient. The results show that under static loading conditions, as the loading depth increases, the load inclination angle corresponding to the maximum bearing capacity decreases from 45° to 0°. As the cyclic load ratio and static load ratio increase, cyclic loading significantly intensifies displacement accumulation and the degradation of ultimate bearing capacity. As the loading depth increases, the failure mechanism transitions from rotation-dominated to translation-dominated behavior. In addition, the ultimate bearing capacity increases monotonically with increasing aspect ratio, interface friction coefficient, and soil undrained shear strength. A normalized V–H bearing capacity envelope was established, which shows good agreement with the experimental and numerical results. By introducing a cyclic bearing capacity degradation coefficient, a modified envelope was proposed to describe the evolution of ultimate bearing capacity under cyclic loading conditions. The bearing capacity evolution patterns and envelope method proposed in this study provide a useful reference for the engineering design of SCF.

To address the critical challenge of ensuring bottom water-inrush stability during the excavation of ultra-deep foundation pits for riverside suspension-bridge anchorages under complex geological conditions involving high-pressure confined groundwater, we investigate the application of D-RJP high-pressure rotary jet grouting pile technology for ground improvement. Its effectiveness is systematically validated through a case study of the South Anchorage Foundation Pit for the North Channel Bridge of the Zhangjinggao Yangtze River Bridge. The D-RJP method led to the successful construction of a composite foundation within the soft soil that satisfies the permeability coefficient, interface friction coefficient, bearing capacity, and shear strength requirements, significantly improving the geotechnical performance of the anchorage foundation. A series of field experiments were conducted to optimize the critical construction parameters, including the lifting speed, water–cement ratio, and stroke spacing. Core sampling and laboratory testing revealed the grout columns to have good structural integrity. The unconfined compressive strength of the high-pressure jet grout columns reached 5.45 MPa in silty clay layers and 8.21 MPa in silty sand layers. The average permeability coefficient ranged from 1.67 × 10⁻⁷ to 2.52 × 10⁻⁷ cm/s. The average density of the columns was 1.66 g/cm³ in the silty clay layer and 2.08 g/cm³ in the silty sand layer. The cement content in the return slurry varied between 18% and 27%, with no significant soil squeezing effect observed. The foundation interface friction coefficient ranged from 0.44 to 0.52. After excavation, the composite foundation formed by D-RJP columns was subjected to static load and direct shear testing. The results showed a characteristic bearing capacity value of 1200 kPa, the internal friction angle exceeded 24.23°, and the cohesion exceeded 180 kPa. This study successfully verifies the feasibility of applying D-RJP technology to construct high-performance artificial composite foundations in complex strata characterized by deep soft soils and high-pressure confined groundwater, providing valuable technical references and practical insights for similar ultra-deep foundation pit projects involving suspension bridge anchorages.

Large-diameter rock-socketed monopiles supporting offshore wind turbines in soft clay strata face significant geotechnical risks from negative skin friction (NFS) induced by construction surcharges. While the effects of NFS on axial drag loads are documented, the critical interaction between horizontal pile loading and NFS development remains poorly understood. This research bridges this gap using a rigorously validated 3D finite element model that simulates the complex coupling of vertical substructure loads (5 MN), horizontal loading, and surcharge-induced consolidation. The model’s accuracy was confirmed through comprehensive verification against field data for both NFS evolution under surcharge and horizontal load–displacement behavior. The initial analysis under representative conditions (10 MN horizontal load, 100 kPa surcharge, 3600 days consolidation) revealed that horizontal loading fundamentally distorts NFS distribution in the upper pile segment (0 to −24 m), transforming smooth profiles into distinct dual-peak morphologies while increasing the maximum NFS magnitude by 57% (from −45.4 kPa to −71.5 kPa) and relocating its position 21 m upward. This redistribution was mechanistically linked to horizontal soil displacement patterns. Crucially, the NFS neutral plane remained invariant at the clay–rock interface (−39 m), demonstrating complete independence from horizontal loading effects. A systematic parametric study evaluated key operational factors: (1) consolidation time progressively increased NFS magnitude throughout the clay layer, evolving from near-linear to dual-peaked distributions in the upper clay (0 to −18 m); NFS stabilized in the upper clay after 720 days while continuing to increase in the lower clay (−18 to −39 m) due to downward surcharge transfer, accompanied by neutral plane deepening (from −36.5 m to −39.5 m) and 84% maximum axial force escalation (12.5 MN to 23 MN); (2) horizontal load magnitude amplified upper clay NFS peaks at −3.2 m and −9.3 m, with the shallow peak magnitude increasing linearly with load intensity, though it neither altered lower clay NFS nor neutral plane position; (3) surcharge magnitude increased overall NFS, but upper clay NFS (0 to −18 m) stabilized beyond 100 kPa, while lower clay NFS continued rising with higher surcharges, and the neutral plane descended progressively (from −38 m to −39.5 m). These findings demonstrate that horizontal loading critically exacerbates peak NFS values and redistributes friction in upper pile segments without influencing the neutral plane, whereas surcharge magnitude and consolidation time govern neutral plane depth, total NFS magnitude, and maximum drag load. This research delivers essential theoretical insights and practical guidelines for predicting NFS-induced drag loads and ensuring the long-term safety of offshore wind foundations in soft clays under complex multi-directional loading scenarios.

With the rapid urbanization and increasing development of underground spaces, foundation pit groups in complex geological environments encounter considerable challenges in deformation control. These challenges are especially prominent in cases of adjacent constructions, complex geology, and environmentally sensitive areas. Nevertheless, existing research is lacking in systematic analysis of construction sequencing and the interaction mechanisms between foundation pit groups. This results in gaps in comprehending stress redistribution and optimal excavation strategies for such configurations. To address these gaps, this study integrates physical model tests and PLAXIS 3D numerical simulations to explore the Nanjing Jiangbei New District Phase II pit groups. It concentrates on deformations in segmented and adjacent configurations under varying excavation sequences and spacing conditions. Key findings reveal that simultaneous excavation in segmented pit groups optimizes deformation control through symmetrical stress relief via bilateral unloading, reducing shared diaphragm wall displacement by 18–25% compared to sequential methods. Sequential excavations induce complex soil stress redistribution from asymmetric unloading, with deep-to-shallow sequencing minimizing exterior wall deformation (≤0.12%He). For adjacent foundation pit groups, simultaneous excavation achieves minimum displacement interference, while phased construction requires prioritizing large-section excavation first to mitigate cumulative deformations through optimized stress transfer. When the spacing-to-depth ratio (B/He) is below 1, horizontal displacements of retaining structures increase by 43% due to spacing effects. This study quantifies the effects of excavation sequences and spacing configurations on pit group deformation, establishing a theoretical framework for optimizing construction strategies and enhancing retaining structure stability. The findings are highly significant for underground engineering design and construction in complex urban geological settings, especially in high-density areas with spatial and geotechnical constraints.

Red mud, a highly alkaline industrial by-product generated during aluminum smelting, poses serious environmental risks such as soil alkalization and ecological degradation. In this study, response surface methodology (RSM) was integrated with advanced microstructural characterization techniques to optimize the performance of red mud–slag composite cementitious materials through multi-factor analysis. By constructing a four-factor interaction model—including red mud content, steel fiber content, alkali activator dosage, and calcination temperature—a systematic mix design and performance prediction framework was established, overcoming the limitations of traditional single-factor experimental approaches. The optimal ratio was determined via multi-factor RSM analysis as follows: the 28-day flexural strength and compressive strength of the specimens reached 12.26 MPa and 69.83 MPa, respectively. Furthermore, XRD and SEM-EDS analyses revealed the synergistic formation of C-S-H and C-A-S-H gels, and their strengthening effects at the fiber–matrix interfacial transition zone (ITZ), elucidating the micro-mechanism pathway of “gel densification–rack filling–strength enhancement.” This work not only enriches the theoretical foundation for the design of red mud-based binders but also offers practical insights and empirical evidence for their engineering applications, highlighting substantial potential in the development of sustainable building materials and high-value utilization of industrial solid waste.

This study investigates the deformation characteristics and base stability of a circular diaphragm wall support system (external diameter: 90 m, wall thickness: 1.5 m) with pit bottom reinforcement for the South Anchorage deep foundation pit of the Zhangjinggao Yangtze River Bridge, which uses layered and partitioned top-down excavation combined with lining construction. Through field monitoring (deep horizontal displacement of the diaphragm wall, vertical displacement at the wall top, and earth pressure) and numerical simulations (PLAXIS Strength Reduction Method), we systematically analyzed the deformation evolution and failure mechanisms during construction. The results indicate the following: (1) Under the synergistic effect of the circular diaphragm wall, lining, and pit bottom reinforcement, the maximum horizontal displacement at the wall top was less than 30 mm and the vertical displacement was 0.04%H, both significantly below code-specified thresholds, verifying the effectiveness of the support system and pit bottom reinforcement. (2) Earth pressure exhibited a “decrease-then-increase” trend during the excavation proceeds. High-pressure jet grouting pile reinforcement at the pit base significantly enhanced basal constraints, leading to earth pressure below the Rankine active limit during intermediate stages and converging toward theoretical values as deformation progressed. (3) Without reinforcement, hydraulic uplift failure manifested as sand layer suspension and soil shear. After reinforcement, failure modes shifted to basal uplift and wall-external soil sliding, demonstrating that high-pressure jet grouting pile reinforcement had positive contribution basal heave stability by improving soil shear strength. (4) Improved stability verification methods for anti-heave and anti-hydraulic-uplift were proposed, incorporating soil shear strength contributions to overcome the underestimation of reinforcement effects in traditional pressure equilibrium and Terzaghi bearing capacity models. This study provides theoretical and practical references for similar deep foundation pit projects and offers systematic solutions for the safety design and deformation characteristics of circular diaphragm walls with pit bottom reinforcement.

In coastal regions with thick, soft soil deposits, bridge pile foundations are susceptible to lateral displacements induced by the construction of adjacent seawalls. This study employs a three-dimensional nonlinear finite element framework to investigate the lateral deformation mechanisms of rock-socketed bridge piles under seawall surcharge loading in soft soils, considering the effects of both immediate construction and long-term consolidation. A parametric analysis is performed to evaluate the effectiveness of deep cement mixing (DCM) piles in mitigating pile displacements, focusing on key design parameters, including DCM pile length, area replacement ratio, and elastic modulus. The results reveal that horizontal pile displacements peak at the pile head post-construction (25 days: 25 mm) and progressively decrease during consolidation, shifting the critical displacement zone to mid-pile depths (20 years: 12 mm). Bending moment analysis identifies persistent positive moments at the rock-socketed interface. Increasing pile stiffness marginally reduces displacements (a < 1 mm reduction for a 22% diameter increase), while expanding the seawall–pile distance to 110 m decreases displacements by 72–84%. DCM pile implementation significantly mitigates short-term (48% reduction) and long-term (54% reduction) displacements, with optimal thresholds at a 30% area replacement ratio and a 40.5 MPa elastic modulus. This study provides critical insights into time-dependent soil–pile interaction mechanisms and practical guidelines for optimizing coastal infrastructure design to minimize surcharge-induced impacts on adjacent pile foundations.