Search Results (75)

Self-drilling hollow bar micropiles (HBMPs), which integrate drilling, grouting, and reinforcement into a single process, have broad application prospects in mountainous transmission lines and offshore wind power projects. However, existing research has focused mainly on friction piles in soil layers, and there is a lack of systematic understanding of the load-transfer mechanism and bearing capacity calculation method for rock-socketed HBMPs. Based on field static load tests of rock-socketed HBMPs, this study systematically investigates the vertical bearing behavior and capacity calculation method of single rock-socketed HBMPs through a combination of test data analysis, finite element numerical simulation, and theoretical analysis. The field test results show that the load-settlement curves of rock-socketed HBMPs are of a slowly varying type, exhibiting mixed friction-end-bearing characteristics. After data screening, the average Q-s curve of Pile No. 1 and Pile No. 5 was taken as the benchmark, and the representative ultimate bearing capacity of a single pile determined by the 40 mm settlement criterion is 5860 kN. The test data of Pile No. 3 and Pile No. 4 were retained as independent validation data. A three-dimensional finite element model considering the cohesive contact behavior at the pile–rock/soil interface was established using ABAQUS. After calibration with the test results, the error between the simulated and measured bearing capacity is −3.4%, demonstrating good model reliability. Parametric analysis indicates that the bearing capacity increases linearly with the grouting volume increase rate $V_{\mathrm{inc}}$, with the expansion effect being the main enhancement mechanism; the improvement amplitude under hard rock conditions is significantly smaller than that in cohesive soils. The effect of uniaxial compressive strength $q_u$ of hard rock on bearing capacity is negligible because the capacity is controlled by the pile–rock interface shear strength. The bearing capacity increases approximately linearly with the rock-socketed depth $L_r$, and a minimum rock-socketed depth of 1.0 m is recommended. Analysis of the load-transfer mechanism shows that rock-socketed HBMPs rely mainly on shaft resistance (accounting for 90.6%), and the axial force decays significantly along the pile length. Elastic compression of the pile accounts for 78% of the pile head settlement, and the limited displacement at the pile tip leads to insufficient mobilization of end bearing. A modified bearing capacity formula considering the grouting expansion effect is established with shaft resistance as the core. A hierarchical validation strategy is adopted to test its predictive ability: for the finite element cases not participating in parameter calibration, the prediction error is within ±2%; for the field test piles, the prediction error is +7.9%; and for Pile No. 3 and Pile No. 4, the errors are +1.7% and −2.1%, respectively. These values are significantly better than those of existing methods (errors ranging from −72.1% to +54.5%). The research results can provide a theoretical basis for the design of single HBMP bearing capacity under rock-socketed conditions. Full article

Prestressed centrifugal concrete hollow square piles often require on-site splicing, and the structural reliability of the pile connection largely governs the performance of the assembled pile. To address the limitations of conventional welded and mechanical joints, a section steel plug-in composite joint combining central grouted steel tube anchorage and peripheral end-plate welding was developed and experimentally evaluated. Flexural and shear tests were conducted on 12 full-scale specimens, including pile shaft specimens and joint specimens with cross-sectional side lengths of 400, 500, and 600 mm. The flexural and shear behavior of the jointed specimens was assessed in terms of bearing capacity, load–deflection response, crack development, and failure mode by comparison with the corresponding pile shafts. Under flexural loading, the pile shaft specimens mainly failed by fracture of prestressing steel bars at midspan, whereas the joint specimens failed near the loading point by prestressing steel fracture, indicating that the critical failure region shifted away from the joint core. The flexural capacities of the joint specimens reached about 92–97% of those of the corresponding pile shafts. Under shear loading, both pile shaft and joint specimens mainly exhibited diagonal compression failure in the flexural–shear region, while no obvious damage was observed in the joint core region. The shear capacities of the joint specimens were about 103–130% of those of the corresponding pile shafts. These results indicate that the proposed section steel plug-in composite joint can effectively maintain flexural resistance while enhancing shear performance. The central steel tube, hardened grout, anchorage reinforcement, and peripheral welds jointly contributed to the integrity and force transfer capacity of the connection, showing favorable potential for engineering application in prestressed centrifugal concrete hollow square pile splicing. Full article

In the assessment of bearing capacity for in-service bridge pile foundations, static load tests are costly, destructive, and difficult to scale. The traditional dynamic formula approach relies heavily on an empirical dynamic–static conversion coefficient that introduces considerable uncertainty. To address these limitations, this study proposes a non-destructive evaluation method for pile foundation bearing capacity based on measured dynamic stiffness and machine learning algorithms. Using data from a highway bridge inspection project, a dataset comprising 680 piles was compiled, including measured dynamic stiffness, geometric parameters, and design load information. An end-to-end binary classification model was constructed to map multidimensional physical features to an engineering decision target, namely, whether the bearing capacity meets the design requirement. The performance of several algorithms was compared, including logistic regression, random forest, and gradient boosting decision tree (GBDT). Among the evaluated models, the GBDT model demonstrated the best capability for capturing the complex nonlinear pile–soil interactions. On an independent test set, it achieved an accuracy of 96.3% and an F1 score of 0.96, with a very low false-negative rate, satisfying the high precision required for engineering safety screening. Feature importance analysis indicates that measured dynamic stiffness contributed approximately 42% to the classification outcome, establishing it as the dominant indicator for detecting capacity deficiencies and reinforcing its physical relevance as a key health indicator for pile foundations. This study demonstrates that data-driven methods can effectively circumvent the uncertainty associated with traditional empirical coefficients, providing a promising approach to the health monitoring and rapid evaluation of in-service bridge pile foundations. Full article

Integral abutment bridges (IABs) eliminate deck joints by rigidly connecting the superstructure to the abutments, reducing maintenance costs but introducing thermal restraint forces. When only one abutment is made integral, all thermally induced longitudinal movement concentrates at the remaining non-integral end, overloading bearings and concrete elements not designed for this condition. This paper investigates IAB behavior and evaluates two repair options for two, three-span continuous steel bridges on Interstate 635 in Kansas City, Kansas, which sustained progressive abutment damage following a unilateral integral conversion in 2005. A 2D finite element model was developed in LARSA 4D, incorporating composite superstructure elements, shell element abutments, beam element piles, and soil-structure interaction via distributed lateral springs. The model was analyzed under dead, live, braking, and thermal load combinations in accordance with AASHTO LRFD. Full integral conversion generates thermal restraint moments of approximately 813.5 kN-m (600 kip-ft) at the abutments, and pile stresses of 383.9 MPa (55.68 ksi) under Service I and 497.4 MPa (72.14 ksi) under Strength I combinations, both exceeding allowable limits. Elastomeric bearing pads at the non-integral abutment satisfied all stress limits without foundation modification and are recommended as a practical repair strategy for bridges in similar conditions. Full article

A rock-socketed pile model load test was conducted for the renovation project of the dangerous old bridge at Shaoping Bridge. The experiment focused on the core parameter of the roughness factor (RF) of the pile body, revealing its influence on the bearing characteristics. The study delved into the load–displacement relationship, ultimate bearing capacity evolution, axial force transmission mechanism, average lateral resistance performance characteristics, and pile–soil relative displacement law of test piles in complex rock formations under different RF values. The research results indicated the following: The test pile exhibited typical brittle failure. At the moment of failure, the load at the pile head dropped abruptly, resulting in a steep drop in its load–displacement curve. Under ultimate load conditions, the average attenuation amplitudes of axial force in the four test piles decreased progressively in Rock Layer I, II, and III, measuring 26.96%, 14.86%, and 10.84%, respectively. The average side resistance distribution along the pile shaft showed a single-peak pattern, peaking in Rock Layer I. Increasing RF effectively enhanced the bearing capacity of test piles. However, a higher RF value does not necessarily yield better results, as it exhibits an inverted U-shaped relationship with bearing capacity. Under the specific conditions of this study, the highest bearing capacity among the tested RF values was observed at RF = 0.168; beyond this threshold, performance actually declined. The pile-top load was primarily shared by side resistance and end bearing resistance. Both components initially increased and then decreased with increasing RF, where the end bearing resistance accounted for 43.64~49.47% of the upper load. Full article

To investigate the bearing characteristics of super-long and large-diameter cast-in-place piles with combined pile-end and pile-side post-grouting, double-load-box self-balanced static-load tests were conducted on two such piles of the Yellow River Bridge Project on Jiaoping Expressway both before and after grouting. This study aims to provide technical insights for the design and construction of similar pile foundations. The test results indicate that, after grouting, the ultimate bearing capacities of test piles SZ1 and SZ2 increased by 123.1% and 72.8%, respectively, with a significant reduction in pile top settlement under the same load level. Under each load level, the axial force of the pile shaft reaches its maximum near the upper load box, presenting a triangular distribution curve. Furthermore, the side frictions of SZ1 and SZ2 enhanced by 87.73% and 83.59%, respectively, after grouting, while their ultimate end resistances are improved by 362.6% and 120.6%. These findings demonstrate that post-grouting effectively optimizes the mechanical properties of the pile–soil interface and enhances the structural stiffness of the surrounding soil. Specifically, the grout hardens at the pile end, solidifies the sediment there, increases the density of the pile-end soil layer, and improves the bearing rigidity of the bearing stratum. This research validates the effectiveness of the combined pile-end and pile-side post-grouting technology in improving the bearing performance of super-long and large-diameter cast-in-place piles, providing valuable technical support for the safe and efficient construction of the Yellow River Bridge on the Jiaoping Expressway and similar engineering projects. Full article

This study evaluates the performance of single concrete-filled frictional Fibre-Reinforced Polymer (FRP) piles embedded in saturated liquefiable sand and subjected to seismic loading using a shaking table. A unidirectional shaking table equipped with a 1000 mm × 1000 mm × 1000 mm laminar shear box with 27 lamina rings was utilized in the study. FRP tubes manufactured from epoxy-saturated Carbon Fibre-Reinforced Polymer (CFRP) and Glass Fibre-Reinforced Polymer (GFRP) fabrics were filled with 35 MPa concrete and allowed to cure for 28 days, serving as model piles for the experimental programme, with cylindrical concrete prisms employed to represent the behaviour of traditional piles. Pile dimensions and properties based on scaling relationships were selected to account for the nonlinear nature of soil–pile systems under seismic loading. Scaled versions of ground motions from the 2010 Val-des-Bois and 1995 Hyogo-Ken Nambu earthquakes were implemented as input motions in the tests. The results show limited variation in the inertial and kinematic responses of the piles, especially before liquefaction. Head rocking displacements were within 5% of each other during liquefaction. Post liquefaction, the concrete-filled FRP piles showed lower response compared to the traditional concrete pile. The results suggests that concrete-filled FRP piles, especially those made from carbon fibre, provide practical alternatives for use. Full article

Rock-socketed piles are a common type of end-bearing pile, but when there is deep sediment or holes at the pile bottom, the load is primarily supported by side resistance. In this study, based on such conditions and considering the influence of pile shaft roughness, model tests were conducted to investigate the bearing characteristics of rock-socketed friction piles. The results show that the failure mode of rock-socketed friction piles is the formation of a penetrating cylinder in the rock layer, with the cylinder diameter directly approximating the pile diameter. The load–displacement curves of the test piles are steeply variable. After reaching the ultimate bearing capacity, the residual bearing capacity of rough test pile is approximately 60% of the ultimate bearing capacity, while that of smooth test pile is 72.4%. The maximum side resistance of the test pile is located within a depth range of 25 mm below the soil–rock interface, and the upper load of 41.0% to 48.9% on the test piles was born by the pile side resistance within this depth range. As the roughness factor (RF) increases gradually from 0.0 to 0.3, the ultimate bearing capacity of the test pile shows nearly linear growth, the ultimate displacement increases sharply first and then decreases slowly, and both the axial force attenuation and the percentage of side resistance within the depth range of 25 mm below the soil–rock interface gradually increase slightly. In this paper, two existing methods are employed to calculate the ultimate bearing capacity of friction piles under the conditions of this study. Based on a comparison of the results, the applicable conditions for each method are proposed. The findings of this study can serve as a reference for the design of rock-socketed piles in similar geological formations. Full article

To reveal the influence mechanism of complex and thick lacustrine sedimentary strata on the bearing performance of large-diameter and ultra-long piles, experimental and numerical simulation studies were carried out in view of the special engineering properties of peat soil in the strata. Three test piles with a diameter of 1 m and lengths of 98 m, 93 m, and 92 m, respectively, were taken as the research objects. The bearing characteristic parameters were obtained through static load tests, and the influence laws of parameters such as the thickness of the weak layer, pile length and pile diameter in the peat soil layer were analyzed in combination with numerical simulation. The results show that in the geological conditions of thick lacustrine sedimentary strata, the Q-s curve of the ultra-long pile is steeply descending, and the ultimate bearing capacity is 28,800 kN, showing the characteristics of end-bearing friction piles. The side friction resistance of the pile shows a typical “triangle” distribution in the upper part and reaches the ultimate value in the middle and lower parts, presenting a multi-hump “R” shape distribution. When the thickness of the weak layer increases from 10 m to 30 m, the settlement increases sharply, and when it reaches 40 m, the settlement almost remains unchanged. The change of pile length has a relatively small impact on the bearing capacity and can be ignored in terms of settlement change. When the pile diameter increases from 0.5 m to 1.0 m, the settlement decreases sharply, and when the pile diameter exceeds 1.5 m, the settlement fluctuates very little. The research confirms that the change of pile diameter has the greatest impact on the ultra-long pile, followed by the thickness of the weak layer, and the pile length has the least impact. The research results can provide theoretical basis and technical support for the design and construction of large-diameter and ultra-long piles in similar complex lacustrine sedimentary strata. Full article

The stability of pile foundation is influenced by many interacting factors, particularly geological conditions. Quantifying the impact of physical and mechanical soil properties on pile stability is critical for achieving optimal design outcomes. This study investigates the sensitivity of key soil parameters and validates the findings with a case study of a university building in Kashkar, Xinjiang, China. A three-dimensional pile–soil model was developed in Abaqus and calibrated with static load test data. Variable control and orthogonal experiments were conducted to examine settlement patterns and ultimate bearing capacity under varying soil parameters. Settlement and ultimate bearing capacity were adopted as stability indicators. Sensitivity analysis was performed through multi-factor variance analysis, sensitivity analysis of factors (SAF), and variance inflation factor (VIF) collinearity analysis. The results show that the most influential parameters are the friction coefficient of the soil above the pile tip, the Poisson’s ratio of the pile-end soil, the Poisson’s ratio of the soil above the pile tip, the friction coefficient of the pile-end soil, and the elastic modulus of the pile-end soil. These findings provide a quantitative basis for optimizing design parameters and improving the efficiency and reliability of pile foundation design in sandy soil regions. Full article

The implantable capsule grouting pile is a novel pile foundation technology in which a capsule is affixed to the side of the implanted pile to facilitate grouting and achieve extrusion-based reinforcement. This technique is designed to improve the bearing capacity of implanted piles in coastal areas with deep, soft soil. This study conducted model tests involving multiple grouting positions across different foundation types to refine the construction process and validate the enhancement of bearing capacity. Systematic measurements and quantitative analyses were performed to evaluate the earth pressure distribution around the pile, the resistance characteristics of the pile end, the evolution of side friction resistance, and the overall bearing performance. Special attention was given to variations in the lateral friction resistance adjustment coefficient under different working conditions. Furthermore, an actual case analysis was conducted based on typical soft soil geological conditions. The results indicated that the post-grouting process formed a dense soil ring through the expansion and extrusion of the capsule, resulting in increased soil strength around the pile due to increased lateral earth pressure. Compared to conventional piles, the grouted piles exhibited a synergistic improvement characterized by reduced pile end resistance, enhanced side friction resistance, and improved overall bearing capacity. The ultimate bearing capacity of model piles at different grouting depths across different foundation types increased by 6.8–22.3% compared with that of ordinary piles. In silty clay and clayey silt foundations, the adjustment coefficient η_s of lateral friction resistance of post-grouting piles ranged from 1.097 to 1.318 and increased with grouting depth. The findings contribute to the development of green pile foundation technology in coastal areas. Full article

In order to study the bearing characteristics of pile groups under the coupling of multiple caves, the influence of the interaction between the crossing cave, the underlying inclined cave, the pile-side cave, and the underlying cave on the ultimate bearing capacity, axial force, lateral friction, and load sharing ratio of the pile group was analyzed based on the model test. The research results show the following: (1) Due to the existence of the underlying cave, the Q-S curves of the pile groups are all steep drop types, and they show the characteristics of end-bearing piles. The influence of other caves is not obvious; the existence of beaded caves, lower crossing caves, underlying inclined caves, upper crossing caves, and pile-side caves will reduce the ultimate bearing capacity of the pile group. The reduction in the ultimate bearing capacity is 7.38%, 4.94% for the lower crossing cave, 2.59% for the underlying inclined cave, 2.27% for the upper crossing cave, and 0.74% for the pile-side cave. (2) When the pile body passes through the cave, the axial force changes slightly in the overburden layer, changes greatly in the limestone layer, and remains unchanged in the cave; under the same load level, the axial force of the pile close to the underlying inclined cave and the pile-side cave is smaller than that of the pile farther away. (3) Under the same load level, the lateral friction of the pile foundation shows a decreasing trend in the sand layer and limestone layer. The friction inside the sand layer is small. After entering the lime layer, the lateral friction increases sharply. The lateral friction is approximately 0 within the cave range. After passing through the cave, the lateral friction increases sharply. (4) The underlying inclined cave and the pile-side cave do not affect the position of the peak point of the pile foundation. The existence of the cave makes the pile foundation increase the peak point at the exit of the cave; under the same load level, the lateral friction of the pile close to the underlying inclined cave and the pile-side cave is larger than that of the pile farther away. (5) The existence of beaded caves, lower crossing caves, underlying inclined caves, upper crossing caves, and pile-side caves will increase the proportion of pile end resistance by 6.95%, 4.23%, 0.94%, 0.77%, and 0.62%, respectively. (6) This study systematically analyzed the differences in the degree of influence of different types of caves (including crossing caves, underlying inclined caves, and pile-side caves) on the bearing characteristics of pile foundations under the condition of the existence of underlying caves. It was found that beaded caves > lower crossing caves > underlying inclined caves > upper crossing caves > pile-side caves, which provides a priority decision-making basis for the optimal design of cave treatment schemes in engineering practice. Full article

The NT-CEP pile is an innovative type of pile that builds upon the conventional concrete straight-hole cast-in-place pile. It primarily consists of two components: the main pile and the bearing plate. The key factors influencing its load-bearing capacity include the pile diameter, the cantilever dimensions of the bearing plate, and the slope of the bearing plate’s foot, among others. The pile spacing significantly influences the bearing capacity of NT-CEP pile group foundations. The overall bearing capacity of an NT-CEP pile group foundation is not merely the sum of the ultimate bearing capacities of individual piles; rather, it results from the interactions among the pile bodies, the cap, and the foundation soil. Advancing the design theory of NT-CEP pile groups and enhancing their practical applications in engineering requires an in-depth investigation of how different pile spacings influence the load-bearing performance of pile group foundations. This objective can be achieved by exploring the soil damage mechanisms around side, corner, and central piles. This exploration helps in clarifying the influence of pile spacing on the load-bearing performance. Based on research findings regarding the bearing capacity of single and double pile foundations, this paper utilizes ANSYS finite element simulation analysis to model six-pile and nine-pile groups. Because these arrangements are universally adopted in engineering practice, they are capable of accounting for the pile group effect under various pile spacings and row configurations. The nine-pile group comprises corner piles, side piles, and a center pile, enabling a comprehensive analysis of stress variations among piles at different positions. As six-pile and nine-pile groups represent common pile configurations, studying these two types can provide valuable insights and direct references for optimizing pile foundation design. The study systematically investigates the influence of varying piles spacings on the bearing capacity of NT-CEP pile group foundations. It concludes that, as pile spacing decreases, The displacement of the top of this pile increases. thereby enhancing the group piles effects. Conversely, increasing the spacing between piles represents an effective strategy for elevating the compressive capacity of the NT-CEP pile-group foundation. Larger spacing also increases the vertical load-bearing capacity of the central piles, enhances the lateral friction resistance of corner piles, and heightens the load-sharing proportion between the bearing plate and the pile end. Furthermore, increasing pile spacing raises the ratio of load sharing by the foundation soil for both the CEP nine-pile foundation and the CEP six-pile foundation. The reliability of the simulation study has been verified by a visualization small scale model test of a half cut pile. Full article

In order to study the influence of pile end sediment on the bearing characteristics of bored piles, the on-site bearing capacity test was conducted on a single pile. A mathematical model of bearing capacity and the settlement response of a single pile considering sediment effects and a finite element model of a single pile with pile end sediment were established. In addition, the influence of sediment thickness on the bearing capacity of bored piles was systematically analyzed. The results show that the compaction of sediment at the pile end could significantly improve the ultimate bearing capacity of the single pile. Compared with the single pile that did not consider the compaction of the sediment at the pile end, the load required to reach the ultimate bearing capacity of the pile after compaction of the sediment increases by 900 KN. The settlement of the pile under a maximum vertical load increases with an increase in the thickness of the sediment. The influence of sediment thickness on axial force transmission is mainly reflected in the linear to nonlinear transformation of axial force distribution from low to high during the process of load. The slight decrease in axial force at the bottom of the pile could also be caused by the increase in the thickness of sediment. The increase in sediment layer thickness means that the transfer efficiency of the pile end resistance decreases. However, with an increase in load, the compression effect of the pile end sediment becomes obvious, which will further change the distribution of load between the pile side resistance and the pile end resistance. Full article

The span of pile foundations beneath metro depots typically ranges from 10 to 20 m, exhibiting a notably large span. This structural characteristic results in the pile foundations bearing a more concentrated upper load, while the interstitial soil between the piles bears minimal force. Concurrently, global climate change and enhanced urban greening initiatives have led to a significant increase in rainfall in northwest China, a region traditionally characterized by arid and semi-arid conditions. This climatic shift has precipitated a continuous rise in groundwater levels. Furthermore, the extensive distribution of collapsible loess in this region exacerbates the situation, as the rising groundwater levels induce loess collapse, thereby adversely affecting the mechanical behavior of the pile foundations. In light of these factors, this study utilized the pile foundations of a metro depot in Xi’an as a prototype to conduct static load model tests under conditions of rising groundwater levels. The experimental results reveal that the load–settlement curve of the pile foundations in the absence of groundwater exhibited a steep decline with distinct three-stage characteristics, and the ultimate bearing capacity was determined to be 5 kN. When the groundwater level is situated below the loess stratum, the settlement of both the pile foundations and the foundation soil, as well as the axial force, skin friction, and pile tip force, remains relatively stable. However, when the groundwater level rises to the loess stratum, there is a significant increase in the settlement of the pile foundations and foundation soil. Negative skin friction emerges along the pile shaft, and the bearing type of the pile foundation transitions gradually from a friction pile to an end-bearing pile. The influence range of the pile foundation on the settlement of the foundation soil is approximately three times the pile diameter. Full article