Super-long Pile Research Articles

Investigation and numerical simulation study on the vertical bearing mechanism of large-diameter overlength piles in water-enriched soft soil areas

Abstract With the development of urbanization, there is an increasing demand for higher land utilization rates, leading to the emergence of high-rise residential and commercial complexes. Additionally, in coastal areas, the presence of soft soil and low bearing capacity of the foundation necessitate higher foundation bearing capacity. Large-diameter, super-long piles have been widely employed in engineering projects to address these challenges effectively. This study analyzes their vertical bearing characteristics through field load tests and determines vertical load distribution and transfer mechanisms by using Brillouin Optical Time Domain Reflectometry. A numerical computation and analysis method based on PLAXIS 3D was established, examining the effects of parameters such as pile diameter, length, and soil modulus on the vertical bearing characteristics. Results indicate that large-diameter, super-long piles mainly bear loads through side friction, with the tip bearing less load. As load levels increase, axial force increases linearly above 40 m depth and becomes nonlinear below. Frictional resistance is significant below 40 m at 3,700 kN load. Parameter analysis shows that increasing pile length and diameter enhances bearing capacity, suggesting this method to improve pile foundation capacity in engineering.

Influence of Settlement on Base Resistance of Long Piles in Soft Soil—Field and Machine Learning Assessments

Understanding the role that settlement can have on the base resistance of piles is a crucial matter in the design and safety control of deep foundations under various buildings and infrastructure, especially for long to super-long piles (60–90 m length) in soft soil. This paper presents a novel assessment of this issue by applying explainable machine learning (ML) techniques to a robust database (1131 datapoints) of fully instrumented pile tests across 37 real-life projects in the Mekong Delta. The analysis of data based on conventional methods shows distinct responses of long piles to rising settlement, as compared to short piles. The base resistance can rapidly develop at a small settlement threshold (0.015–0.03% of pile’s length) and contribute up to 50–55% of the total bearing capacity in short piles, but it slowly rises over a wide range of settlement to only 20–25% in long piles due to considerable loss of settlement impact over the depth. Furthermore, by leveraging the advantages of ML methods, the results significantly enhance our understanding of the settlement–base resistance relationship through explainable computations. The ML-based prediction method is compared with popular practice codes for pile foundations, further attesting to the high accuracy and reliability of the newly established model.

Predicting base resistance of super-long piles using a random forest model: A case study from Ho Chi Minh city

The prediction of the base resistance for long piles is usually challenging because of the impact of material characteristics and the influence of the nature of the surrounding soil. Artificial intelligence models have been applied in various geotechnical engineering fields, and significant results have been achieved. Based on a well-instrumented static load test dataset (1131 data points) from various projects in the soft soil area of Ho Chi Minh City, this study established a random forest (RF) model considering five input parameters, including the applied load, load point displacement, axial stiffness, standard penetration test value of the soil beneath the pile toe, and the distance from the load point to the pile toe. Twenty percent of the data was designated as the test set, which was used to make predictions using the established model. The results show that the RF model has good predictive performance in terms of prediction accuracy and reliability. A sensitivity analysis of the input factors provided a deeper understanding of the base resistance mechanism, which is important in pile foundation design practice.

Field experimental investigation on bearing characteristics of super-long pile under dynamic compaction in land reclamation area

To investigate the bearing mechanism of super-long piles in reclamation, this study performed a series of field tests on super-long cast-in-place piles. The load distribution and bearing characteristics of the piles were analyzed, and the bearing capacity calculation method was evaluated. The findings revealed that dynamic compaction (8000 kN·m) improved the compactness and bearing capacity of the filled soil, and was capable of avoiding negative friction around the pile. The pile bearing characteristics were friction type, and bearing capacity was mainly borne by axial friction resistance. The pile end resistance accounted for 15.7 %–35.1 % of the bearing capacity, and the peak friction resistance was distributed in the upper soil at a depth range of 0.13–0.31 times pile length. The measured value of friction resistance was 9.8 %–90.2 % higher in the upper soil layer and 46.7 %–60.8 % lower in the rock layer than the value recommended by the current Chinese code. The hyperbolic model overestimated the bearing capacity by 26.8 %–50.4 % and the exponential model error was -4.5 % – +18.9 %. The hyperbolic model was corrected, and the correction factor was determined as 0.73. This study provides guidance for the structural design and theoretical calculation of super-long piles in reclamation areas.

Soft computing for determining base resistance of super-long piles in soft soil: A coupled SPBO-XGBoost approach

Prediction of base resistance for long piles is usually challenging because of the complex mobilization of load over the depth. This study hence proposes a novel machine learning (ML) method where the eXtreme Gradient Boosting (XGBoost) algorithm is coupled with the state-of-the-art Student Psychology-Based Optimization method (SPBO) to predict base resistance of piles. A dataset (796 datapoints) from well-instrumented static load tests among various projects in the soft soil region of Ho Chi Minh City is used to develop the model. Five input parameters including the applied load, loading point displacement, axial stiffness, SPT value of the soil beneath the pile toe, and the distance from the loading point to the pile toe are considered. Performance of the coupled SPBO-XGBoost model is comprehensively assessed by comparing it with other optimized XGBoost models including the Teaching-Learning Based Optimization, Bayesian Optimization, and other well-known ML techniques. The established model is used to predict data obtained from independent studies and compared with practice design codes of pile foundation. The results show that the proposed SPBO-XGBoost model outperforms all other models considering the prediction accuracy and reliability. Sensitivity analysis and Partial Dependence Plots are conducted to gain insights into the base resistance mechanism, giving considerable implications to design practice of pile foundation.

Buckling failure mechanism and critical buckling load prediction method of super-long piles in soft-clay ground in deep water

Buckling may be a possible failure mode of super-long piles in soft-clay ground in deep water as concerned by several researchers. Although there are many studies on the failure mode of super-long piles in sands or silts deposits, few studies have been carried out thoroughly on mechanical mechanism of buckling instability in soft-clay ground in deep water and the corresponding calculation method. In this paper, first, the Riks Arc-Length method in ABAQUS was evaluated and verified using the FEM simulation and the analytical results from Bhattacharya. It is observed that buckling failure is indeed one of failure modes of super-long piles in soft-clay ground. And then, the buckling capacities of super-long piles were assessed due to various influential parameters. The results show that the critical buckling load of super-long piles increases with the increase of elastic modulus and embedding rates of piles, and the increase of elastic modulus and cohesion of soil, while decreases with the increase of slenderness ratios of piles and horizontal loads. Finally, a prediction method of the critical buckling load for super-long piles was presented based on the above sensitivity analysis for influential parameters and the Euler buckling formula, and the criterion for buckling instability for super-long piles was proposed as well.

Experimental study on wall-protecting mud modification of super-long bored pile in the alluvial plain region of the Yellow River

In this study, the effect of different additives (sodium carbonate, carboxymethyl cellulose, polyacrylamide, and barite powder) on modified mud performance and microstructure has been evaluated and the optimal proportion of the modified mud was determined. The specific gravity, viscosity, sand content, colloid rate, and water loss of modified mud has been investigated. The characteristics of mud cake in microstructure of modified mud have been also analyzed and evaluated by the scanning electron microscopy images (SEM). The results showed that adding bentonite dosage increased the specific gravity, the adding carboxymethyl cellulose increased the viscosity and dropped the water loss rate, and the pH value experienced a growth with the increasing sodium carbonate content. SEM tests revealed the additives made the overall structure of the mud cake more compact and enhanced the modified mud's wall protection effect and filtration loss reduction effect. The modified mud was successfully applied to the super-long pile of a project in the Yellow River alluvial plain area, which provided support for the dynamic adjustment of the mud proportion in the field construction.

A Novel Method for Testing the Effect of Base Post-Grouting of Super-Long Piles

The base post-grouting technology is widely used in pile foundation engineering to improve its behavior. For super-long piles, due to the large compression of pile shaft, conventional static load tests cannot effectively test the effect of post-grouting at the pile base. This study introduced a novel method to test the effect of base post-grouting of three 88 m long piles. A total of three test piles were grouted at the bottom of the piles. The grouting amount of the No. 1 pile was 500 kg, the No. 2 pile was 250 kg, and the No. 3 pile was 0 kg. The transmitted wave method in the pile and dynamic penetration method were used in combination to quantitatively evaluate the effect of base post-grouting on the soil below the pile base. The influencing range of base post-grouting was firstly evaluated by the transmitted wave method in the pile, and the strength of the soil before and after grouting under the same pile base was measured by the dynamic penetration method. The test results showed that: the depth of the pile base reinforcement was 2 m for No. 1 pile, and 1 m for No. 2 pile, which was consistent with the grouting amount; the elastic modulus of the soil below the pile base after base post-grouting was about twice that of the soil before base post-grouting; the strength of the soil below pile base was largely improved after grouting; and the bearing capacity of the base soil layer increased from 35.5% to 41.7% when the grouting amount increased from 250 kg to 500 kg.

Monitoring Axial Force Development in a Super-Long Pile during Construction Using BOFDA and Data Interpretation Approaches: A Case Study

Long-term monitoring data for super-long piles are scarce and valuable. This paper reports axial strain measurements of a cast-in-place large-diameter pile embedded 76.7 m into a “weathered trench” of granite in Nanshan District, Shenzhen, China, using BOFDA monitoring technology. An approach based on the load-transfer method to interpret data is proposed, in which the axial load at the pile head and the shear behavior at the pile–soil interface can be analyzed. Results show that these data can well reflect the increase in axial strain as the number of floors built increases, although there is deviation related to fiber cable bending due to the installation and compaction of concrete, and the complex loading condition at the pile head. Sensitivity analysis of parameters disclosed that the friction angle between the soil and the pile was approximately 10° for the cast-in-place pile monitored in this study, which is approximately one third of the interface friction angle, considering the slurry cake effect. The average axial force exerted on the pile head induced by building one floor ranged from 116.00 kN to 297.43 kN; this increased with the number of floors built and the total loads of the superstructure. This implies that the raft carried a large portion of the structural load during the early construction stage; piles gradually carried a major portion of the increased load due to continuous construction. The overall mobilized percentage of skin friction was approximately 40.8% when 40 floors were built, and the pile had the potential to carry more axial load.

Study on dynamic stability for the superlong pile foundation of marine engineering structures in service

To fully utilize the earth’s resources, the offshore structure is a growing trend for human development. Pile foundations are widely used to support offshore structures. In the ocean, wave loads and ship impacts (SIs) may cause the instability or failure of the pile-soil system. This paper studies the dynamic stability of the pile foundation of offshore structures under both wave loads and SIs. The two-parameter foundation model and Galerkin integral method are used to establish offshore structures' nonlinear dynamic differential control equations. The displacement function of the pile body is obtained by using the fourth-order Runge-Kutta method, and the instability load of the pile body is obtained from the displacement time-history curve. The influence of pile length, stiffness, and soil properties on pile performance is investigated. Through the study of this paper, it can be found that the constraint effect of the soil on the side of the pile can make the pile experience a period of steady-state vibration before the instability; that is, the instability of the pile is delayed. Compared with the traditional Winkler model, the instability load and displacement time history curve calculated by the two-parameter model are more in line with the engineering practice.

Experimental Study on the Force Characteristics of Superlong Pile Groups in Silty Sand

Laboratory model testing of single pile and superlong pile groups in saturated silty sand was conducted to investigate the response and bearing behavior of superlong pile groups with a high or low cap under vertical loads. The load transfer mechanism and bearing behavior of the pile shaft were discussed in detail. The load‐settlement curve of the loaded superlong pile groups belongs to the type of gradual descent in silty sand. The transferred load decreased along the pile length during loading, but the gradients differed in different positions of the superlong pile group foundation with a high or low cap. The maximum shaft friction of the superlong pile groups with a high and low cap is about 2.5 times and 1.8 times, respectively, than that of the single pile. In addition, the tip resistance of the piles in the pile group foundation is about 2–3.5 times that of the single pile. The friction resistance of the superlong pile group foundation with a low cap was slightly larger than that of the high cap in the entire pile length, and two peaks and one peak, respectively, were observed. Under the ultimate load, the pile‐soil maximum relative displacement of the friction on the pile side in the silty sand stratum was about 3% of the pile diameter. Under the ultimate load, the load sharing ratio of the pile side resistance of the two types of pile group foundations was about 60% of the total load. The load sharing ratios at the pile tip of the superlong pile groups with high and low caps are 40% and 33%. Furthermore, equations were proposed to determine the axial capacity of the superlong pile group based on the single pile bearing capacity and were applied to analyze the test pile. The calculated ultimate bearing capacity was similar to the measured value, with a maximum error of only 4.88%, thus validating the proposed method.

A New Load Transfer Model of Skin Friction for Super-long Pile Under Axially Load

A New Load Transfer Model of Skin Friction for Super-long Pile Under Axially Load

Wireless Monitoring System of Super-long Pile Foundation in Bridge Engineering

The load-carrying properties of super-long pile were important to bridge engineering in construction and operation phases, however, the research of its load-carrying properties was imperfect. The wireless monitoring system of super-long pile foundation in bridge engineering was presented in this paper. The intelligent vibrating wire steel stress gauges were used to reflect the axial stress of pile foundation, then the sensor data was acquired using wireless intelligent acquisition instrument. The LoRaWAN was used to establish the local area network, which was long range wide area network. Thus the monitoring data of each sensor was collected by Lora modules. Furthermore, the collected data was uploaded to the cloud platform using 4G wireless communication technology. Finally, the wireless monitoring system was applied in actual bridge engineering. The results indicated that the change of monitoring data was synchronized with the progress of actual working conditions, which could reflect the dynamic process of construction to a certain extent. It could provide an important reference for similar projects in the future.

Field test research on post-grouting effect for super-long cast-in-place bored pile in thick soft foundation

The field static load test method was adopted for two test piles in a project in Zhejiang area of China, analyze the effect of post-grouting technique on super-long cast-in-place bored pile and its internal mechanism. Q-s curves and pile axial force curves were drawn based on test data to show that post-grouting technique played a prominent role in improving bearing capacity of single pile and pile skin friction. In addition, pile skin friction at upper soil layer around super-long pile was fully used, but that at lower soil layer was not. For the design of super-long pile, the compression of pile body should be considered to make the best use of pile skin friction caused by relative displacement between pile and soil.

Improved approach of super-long pile group considering multiple interaction under axial load

This work presents an improved approach to consider the effect of any intermediate piles between the two piles in pile groups. Based on the thin ring shear model of concentric cylinder and elastic theory, a multi-pile analysis model under axial load is proposed with full consideration of the mutual reinforcing effect between super-long piles. A theoretical derivation of interaction coefficient and the deformation of super-long pile groups which simultaneously consider the reinforcing effect and the existence of other piles in pile groups was carried out. Example analysis shows that the analytical solution agrees well with the experimental results. In this work, the error between the calculated result of the presented model and the measured value is 25% less than that in the traditional model. The multi-pile model fully analyzes the interaction mode between multiple piles and considers the mutual reinforcing effect among the piles, which makes up for the deficiency in traditional settlement calculation of pile groups. Consequently, the model proposed can accurately reflect the working characteristics of super-long pile groups in actual engineering.

Buckling analysis of super-long rock-socketed piles based on double-parameter foundation model

The Winkler foundation model is widely used in soil calculation for the analysis of buckling failure of super-long piles. The model regards the foundation soil as an elastic body composed of discrete spring elements with the continuity of the foundation soil ignored, which is inconsistent with the actual engineering. In this paper, the double-parameter foundation model which conforms to the actual deformation characteristics of soil is introduced. The model uses two independent parameters to reflect the characteristics of the foundation soil, and the shearing effect between the soil springs is considered to reflect the continuity of foundation soil. Based on this, the total potential energy function of the pile-soil system is established. According to the principle of stationary potential energy and the variational method, the analytical solution of the buckling critical load of the pile foundation can be calculated by the m-method and the combination method respectively. The results of the example show that the error between the calculated method and the test piles is within 5%, which indicates that the double-parameter can accurately reflect the reaction behavior of the foundation soil during the buckling failure of super-long pile in the actual engineering. Meanwhile, the soil resistance model of the combination method is closer to the actual pile body constraint.

Influence of Karst Caves at Pile Side on the Bearing Capacity of Super-Long Pile Foundation

The differences in development and situation of karst caves lead to two kinds of karst caves, and the karst cave may be on the pile side or at the pile bottom, which has a different influence on the bearing capacity of pile foundation. The paper presents a numerical analysis of the influence of karst caves at pile side on the bearing capacity of super-long pile foundation in karst areas. According to the size of pile foundation of a real bridge project, this paper modelized karst caves and investigated the karst cave from the effect of length, height, and thickness of roof on horizontal and vertical bearing capacity of pile foundation. The main conclusions can be drawn as: when the horizontal displacement at the top of pile foundation is greater than 0.05 m, the horizontal load is correlated positively with the length of karst cave; when the vertical displacement is greater than 0.07 m, the vertical load is correlated negatively with the thickness of the roof of karst cave. However, the height of karst cave has little effect on the bearing capacity; also the existence of karst cave has little influence on the dynamic response of pile foundation. The results of this study can be important with reference to the design and construction of pile foundations in karst areas.

The analysis model of shaft resistance softening of a new type of static drill rooted super-long nodular pile

The static drill rooted nodular pile is a new type of pile foundation which consists of precast nodular pile and the cemented soil along the pile shaft. The pile foundation not only reduces the compacting effects and mud pollution, but also combines the advantages of the relatively high skin friction of the cemented soil and the higher strength of the precast pile. As the mechanism of the shaft resistance softening of such super-long piles is not clear, existing general finite element software can not reflect the softening of soil. Therefore, a new hyperbolic load transfer model considering the shaft resistance softening is proposed in this paper and corresponding UMAT subprogram is developed in FORTRAN language, which forms the integration model of load transfer model and subprogram. The curves of Q-s relationship of pile’s top, the distribution of pile’s skin friction and the relationship of pile’s skin friction versus pile-soil relative movement calculated by the proposed model are highly consistent with the field test data. In this paper, the mechanical behavior can be calculated by the proposed model accurately and effectively. At the same time, this new type of static drilling rooted nodular pile can make the bearing capacity of super long piles to be mobilized effectively.

Deformation of Compressible Layers Below Pile Foundations of a High-Speed Railway

New installation methods and techniques using single-point settlement gauges to monitor the compression deformations of pile shafts and soil layers below the pile bottom were developed. A field study was conducted at a test site of the Beijing–Shanghai high-speed railway, and a 3D finite-element analysis was performed to analyze the deformation behavior of the pile foundation. The measured compression deformations of the pile shafts from the single-point settlement gauges agreed well with the values obtained from the strain gauges, which validated the effectiveness and feasibility of the developed methods and techniques. During the construction of the high-speed railway, the pile top settlement–time curve presented a ‘ladder’ shape and then increased with time before gradually stabilizing. The compression deformations of the subsoil layers within 20–30 m below the pile bottom were not significantly different, and the average unit compression deformation under 20 m below the pile bottom remained approximately constant after construction. The axial force and tip resistance of the pile shaft increased during the consolidation of the soils surrounding the pile shaft and might result in an increase of the compressions of the pile shaft and subsoil layers. The thicknesses of compressible layers (hc) of the tested pile foundations were determined using a deformation ratio method and were approximately 15.0 m. These results demonstrate that the developed methods and techniques are useful for studying the compression deformations of subsoils below the pile bottom (especially for super-long piles).

Improved Calculation Method of Super-Long Pile in Deep Soft Soil Area

Improved Calculation Method of Super-Long Pile in Deep Soft Soil Area