Pile End Research Articles

Theoretical Analysis of Deformation and Internal Forces of Used Piles Due to New Static-Pressure Pile Penetration

Evaluation of the impact of new static-pressure pile penetration on used piles is vitally important for the reutilization of the used piles. The cavity expansion theory in semi-infinite soil is adopted to obtain the displacement field of the surrounding soil caused by new pile penetration, and then the displacement is applied to the used pile based on a two-stage method to analyze the deformation and internal force of the used pile. The effects of constraint conditions of the used pile, the pile rigidity and the soil modulus on the response of the used pile are considered. Meanwhile, numerical analysis is adopted to verify the effectiveness of the theoretical method. The influence of the distance between the new and used piles and the radius of the new pile is analyzed, and the measures to reduce the influence of new pile penetration on existing piles are proposed. The results show that the form of pile end only affects the deformation near the pile end. With the increase in pile diameter, the existing pile deformation gradually increases. As the distance between the existing pile and new pile increases, the existing pile deformation decreases significantly.

Influence of pile end soil on torsional vibration of pile considering the stress diffusion

This study focuses on the influence of the pile end soil on the torsional vibration of a pile based on a fictitious soil pile model considering stress diffusion. The pile end soil is simulated as a cone-shaped fictitious soil pile whose parameters remain the same as the soil. The dynamic equilibrium equations for the pile–soil system subjected to torsional dynamic loadings are developed and solved to obtain the corresponding analytical solution. Then, comparisons with other solutions are conducted to verify the reliability of the proposed solution. Finally, parametric analysis results are presented based on the proposed solution to portray the influence of the stress diffusion angle, thickness and shear wave velocity of the pile end soil.

Study on Mechanical Properties of Composite Foundation with Rigid Pile Based on the Cushion and Strength Adjustor Control

Settlement of the pile ends in end-bearing rigid pile composite foundations is generally minimal, so only relying on the cushion to coordinate the pile soil deformation may result in insufficient deformation adjustment capacity. Using a deformation adjustor with a specific stiffness on the top of the pile is a method to coordinate pile–soil deformation, and the stiffness value of the deformation adjustor depends on the accurate calculation of soil deformation; however, the calculation of soil deformation is not mature at present. A new deformation adjustor based on strengths used in composite foundations is proposed, in which foam slabs with different yield strengths are placed on the top of the pile to coordinate the pile–soil deformation. Five tests are used to study the mechanical and deformation properties of a composite foundation with a foam slab. The test results show that when the stress at the top of the pile is less than the yield strength of the foam slab, the coordination of the pile–soil deformation mainly depends on the cushion. When the stress of a rigid pile exceeds the yield strength of foam concrete, the foam slab begins to yield and coordinate the deformation of pile and soil, and the settlement coordination ability of a rigid pile composite foundation with a foam slab is significantly improved. Finally, an engineering case is used to simulate the pile–soil stress sharing when the actual settlement is greater than the calculated settlement. The case analysis shows that the pile-top stress can be well controlled by the successive yielding of foam concrete slabs of different strengths, which reduces the influence of settlement error on the pile–soil stress sharing, and further promotes the engineering application of end-bearing rigid pile composite foundations.

Experimental study on penetration characteristics of an open-ended pile under static and dynamic driving methods

Open-ended piles have been widely adopted in both offshore and onshore geotechnical engineering projects. However, the influence mechanism of pile driving methods on the penetration characteristics of the open-ended piles is not clear. Therefore, this paper presents simulations of an open-ended pile penetrating through a silt layer to a fine sand bearing stratum using static and dynamic driving methods respectively in a series of indoor model tests. Properties including pile sinking resistance, pile end resistance, pile side friction resistance, pile load transfer law and soil plug formation during penetration are analyzed. The results show that the PLR value of the hammered pile is always greater than that of the static pressure pile in the stable sinking stage, while the IFR value of the latter in the sand layer is significantly higher than that of the former. The resistance of the hammered pile under the same penetration depth is less than that of the static pressure pile and increases sharply when entering the holding layer. In addition, the final pile driving resistance of the hammering method is 66.68% of the static pressure pile driving resistance, and the pile end resistance from the static pressure method and hammering method account for 75.4% and 77.6% of the total resistance respectively In the choice of construction method, the static pressure method is more suitable for silty soil, and the hammering method can show its advantages in fine sand formation.

Study on the thermal response of spiral energy piles based on field test

Field tests of spiral energy piles under the combined effect of temperature and loading are relatively few. Based on the field test, the heat transfer efficiency, pile strain, axial force and shaft friction of two spiral energy piles were studied. The major findings of the experimental studies were: First, when the double spiral energy pile was heated, the temperature distribution was more uniform; the total heat transfer and the heat transfer rate were higher than those of the single spiral energy pile. Second, the pile strain distribution was such that smaller values were noticed at both pile ends while larger values were in the middle part of the pile. The additional tensile stresses of the two piles generated during cooling reached 4.06 MPa and 4.75 MPa, which exceeded the tensile strength of concrete. Finally, during heating, the shaft friction was negative in the middle and upper pile and positive in the middle and lower pile. The single spiral energy pile showed two neutral points. The downward load generated by the single spiral energy pile was about 885 kN higher than that generated by the double one. The aforementioned changes should be focused on in the actual project.

Analysis of the behaviour of the floating pipe pile foundation on soft soil

Planning the foundation according to the characteristics of the soil is the most important thing before starting construction. In recent studies, the use of PVC pipe pile foundations has been introduced. These foundations, called floating foundations, offer a lighter and more economical solution for simple buildings on soft soils. The study involved variations in pipe diameters of consistent lengths. In laboratoryscale axial loading tests, the behaviour of pipe stacks against load drop is observed. The bearing capacity of pipe piles is evaluated through a static loading test, with one end of the pipe pile hermetically sealed and aspirated in an alternating pile in a glass box filled with soft soil with a content of 98%. The results of this test are then analyzed with the Chin Method Interpretation to determine the carrying capacity and behaviour of pipe piles against soft soil, The results of the analysis show that the diameter of the pipe piles has a significant effect on the bearing capacity of the foundation. Although the results are promising, further research is still needed, especially in the development of foundation construction suitable for multi-storey buildings in areas with soft soil or peat

水平力を受ける中詰めコンクリート二重鋼管杭の変形特性とその算定法

Loading tests and numerical analyses using the finite element method were carried out to clarify the deformation characteristics of a concrete-filled double steel pipe pile subjected to a horizontal load. The diameter and length of the concrete-filled double steel pipe pile were about two times and ten times the inner pipe diameter, respectively. The results indicated that the steel pipe and concrete behaved as a single unit from the pile head to the central part of the concrete-filled double steel pipe pile. On the other hand, this unity was lost in the length range of 3 to 4 times the diameter of the inner steel pipe from the bottom end of the concrete-filled double steel pipe pile. This is due to slippage at the boundary between the steel pipe and the concrete, and local compressive deformation of the concrete. We attempted to simply calculate the deformation of this pile when subjected to a horizontal load by constructing a beam spring model that combines the interface springs between the steel pipe and concrete and Timoshenko beams. The applicability of this calculation method was verified by comparing the results with those of loading tests.

Research on Mechanical Characteristics of Slope Reinforcement by Spatial Arc Crown Beam Composite Supporting Structure

To effectively optimize the mechanical behavior of a traditional anti-slide pile and reduce environmental destruction, a new method for slope reinforcement by a spatial arc crown beam composite supporting structure was proposed. First, a numerical model was validated through lab-scale model test data obtained herein, and then a full-scale numerical model was created for an in-depth understanding of the distribution regularity of displacement along the pile, the soil pressure, the crown beam stiffness, and so on. The results demonstrated that: (1) The spatial arc crown beam is simplified to a two-hinged arch, and the maximum value of the bending moment in the arc crown beam is about one-third of the straight crown beam through theoretical calculation. (2) The spatial arc crown beam redistributes the load sharing among different piles, and the extreme bending moment of other piles varies within 10% along the downhill direction except for the piles at the slope foot. (3) Bending moments are close to zero at the pile end, and the anti-slide pile can be simplified as a vertical beam with one end fixed and the other end hinged. (4) The axial force in the spatial arc crown beam is always presented as pressure, so the crown beam can make full utilization of the compression resistance of concrete. (5) The distribution characteristic of soil pressure in front of the pile at the arch foot is different from that in other positions, and the stable soil at the slope foot provides greater soil resistance for anti-piles. (6) As the crown beam stiffness is above five times the reference value, the axial force of the crown beam tends to be stable, and as the crown beam stiffness increases continually, the maximum value of My is −1013.13 kN·m, and the constraining effect of the crown beam is gradually weakened.

Study on time effect of residual pressure and bearing characteristics of jacked pile in saturated silt foundation

The history of stress in soil mass and pile surface roughness significantly impacts the time effect of residual pressure at the pile end and bearing characteristics of the jacked pile. In this study, the impacts of soil over-consolidation ratio and pile surface roughness on the time effect of residual pressure and bearing characteristics of jacked pile end in saturated silt foundation are explored. Through the independently developed model test device for the vertical bearing characteristics of jacked pile, the driving of jacked pile with different pile surface roughness and static load tests at different resting phases are carried out on saturated silt foundations with different over-consolidation ratios. The model box is cylindrical in shape with a size of 40 cm × 48 cm (inner diameter × height) and is made of transparent tempered glass. The results show that: the increase in surface roughness of jacked pile in saturated silt foundation causes not only the increase in the pile side friction but also the increase in the pile end resistance during the static pressure sinking pile; the change laws on the residual pressure of pile end and limit friction resistance of pile side for jacked pile in saturated silt foundation vary with over-consolidation ratio of soil mass and the pile surface roughness.

Soil Resistance during Driving of Offshore Large-Diameter Open-Ended Thin-Wall Pipe Piles Driven into Clay by Impact Hammers

To investigate soil resistance during driving (SRD) of large-diameter open-ended thin-walled piles (LOTPs) installed in clay by impact drivers, a finite element method (FEM) analysis incorporating the dynamic remeshing and interpolation technique with small strain (RITSS) was conducted. This method was verified using a field case, and its SRD was compared with that of the empirical driving formulas. The dynamic penetrations of LOTPs with three pile inner diameters (2, 4, and 6 m), and three ratios of wall thickness to pile inner diameter (1%, 1.5%, and 2%) were then numerically analyzed. The pile load-bearing performance had a significant dimensional effect, which is characterized that an increase in diameter resulting in a decrease in SRD. The LOTP shaft frictional resistances during driving was generally held in the range of around 0.3–0.5 times the undrained shear strength of the soil (Su), and it was about 6 - 8 times for the pile end resistances. The prediction equation was proposed for SRD behavior with depth, which considered the pile diameter. Then, the curves of the equation were compared with the numerical results. This study can serve as a reference for the dynamic penetration analyses of piles, including the SRD and pile-soil interaction.

Research of jacked pile penetration test based on airbag loading

In view of the problem that the model test of pile jacking cannot reproduce the soil's self-gravity and thus cannot truly reflect the pile jacking force, this paper developed an airbag loading system. The airbag is used to pressurize and simulate the overburden soil weight. The method of subsection pile jacking and coupling is ingenuously adopted to achieve long pile penetration. Because the airbag and pressure can be controlled well, it is convenient to simulate the cohesive soil foundation. The size of the model pile used in the test is larger than that of the usual centrifugal model test pile, so it belongs to large scale model test. The experimental results using this system show that the pile jacking force increases with the increase of penetration depth. The degradation of pile lateral resistance has little impact on pile jacking force. After modification based on the degradation theory of pile lateral resistance, the overall variation law of pile lateral resistance is good, and the difference value generated at the joint of pile segments is basically 0. With the increasing of pile depth, the pile lateral resistance is gradually exerted. The pile end resistance fluctuates with the depth, and accounts for a relatively high proportion of pile jacked force, which is maintained above 50%. The method of pile jacking with airbag loading in this paper provides a new idea for the study of the penetration characteristics of jacked pile.

Experimental Study on Seismic Performance Improvement of Pile-Cap Joints in a Prestressed Concrete Solid Square Pile Using a Snap-In Mechanical Connection

ABSTRACT To improve the seismic performance of prestressed concrete solid square piles with a snap-in mechanical connection, effects of the prestressed tension control stress of PC steel rods, anchoring method and reinforcement ratio of hot-rolled steel bars on the seismic performance of the pile-cap joints in the prestressed concrete solid square piles using a snap-in mechanical connection were investigated in this article. Considering the above-mentioned effect factors, the quasi-static test was first carried out on the cap pile-cap joint specimens. Then, the seismic performance of the specimens was investigated. The results indicated that the yield load and ultimate load changed slightly, the displacement ductility increased, the stiffness decreased and the damage development rate slowed down as the prestressed tension control value decreased. When the prestressed tension control stress was reduced to 0.4 times of the standard value of tensile strength of the PC steel rods, the crack resistance moment could meet the requirements of construction and hoisting at the same time. In the process of pile cutting and anchoring, the 135° hook treatment on the end of the PC steel rods could reduce the strength degradation of the pile-cap joints during the loading process, improve the energy-dissipation capacity and slow down the damage development rate of the pile-cap joints. For the specimens with ordinary hot-rolled deformed steel bars added to the pile end, its ductility was improved, the horizontal bearing capacity, displacement ductility and energy dissipation capacity were all greatly improved, and the damage development rate was slowed down. However, the reinforcing ratio of the ordinary hot-rolled steel bars should be kept within a reasonable range. Finally, the improvement measures were proposed to improve the mechanical properties of the prestressed concrete solid square piles and pile-cap joints.

Thermomechanical Behaviors of an Energy Pile–Raft Foundation under Intermittent Operation with Forced Heat Recharge

This paper presents an experimental investigation of the thermal and mechanical behaviors of an energy pile–raft foundation under intermittent heating operation with forced heat recharge, with 16 h of pile cooling and 8 h of forced heat recharge (referred to as 16F, where F indicates forced). The forced heat recharge was found to increase the daily heat exchange rate to 180 W/m, which was 50% higher than the value under general intermittent heating operation without forced heat recharge. The raft and building load at the pile head imposed a significant restraint on the upper part of the energy pile, leading to a minimum degrees of freedom (0.27) near the pile head. The mechanical behaviors of the pile, including strains, stresses, and pile end displacements and stresses, followed reversible paths when plotted against the daily temperature cycles. A slight residual pressure stress (10 kPa) was observed at the bottom of the raft during the initial thermal cycles.

Analysis of the Response of a Single Pile Using the Disturbance State Concept Theory

Based on disturbed state concept (DSC) theory, this paper reports an analysis of the progressive failure mechanism of a pile–soil system from a new perspective. Assuming that the strengths of the pile–soil interface elements and the soil elements at the pile end follow an exponential distribution, the load transfer models of skin friction and end resistance based on DSC theory are established, and the significance and value method of the parameters in the models are clarified. The analysis of the relevant parameters shows that the proposed DSC load transfer models can simulate the nonlinear hardening or softening behavior of skin friction and end resistance. Based on the proposed DSC load transfer models, a simple and efficient iterative calculation method for the analysis of the bearing behavior of a single pile is established. It is demonstrated from case studies that, compared with calculated results using other methods, the load–settlement curves at the pile head obtained from the method proposed in this paper are in better agreement with the measured values, and could reflect the skin friction and end resistance softening characteristics of the tested piles in a more realistic manner. Furthermore, a parametric study is carried out to analyze the influence of the relevant model parameters on the response of a single pile.

Prediction and Optimization of Pile Bearing Capacity Considering Effects of Time

Prediction of pile bearing capacity has been considered an unsolved problem for years. This study presents a practical solution for the preparation and maximization of pile bearing capacity, considering the effects of time after the end of pile driving. The prediction phase proposes an intelligent equation using a genetic programming (GP) model. Thus, pile geometry, soil properties, initial pile capacity, and time after the end of driving were considered predictors to predict pile bearing capacity. The developed GP equation provided an acceptable level of accuracy in estimating pile bearing capacity. In the optimization phase, the developed GP equation was used as input in two powerful optimization algorithms, namely, the artificial bee colony (ABC) and the grey wolf optimization (GWO), in order to obtain the highest bearing capacity of the pile, which corresponds to the optimum values for input parameters. Among these two algorithms, GWO obtained a higher value for pile capacity compared to the ABC algorithm. The introduced models and their modeling procedure in this study can be used to predict the ultimate capacity of piles in such projects.

An analytical solution of a horizontally vibrating pile considering water-pile-soil interaction on viscoelastic half-space

The water-pile-soil model on viscoelastic half-space is developed to investigate the dynamic responses of an end-bearing pile subjected to the horizontal dynamic loads. The pile is assumed to be viscoelastic material with a circular cross-section and can be divided into several pile segments in a water-pile-soil system, in which the soil and water are modeled as linear viscoelastic media and linear acoustic media, respectively. The pile-soil with viscoelastic half-space and pile-water with rigid ground vibration are solved by the potential function decomposition and the variable separation method, respectively. The displacement response of the pile is then obtained based on the continuity conditions between the pile and soil and water. The predictions of the present solution are compared with numerical method. Moreover, the present solution can be degenerated into a rigid foundation and verified the rationality with existing analytical solutions and numerical methods. Parametric analyses are further conducted by using the present solution to evaluate the relationships between water and pile-soil parameters, especially focus on the influence of the soil viscoelastic half-space shear modulus. The results indicate that it is necessary to consider the influence of the viscoelastic half-space at the pile end soil when the pile elastic modulus is relatively small in the design of offshore cast-in-place piles.

Analysis of Load Transfer Law of Short Pile-Net Composite Foundation for High-Speed Railway

Although pile-net composite foundation has been widely used in the construction of high-speed railway, load transfer characteristics of its structural system during the filling of roadbed are seldom studied, so field studies are conducted in the Gan-Long (Ganzhou-Longyan) high-speed railway DK86 + 998.0–DK87 + 191.2 test section, aiming to explore the bearing characteristics of a short pile-net composite foundation over low to medium compressible soil. The study encompasses a field measurement of Earth pressure and pore water pressure which lasted almost two years, and a systematic analysis of variation law of soil pressure between piles and their tops and that of pore pressure in reinforcement zone and retaining layer in medium and low compressibility soil area. The results suggest that the soil in the test section bears the characteristic of a low water content, low porosity ratio, and high liquid limit, with compression factor being approximately 0.25, and test values of the natural soil pressure and the lateral distribution are close to the theoretical values. Soil pressure at the top of the piles is higher than that between the piles by approximately 2.4 times because of the differences in pile-net rigidity. Besides, the soil pressure of the pile-net composite foundation follows the pattern of “jagged” in the transverse direction of the roadbed. The filling load of the composite foundation soil arch is 80 kPa and the composite foundation reaction coefficient is 6.4 kN/m3 when the filling is stable. The pile-soil stress ratio at the shoulder and the center of the line is 3.2 and 2.8, respectively. The change of the hole pressure at the pile end point is larger than that at the reinforcement area, and the side verification short pile can effectively transmit the additional stress of the upper part to the pile end soil layer.

Physical Model Test on the Influence of Karst Cave under and in front of Pile on the Stability of Embedded End of Antislide Pile

With the extensive use of antislide piles in engineering, the stability of the embedded end of antislide piles in karst areas has become increasingly prominent. When there is a cave in front of the antislide pile, the cave may be deformed under horizontal load, causing the stability of the embedded end of the antislide pile to lose its stability. Based on the similarity model theory, this study establishes two different shapes of caves (horizontal in the longitudinal direction and vertical in the longitudinal direction) beneath and in front of the rock-socketed section of the antislide pile. Using horizontal loading tests, the influence of the karst cave beneath and in front of the elliptical pile on the stability of the antisliding pile embedded end was studied, and the following relevant conclusions were obtained. (1) Compared with the model without cavities, the presence of cavities in front of and under the pile significantly reduces the horizontal bearing capacity of the antislip pile. The fracture trace initially develops from the bottom of the pile against the side of the active zone, roughly at 90° to the horizontal direction through the top plate of the cave and continues to develop under the cave until the boundary. (2) The horizontal bearing capacity of horizontal bearing pile is greatly reduced by karst cave. The karst cave under the pile has a greater influence on this property than that in front of pile and the influence of the two karst caves has a common effect. (3) Both sides of the bottom for pile and the roof of karst cave are prone to damage. Besides, the roof of the karst cave under the pile is more prone to damage for more obvious stress concentration. (4) The loading failure processes of all model can be roughly divided into three stages: gentle deformation, stable expansion of deformation, and accelerated expansion of deformation. (5) Compared with the case where there is no karst cave in front of pile, the karst cave in front of pile will increase the deformation of pile under horizontal load and the same level of load. (6) Based on the limit equilibrium method, a check calculation method for the bearing capacity of the anchored end when a cave existing in front of the pile is proposed. The verification method was verified.

Vertical Dynamic Impedance of a Viscoelastic Pile in Arbitrarily Layered Soil Based on the Fictitious Soil Pile Model

The vertical vibration of a viscoelastic pile immersed in arbitrarily layered soil is investigated by taking the interaction among pile, pile surrounding soil (PSS) and pile end soil (PES) into account. Firstly, considering both the stratification and stress wave effect of soil, a mathematical model of the pile–soil system is established based on the fictitious soil pile (FSP) model. Then, utilizing the impedance function transfer method and Laplace transform technique, the analytical solutions of the vertical dynamic impedance of pile are derived in the frequency domain. The analytical solutions are validated by comparing them with other existing solutions. Finally, a parametric study is put forward to investigate the properties of PES on the vertical dynamic impedance of pile. The results reveal that the properties of PES have a significant effect on the vertical dynamic impedance of pile, but there is a critical influence thickness for this effect. For the cases of the PES thickness exceeding the critical influence thickness, further increase of PES thickness will not affect the dynamic behavior of the pile–soil system.

Laboratory Model Test of Fully Buried Portal Frame-Shaped Slope-Stabilizing Piles

Abstract Fully buried portal frame-shaped piles have been widely used to control and stabilize large-scale landslides or slopes. However, the working mechanism of portal frame piles, such as the soil–pile interaction and load transfer behavior, is still not fully understood, which has limited the development of effective pile designs and construction. In this study, a well-designed laboratory model test was conducted in a sandy soil slope with a man-made slip surface. Nine portal frame piles were installed, synthetically considering the pile spacing between the rear and front piles as well as the support condition of the pile ends. To make the test closer to reality, in addition to a small thrust due to the excavation of the slope, greater landslide thrusts were imposed on the rear side of the portal frame piles by gradually applying static loads on the top of the slope. It is found that in addition to a concentrated thrust transferred via the cap beam, a landslide thrust accounting for 30 to 60 % of the landslide thrust on the rear pile was transmitted to the front pile by the soil sandwiched between the rear and front piles, which should not be ignored in designs. Meanwhile, to evenly mobilize the bearing capacity of both the rear and front piles, the reasonable spacing between them should be 1.0 to 1.5 times the diameter of the pile. The study also shows that portal frame piles should be embedded in a stable soil layer rather than the bedrock if they can meet the requirements for bearing capacity.