Pile-soil Relative Displacement Research Articles

Study of the dynamic interactions between helix-stiffened cement mixing piles and soft soil under oblique incidence of SV waves in arbitrary space

Helix stiffened cement mixing (HSCM) piles are extensively utilized in soft soil sites due to their excellent seismic and pullout resistance. Initially, the equivalent nodal force formulas for each face under the oblique incidence of SV waves in any spatial direction are derived based on the viscous-spring artificial boundary. The method's applicability in studying pile–soil interactions is verified based on existing pile–soil tests. Subsequently, the HSCM pile–soil model is constructed using finite element software through a Python program. Various analyses of accelerations, relative displacements, soil reactions, and pile–soil dynamic p-y curves are conducted using two different directions of incidence angles (horizontal and vertical) and various types of seismic waves. Subsequently, the backbone line of the dynamic p-y curve is plotted, revealing the pile–soil interaction mechanism under spatial oblique incidence. Finally, Matlock's formula is modified to ensure its applicability to pile–soil interactions under spatially oblique incidence. The results demonstrate that loading the Kobe wave causes the peak acceleration in the x direction, the peak pile–soil relative displacement, and the peak pile-side soil reaction force to increase by 22.64 %, −18.90 % and 18.42 %, respectively, compared to those of the El Centro wave.

Research on dynamic p-y curves in liquefied sand under cyclic loading conditions

Under dynamic cyclic loads, the pore water pressure in saturated sand rises continuously, which leads to the reduction of horizontal bearing capacity or the damage of the pile foundation. Under dynamic cyclic loads, the existing calculations methods of horizontal bearing capacity of pile foundations were obtained by proposing a reduction factor for the static p-y curve model mostly. Compared with the test results, the soil resistance around the pile obtained by this method is often overvalued and the pile-soil relative displacement has a relative underestimation. As a result, this method is not suitable for pile foundation design. Therefore, the variations of soil resistance around the pile and the pile-soil relative displacement are obtained by analyzing the effect of pore pressure ratio on soil strength around the pile in saturated sand in this paper. The theoretical formulas related to the cyclic vibration times and pore pressure ratio are proposed. The proposed formulas are introduced into the dynamic p-y curve model to obtain the dynamic p-y backbone curve in the process of sand liquefaction. The research results show that the variations of the soil resistance around the pile and the pile-soil relative displacement are related to the pore pressure ratio and the cyclic vibration times during the sand liquefaction. As the cyclic vibration times increases, the pore pressure increases, the soil resistance around the pile decreases and the pile-soil relative displacement increases.

Centrifuge model test study on micro-pile reinforcement of shallow foundation

Micro-piles are an effective reinforcement measure for existing foundations in practice. A series of centrifuge model tests were conducted on different combinations of shallow foundation and micro-piles to investigate the reinforcement effect of micro-piles. The interaction mechanisms between the shallow foundation and micro-piles were analyzed on the basis of full-field image-based measurements of soil displacement. The micro-piles were proven to provide an effective reinforcement on the shallow foundation by decreasing the settlement, increasing the bearing capacity and making the soil deformation more uniform. The loading induces soil deformation within an area that expands gradually with a similar boundary shape and is widened by the micro-piles. The shallow foundation and micro-piles exhibit a significant nonlinear coupling effect on the bearing ability due to their interaction through the soil with the mechanism summarized with the strengthening effect and weakening effect. The micro-piles have a strengthening effect on the shallow foundation to increase the bearing resistance by constraining the horizontal deformation of the neighboring soil. The shallow foundation has a weakening effect on the micro-piles to decrease the bearing resistance by reducing the pile-soil relative displacement. The weakening effect and strengthening effect play the dominant role in turn as the total resistance increases.

Study on Anti-Uplift Effect of Micro-Steel-Pipe Pile on Red-Bedded Soft Rock Subgrade

To investigate the treatment effect of micro-piles on uplift deformation of red-bedded soft rock subgrade, an in-situ static load test of slurry injected steel pipe micro-piles with different length was carried out, the uplift bearing capacity and deformation characteristics of micro-piles were analyzed, and the load transfer function of pile lateral friction resistance was modified with the consideration of pile length. A numerical simulation method considering the variable shear stiffness at the pile-soil interface was established, and the inversion of the relevant material parameters was carried out based on the in-situ test results. Through numerical simulation, the effect of single piles with different pile lengths and group piles with different pile spacing on the treatment of the uplift deformation of the subgrade was investigated. Finally, the anti-uplift design method of micro-piles in red-bedded soft rock was proposed. The results show that the uplift bearing capacity increased nonlinearly with the increase in pile length, and the variation curve of pile lateral friction resistance with pile-soil relative displacement showed a hardened type. The predicted pile lateral friction resistance shows a good correlation with the measured result; all the correlation coefficients were greater than 0.81. The uplift deformation of subgrade without piles was radially distributed with the maximum value of 5.12 mm as the center. A single micro-pile with a length of 7 m or a rectangular array of group piles with a length of 7 m and a spacing of 3D could effectively decrease the maximum uplift deformation to less than 4.0 mm, which can meet the requirement of specification. Thus, the micro-piles could be used for controlling the uplift deformation of red-bedded soft rock subgrade, and this study can provide a reference for anti-uplift design in the distributed area of red-bedded soft rock.

Analysis of Load-Settlement Curve Based on Load Transfer at Pile-Soil Interface

A field pile loading test was carried out on the Peshawar–Karachi Motorway (PKM) project in Pakistan to show the settling mechanism of bored pile foundation in pulverized soil and the force characteristics of frictional resistance at the pile-soil interface. The changes in pile lateral frictional resistance and pile settlement during the loading-unloading process of test piles were measured and analyzed, as well as the load-settlement distribution characteristics of test piles in different soil layers, the distribution of test pile internal forces, and the changes in pile-soil relative displacement. It was established that there was considerable deterioration of pile lateral frictional resistance and residual deformation of pile tip displacement throughout the test pile load-settlement process, and the association between the pile-soil interface frictional resistance and pile-soil relative displacement was addressed. The results reveal that the frictional resistance at the pile-soil interface is directly connected to the nature of the soil layer, with a positive connection between the natural density, specific gravity, compression deformation, and the plastic index under immediate load, and a negative correlation between the natural moisture content, compression coefficient, and settlement variations after unloading. The load-settlement of the pile rose in a non-linear proportion during the loading-unloading operation, with a maximum settlement value at the pile top of 8.14 mm and a residual deformation at the pile bottom of 1.94 mm. The frictional resistance of the pile perimeter was distributed non-linearly throughout the pile depth, and the frictional resistance of the pile-soil interface was severely deteriorated at an embedded depth of 15 m, with the degradation degree of the silty soil layer being significantly smaller than that of the silty clay soil. The relative pile-soil displacement was positively linked with the lateral frictional resistance of the pile under the same load, and the correlation coefficient in silty soil was much greater than that in sandy soil.

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 simple method for analyzing thermomechanical response of an energy pile in a group

This study develops a simple method for rapid estimation of the thermomechanical response of an energy pile in a group. Based on the equilibrium condition of the force, the governing equations for a single energy pile are established using the finite difference method. An exponential model is used to describe the relationship between the unit skin friction and pile–soil relative displacement as well as the relationship between the unit end resistance and displacement at the pile end. Based on the Pyke model for the loading–unloading behavior of the pile–soil interface, the incremental relationship between the unit skin friction and pile shaft displacement, and the incremental relationship between the unit base resistance and pile end displacement, are presented in the unloading and reverse loading stages. Considering the mechanical and thermal loads and the reinforcing effect of neighboring piles, a simple method is proposed to analyze the thermomechanical response of an individual energy pile in a group. Comparisons between the measured and computed results of six well-documented cases are presented to verify the reliability of the proposed method.

Application of Fiber Bragg Grating Sensing in Bidirectional Tests of Pile Foundations

Abstract The fiber Bragg grating (FBG) technique has emerged as a new strain monitoring technique in civil engineering applications. In this paper, based on the principle of the FBG technique and the bidirectional test method, FBG sensors and conventional vibrating wire strain gauges (VWSGs) were employed to monitor the strain variation law of a pile under different loads and analyze the relationship between the side resistance and pile-soil relative displacement of soil layers in bidirectional field testing of a large-diameter cast in situ pile. Four field bidirectional test cases of cast in situ piles, prestressed high-strength concrete (PHC) and steel pipe pile, and pile-base post-grouting pile are presented. The applicability and stability of the FBG sensing technique were verified by analyzing the side resistance distribution and bearing capacity. The results showed that FBG sensors can remove the influence because of residual force and the effect of temperature change and concrete expansion photosensitive material provides an improved sensitivity to the external environment compared with a VWSG. The strain and side resistance variation tendency of the cast in situ pile were consistent between the two kinds of sensors. The FBG sensing technique had high test accuracy in the four bidirectional O-cell field test cases of the cast in situ pile, PHC and steel pipe pile, and pile-base post-grouting pile. The pile-base post-grouting technique is an effective procedure that was implemented to increase the ultimate bearing capacity. The research results of this paper reveal a new strain monitoring technique for the bidirectional test method of pile foundations and represent a new test method for measuring the bearing capacity of PHC and steel pipe piles that has a remarkable application value in practical engineering.

Field Tests on Axial Behavior of Grouted Steel Pipe Micropiles in Marine Soft Clay

This paper presents full-scale field tests of grouted steel pipe micropile embedded in marine soft clay under compressive and tensile loads. The load-displacement responses, axial forces, tip resistance, and unit skin friction were measured and evaluated. The relationship between skin friction and pile–soil relative displacement in each soil layer is discussed. The bearing capacity of the micropile was improved by the post-grouting technique. After post-grouting, the ultimate skin friction of the soft clay under compression and tension was improved by 48.7% and 30.8%, respectively. The ultimate bearing capacity of the grouted steel pipe micropile under a compression load is approximately twice that under a tension load.

Mechanism of Pile-Soil Relative Vertical Displacement under the Freeze-Thaw Action

Abstract To effectively mitigate the risk of freeze-thaw disasters, the lines, poles, and piles in point-line constructions need be deployed with careful consideration of the compaction degree control of backfilled soil. Vertical displacements of soil and piles occur at the same time under freeze-thaw action, thereby making it difficult to determine the frost heave and thaw subsidence of piles. To solve this problem, we monitored the vertical displacements of piles and soil simultaneously and found that the pile-soil interaction differs, depending on the variances in backfill density. Such displacement also ultimately results in “pile-soil” system modification synchronization. To determine the vertical displacement of the pile foundation, one must first determine the relative displacement of the pile and soil. This work presents serial experimental results and verifies experimentally that an optimal degree of soil compaction exists in which the pile shows minimal changes in its vertical displacement.

TIME EFFECT OF PILE-SOIL INTERACTION AND ITS ELASTIC-VISCOPLASTIC CONSTITUTIVE MODEL

Pile-soil interaction is considered to be one of the most important problems in the study of the mechanical behaviour of pile foundation. In this paper, the lateral friction resistance and pile-soil relative displacement of bridge piles in deep soft soils are monitored for an extended period by using concrete strain gauges that are embedded in the test pile. Field test results show that both the pile lateral frictional resistance and pile-soil relative displacement increase along with time, while the pile-soil interaction demonstrates a time effect. Therefore, an elastic-viscoplastic constitutive model must be established to better simulate the time-dependent mechanical behaviour of the pile-soil contact. Based on the Goodman model, we developed an elastic-viscoplastic interface constitutive model of the pile-soil interface into the FRIC subroutine through ABAQUS software, which is one of the most commonly used finite element analysis softwares in the world, to simulate a well-recorded pile test in deep soft soils. The calculated pile lateral frictional resistance and pile-soil relative displacement are close to the measured values, and the ability of the interface model to describe the changes in the shear stress-strain of the pile-soil interface along with time is validated.

Load displacement analysis of a single pile in an unsaturated expansive soil

A three-step computer program is developed for simulating the load-displacement response of a single pile in unsaturated expansive soil taking account of influence of water infiltration. In the first step, peak and residual pile-interface shear strength variation with suction are estimated. The relationship between interface shear strength and pile-soil relative displacement and pile base resistance and pile base settlement is achieved by modifying universal transfer curve model, in the second step. Lastly, traditional load transfer method was modified for analyzing the load-displacement response. The validity of program is verified using a laboratory model test and a field investigation case study.

Evaluation of the dynamic p–yp loops of pile-supported structures on sloping ground

A key issue in the design of pile-supported structures on sloping ground is soil–pile interaction, which becomes more complicated in case of dynamic loading. This study aimed to evaluate the effect of slope on the dynamic behavior of pile-supported structures by performing a series of centrifuge tests. Three models were prepared by varying the slope and soil density of dry sand grounds. The mass supported on 3 by 3 group piles was shaken applying sinusoidal wave with various amplitudes. Test results showed that the location of maximum values and distribution shape of the bending moment below the ground surface varied noticeably with the pile position in the slope case. The relationship between the soil resistance and pile deflection (p–yp loops) was carefully evaluated by applying the piecewise cubic spline method to fit the measured bending moment curves along piles. It was found that the shape of the p–yp loops was irregular due to the effect of slope, and immensely influenced by the movement of the unstable zone. In addition, the effect of the pile group in the horizontal case was evaluated by comparing with the previously suggested curves that represent the relationship between the soil resistance and pile–soil relative displacement (p–y curves) to propose the multiplier coefficients.

A Simplified Calculation Method for Stress Concentration Ratio of Composite Foundation with Rigid Piles

Piles in a composite foundation share loads with the surrounding soil instead of carrying all loads alone as a pile foundation. Stress concentration ratio, defined as the ratio of the stress on the top of piles to the stress on the surrounding soil, is essential for the design of piles in the composite foundation. An equal strain condition is widely accepted and adopted in the existing methods of calculating the stress concentration ratio. In other words, the piles should have identical relative displacement with the surrounding soil in an equal strain condition. This assumption may work for flexible and semi-rigid piles; however, it may not be suitable for rigid piles considering the significant stiffness difference of pile and soil. The relative displacement of rigid piles and surrounding soil can significantly change the stress distribution on piles and soils due to the pile-soil interaction (i.e., pile side friction). Therefore, the pile-soil interaction should be considered in the design of the rigid piles in the composite foundation. In this study, a simplified theoretical solution of the stress concentration ratio was derived based on the deformation coordination between rigid pile and soil to consider the pile-soil relative displacement and thus the pile-soil interaction. The pile side friction was assumed as linearly increased with the pile-soil relative displacement and then the stress concentration ratio was derived based on the force and deformation equilibrium in the composite foundation. A neutral point (i.e., location with zero pile-soil relative displacement) was included in the derivation to take the negative skin friction along the pile into account. Lab-scale and full-scale field tests were conducted to verify the effectiveness of the proposed method. Comparison of measured stress concentration ratios with calculated ones using the proposed method demonstrates the effectiveness of the proposed method.

Analysis on response prediction of a single pile and pile groups based on the Runge-Kutta method

The conventional load-transfer approach can not consider the interaction of piles, and can not be directly used in the analysis of the response of pile groups. The interactive effects among piles can be taken into account in the shear displacement method based on the principle of superposition. In the present analysis, the shaft displacement of a single pile at a given depth is assumed to be composed of the pile-soil relative displacement developed at the disturbed soil around pile and the elastic vertical soil displacement developed in the soil mass. A new load-transfer function which is very similar to a hyperbolic model is then established to describe the relationship between unit skin friction and shaft displacement at a given depth. Furthermore, a hyperbolic model is used to simulate the relationship between unit end resistance and pile end displacement. As to the analysis of the response of pile groups, the interactive effects among piles are considered by using the shear displacement method. For an individual pile in pile groups, new load-transfer functions are established to capture the relationship between unit skin friction and shaft displacement and the relationship between end resistance and pile end displacement. Based on the fourth order Runge-Kutta method, a modified load-transfer method is proposed to analyze the response of pile groups considering the interactive effects among piles. The reliability of the present method is checked by using comparisons of the present calculated results, the measured results and the computed values derived from other methods.

A practical method for construction of p-y curves for liquefiable soils

In practice, the analysis of laterally loaded piles is often carried out using a “Beam on Non-linear Winkler Foundation method” whereby the lateral pile-soil interaction is modelled as a set of non-linear springs (also known as p-y curves). During seismic liquefaction, the saturated sandy soil changes its state from a solid to a viscous fluid like material, which in turn alters the shape of the p-y curve. Typically, p-y curves for non-liquefied soil looks like a convex curve with an initial stiff slope that reduces with increasing pile-soil relative displacement (y) i.e., elasto-plastic softening response. However, recent research conclusively showed that p-y curve for liquefied soil has a different shape, i.e., upward concave with practically-zero initial stiffness (due to the loss of particle to particle contact) up to a certain displacement, beyond which the stiffness increases due to reengaging of the sand particles. This paper presents a practical method for construction of the newly proposed p-y curves along with an example.

Analysis on Modification of the Measured Results of Test Pile Due to the Influence of Reaction Piles in Static Loading Test

Abstract In static load testing, the measured displacement of the test pile is different from that of an individual pile loaded by the same load. This is because the transferred load of the reaction piles through the soils causes an opposite movement of the test pile. To modify the measured results of test pile due to influence of reaction piles in static loading test conducted by using reaction piles as the reaction system, this paper presented a simple analytical approach for the analysis of the influence of reaction piles on the test pile response in static load testing. This was done by using the load-transfer approach. In this paper, the shaft displacement at a given depth was assumed to be composed of the pile-soil relative displacement developed at the disturbed soil around pile and the elastic vertical soil displacement developed in the soil mass. A load-transfer function which is similar to a hyperbolic model was then established to simulate the relationship between unit skin friction and shaft displacement at a given depth. Furthermore, a hyperbolic model was used to describe the relationship between unit end resistance and pile end displacement. Considering interactive effects between reaction piles and test pile, the load transfer function of the test pile in static load testing conducted using reaction piles as the reaction system is proposed. As to the analysis of the response of an influenced test pile considering progressive deformation of pile-soil system, a highly effective iterative computer program was developed to modify the response of an influenced test pile in static load testing. Comparisons of the load-settlement responses of a single pile and an influenced test pile in static load testing demonstrated that the proposed method is generally in good agreement with the observed behavior and the calculated results derived from other approaches.

Simplified non-linear approaches for response of a single pile and pile groups considering progressive deformation of pile–soil system

This work presents a simplified approach for the nonlinear analysis of the load–displacement response of a single pile and pile groups embedded in multilayered soils. A hyperbolic model is used to capture the relationship between the skin friction and the pile–soil relative displacement developing along the pile–soil interface. The shaft displacement is assumed to be composed of the pile–soil relative displacement developing at the disturbed soil around the pile shaft and the elastic vertical soil displacement developing in the soil mass. The relationship between the shaft displacement and the skin friction is then presented. Moreover, a hyperbolic model is also used to capture the relationship between end resistance and pile end displacement. Considering the interactive effect among piles, hyperbolic models of an individual pile in pile groups are proposed. As to the analysis of the response of a single pile and pile groups, considering the progressive deformation of the pile–soil system, a highly effective iterative computer program is developed using the proposed hyperbolic models. Comparisons of the load–settlement responses demonstrate that the proposed method is generally in good agreement with the field-observed behavior and the calculated results derived from other approaches.

Some observations of the influence factors on the response of pile groups

A simplified approach for nonlinear analysis of the load-displacement response of pile groups embedded in multilayered soils is presented in this work. A hyperbolic model is used to capture the relationship between unit skin friction and pile-soil relative displacement developed along the pile-soil interface and the stress-displacement relationship developed at the pile end. Considering interactive effect among piles, the parameters related to the hyperbolic model of an individual pile in a group can be computed. As to the analysis of the response of pile groups, a highly effective iterative computer program is developed using the hyperbolic model of an individual pile in a group. The efficiency and accuracy of the present method is verified using a well-documented field test. Furthermore, a parametric study is conducted to capture the influence of pile spacing and number of piles on the load-settlement response of the pile groups connected to a rigid cap. The pile-group settlement ratio and the pile-group resistance ratio are analyzed to assess the interaction effect among individual piles.

Analysis on Response of a Single Pile Subjected to Tension Load Using a Softening Model and a Hyperbolic Model

A softening model and a hyperbolic model are adopted in the proposed approach to simulate the relationship between pile-soil relative displacement and unit skin friction developed at the pile-soil interface. Based on the proposed two models, two simple analytical approaches are presented for nonlinear analysis of the load-displacement response of a single pile subjected to tension load. Furthermore, a highly effective iterative computer program is developed using the two proposed analytical approaches. To demonstrate the effectiveness and accuracy of the proposed analytical methods, comparisons of the results derived from the present methods, other analytical methods and well-documented field experimental pile loading cases are made. Further study is conducted to analyze the influence of the parameters related to the proposed softening model and hyperbolic model on the load-displacement response of a single pile subjected to tension load. The proposed simple analytical approach is economical and efficient, and suitable for the analysis of a tension pile embedded into layered soils by using a softening model or a hyperbolic model.