Showing papers on "Dynamic load testing published in 2022"

Response of energy pile-soil structure and pile group effect: An indoor similarity simulation study

Abstract: Energy pile has dual functions of coupled heat transfer and load-bearing. The research on its bearing characteristics has attracted much attention. This research is based on the indoor scale model test of energy pile to simulate the thermo-mechanical response of single pile and pile group of spiral energy pile under summer and winter conditions. The thermal response of the middle part of the energy pile is the largest. Under the action of circulating water, additional tension is generated in the pile section in winter and additional pressure is generated in summer. Through comparative analysis, it is proved that when multiple piles are operated simultaneously, the heat transfer efficiency is almost the same as that of single pile in the initial operation, and the heat transfer efficiency of single pile is reduced in the long-term operation, the temperature response of soil between pile and pile is higher, and the quasi-steady state time is shortened. The axial force distribution of single pile and multiple piles is compared and analyzed. The results show that in the service process of multiple piles, when the upper load is 0.8 Pu in winter, the axial force in the middle of the pile group is reduced by 0.73 kN compared with that of single pile. In summer, when the upper load is 0.8 Pu, for example, the circulating water temperature is 35 °C, 45 °C and 55 °C, and the maximum axial force in the middle of the pile increases by 0.5 kN, 0.79 kN and 0.8 kN respectively. It is proved that the pile group effect does exist when the energy pile group works, which will reduce the heat exchange efficiency of single pile, advance the quasi steady state time, increase the single pile axial force in summer, reduce the single pile axial force in winter, and even generate tension in the pile section. The energy pile group effect should be fully considered in the design and application of energy pile. • An indoor similar simulation experiment platform of energy pile is built. • Multiple model piles were set up to study the response of energy pile-soil structure. • The heat transfer law and stress characteristics of energy piles are defined. • Experimental results showed that the proposed approach was efficient.

Pile group in clay subjected to cyclic lateral load: Numerical modelling and design recommendation

Abstract: Major structures like offshore platforms, wind turbines, transport infrastructure, tall buildings, etc., resting on soft compressible clays, are often supported by pile foundations. Apart from usual vertical loading (dead load, live load, etc.), these piles are subjected to significant cyclic loads arising from actions of waves, ship impacts, winds or moving vehicles. Under such circumstances, the lateral mode of cyclic loading is predominant and affects the overall foundation stability. Such repetitive loading leads to stress reversal in adjacent soft clay initiating progressive degradation in soil strength and stiffness, deteriorating the pile capacity with unacceptable displacements. Although several past studies investigated the response of single pile under lateral cyclic loading, a detailed investigation on pile group in clay subjected to cyclic lateral loading, which is of immense practical interest to field engineers, is yet to be carried out. In this paper, in-depth study has been carried out by developing a three-dimensional dynamic finite element model. Comparison of the computed results with available test data validates the numerical model. Extensive parametric studies with field data indicate that both the axial and lateral pile capacities and displacements have been significantly influenced by the cyclic loading parameters. Relevant design curves are also constructed.

Experimental Study on the Vertical Bearing Capacity of Stiffened Deep Cement Mixing Piles

Abstract: A stiffened deep cement mixing (SDCM) pile is a type of composite pile that has a rigid pile at the core that reinforces the deep cement mixing (DCM) pile. An SDCM pile combines the advantages of both a DCM pile (i.e., high lateral friction resistance) and a rigid pile (i.e., high stiffness and strength). This work presents four sets of laboratory model tests that were aimed at investigating the vertical bearing capacity of SDCM piles with different cement contents by monitoring the pile top load, the axial force, the pile settlement, and the pile end resistance. Two failure modes of the SDCM piles were observed during the tests––pile failure (the pile ruptured at the section below the core pile) and punching failure (the pile penetrated into the underlying soil). It was found that the vertical bearing capacity of the SDCM piles increased with cement content, up to the point of punching failure. The pile end resistance increased with the pile top load. When the ultimate bearing capacities of the SDCM piles were reached in the model tests, the ratio between pile end resistance and pile top load ranged from 19.4% to 54.2%, indicating that pile end resistance cannot be ignored.

Numerical Simulation Study on Lining Damage of Shield Tunnel under Train Load

Abstract: Under the long-term dynamic load influence of trains, shield tunnel structures are damaged. With the increase in operating number, cumulative damage gradually increases. When cumulative damage increases to a certain value, the tunnel lining produces cracks and loses tensile strength, which leads to tunnel deformation, damage, etc. In serious cases, the tunnel ceases operation, causing traffic accidents and casualties. Based on the finite element software ABAQUS, this paper analyses the change rule of tunnel lining damage under long-term dynamic train load and explores the influence of tunnel buried depth on the change rule of tunnel lining damage. The excitation force function is used to generate a series of dynamic and static loads superimposed by sine functions to simulate the dynamic loads of trains. Load is applied above the tunnel by writing DLOAD subprogram. The results show that the damage of tunnel lining mainly occurs at the arch foot and the structural damage in other places can be neglected. Under the same loading condition, the greater the tunnel lining damage is. Under the same loading conditions, the tunnel lining damage increases with the increase in buried depth. According to the test results, the mathematical expressions of cumulative damage value versus loading times at the location prone to fatigue damage. It provides theoretical reference for safety evaluation and protection of tunnel structure under long-term train load.

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

Abstract: 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.

Experimental Study on Mechanical Properties and Failure Laws of Granite with Artificial Flaws under Coupled Static and Dynamic Loads

Abstract: Rock is the main construction material of rock engineering, such as the engineering of mines and tunnels; in addition, its mechanical properties and failure laws are of great significance to the stability evaluation of rock engineering, especially under the conditions of coupled static–static stresses. In this study, granite specimens were manufactured with artificial flaws. Coupled static and dynamic loads tests were carried out with a modified split Hopkinson pressure bar (SHPB) apparatus; and six typical levels of axial pre-stresses and three crack inclination angles were designed. Three-dimensional digital image correlation (3D-DIC) was also applied to record and analyze the fracturing process and damage evolution of the specimens. The test results show that there was no compaction stage in the stress–strain curve under combined dynamic and static loading. The dynamic strength of the specimens increased first and then decreased with the increase in the static pressure; moreover, the specimens reached the maximum dynamic strength when the static pressure was 10% UCS. The dynamic strength decreased first and then increased with the increase in the crack inclination angle; and the lowest strength appeared when the inclination angle was 45°. The change in axial compression had a significant influence on the failure mode, and the failure mode gradually transformed from shear–tensile failure to shear failure with the increase in the pre-stress. The tensile strain was usually generated at the end of the fractures or near the rock bridge. When the axial pressure was small, the tensile strain zone parallel to the loading direction was easily generated; and when the axial pressure was large, a shear strain zone developed, extending along the diagonal direction. The research results can provide a theoretical reference for the correct understanding of the failure mechanisms of granite and its engineering stability under actual conditions.

A Calculation Method of Bearing Capacity of Single Squeezed Branch Pile Based on Load Transfer Method

Abstract: A calculation method of the bearing capacity of single squeezed branch pile is established based on the load transfer method. In the method, the hyperbolic model is used to describe the nonlinear load-displacement relationship of pile-soil interaction at the pile tip, the pile skin, and the squeezed branch, and the theoretical expression of six load transfer coefficients of squeezed branch pile is given. The correctness of the method is proved by a small-scale model test in homogeneous soil and a large-scale model test in stratified soil. The results show that the calculation based on the load transfer method is applicable to predict the ultimate load in engineering application.

Dynamic Characteristics and Damage Constitutive Model of Mudstone under Impact Loading

Abstract: The mechanical response characteristics of mudstone from the ingate roadway of the west ventilation shaft in Yuandian No. 2 coal mine, Huaibei City, Anhui Province, China to dynamic loads were quantified in single- and cyclic-impact compression tests, using the split-Hopkinson pressure bar test device. The dynamic stress–strain relationships and the failure characteristics of mudstone samples under different impact loads were analyzed systematically. Considering the “rate effect” of the mudstone dynamic strength, the dynamic strength criterion of mudstone was proposed, and the dynamic damage constitutive model of mudstone was established, based on the statistical damage theory. In response to single-impact loads, with increasing impact pressure, the mudstone peak stress and strain gradually increased, and the peak stress and average strain rate increased nonlinearly. In response to cyclic-impact loads, with an increasing number of impacts, the mudstone peak stress first increased and then decreased, and the peak strain increased gradually. With increasing impact pressure, the number of impacts to the samples’ failure decreased gradually. By parameter identification and comparative analysis of the test results, the proposed dynamic damage constitutive model of mudstone was validated. The model can be used for stability analysis of roadway-surrounding rock under dynamic loads.

Static and dynamic lateral non-linear pile–soil–pile interaction

Abstract: Three-dimensional finite-element analyses are performed to compute the interaction factors between two neighbouring piles, under both static and dynamic horizontal displacements. The fixed-head elastic piles are flexible, embedded in a homogeneous saturated clay stratum, and loaded under undrained conditions. Soil inelasticity and soil–pile interface separation are modelled in a rational way. With the aim of improving fundamental understanding of pile–soil–pile non-linear interaction mechanisms, it is found that the effect of a loaded (‘source’) pile on an adjacent (‘receiver’) pile diminishes rapidly with increasing amplitude of imposed displacement, at a rate which depends on the angle of departure from the direction of loading between source and receiver piles. Gap formation at the back of a displaced pile affects the response of the group. Using the interaction factors developed, the behaviour of 2 × 2 and 3 × 3 pile groups is analysed and compared with the three-dimensional analysis of the whole group. Under static conditions the differences between front (‘leading’) and back (‘trailing’) piles are illustrated. Under cyclic dynamic conditions, the separation gap forms on each side of a pile alternately, leading to the peaks in stiffness diminishing, and leading to larger (than the elastic) group efficiencies. Superposition using proper non-linear interaction factors offers reasonable approximation, but only for moderate amplitudes of load, smaller than about one-half of the ultimate lateral pile capacity.

Numerical modeling of single closed and open-ended pipe pile embedded in dry soil layers under coupled static and dynamic loadings

Abstract: Abstract For the design of a deep foundation, piles are presumed to transfer the axial and lateral loads into the ground. However, the effects of the combined loads are generally ignored in engineering practice since there are uncertainties to the precise definition of soil–pile interactions. Hence, for technical discussions of the soil–pile interactions due to dynamic loads, a three-dimensional finite element model was developed to evaluate the soil pile performance based on the 1 g shaking table test. The static loads consisted of 50% of the allowable vertical pile capacity and 50% of the allowable lateral pile capacity. The dynamic loads were taken from the recorded data of the Kobe earthquake. The current numerical model takes into account the material non-linearity and the non-linearity of pile-to-surrounded soil contact surfaces. A lateral ground acceleration was adapted to simulate the seismic effects. This research emphasizes modeling the 1 g model by adapting MIDAS GTS NX software. This will, in turn, present the main findings from a single pile model under a combined static and dynamic load. Consequently, the main results were first validated and then used for further deep investigations. The numerical results predicted a slightly higher displacement in the horizontal and vertical directions than the 1 g shaking table. The shear stress–shear strain relationship was predicted. Positive frictional resistance for the closed-ended pile was captured during the first 5 s when low values of acceleration were applied and, consequently, the pile resistance decreased and became negative. Internal and external frictional resistance was captured for the open-ended pipe pile. Overall, frictional resistance values were decreased with time until they reached the last time step with a minimum value. As a result, the evaluation of the current study can be used as a guide for analysis and preliminary design in engineering practice.

A Numerical Investigation to Calculate Ultimate Limit State Capacity of Cable Bolts Subjected to Impact Loading

Abstract: As rock bursts are unavoidable in deep mines and excavations with high in-situ stresses, ground support systems are implemented to manage and mitigate rock bursts. Cable bolts are commonly used as reinforcing elements in ground support systems, which are subject to dynamic loads in burst-prone excavations. To design an efficient cable bolt in burst-prone conditions, shear and energy absorption capacity must be considered. Numerical modelling is an advantageous method of repeatable testing and it is inexpensive and non-destructive. This study develops a statically and dynamically loaded numerical model of a double shear test in ABAQUS/Explicit. A total of 36 static and 576 dynamic tests are carried out, which examine the influence of bolt diameter, steel yield and ultimate strength, dynamic load velocity and dynamic load mass on the displacement, shear force and energy absorption capacity of cable bolts. As bolt diameter and steel yield strength increases, the maximum shear force resisted and bolt displacement increases. Similarly, as the mass and velocity of the dynamic load increases, the amount of energy absorbed by the cable bolt increases. The main novelty of the current research is to suggest a reliable computational tool to investigate the influence of the different key parameters in the cable bolts on the ultimate capacity. The suggested method is a significantly cost-effective technique compared with the experimental investigations.