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To better understand the characteristics of the seismic response of nuclear power stations with fixed-base and pile-raft foundations, a series of shaking table tests were performed. The shaking table tests included three cases: free-field soil, fixed-base structure without soil, and superstructure with a pile-raft foundation embedded in the soil. One white noise excitation and three earthquake motions with different earthquake intensity were selected as the ground motion to identify the seismic response of the structure, raft, piles, and soils. The effects of earthquake intensity, earthquake frequency, soil-pile-structure interaction, and soil nonlinearity on the system dynamic responses were analysed. The test results show that the natural frequency and the damping ratio of the superstructure for the pile-raft foundation and the fixed-base structure are different, owing to the soil-pile-structure interaction effect. The acceleration amplification ratio of the fixed-base superstructure shows a significantly higher value than that of the superstructure with a pile-raft foundation. The average peak acceleration ratio of the raft for the pile-raft foundation (raft/soil surface) is 1.2. Under the long period wave excitations, the bending moment of the pile is greater than that of short-period wave excitations, and the peak bending moment occurs at the pile head. The vertical and horizontal displacement and residual displacement increases with the earthquake intensity. These observations suggested that the design of nuclear power station with a pile-raft foundation embedded in clay cannot be simplified as a fixed-base structure. Moreover, adopting pile-raft foundation for nuclear power station will extend the choice of finding suitable construction sites for the nuclear power stations, and the test results could provide references for engineers.

Surcharge due to improper soil dumping and irregular backfilling often causes excessive deformation along the adjacent shallow tunnel in soft-soil areas. Tunnel structures are usually modelled with beam elements where the bending of the concrete segments is considered while the shearing dislocation between segmental rings is usually ignored. In this work, a Timoshenko beam was used to consider such dislocation of a shield tunnel under surcharge and a Pasternak foundation was used to account for the tunnel–soil shear effect. A numerical method was then developed to assess the effect of surcharge loading on the longitudinal responses of the underlying tunnel based on a two-stage method. The proposed solutions were verified using a case history from Shanghai and a three-dimensional numerical simulation from the literature. Results of a further parametric study showed that the deformation induced by surcharge loadings would be underestimated without the shearing-induced dislocation of tunnel segments, which could overpredict the serviceability of the underlying tunnel in surcharge accidents.

Evaluation of scour depth is essential but challenging for the design and surveillance of bridges. The current engineering practice is primarily based on empirical equations. However, without consi...

A simplified added-mass model is proposed in this paper for evaluating the seismic response of rectangular hollow piers in deep water, which are widely used in railway bridges. For this pu...

This paper analytically investigates the postconstruction load-carrying capacity of a cast-in-place pile in clayey soil. The stress release induced by borehole excavation, the slurry suppo...

The influences of thermal gradient on the fluid flux (thermo-osmosis effect) and pore-pressure gradient on the heat flux (mechano-caloric effect) are usually ignored in the thermo-hydro-mechanical (THM) analysis of porous media. However, this assumption could have significant impacts on the analyses of geomaterials of relatively low permeability. In this paper, a point heat source was deeply buried in a saturated porous medium where the thermo-osmosis and mechano-caloric effects were considered. Laplace-Hankel transforms were applied to the linear equations governed by porothermoelasticity and to obtain the analytical solutions, the volumetric strain connected the displacements with the pore pressure and the temperature change. By applying the Laplace-Hankel inversion, the solutions in the real domain were obtained and verified by the existing results. The numerical results showed: (1) in geomaterials of low permeability, the mechano-caloric effect on the temperature change, heat flux and thermal gradient was negligible but the thermo-osmosis effect caused negative pore pressures which increased the effective stress of porous medium; (2) the thermo-osmosis effect pronouncedly drove the outflow of pore fluid away from the heat source which would be balanced by an inflow of pore fluid toward the heat source caused by the negative pore pressure.

Multiple shaking table tests on an integral long-span cable-stayed bridge model were carried out to study the transverse response of pile group foundations. An elaborate bridge model was fabricated according to an assumed bridge with a total length of 2672 m and a geometric similarity ratio of 1/70. The superstructure of the bridge model consisted of a bridge girder, cables, two transition piers, four auxiliary piers and two pylons. The substructure of the model consisted of eight groups of piles embedded in synthetic soil. Accelerations, displacements and bending moments of the piles that supported the transition piers, auxiliary piers and pylons, respectively, were analyzed when the bridge was excited uniformly and nonuniformly in the transverse direction. The nonuniform excitation test was implemented by inputting the same motion to different shaking tables at different times. Three types of earthquake waves with different predominant frequencies were applied to the integral bridge model from low to high intensity. The test results indicated that distinct differences arose between the motions at the top of the pile groups supporting different parts of the superstructure despite the integral bridge model being subjected to uniform excitation. The large displacement difference between pile groups may be one of the origins of damage or even collapse of the superstructure. The amplification ratio of acceleration at the top of the pile group supporting the transition piers increased when the bridge model was excited by waves of low predominant frequency with increasing intensity. The transfer functions of accelerations (Fourier spectra of output accelerations over that of shaking table accelerations) at the top of the pile groups supporting the auxiliary piers were bimodal, which is different from the results of shaking table tests simulating the superstructure with a single degree of freedom (SDOF) system, in which only unimodal transfer functions can be obtained. The influence of the wave passage effect on the seismic response of the pile groups is detectable when the bridge model is excited longitudinally, while the influence is undetectable when the bridge model is excited transversely.

Fluid injection in a borehole into the natural geological formation often encounters the contrasting pore pressures in the matrix and fractures, which can be described by the double-porosity model. However, the fluid-fluid interactions, in terms of the volume change of fluid caused by pore pressures in two porosities reciprocally, were usually neglected. The present solution showed that such simplifications resulted in a pronounced negative pore pressure in the matrix and an overestimation of the stability of borehole. Then, the volume and permeability of fractures and the mass communication between two porosities were analyzed for the interactions of pore pressures.

A series of seismic centrifuge tests were performed on an RC girder-type bridge with a 3 × 3 pile-group foundation embedded in dry sand to investigate the seismic behavior under various sc...

The construction of a 100-km road network is planned on a land reclamation area in the Oujiang Estuary in China. The embankment had a height of 4 m and a base width of 60 m. The reclamation area is...

This paper presents and verifies a novel Cartesian impedance control of the flexible joint manipulator for physical human–robot interaction application based on virtual decomposition control approach. Firstly, the Cartesian impedance control based on virtual decomposition control (VDC) is presented, and the asymptotical stability of the controller is proven by Lyapunov stability theorem. Compared with traditional methods based on singular perturbation, this method can greatly reduce the computational loads and is more suitable for real-time application. Then, a Cartesian force-feedback path planning combined with Cartesian impedance control based on VDC was used to keep the real contact force within the desired value to protect the manipulator and objects as a force, position, velocity and acceleration sensor, and the sensor can be configured freely by regulating the stiffness, damping and inertia, so the manipulator can interact with human (or unknown environment) in a friendly manner. The experimental results illustrate the validity of the developed VDC-based impedance control approach.

Scour induces the loss of soil support around the pile foundations for the offshore and coastal structures, and thus increases their bending moments and lateral deflections, which may change the vertical loads effect on the lateral response of piles applied in such cases. In this paper, the stress changes of the remaining soils at the corresponding pile location due to the three-dimensional scour-hole formation are first determined based on the Mindlin's elastic solutions. Then, the calculated new soil stresses are adopted to re-examine the Young's modulus of the remaining soils undergoing a stress history due to scour. With this key soil parameter that have been modified appropriately to reasonably reflect the effects of three-dimensional scour-hole geometry and the stress history of the soils simultaneously, an analytical model based on the principle of minimum potential energy is further developed to investigate the influence of vertical loads on the lateral responses of offshore piles in marine clay under scour conditions. The influence of related parameters including the lateral displacement level and the vertical load level with different scour-hole dimensions (i.e., scour depth, scour width and scour-hole slope angle), considering or ignoring stress history effect, have also been analyzed based on a case study. The results show that scour depth plays the most important role among the various scour-hole dimensions in influencing the responses of scoured piles under combined vertical and lateral loading. Although still possessing some limitations (e.g., the simplistic assumption of linear-elastic behavior for the soil and the approximate allowance made to account for the gapping effect), the simplicity and relative ease of utilization offered by this proposed analytical model make it a good alternative approach for estimating the deflection and moment responses of the scoured pile foundations subjected to combined lateral and vertical loading.

Local scour has been regarded as one of the key factors that cause bridge failures. The complex interactions between flow and bed materials will result in a scour hole around structures. This proce...

Pile foundations of the coastal and offshore structures always have strong nonlinear response due to marine environmental loads. Derived from the specific hyperbolic p-y curve and the soil stiffness degradation with the average shear strain, the quasi-static p-y hysteresis loop is modeling to develop the BNWF approach for analysis of the soil-pile interaction under low cycle number and irregular cyclic loads. The hysteresis loop simulates various important features, such as the soil-pile nonlinearity, the soil degradation, and the soil-pile gap formation. The developed hysteresis loop is consisted of the elastoplastic and drag-gap components, which the elastoplastic component simulates the nonlinear characteristics of soil-pile system and the gap-drag component considers the repeated separation and closure effects of the soil-pile interface. As the quasi-static p-y hysteresis loop subjects to the constant, variable and mixed amplitude displacement loadings, loop patterns can reflect the reasonable soil-pile interaction mechanism correctly, including the soil-pile separation and the soil degradation. The p-y hysteresis loop shape reflects the trend of irregular loads and the soil-pile gap affects the pattern of the hysteresis loop. The developed quasi-static p-y hysteresis loop incorporated in the generalized BNWF method to analyze the soil-pile interaction under the cyclic loads.

Commonly used hydraulic models that describe the water retention and transport properties of variably saturated soils mainly account for the capillary component of matric potential but ignore the adsorption component. Experimental evidence suggests that soil hydraulic conductivity may be underestimated by several orders of magnitude in the low saturation range for models ignoring the adsorbed film flow. Thus, this paper introduces a fractal method to characterise the effects of capillary and adsorption components of matric potential on soil hydraulic properties considering a simplified pore water transport process. Furthermore, the concept of specific thickness for the adsorbed film was defined to describe the adsorption and adsorption capacity of the soil. Based on these, a complete hydraulic model, including the soil water retention curve (SWRC) and relative hydraulic conductivity curve (RHCC), was derived to describe the water retention and transport properties of the soil in the entire matric head range. The performance of the proposed model was then evaluated using 13 sets of test data, from clean sand to bentonite, showing that it can capture the characteristics of water retention and transport properties well. The model parameters calibrated from the SWRC dataset could predict RHCC over the entire suction range without introducing any additional parameters. Additionally, the influences of the model parameters on soil hydraulic behaviours are also discussed in the paper. It was found that the pore size distribution of the soils mainly controlled the transport characteristics of capillary water, but had a slight influence on the hydraulic properties of adsorbed water, which was dominated by the type and content of minerals and specific surface area of the soil.

Long-span cable-stayed bridge is one of the main structures for highways and railways crossing wide rivers. Seismic response of an integral 1:70 scaled model for a long-span cable-stayed bridge was studied with multifunctional shake tables. The bridge model includes synthetic soil, pile groups and long span cable-stayed bridge structure. Seismic responses of pile groups supporting bridge structural systems of different stiffness, namely the floating system, the elastically constrained system and the supporting pier system are studied considering the seismic soil-pile-structure interaction (SSPSI). The accelerations, bending moments and displacements of piles supporting pylons, auxiliary piers and transition piers are discussed. The wave passage effect on pile foundation supporting long-span cable-stayed bridge is studied by tests for the first time. Results showed that compared with the floating system and the elastically constrained system, the bridge girder of long natural period has larger impact on pile foundations in the supporting pier system under uniform excitations. When subjected to non-uniform excitations, different seismic load generates from the superstructure and acts on the substructure. As a result, seismic response of pile foundation supporting long span bridge under non-uniform excitations is different from that under uniform excitations. The wave passage effect has little influence on seismic response of piles in the floating system and the elastically constrained system. However, in the supporting pier system, with shear wave velocity decreased from infinity to 1000 m/s, the Fourier amplitudes of pile accelerations, bending moments and relative displacements at 3.50 Hz decreased dramatically. The peak bending moments at the head of piles supporting auxiliary piles decreased about 50%.

To provide appropriate uses of nonlinear ground response analysis for engineering practice, a three-dimensional soil column with a distributed mass system and a time domain numerical analysis were implemented on the OpenSees simulation platform. The standard mesh of a three-dimensional soil column was suggested to be satisfied with the specified maximum frequency. The layered soil column was divided into multiple sub-soils with a different viscous damping matrix according to the shear velocities as the soil properties were significantly different. It was necessary to use a combination of other one-dimensional or three-dimensional nonlinear seismic ground analysis programs to confirm the applicability of nonlinear seismic ground motion response analysis procedures in soft soil or for strong earthquakes. The accuracy of the three-dimensional soil column finite element method was verified by dynamic centrifuge model testing under different peak accelerations of the earthquake. As a result, nonlinear seismic ground motion response analysis procedures were improved in this study. The accuracy and efficiency of the three-dimensional seismic ground response analysis can be adapted to the requirements of engineering practice.

Bridge pile foundations in the midst of water current are often subjected to scour, which induces the loss of soil support around the piles and thus results in a significant decrease of foundation capacities or even the failure of bridges. In the current practice, when the scour-affected behavior of the pile foundations is analyzed, either the whole scour-hole geometry or the possible changes in the stress history of the remaining soils is often ignored. In reality, however, scouring creates scour holes with certain dimensions around the pile foundations and the remaining soils that are not scoured away undergo an unloading process at the same time, which will increase the over-consolidation ratios of the remaining soils and accordingly changes of their mechanical properties. This paper examines the behavior of laterally loaded piles in soft clay under scour conditions by using the p-y method. The conventional p-y curves have been modified appropriately to reasonably reflect the effects of three-dimensional scour-hole geometry as well as the stress history of the soils, with the aid of integration of Mindlin’s fundamental solutions. A field test is used to serve as a reference case and to investigate the effects of stress history, scour depth, scour width, and scour-hole slope angle on the responses of laterally loaded piles in soft clay. The results indicate that neglecting the stress history effect can be unconservative for scour-affected pile foundations in soft clay, whereas neglect of the scour-hole dimensions and geometry would lead to over-conservative predictions/design of the laterally loaded piles under scour condition.

Scour of sediments around bridge foundations by the stream is the most significant contributing factor for bridge failures. The scour failures tend to occur without prior warning and have led to fatalities and economic loss every year. A significant amount of work has been conducted on bridge scour. Such efforts can be broadly classified into two major categories, namely science driven and engineering driven. The science-driven research focuses on understanding the scour mechanism and aims to explain the cause of scour due to different factors. Meanwhile, engineering-driven research focuses on the estimation, monitoring and countermeasures of bridge scour. This paper presents a comprehensive and up-to-date literature review of bridge scour research and practice. Firstly, a brief introduction is given which includes recent cases of failures caused by bridge scour. Then, both scientific and technical research on bridge scour is reviewed, which are categorized into four aspects: macroscopic and microscopic mechanism, scour depth prediction carried out by experimental and field data, direct and remote monitoring methods and active and passive countermeasures. Finally, a summary is provided covering both experimental and computational methods for scour research. Discussion is also provided on emerging ideas to investigate bridge scour from both science and engineering perspectives.

The scour behavior of cushioned caisson constructed on reinforced ground, which is used to support superstructure constructed in deep water in seismic zones, was investigated by experimental and numerical methods. Flume tests under nine different flow velocities between 18 and 48 cm/s were performed based on hydraulic similarity design. Complementary numerical simulations were also conducted for the flow velocities ranging from 16 to 46 cm/s. Five typical working modes of the foundation under erosion, namely, ideal working, well working, edge failure, shear failure, and total failure, are analyzed together with their potential impacts on seismic-designed foundation. The critical shear stress, local flow structures, and streamlines were used as the key factors to analyze the change of bed materials and the scour characteristics. Fluid–solid interaction model was built by computational fluid dynamics with sediment transport model, and k–e turbulent model has been implemented to describe the turbulen...

Bridge scour is a phenomenon that describes the loss of riverbed sediments around bridge supports because of flow, and it has been identified as one of the leading causes of bridge failures...

For robots with flexible joints, the joint torque dynamics makes it difficult to control. An effective solution is to carry out a joint torque controller with fast enough dynamic response. This article is dedicated to design such a torque controller based on sliding mode technique. Three joint torque control approaches are proposed: (1) The proportional-derivative (PD)-type controller has some degree of robustness by properly selecting the control gains. (2) The direct sliding mode control approach which fully utilizes the physical properties of electric motors. (3) The sliding mode estimator approach was proposed to compensate the parameter uncertainties and the external disturbances of the joint torque system. These three joint torque controllers are tested and verified by the simulation studies with different reference torque trajectories and under different joint stiffness.

Bridges are critical nodes in the transportation system, especially those crossing wide rivers and seas. Once in service, bridge foundations and the surrounding soils will act as a system together and are subjected to erosion caused by the flows. For more than one century, researchers and engineers have made significant efforts to understand the scour process and its mechanisms to protect bridge foundations from failures. Past studies on bridge scour mainly started from treating scour a hydraulic problem and focused on the flow field due to the interactions between flow and structure. Actually, for a given flow condition, the erodibility of bed materials plays an important role for scour initiation and propagation. For this reason, this study developed a new laboratory apparatus, Simplified Scour Resistance Test (SSRT), that can quickly evaluate the erodibility properties of soil samples and assist in identifying the critical conditions for scour initiation and development. It was firstly validated by Toyoura sand samples with a median particle size of 0.17 mm. Additional tests were then conducted on samples sieved to be populated by single-size particles (d = 0.075 mm, 0.25 mm, 0.5 mm, and 2.0 mm, respectively) and four different soil mixtures under different flow conditions. Results indicate that the grain composition has a significant influence on the scour resistance, while the compaction or density has less influence than the grain size. Granular materials with the same median particle size showed very different scour resistance, i.e., well-graded samples have much higher scour resistance than poorly graded ones. Scour characteristics of the mixed soils can be related to the particle size distribution curve. With this new testing apparatus and the results obtained, a correction factor ks is proposed to improve the traditional predictive methods considering soil composition, which overcomes the limitation of traditional scour prediction methods that often only considered the median particle size, D50.

Earthen levees along rivers may break down because of piping; this is especially the case for levees built on two-stratum levee foundations with an impervious stratum and an underlying pervious substratum. However, the initial stage of this piping is difficult to recognize because finding the piping outlets along the whole levee is challenging. Practical experience confirms that weak spots, such as holes or thickness reductions in the blanket stratum, usually occur at the levee toe. Some of these are harmless, whereas others are not. This study takes weak spots into consideration and provides a detailed description and evaluation of the piping features. Experiments were conducted on three models with different pervious conditions in a flume. Weak spots caused by holes, a thickness reduction, or exposure of the substratum at the levee toe were tested. The results show that the thickness reduction influences the formation of continuous sand boiling once the confined water bursts the blanket stratum. The potential for piping is reduced when the pervious substratum is composed of thin and uniform soil. In the absence of a covering stratum, piping happens with sand boiling. Three typical phenomena, namely sand boiling near the levee toe, a pressure decrease of the confined water, and a rise in the hydraulic gradient in the pervious substratum, could be used as indicators of the occurrence of piping.

Cushioned pile-raft foundation (Cushioned-PR) is a new type of foundation for sea-crossing bridges in deep water (more than 50 m in depth). Cushioned-PR is able to tolerate a certain amount of slide that occurs between the raft and the piles under strong earthquakes. This characteristic could reduce the seismic motion transmitted to the superstructure to maintain the integrity of the bridge. Thus, Cushioned-PR has become popular for bridges under the threat of strong earthquakes. However, the dynamic responses of the Cushioned-PR under various earthquake intensities in soft clay have not been thoroughly understood. It is essential to better evaluate the isolation effect of Cushioned-PR to improve the design. In this study, the non-linear behavior of Cushioned-PR and the isolation effect of its interposed layer are evaluated using centrifuge tests. A series of dynamic centrifuge tests are performed with a 3 × 3 pile group foundation embedded in soft clay under different earthquake motions. Both the cushioned pile raft foundation system (Cushioned-PR) and the connected pile raft foundation system (Connected-PR) are tested in the centrifuge tests. Acceleration and residual displacement of the super-structure, and bending moment of columns and piles, are monitored during the experiments. The results show that the interposed layer in Cushioned-PR has a significant impact on the bending moment and residual displacement of the pile-raft system. Based on the observation from this study, the maximum bending moment of the column in the Cushioned-PR is around 28%–54% of the values from the Connected-PR. On the other side, the maximum horizontal displacement of Cushioned-PR is about 2–10 times larger than that of the Connected-PR. The results show that Cushioned-PR can effectively reduce the seismic excitation transmitted to the super-structure if the seismic intensity is high. However, the benefit of Cushioned-PR is marginal under low-intensity earthquakes.

Compressibility and permeability are two major coupled factors for the engineering properties of soils. Such hydro-mechanical behaviors of soil greatly rely on the composition and the structure of soil which is often expressed in terms of porosity. In this paper, the effect of soil porosity on the soil mechanisms is studied in the research of Biot's consolidation for soft soil. Assuming a constant porosity in a finite layer, the drained bulk modulus and the permeability are porosity-dependent, and the fluid and the soil skeleton of highly saturated soil are assumed to be compressible. The expressions of displacement, stress and pore pressure are obtained with the aid of Laplace-Hankel transforms for the axisymmetric problem. The continuity of multilayered soil yields the global stiffness matrix in the transformed domain. Meanwhile, by introducing the stiffness matrix of the soil base, the compressible and permeable boundary conditions are more acceptable in practical engineering and the global stiffness matrix is then solved in the transformed domain. Taking the inverse of the Laplace-Hankel transforms gives the real answers for Biot's consolidation. The results show that the increase in porosity leads to a higher compressibility and permeability of soil, and consequently, causes a higher settlement and accelerates the process of consolidation. However, the assumption of the constant porosity of soil has some limitations due to ignoring the nonlinear development of soil structure during consolidation and the further works about the variation modes of porosity need to be done for a better understanding of soil consolidation.

This paper presents both experimental study and numerical simulation to investigate the effects of an innovative sacrificial piles concept to reduce local scour surrounding service piles. These sacrificial piles are placed upstream of the pile groups, which serves to change the flow field and reduce the erosive force before it reaches the service piles. Four types of pile group arrangements were considered in the experimental and simulation analyses, including single pile, double pile in tandem arrangements, double pile with side-by-side arrangements, and pile array of 2 × 2 arrangements. Scaled experiments were conducted in flume for each group of design, and local scour around piles with and without sacrificial piles was monitored. It was found that the sacrificial piles effectively reduced the development of local scour surrounding the service piles, especially at their heels. The results also showed that the use of sacrificial piles reduced the time required for the scour depth to reach the equilibrium conditions than those without the sacrificial piles. To corroborate the experimental observation, numerical simulation with three-dimensional computational fluid dynamics model has also been carried out with commercial code with verified erosion model. The development of dynamic scour process and maximum scour depth were obtained by the computational simulations. Comparison of experimental and computational results both indicates that the use of sacrificial pile is effective in reducing the scour depth. The extent of reduction in the maximum scour depth varied with the arrangement of sacrificial piles.