Results Of Physical Model Tests Research Articles

Stability of Wide-Graded Rubble Mounds

The reshaping of temporary rubble mounds like the core of breakwaters or reclamation bunds is often a concern for contractorsi n the construction stages of marine structures. The formulas found in literature for the prediction of such behavior are few, and they do not provide clear insight on the influence of relevant parameters, in particular the small dimensions and wide stone-size gradation of the material involved, usually consisting of quarry run or resulting from dredging. The previous research in the field of dynamic stability focused on berm breakwaters and gravel beaches. These two typologies of structures define the range to which the rubble mounds considered in this study generally belong. An overview on the design tools provided by the technical literature shows that, whenever the grading was included as a governing parameter, some influence was recognized in the characteristics of the structure (e.g. the permeability) and in the dynamism of the different fractions of stone sizes. However, very wide ranges of the parameter grading were never investigated and a specific analysis in this direction constitutes the main significance of this study. The Delft University of Technology provided the laboratory facilities to carry out physical model tests on a wide graded rubble mound structure representative of the core of a breakwater. The parameter D85/D15, describing the stone-size gradation of the construction material, was varied between the values 2.71 and 17.7, and two different seaward slopes of the model structure were also tested. The reshaped cross-shore profiles measured during the tests showed how if the grading increases the stability of the structure is reduced. This is not always in accordance with the findings of previous researchers, showing how the extrapolation of existing empirical formulas to structures with high values of the ratio D85/D15 do not give reliable results. Instead, the formulas given by van de Meer (1992) to estimate the whole reshaped profile of a dynamic slope predict with good agreement the shape of the measured profiles, although the physical model shows a larger horizontal extension of the displacements. This difference is governed by the grading, being more noticeable as this parameter increases. This result leads to the definition of new formulas, some of them being modifications of the ones given by van der Meer, to describe the geometry of a reshaped profile. The formulas, all including the parameter grading, are derived through curve fitting of the measured data. Also a formula for the direct estimation of the crest recession is given. As a final step, a simple numerical model is proposed in which the new formulas are implemented, constituting a quick way to assess the shape of a slope after a wave attack. As a suggestion for further utilization of the results of physical modeling, a brief comparison is also carried out between the output of the tests and the prediction of the numerical model XBeach (developed mainly at UNESCO-IHE). In conclusion, this research points out how the formulas provided by the technical literature are not reliable in representing the effects of a very wide stone-size gradation in the stability of a rubble mound structure. Physical model tests proved to be a suited way to investigate these effects, as the nature of the phenomena who play a role in the stability does not allow a simple analytical representation. The tests carried out within the present study lead to the implementation of a numerical model of practical use for engineers and contractors: further investigations through laboratory tests are recommended to validate and extend the findings of this study. Another proposed direction for further research is the comparison between the results of physical model tests and the output of numerical models.

An improved method of structure damage diagnosis for jacket platforms

In the exploitation of ocean oil and gas, many offshore structures may be damaged due to the severe environment, so an effective method of diagnosing structural damage is urgently needed to locate the damage and evaluate its severity. Genetic algorithms have become some of the most important global optimization tools and been widely used in many fields in recent years because of their simple operation and strong robustness. Based on the natural frequencies and mode shapes of the structure, the damage diagnosis of a jacket offshore platform is attributed to an optimization problem and studied by using a genetic algorithm. According to the principle that the structural stiffness of a certain direction can be greatly affected only when the brace bar in the corresponding direction is damaged, an improved objective function was proposed in this paper targeting measurement noise and the characteristics of modal identification for offshore platforms. This function can be used as fitness function of a genetic algorithm, and both numerical simulation and physical model test results show that the improved method may locate the structural damage and evaluate the severity of a jacket offshore platform satisfactorily while improving the robustness of evolutionary searching and the reliability of damage diagnosis.

Analysis on Variable Bond Strength Failure Criterion of Layered Rock Mass Based on Numerical Simulation and Model-Test

The layered rock mass consists of kinds of stratifications whose mechanical properties are not wholly identical with each other. In order to figure out its strength failure criterion, the variable bond strength failure criterion is proposed depending on Mohr-Coulomb yield criterion and the change law of the strength parameters varying with the dip angle of stratification plane (that is the angle between the maximum principal stress and the stratification plane). What’s more, the criterion is verified by physical model test and numerical simulation adopting assembly language VC++6.0. Compared with the results of physical model tests and numerical simulation, it is shown that the stress distributions and failure regions are elliptic in shape and that the maximum failure regions are vertical to the stratification planes. That the results obtained in the physical model test are compatible with those numerically obtained verifies the correctness of variable bond strength failure criterion.

PHYSICAL MODEL EXPERIMENTS ON RECLAMATION PROJECT OF FANGCHENG PORT

Tidal flat reclamation works is an effective way to relieve the contradiction between supply and demand of land in the coastal region. The reclamation project in the bay will decrease tidal current velocities, change tidal currents field, then slowdown the water exchange in the bay ,which will influence water quality in the bay and increase sediment deposit at navigation channels. To study impacts of hydrodynamic environmental and sediment movement due to reclamation projects in Fangcheng Port, the physical model for tide-current and sediment was constructed in NHRI. The horizontal scale and vertical scale of the Fangcheng Port model is 1/660 and 1/100 respectively. The siltation in the bay and channels mainly caused by suspended load transport under the action of tidal currents and waves.The ability of water exchange in the bay is discussed by simulating sewage diffusion characteristics before and after reclamation projects in the bay. The physical model test results are shown the total tidal volume is decreased by 20% after the reclamation plan, of which 17.5% reduction in the East Bay, 2.5% in West Bay. The tidal currents field in the channel are discussed. The sedimentation rate increases 3-10cm/a in navigation channels and basins of Fangcheng Port.

Horizontal pressure on barricades for backfilled stopes. Part I: Fully drained conditions

Stope backfilling is used in underground mines to provide a safer working space for personnel and machinery. To ensure a secure application of backfill, it is critical to accurately evaluate the pressures exerted on the walls and barricades placed near the base of the stope to retain the fill materials. In this paper, existing solutions for evaluating the stresses on barricades are reviewed. The authors then propose a new three-dimensional analytical solution that takes into account the stress transfer from the backfill to the walls in both the stope and access drift. In particular, the solution considers the influence of the barricade position within the drift on the stress state. Comparisons are made between the proposed solution, physical model test results, and numerical simulations. The good agreement between various results indicates that the proposed formulation provides a good estimate of the pressure on a barricade. The solution presented here is applicable to fully drained backfill (without water-pore pressure); the solution for saturated backfill is given in a companion paper (see Part II, this issue).

Mobilisation of Earth Pressure Acting on Pile Caps Under Cyclic Loading

ABSTRACT In lateral resistance of piles with a pile cap, marked contribution of the pile cap resistance can be expected. For seismic performance assessment of pile foundations, mobilisation of the earth pressure acting on pile caps, induced by interactions between the pile cap and surrounding soils, has to be properly considered. In this study, a series of centrifuge model tests were conducted (1) to examine the effect of strain history on the mobilization of lateral earth pressure acting on pile caps and (2) to show the importance of considering strain history when modelling the interaction between a surface soil layer and a pile cap. Observations into the physical model tests reveal that mobilisation of the earth pressure acting on pile caps under cyclic loading can drastically change depending on soil type and/or conditions. Especially, relocation of soil adjacent to the pile cap in unload-reload phase plays an important role for the earth pressure mobilisation, as it completely alters the shape of the earth pressure-displacement curves. Based on the physical model test results, a simple empirical model that can be used for the beam on non-linear Winkler foundation type analysis is proposed and compared to the test results.

Analysis of sand slopes stabilized with facing of soil bags with extended reinforcement strips

A limit-equilibrium-based stability analysis method was developed to simulate the stability of unreinforced sand slopes and geosynthetic-reinforced sand slopes that had been stabilized with soil bags stacked on the slope face. The stability of unreinforced and reinforced sand slopes when loaded with a rigid strip footing placed on the slope crest was evaluated by performing physical model tests. The stabilizing effects of an inclined stacked soil bag facing with extended reinforcement strips (tails) were confirmed. The results of limit-equilibrium analyses showed good agreement with the physical model test results. A parametric study performed using the proposed method also revealed that the effects of the soil bag inclination angle, αb, and internal friction angle of the backfill soil, ϕs, on the slope stability of the unreinforced and reinforced sand slopes are significant. The ultimate footing loads observed in five physical model tests on reinforced and unreinforced slopes can be simulated by the proposed analysis method when using a single ‘operational’ friction angle (ϕop) that is 82 to 86% of the peak internal friction angle obtained from conventional drained plane strain compression tests (namely, ϕp) under low pressure levels as in the unloaded model slope. This ratio of ϕop/ϕp is consistent with the value found in stability analyses of reinforced and unreinforced shallow sand slopes reported in the literature.

A Comparison of Empirical, Semiempirical, and Numerical Wave Overtopping Models

Abstract Output is compared from four methods used to estimate the overtopping rate at seawalls subject to random wave action: two empirical models, a semiempirical model, and a numerical model. The empirical models were developed by fitting dimensionless groups to data derived from physical model tests. The semiempirical model was derived from consideration of the unsteady flow of water over a weir. However, like the empirical models, it was calibrated with the results of physical model tests. In contrast, the numerical model AMAZON is a high-resolution two-dimensional finite volume model based on the nonlinear shallow-water equations. In this study, we calculated the mean overtopping discharge for a range of seawalls with front slopes from 1 : 1 to 1 : 20 and for incident wave steepnesses from 0.01 to 0.03. The results are considered alongside four sets of data from physical model tests. They show general agreement between the output from the numerical and semiempirical models and the data. Agreement wi...

Theoretical Research on Hydrodynamics of a Geometric Spar in Frequency- and Time-Domains

Considering the coupling effects of the vessel and its riser and mooring system, hydrodynamic analyses of a geometric spar were performed both in frequency- and time-domains. Based on the boundary element method, the 3-D panel model of the geometric spar and the related free water surface model were established, and the first-order and second-order difference-frequency wave loads and other hydrodynamic coefficients were calculated. Frequency domain analysis of the motion Response Amplitude Operators (RAO) and Quadratic Transfer Functions (QTF) and time domain analysis of the response series and spectra in an extreme wave condition were conducted for the coupled system with the mooring lines and risers involved. These analyses were further validated by the physical model test results.

Modelling of submerged vanes

An overview is given of the research activities carried out on submerged vanes at WL∣ Delft Hydraulics. These activities are concentrated on the development of a morphological simulation system capable to simulate the effect of submerged vanes on an alluvial bed. The results of the simulations are shown and compare reasonably well with experimental results. Furthermore, physical model test results are presented and discussed in relation to the mathematical model. The measurement outcomes suggest a revision of the theoretical base of the model. Most experiments concern tests in a flume with a fixed bed and one or more submerged vanes. The measurements concern the flow velocity mostly downstream of the vanes and the lift and drag forces on the vane

Ground settlement and wall deformation from a large scale model test on a single strutted sheet pile wall in sand

The ground settlement and wall deformation of a sheet pile wall during different stages of excavation have been studied using a large scale model test in a bin with dry sand at the Norwegian University of Science and Technology, NTNU. The model sheet pile wall was built prior to raining in the sand in a bin (4 m × 4 m in plan and 3 m deep). This paper deals with results from tests on a relatively loose sand with 43.2% porosity. Strain gauges, load cells and potentiometers were used to monitor the wall movement, the bending moments in the wall, the strut loads and the ground settlements. The large scale model test results were compared with finite element simulations based on an elastoplastic soil model calibrated only against conventional oedometer and triaxial test results performed on the sand used in the model tests. Discrepancies between the physical model test results and the simulations are discussed trying to identify the cause of the discrepancy when a discrepancy may have been observed and suggest improved procedures for parameter determination. However, in conclusion, the comparison generally shows good agreement, and illustrates how a finite element analysis can be used to predict the behaviour of a supporting wall.

Interaction behaviours between parallel tunnels in soft ground

This paper describes a study of the influence of shield tunnel construction on the displacements and stresses induced in the linings of existing nearby parallel tunnels. The paper presents a brief review of a set of laboratory scale model test research programme investigated the influence of tunnel proximity and alignment, liner stiffness on the nature of the interactions between closely spaced tunnels in clay. A total of two sets of carefully controlled physical model tests were performed. A cylindrical test tank was developed and used to produce clay samples of Speswhite kaolin. In each of the tests, three model tunnels were installed in order to conduct two interaction tests that have been carried out to investigate the interaction problem between parallel tunnels. The results of these tests are compared with the results of finite element analysis to investigate the techniques that must be used to obtain reliable numerical solutions to this type of problem. The results presented here are limited to the comparison between the results of physical model test and the corresponding finite element back-analysis. The results of the physical model tests generally indicate that interaction effects are greatest on the pillar springline and crown of the instrumented tunnel. It was also generally observed that the total stresses acting on the instrumented tunnel decreased immediately after a non-instrumented tunnel had been installed. These stresses tended to increase in the long term as pore-pressure dissipation occurred. These results also indicate that significant bending moments were induced in the instrumented tunnel by installation of the new tunnels. In both cases, the numerical and model test data agree well. It is also found that the closer new tunnels are constructed to an existing tunnel, the larger additional moment is developed at the pillar springline of the existing tunnel liner. The physical model tests show that for closely spaced tunnels the distortions and moments of the liner may be important. The values of the bending moments induced in the liners appear to be particularly significant. It is shown that interaction between tunnels is unlikely to be significant unless the spacing between the tunnel centres is less than about two tunnel diameters. Elastic finite element analysis with a hydrostatic gap modelling procedure gives results that appear to be in good agreement with the results of the physical model tests. Based on the results of finite element analyses performed to verify experimental procedure, it is suggested that the procedures adopted in the model tests of using one clay sample to carry out two interaction experiments is reasonable. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.

Stability assessment of the Three-Gorges Dam foundation, China, using physical and numerical modeling—Part I: physical model tests

Abstract Sliding in a dam foundation along potential sliding paths is generally caused by two kinds of external factors: one is the overloading of the designed upstream hydrostatic load due to flooding; and the other is the gradual degradation of the shear strength of joints due to seepage, deformation, damage, and geochemical reactions between water and joint surface minerals. Based on the conceptualized geomechanical model of the Three-Gorges Dam, described in the Part I paper, in this Part II paper the limit equilibrium method and finite element method are used to study the effects of gradual degradation of the shear strength of joints on the stability of the dam foundation. The numerical modeling focuses on the stability conditions of the no. 3 powerhouse-dam section which are estimated to be the most critical. The constraint influences from the adjacent no. 2 and no. 4 powerhouse-dam sections are also included. The failure mechanisms, factors of safety and critical displacements of these dam sections are derived numerically as the measures for stability evaluation. The factor of safety is defined as the ratio between the combined shear strength of joints and intact rock bridges, and the mean shear stress along potential sliding path required for limit equilibrium under the designed external loads. All the results obtained from these different numerical models, together with the results of physical model tests as presented in Part I, are compared in this paper. The comparisons show that both the numerical modeling and physical modeling results support each other and demonstrate the stability of the Three-Gorges Dam foundation as designed. Nevertheless, considering the overall engineering and social–economical importance of the Three-Gorges Dam complex, some additional treatment and reinforcement measures are recommended in this paper.

The viscoplastic behaviour of a geogrid-reinforced model wall

The paper deals with the time-dependent behaviour of retaining walls reinforced with polypropylene geogrids and subjected to increasing loads applied to the soil surface behind the wall. The experimental programme involved fully automatic small-scale 1g physical models of a geogrid-reinforced wall. Long-term model tests were carried out using different plate loading rates in order to investigate the time-dependent response of the reinforced walls. The paper presents details of the physical model, testing procedure and principal test results. An analytical interpretation of the time-dependent viscoplastic behaviour using a simple three-parameter time-rate equation is shown to fit the experimental data, and is then proposed to predict the long-term displacements of the reinforced walls.

The viscoplastic behaviour of a geogrid-reinforced model wall

The paper deals with the time-dependent behaviour of retaining walls reinforced with polypropylene geogrids and subjected to increasing loads applied to the soil surface behind the wall. The experimental programme involved fully automatic small-scale 1g physical models of a geogrid-reinforced wall. Long-term model tests were carried out using different plate loading rates in order to investigate the time-dependent response of the reinforced walls. The paper presents details of the physical model, testing procedure and principal test results. An analytical interpretation of the time-dependent viscoplastic behaviour using a simple three-parameter time-rate equation is shown to fit the experimental data, and is then proposed to predict the long-term displacements of the reinforced walls.

Model Tests and Fem Simulation of Some Factors Affecting the Bearing Capacity of a Footing on Sand

A series of model tests was carried out on a particular type of sand (Toyoura sand) with a target void ratio of 0.66 to study into the effects of the shape and depth of a footing and the load inclination on the bearing capacity of a rough and rigid footing on sand. The test results were numerically simulated by two-and three-dimensional FEM analysis using a constitutive model that has been developed for Toyoura sand based on the results from an extensive series of laboratory stress-strain tests. The effects of the following factors, among others, on the strength and deformation characteristics of sand are taken into account; a) pressure level; b) the intermediate principal stress; c) inherent anisotropy; d) non-linear strain-hardening and softening; e) non-associated dilatancy; and f) strain localisation into a shear band(s). The material model is coupled with an isotropically hardening, non-associated, elasto-plastic material description. The FEM simulation is consistent with the physical model test results. In some important aspects, the results from both the physical model tests and FEM simulation are not consistent with some of the previous proposals that have been widely used in practice: i.e., in the results of the present study, the bearing capacity of a rectangular footing is noticeably larger than that under purely plane strain conditions, and negative effects of load inclination and positive effects of footing depth on the bearing capacity are generally larger than those predicted by some previous proposals.

Numerical Simulation of Bearing Capacity Characteristics of Strip Footing on Sand

This study aims at numerically simulating the bearing capacity characteristics of strip footing on sand, and consequently explaining the scale effects observed in a series of plane strain model tests carried out on a particular type of sand (Toyoura sand). The model tests were performed using different sizes of footing, with the largest width being 50 cm, under normal gravity and in a centrifuge. A constitutive model developed for Toyoura sand based on the results from an extensive series of plane strain compression tests is used. A number of factors which affect the strength and deformation of sand are taken into account, including; 1) confining pressure; 2) anisotropy; 3) non-linear strainhardening and strain-softening; 4) dilatancy; and 5) strain localization into a shear band(s). This material model is coupled with an isotropically hardening, non-associated, elasto-plastic material description. A widely used numerical technique, FEM, is applied to solve the non-linear equations. A parametric study is performed to evaluate the effects of using different assumptions for the material model. The simulation of bearing capacity compares well with the physical model test results. It is shown that the isotropic perfectly plastic modelling of soil property, assumed in most of the classical bearing capacity theories, is an overly simplified approximation to be used in FEM analysis of this issue. It is explained that the scale effect consists of the pressure level effect and the particle size effect.

A study of gate-type wave attenuators

A new type of gate structures for harbour entrances is proposed in this paper. The principal objective of the proposed structures is provide protection from incident wave fromthe outer harbour in rough condition, thus creating a inner area of calm water for the mooring of small boats. The bottom hinged flap type was proposed as one of the typical structures and the side hinged double flaps type was proposed for shallow water for not be an obstacle of boat passage. It was evaluated the wave attenuation characteristics and the forces from wave loading of the structures by numerical analysis comparing with physical model test results.

Displacement discontinuities and stress changes between roof strata and their influence on longwall mining under aquifers

The results of physical model tests are analysed with special attention to the development of displacement discontinuities (bed separations) at the two stages of roof stratum movement over a longwall coal face. The relationships between roof displacement and stress concentration or relief are explored. The stress-displacement relationship is further examined by an analytical model, based on the assumption of laminated rock beams, which is used to identify possible displacement discontinuities in the roof strata which would form water inflow passages from an ovelying aquifer. Also demonstrated is the potential of the analytical model for serving as a theoretical approach or, at least, a complement to empirical methods in estimating the critical and the safe distance of mining under an aquifer.