Showing papers on "Embedment published in 2012"

Sand Deformation around an Uplift Plate Anchor

Abstract: This paper presents an experimental investigation on soil deformation around uplift plate anchors in sand by using digital image correlation (DIC). The experimental setup consists of a camera, loading frame, plexiglass mold, and computer, which is developed to capture soil deformation during anchor uplifting. A series of model tests are performed to investigate the influence of particle size, soil density, and anchor embedment depth on soil deformation. A set of images captured during anchor uplifting are used to calculate soil displacement fields by DIC. The failure surface is studied by tracking the points with maximum shear strain values. On the basis of this study, it is found that soil deformation and the pullout resistance of plate anchors are substantially influenced by soil density and anchor embedment depth, whereas particle size within the studied range has limited influence. In dense sand, the shape of the failure surface changes from a truncated cone above a shallow anchor to a combine...

The effects of penetration rate and strain softening on the vertical penetration resistance of seabed pipelines

Abstract: Offshore pipelines in deep water are generally laid directly on the seabed, without any additional stabilisation measures. Design parameters that determine the soil resistance to lateral and axial motion of the pipeline are a function of the amount of vertical embedment. However, this latter quantity is difficult to estimate, partly because of the effects of soil heave around the pipeline as it penetrates, and partly because the soil shear strength depends on the strain rate and the degree of softening as the soil is sheared and remoulded. In this paper, a large deformation finite-element approach was adopted to study pipe–soil interaction during vertical embedment of pipelines on the seabed. The simple Tresca soil model was modified to incorporate the combined effects of strain rate and softening. The present large deformation finite-element method was validated by comparing the results with data from centrifuge model tests. A parametric study was then performed, varying the strain rate and softening par...

A plasticity model to assess the keying of plate anchors

Abstract: Suction-embedded plate anchors (SEPLAs) have been developed to answer the growing need for anchors to withstand significant vertical loading. The concept combines the advantage of suction caissons (known penetration depth and location) and ‘drag-embedded' plate anchors (efficiency and low cost). The main issue associated with SEPLAs relates to the keying process, as the anchor is first loaded, and the associated loss of embedment and reduction in capacity. The paper presents a plasticity model developed to predict the trajectory and load development during anchor keying, and up to peak load. Rigid plasticity is assumed, allowing the kinematics of the anchor to be determined from a yield surface and associated plastic potential. The trajectory and performance of a typical SEPLA are predicted using the model, and are compared with results from centrifuge tests and large-deformation finite-element analysis. The anchor loss of embedment ranged from ∼0·2 to 1·5 times the anchor height for loading inclinations ...

Base Capacity of Open-Ended Steel Pipe Piles in Sand

Abstract: This paper presents a new method for estimating the base capacity of open-ended steel pipe piles in sand, a difficult problem involving great uncertainty in pile foundation design. The method, referred to as the Hong Kong University (HKU) method, is based on the cone penetration test (CPT), and takes into consideration the mechanisms of annulus and plug resistance mobilization. In this method the annulus resistance is properly linked to the ratio of the pile length to the diameter—a key factor reflecting the influence of pile embedment—whereas the plug resistance is related to the plug length ratio, which reflects the degree of soil plugging in a practical yet rational way. The cone tip resistance is averaged over a zone in the vicinity of the pile base by taking into account the failure mechanism of the piles in sand, the condition of pile embedment (i.e., full or partial embedment), and the effect of soil compressibility. The predictive performance of the new method is assessed against a number ...

Numerical simulations of pipe–soil interaction during large lateral movements on clay

Abstract: The soil resistance during large lateral movements of pipelines across the seabed is an important input to design solutions for the management of thermal and pressure-induced expansions. To investigate this behaviour, a large-deformation finite-element (LDFE) analysis method involving frequent remeshing was employed. The LDFE method allows the changing geometry of the seabed when disturbed by the pipeline to be incorporated. Also, the effects of strain rate and strain-softening on the undrained shear strength of the soil were accounted for. A back-analysis of a centrifuge modelling simulation was first performed, for validation, and then a parametric study varying the pipe weight and initial embedment was undertaken. The results show that a steady state is generally reached at large displacements, reflecting a balance between the growth of a soil berm ahead of the pipe and the softening of the disturbed soil. The initial breakout response matched well with previously established failure envelopes, and a n...

Behavior of suction embedded plate anchors during keying process.

Abstract: Suction embedded plate anchors (SEPLAs) allow for accurate positioning, thus providing an attractive alternative to traditional drag embedment anchors. This paper presents an analytical model for predicting the behavior of SEPLAs during the keying process, with a specific focus on predicting the loss of embedment depth as the anchor rotates from its initial vertical position to its target orientation perpendicular to the direction of loading. The soil is idealized as an incompressible, rigid-plastic material obeying an associated-flow rule. A generalized plastic limit analysis is employed to estimate the trajectories and corresponding capacities of SEPLAs under different loading conditions. The effects of soil resistance on the shank and anchor interaction with the anchor chain are also considered in the model. The SEPLA design commonly features a hinged flap; the effect of the flap is examined theoretically by comparing the solutions for the SEPLA with and without flap. Predicted solutions are shown in r...

Modeling Hysteretic Behavior of Wood Shear Walls with a Protocol-Independent Nail Connection Algorithm

Abstract: This paper presents an extension to an algorithm called HYST to develop the hysteresis characteristics of a nail connection. The paper also discusses the implementation of the algorithm in a finite-element model of a wood shear wall, called WALL2D, to study the hysteretic wall response. The HYST algorithm is a protocol-independent and mechanics-based procedure that considers the nail shank as steel beam elements and the wood embedment medium as compression-only spring elements smeared along the nail shank. By accounting for the stiffness degradation of the wood embedment medium under cyclic loading, HYST can fully address strength/stiffness degradation and the pinching effect in the hysteresis of typical nail connections. HYST was verified by the load-slip hystereses from nail connections tested with two different loading protocols. The WALL2D application model consists of linear elastic beam elements for framing members, orthotropic plate elements for sheathing panels, linear springs for framing connecti...

Ultimate bearing capacity of shallow strip foundation under eccentrically inclined load, Part II

Abstract: Results of laboratory model tests to determine the ultimate bearing capacity of shallow strip foundation supported by sand and subjected to eccentrically inclined load are presented. The tests were conducted in dense and medium dense sands. The embedment ratio (ratio of the depth of embedment Df to the width of the foundation B) was varied from zero to one. The load eccentricity e was varied from zero to 0.15B as well as the load inclination from zero to 20 degrees. Based on the results of the present study, an empirical nondimensional reduction factor has been developed. This reduction factor is the ratio of the ultimate bearing capacity of the foundation subjected to an eccentrically inclined load to the ultimate bearing capacity of the foundation subjected to a centric vertical load.

Bond of reinforcement in self-compacting steel-fibre-reinforced concrete

Abstract: Crack control, one of the main benefits of using fibre reinforcement, depends to a large extent on the concrete-rebar bond. Pull-out tests of specimens with short embedment length were carried out and the results showed no effect from the fibres on the normalised bond-slip behaviour before peak load. After this, the fibre reinforcement provided extra confinement, changing the failure mode from splitting to pull-out failure. The test results were used to calibrate a finite-element bond model that considers both tangential stresses and stresses in the radial direction from the rebar. Splitting cracks may be thus considered in the finite-element analyses. The model proved to yield results in good agreement with the experimental results regarding failure mode, load-slip relation and splitting strains on the surfaces of the pull-out specimens. The analyses revealed that two types of action were active in the cracking process. In addition, the confinement effect of the fibre reinforcement was compared with the confinement of conventional stirrups using the bond model in CEB-FIP model code 2010.

The effect of fatigue cracks on fastener flexibility, load distribution, and fatigue crack growth

Abstract: : Fatigue cracks typically occur at stress risers such as geometry changes and holes. This type of failure has serious safety and economic repercussions affecting structures such as aircraft. The need to prevent catastrophic failure due to fatigue cracks and other discontinuities has led to durability and damage tolerant methodologies influencing the design of aircraft structures. Holes in a plate or sheet filled with a fastener are common fatigue critical locations in aircraft structure requiring damage tolerance analysis (DTA). Often, the fastener is transferring load which leads to a loading condition involving both far-field stresses such as tension and bending, and localized bearing at the hole. The difference between the bearing stress and the tensile field at the hole is known as load transfer. The ratio of load transfer as well as the magnitude of the stresses plays a significant part in how quickly a crack will progress to failure. Unfortunately, the determination of load transfer in a complex joint is far from trivial. Many methods exist in the open literature regarding the analysis of splices, doublers and attachment joints to determine individual fastener loads. These methods work well for static analyses but greater refinement is needed for crack growth analysis. The first fastener in a splice or joint is typically the most critical but different fastener flexibility equations will all give different results. The constraint of the fastener head and shop end, along with the type of fastener, affects the stiffness or flexibility of the fastener. This in turn will determine the load that the fastener will transfer within a given fastener pattern. However, current methods do not account for the change in flexibility at a fastener as the crack develops. It is put forth that a crack does indeed reduce the stiffness of a fastener by changing its constraint, thus lessening the load transfer.

Upper Bound Solution for Pullout Capacity of Vertical Anchors in Sand Using Finite Elements and Limit Analysis

Abstract: The horizontal pullout capacity of vertical anchors embedded in sand has been determined by using an upper bound theorem of the limit analysis in combination with finite elements. The numerical results are presented in nondimensional form to determine the pullout resistance for various combinations of embedment ratio of the anchor (H/B), internal friction angle (ϕ) of sand, and the anchor-soil interface friction angle (δ). The pullout resistance increases with increases in the values of embedment ratio, friction angle of sand and anchor-soil interface friction angle. As compared to earlier reported solutions in literature, the present solution provides a better upper bound on the ultimate collapse load.

Beam tests of composite steel-concrete members: A three-dimensional finite element model

Abstract: Modern construction makes frequent use of composite steel-concrete beams for bridge and building applications. This paper describes a three-dimensional finite element model in which all components forming the composite member are modelled by means of solid elements. The proposed approach is developed using the commercial software Abaqus and is able to model the composite response without requiring information from push-out tests commonly performed to define the constitutive relationship for the shear connectors. All materials are assumed to behave in a nonlinear fashion. Contact between the elements is simulated using surface-to-surface and embedment techniques. The adequacy and accuracy of the proposed modelling approach are validated against experimental results available in the literature on simply-supported and continuous beam tests with both solid and composite slabs.

Unusual Surface Mechanical Properties of Poly(α-methylstyrene): Surface Softening and Stiffening at Different Temperatures

Abstract: We report results from experiments based on the spontaneous particle embedment technique to determine the surface compliance of poly(α-methylstyrene) (PαMS) for temperatures from room temperature t...

Field observations of as-laid pipeline embedment in carbonate sediments

Abstract: Reliable prediction of the embedment of untrenched subsea pipelines is of increasing importance as hydrocarbon developments progress into deeper waters, located further from shore. Pipeline design issues such as hydrodynamic stability, lateral buckling and axial walking require accurate assessment of the pipe embedment, in order to assess correctly the pipe–soil resistance forces and the thermal insulation provided by the soil. This study presents a detailed back-analysis of the laying process and the as-laid condition of a pipeline on carbonate sediments. The pipe embedment is linked to the relevant soil properties, metocean conditions, vessel motions, and lay geometry along the route. A cycle-by-cycle framework is proposed for the development of embedment as the pipe is subjected to oscillations during laying. The calculations use parameters obtained from standard in situ tests, and are applied across a range of soil and lay conditions along this particular pipeline route. The proposed calculation frame...

Determination of Embedment Depth of Timber Poles and Piles Using Wavelet Transform

Abstract: This paper presents an investigation on the wave propagation in timber poles with Wavelet Transform (WT) analysis for identification of the condition and underground depth of embedded timber poles in service. Most of non-destructive testing (NDT) applications for timber poles using wave-based methods consider only single wave mode and no dispersion. However, for wave propagations in timber poles (damaged/undamaged), such simplification may not be correct, especially for broad band excitation using impulse impact. To investigate the problem, a 5m timber pole was investigated numerically and experimentally. A dispersion curve is generated from the numerical results to provide guidance on the velocity and wave mode selection. Continuous wavelet transform (CWT) is applied on the same signal to verify the presence of modes and to process data from experimental testing. The results are presented in both time domain and time-frequency domain for comparison. The results of the investigation showed that, wavelet transform analysis can be a reliable signal processing tool for NDT in terms of condition and embedment length determination.

Pullout Capacity of a Vertical Plate Anchor Embedded in Cohesion-less Soil

Abstract: In this paper, the ultimate pullout capacity of a shallow laid vertical plate strip anchor in cohesion-less soil is analyzed with the consideration of active and passive states of limit equilibrium in the soil. Kotter’s equation is used to compute the active and passive thrusts, which are subsequently used in the analysis in which, all the equation of equilibrium are properly interpreted. The unique failure surfaces under active and passive states of limit equilibrium are identified on the basis of force equilibrium conditions. One distinguishing feature of the proposed method is its ability to compute the point of application of active/passive thrust using moment equilibrium. Another distinguishing feature is the prediction of distribution of soil reactions on the failure surface. Comparison of the results of the proposed method with the available experimental results vis-a-vis other theoretical methods shows that, up-to embedment ratio of 3.0, the proposed method is capable of making reasonably good predictions.

Micromechanical constitutive models forcementitious composite materials

Abstract: A micromechanical constitutive model for concrete is proposed in which microcrack initiation, in the interfacial transition zone between aggregate particles and cement matrix, is governed by an exterior-point Eshelby solution. The model assumes a two-phase elastic composite, derived from an Eshelby solution and the Mori-Tanaka homogenization method, to which circular microcracks are added. A multi-component rough crack contact model is employed to simulate normal and shear behaviour of rough microcrack surfaces. It is shown, based on numerical predictions of uniaxial, biaxial and triaxial behaviour that the model captures key characteristics of concrete behaviour. An important aspect of the approach taken in this work is the adherence to a mechanistic modelling philosophy. In this regard the model is distinctly more rigorously mechanistic than its more phenomenological predecessors. Following this philosophy, a new more comprehensive crack-plane model is described which could be applied to crack-planes in the above model. In this model the crack surface is idealised as a series of conical teeth and corresponding recesses of variable height and slope. Based on this geometrical characterization, an effective contact function is derived to relate the contact stresses on the sides of the teeth to the net crack-plane stresses. Plastic embedment and frictional sliding are simulated using a local plasticity model in which the plastic surfaces are expressed in terms of the contact surface function. Numerical simulations of several direct shear tests indicate a good performance of the model. The incorporation of this crack-plane model in the overall constitutive model is the next step in the development of the latter. Computational aspects such as contact related numerical instability and accuracy of spherical integration rules employed in the constitutive model are also discussed. A smoothed contact state function is proposed to remove spurious contact chatter behaviour at a constitutive level. Finally, an initial assessment of the performance of the micromechanical model when implemented in a finite element program is presented. This evaluation clearly demonstrates the capability of the proposed model to simulate the behaviour of plain and reinforced concrete structural elements as well as demonstrating the potential of the micromechanical approach to achieve a robust and comprehensive model for concrete.

Analytical Prediction of Pipeline Behaviors in J-Lay on Plastic Seabed

Abstract: J-lay is widely accepted as a favorable method for deepwater pipeline installation. The configuration and internal force of the pipeline during the laying process are key factors for its safety, and the embedment induced during pipeline installation plays an important role for pipeline stability in service. Traditionally, the seabed in the analysis of J-lay is assumed to be elastic, whereas deepwater deposits are usually very soft, exhibiting low strength and obvious plasticity. This paper presents an analytical model for pipelaying on plastic seabed. This model simplifies the pipeline as the combination of four segments: a natural catenary in water, a boundary-layer segment, a beam on the seabed (touchdown zone) where the soil deforms plastically, and a freely laid horizontal segment. The comparison with a traditional elastic-seabed model reveals that both models have similar pipeline configurations, although the elastic-seabed model slightly underestimates the maximum bending moment. In contrast, the pipeline embedment depths in the two models differ with deeper ultimate penetration predicted in the plastic-seabed model.

Seismic interference of two nearby horizontal strip anchors in layered soil

Abstract: In the present analysis, an attempt is made to explore the seismic response of two nearby horizontal strip anchors embedded in non-homogenous c-ϕ soil deposit at different depths. The analysis is performed by using two-dimensional finite-element software PLAXIS 2D. Each anchor carries equal static safe-working load without violating the ultimate uplift capacity under static condition. The soil is assumed to obey the Mohr–Coulomb failure criterion. The behavior of single isolated anchor subjected to an earthquake loading is determined first to study the interference effect between two anchors. The horizontal acceleration response obtained from the Loma Prieta Gilroy Earthquake (1989) is considered as the input excitation in the analysis. A parametric study is performed by varying the clear spacing (S) between the anchors at different embedment ratios (λ). The magnitude of vertical displacement, shear stress, and shear strain developed at different locations of the failure domain is determined for different clear spacings between the anchors.

Insight to failure mechanisms of caisson foundations under combined loading: a macro-element approach

Abstract: The undrained response of massive caisson foundations to combined horizontal, vertical and moment loading is investigated through a series of three-dimensional finite element analyses. The ultimate limit states are presented by failure envelopes in normalized form. The effects of embedment ratio and vertical load on the bearing capacity are investigated in detail. Closed-form expressions are proposed for (a) the yield surface in M-Q-N space, and (b) the lateral capacity under pure horizontal and moment loading. Finally, the use of an associated flow rule to define the plastic deformation of the caisson at near failure conditions is parametrically examined. The results of the analysis could serve as a basis for a macro-element approach.