1,721,084 research outputs found
Consolidation beneath circular skirted foundations
No full textThe effect of foundation embedment on consolidation has not been considered previously in a systematic manner, although this is of particular interest for offshore foundations, where embedment is provided by skirts that enclose a compressible soil plug. For skirted foundations, critical uncertainties include what to assume in terms of the degree of drainage at skirt tip level, and the relative time scales of consolidation within the soil plug and beneath the foundation. In this paper, results from small strain finite-element analyses are used to quantify the immediate and time-dependent response of circular skirted foundations to uniaxial vertical loading. Foundations with frictionless and fully rough skirt-soil interfaces with varying ratio of embedment depth to foundation diameter are considered and the responses compared with those for surface foundations. The findings show that both skirt-soil interface roughness and embedment ratio have a significant effect on the consolidation response.</p
Coefficient of consolidation for soil - that elusive quantity
Full text linkAlthough it is accepted that the coefficient of consolidation for soil is not a true material property, but reflects the net effect of permeability and compressibility, it is a very useful parameter in day to day design. Design calculations make extensive use of elastic solutions for consolidation, such as beneath a shallow foundation or around a driven pile, but an important consideration is how to measure or estimate an appropriate coefficient of consolidation to use in those solutions. Typically the quantity is determined either from laboratory oedometer tests (generally then referred to as cv) or from field dissipation tests using a piezocone or piezoball penetrometer (generally then referred to as ch). Since the latter form of test includes a mix of stress paths, for some of which the soil has a stiffness associated with unloading and others of which involve plastic compression, the magnitude of ch for a given soil is typically 3 to 10 times the value of cv from virgin compression in laboratory oedometer tests. The paper explores the relationship between cv and ch for different boundary value problems, within the confines of soil modelled as Modified Cam Clay, for both isotropic and anisotropic permeability. Problems range among: simulated oedometer testing, field dissipation testing and pore pressure response beneath a shallow foundation. Results of finite element analysis of this range of problems are used to develop guidelines for different classes of problem, comparing the relevant coefficient of consolidation against a benchmark cv value associated with virgin compression in an oedometer. The normalised values of consolidation coefficient are expressed as functions of fundamental soil parameters used within Modified Cam Clay.</p
Parametric solution of lateral buckling of submarine pipelines
No full textLateral buckling analysis of on-bottom submarine pipelines is of particular interest in the offshore industry due to the complexities involved in the analysis, and the potential design efficiencies that can be unlocked. Classical buckling theories by previous researchers and recent joint industry projects provide a basis for estimation of the critical buckling load of a straight, or in some cases imperfect, pipe on either a rigid or elastic seabed. However, systematic solutions for the combined effects of nonlinear soil properties and the as-laid geometry – specifically the out-of-straightness – on the buckle initiation behaviour have not been developed previously. This paper reports an investigation of the buckling problem of an imperfect (non-straight) on-bottom pipeline subjected to axial compressive loading. The seabed was modelled with lateral and axial nonlinear, springs to idealise the load-displacement behaviour of the soil and the pipe was modelled with pipe elements. Buckling was performed by a displacement controlled finite element method with the modified Riks algorithm that is available in the commercial software ABAQUS. This numerical tool was used to develop a parametric solution for the present problem in terms of the various pipe material and geometry parameters and the lateral and axial pipe-soil interaction parameters. In particular, the influence of the magnitude and stiffness of the lateral pipe-soil response was investigated, highlighting the sensitivity of the pipeline response to the geotechnical inputs. The results have been synthesised in a generic non-dimensionalised design chart to estimate the buckling load, valid for the range of inputs covered by the parametric study.</p
Interaction forces between pipelines and submarine slides - A geotechnical viewpoint
No full textAssessment of interaction forces between deep water pipelines and potential submarine slides, debris flows and turbidity currents is an important aspect of geohazard studies. Historically, interaction forces have tended to be expressed in terms of drag factors, within a traditional fluid mechanics framework, with the drag factors depending strongly on an equivalent Reynolds number for the non-Newtonian debris material. Here, we have followed a more geotechnical approach, allowing the interaction forces to be expressed in terms of a strain-rate dependent shear strength of the debris material, and with the inclusion of a drag term (with fixed drag coefficient) for high velocity, low strength, combinations. This superposition approach treats separately the interaction forces that arise from the strain-rate dependent strength and the inertia of the debris, rather than combining them into a single drag force. A failure envelope is proposed, allowing axial and normal interaction forces to be estimated for any angle of attack of the debris flow.</p
Inclined pull-out capacity of suction caissons
No full textThis paper reports a finite element study of suction caissons in normally consolidated clay subject to inclined loading. The optimum load attachment position and the corresponding maximum load capacity for caissons with length to diameter ratios of 1.5, 3, and 5 and a range of shaft friction coefficient are reported. Limit states are presented in terms of vertical and horizontal load (V-H) interaction diagrams and a simple unique expression is proposed to describe the shape of the envelopes.</p
An image-based deformation measurement system for the geotechnical centrifuge
No full textNew techniques of image capture and analysis have been recently applied to geotechnical centrifuge modelling. These techniques improve the utility of the geotechnical centrifuge by increasing the detail and precision of deformation measurements. This paper describes the use of a simple image-based deformation measurement system on a drum centrifuge. Precise deformation fields comprising many thousands of measurement points can he evaluated from images of plane strain tests using small soil models typically comprising 2 litres of soil. Digital still photography is used to capture high resolution images. A weighted shutter allows a continuous sequence of images to he captured without the need for an onboard PC or communication with the camera via sliprings. Particle Image Velocimetry (PIV) and close range photogrammetry are used to process the resulting images. Target markers are not needed since PIV operates on the spatial variation in image brightness to measure displacement. Close range photogrammetry allows image distortion to be corrected. Distortion leads to a 5% variation in image scale (or pixel size) for this test geometry. An example test of a skirted strip foundation under eccentric vertical load is analysed. The instantaneous velocity field at failure is manipulated to allow the realism of simple upper bound failure mechanisms to be examined. It is interesting to note that although the measured failure load matches almost exactly the optimal upper hound solution, the observed deformation mechanism differs significantly
Undrained bearing capacity of square and rectangular footings
No full textThe uniaxial vertical bearing capacity of square and rectangular footings resting on homogeneous undrained clay is investigated with finite element analyses, using both Tresca and von Mises soil models. Results are compared with predictions from conventional bearing capacity theory and available analytical and numerical solutions. By calibrating the finite element results against known exact solutions, best estimates of bearing capacity for rough-based rectangular footings are derived, with the shape factor fitted by a simple quadratic function of the footing aspect ratio. For a square footing, the bearing capacity is approximately 5% lower than that based on Skempton's shape factor of 1.2.</p
Seabed characterisation and models for pipeline-soil interaction
No full textPipelines and flowlines represent major cost items in the development of deepwater fields. Accurate modelling of the axial and lateral pipe-soil resistance can lead to significant cost reductions by optimising design. Critical design issues include axial motion, or walking, of pipelines due to cyclic thermal changes, lateral buckling due to thermal expansion, and fatigue damage to risers in the touchdown region. Traditionally, interaction between a pipeline and the seabed has been simplified into frictional models for axial and lateral resistance during walking or buckling. Improving these models is a priority, but is hampered by difficulties in characterising the behaviour of very low strength, near-surface, seabed soils, and by a lack of detailed understanding of the soil mechanics of pipe-soil interaction. The cylindrical geometry of a pipeline invites comparison with the behaviour of tubular piles. Recent advances in pile design methods generated by considering the underlying soil mechanics indicate that the same potential exists for improving the understanding of pipeline behaviour. This paper describes recent advances in measuring the low shear strengths associated with near-surface seabed soils, using both in situ methods in the form of cylindrical (T-bar) and spherical penetrometers, and laboratory shear tests at very low effective stresses. The relationship between penetration resistance and the vertical and lateral resistance of pipelines is explored, taking account of the depth of burial and the cycles of movement. New approaches for assessing the axial and lateral resistance of on-bottom pipelines are described. Future trends and recent developments are summarised.</p
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