1,721,001 research outputs found
Effect of foundation embedment on consolidation response
No full textEmbedment will generally reduce the magnitude and rate of consolidation settlement of shallow foundations due to the resistance of the soil above foundation level, the reduction in foundation load due to side friction and the longer drainage paths. Conversely, embedment provided by foundation skirts may increase the magnitude and rate of consolidation settlement due to additional one-dimensional compression within the soil plug. This paper presents an investigation into the consolidation response of surface and variously embedded foundations in an isotropic elastic half-space. The results show that embedment, the type of embedment and interface roughness can have a marked effect on consolidation response.</p
Optimal skirt spacing for subsea mudmats under loading in six degrees of freedom
Full text linkTwo- and three-dimensional finite element analyses are performed to identify the optimal internal skirt spacing for the maximum undrained capacity of subsea skirted mudmats. Fully three-dimensional loading (vertical, biaxial horizontal, biaxial moment and torsion) is considered for subsea mudmats with skirt embedment ranging from 5% to 20% of the foundation breadth in soil with a range of linearly increasing strength with depth. The results have identified the governing case for determining the optimal skirt spacing for mudmats subjected to fully three-dimensional loading. It is also shown that optimal skirt spacing for rectangular or square mudmats can be determined in plane strain conditions using the equivalent foundation embedment ratio. The number of internal skirts required to ensure soil plug rigidity under fully three-dimensional loading is presented as a function of skirt embedment ratio, soil heterogeneity index and vertical load mobilisation. Results also indicate that effects of skirt roughness become negligible as foundation embedment ratio increases in terms of determining the optimal skirt spacing.</p
Three-dimensional finite element analysis of combined loading of skirted foundations on non-homogeneous clay
No full textCombined loading of shallow foundations is particularly relevant in offshore engineering as wind and wave loading cause significant lateral and moment forces to the foundations of offshore structures. Research to date has addressed either combined loading of strip footings on non-homogeneous soils (e.g. Bransby & Randolph, 1998) or of circular footings on homogeneous soils (e.g. Taiebat & Carter, 2000). The present paper contributes towards this database by presenting the results of three-dimensional finite element analyses, which have addressed the issues of combined loading, soil strength non-homogeneity and three-dimensional foundation geometry integrated in single analyses. Attention is focused on the effect of the degree of strength non homogeneity on the size and shape of the failure loci under combined vertical, moment and horizontal loading.</p
Experimental study of uplift resistance of shallow skirted foundations in clay under transient and sustained concentric loading
No full textWind, wave and current forces cause significant overturning loads to be transmitted to foundation systems of fixed-bottom offshore structures, while buoyancy forces, in conjunction with environmental loading, impart direct tensile loads to foundations of floating facilities. Shallow skirted foundations are a potentially attractive foundation solution when overturning or tension loading is significant, as suctions mobilised within the soil plug provide tensile resistance. While passive suctions are maintained, undrained reverse end bearing will govern failure providing enhanced uplift resistance compared with vented pull-out of the foundation unit (with or without the soil plug), governed by skirt/soil friction. Currently uncertainty exists over the limiting ratio of skirt depth to foundation diameter to mobilise undrained reverse end bearing and the duration over which passive suctions can be sustained. This paper reports results from beam centrifuge tests investigating the response of shallow skirted foundations in lightly over-consolidated clay to concentric transient and sustained uplift.</p
A numerical study of the vertical bearing capacity of skirted foundations
No full textFinite element analysis is used to investigate the vertical bearing capacity of circular skirted foundations considering the effect of embedment ratio, foundation-soil interface roughness and soil strength heterogeneity. The effect of idealising a skirted foundation as a solid rigid plug and idealising geometry to conditions of plane strain are also addressed through comparison of bearing capacity factors and kinematic mechanisms accompanying failure. A closed-form expression is presented that enables prediction of bearing capacity factors for circular skirted foundations over a practical range of embedment ratio, skirt-soil interface roughness and soil strength heterogeneity, to within ±2.5% of the finite element calculations.</p
Design approach for rectangular mudmats under fully three-dimensional loading
Full text linkMudmats are used in the offshore oil and gas industry to support subsea infrastructure for pipeline terminations and well manifolds. Expansion and contraction of connected pipelines and jumpers due to changing thermal and pressure conditions impose fully three-dimensional loading on the foundations, dominated by horizontal, moment and torsional loading rather than high vertical loads. The mudmat foundations are rectangular, and include shallow skirts in order to increase capacity, particularly for sliding. Offshore design guidelines for shallow foundations tend to excessive conservatism; optimisation of mudmat capacity under general loading has thus become critical in order to keep foundation footprints within the limits of current installation vessels. The paper presents an alternative design method, based on failure envelopes derived from an extensive programme of three-dimensional finite-element analyses, focusing on the sliding and rotational capacity of the foundation. Starting from expressions that quantify the uniaxial capacity under each of the six degrees of freedom, failure envelope shapes for different biaxial combinations are developed. Ultimately, the allowable capacity under the six-degree-of-freedom loading is expressed in terms of a two-dimensional failure envelope for the resultant horizontal and moment loading, after due allowance for the vertical and torsional components of load.</p
Observations of shallow skirted foundations under transient and sustained uplift
No full textThis paper summarises results from a four-year experimental programme of centrifuge tests investigating installation resistance and uplift capacity of shallow skirted circular foundations in lightly over-consolidated clay. Centric and eccentric, monotonic, transient and sustained loading were investigated and the effect of gapping along the foundation-soil interface was explored. The results from this study showed that for a skirted foundation with embedment ratio d/D=0.3, reverse end bearing could be mobilised under transient uplift and loads of up to 50% of the undrained soil resistance could be sustained for months without significant displacement - provided nominal contact along the skirt-soil interface was maintained. The presence of gapping along the skirt-soil interface and eccentric loading reduced undrained capacity by up to 40%, and increased displacement rates under sustained uplift loading by up to a factor five.</p
A novel technique to mitigate the effect of gapping on the uplift capacity of offshore shallow foundations
Full text linkA flexible mat was provided around the periphery of skirted shallow foundations - a so-called 'gap arrester' - to assess the potential to mitigate the effect of gapping on uplift capacity. Results are presented from a series of drum centrifuge tests on skirted foundations with an intact skirt/soil interface, a gapped skirt/soil interface, and a gapped interface with gap arrester, subjected to undrained and sustained uplift. The results are promising, showing that the provision of an effective gap arrester preserves suction to larger foundation displacements and reduces the rate of displacement under sustained uplift, compared with the case of a gapped interface without arrester.</p
Centrifuge study of capacity of a skirted foundation under eccentric transient and sustained uplift
No full textEnvironmental and buoyancy forces impart overturning moments and potentially direct uplift to offshore foundation systems. Shallow skirted foundations are an attractive design solution, as negative excess pore pressures developed within the soil plug provide temporary uplift resistance. This paper presents the results of beam centrifuge tests on a shallow skirted foundation in a lightly overconsolidated clay under eccentric transient and sustained uplift. When compared with the response under concentric uplift, the experimental results show a reduction in capacity and an increase in the rate of degradation of capacity under sustained loading.</p
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