Reliable estimates for the maximum available uplift resistance from the backfill soil are essential to prevent upheaval buckling of buried pipelines. The current design code DNV RP F110 does not offer guidance on how to predict the uplift resistance when the cover:pipe diameter (H/D) ratio is less than 2. Hence the current industry practice is to discount the shear contribution from uplift resitance for design scenarios with H/D ratios less than 1. The necessity of this extra conservatism is assessed through a series of full-scale and centrifuge tests, 21 in total, at the Schofield Centre, University of Cambridge. Backfill types include saturated loose sand, saturated dense sand and dry gravel. Data revealed that the Vertical Slip Surface Model remains applicable for design scenarios in loose sand, dense sand and gravel with H/D ratios less than 1, and that there is no evidence that the contribution from shear should be ignored at these low H/D ratios. For uplift events in gravel, the shear component seems reliable if the cover is more than 1-2 times the average particle size (D 50 ), and more research effort is currenty being carried out to verify this conclusion. Strain analysis from the Particle Image Velocimetry (PIV) technique proves that the Vertical Slip Surface Model is a good representation of the true uplift deformation mechanism in loose sand at H/D ratios between 0.5 and 3.5. At very low H/D ratios (H/D < 0.5), the deformation mechanism is more wedge-like, but the increased contribution from soil weight is likely to be compensated by the reduced shear contributions. Hence the design equation based on the Vertical Slip Surface Model still produces good estimates for the maximum available uplift resistance. The evolution of shear strain field from PIV analysis provides useful insight into how uplift resistance is mobilized as the uplift event progresses.

Upheaval buckling (UHB) is a common design issue for high temperature buried pipelines. This paper highlights some of the key issues affecting out-of-straightness (OOS) assessment of pipelines. The following factors are discussed; uplift resistance soil models, uplift resistance in cohesive soils, uplift mobilisation, ratcheting, uplift resistance at low H/D ratios and the correct methodology for load factor selection. A framework for determining ratcheting mobilisation is proposed. Further research is required to verify and validate this proposed framework. UHB assessment of three different diameter pipelines were carried out using finite element SAGE PROFILE package incorporating pipeline mobilisation and the results are compared with semi-analytical formulation proposed by Palmer et al. 1990. The paper also presents a summary of as-laid pipeline features based on projects over the past 10 years.

Geotechnical survey and the resulting soil classification is one of thefundamental design inputs for any subsea structure or pipeline design. Yet, details of soil classification and its limitations for predicting soilbehaviour under various scenarios are not fully understood by pipeline designengineers. As soil classification is often used by pipeline engineers topredict pipesoil interaction behaviour for a given scenario, lack offundamental understanding of soil classification often leads to problems laterin projects. This paper aims to provide some soil mechanics fundamentals topipeline engineers. This paper presents a comprehensive summary of how soilclassification is carried out based on commonly used standards; ASTM D- 2487,BS 5930 and ISO 14688. The paper highlights the fundamental limitations in theclassification systems and shows how the use of these different standards canresult in different soil classification for very similar soils. The paperbrings out an important point that the soil behaviour in a given application isnot always in accordance with its soil classification. Examples such asploughability assessment results and pipeline on-bottom stability assessmentresults are highlighted to show that when particle size distribution falls near the classification boundary ofcoarse/fine soils, then soil classification alone may not fully capture thesoil behaviour for particular aspects of design and operation. Introduction Seabed soil classification is a key step in any offshore project. The soilclassification is then used by the pipeline design engineers to assignappropriate design parameters for soil/structure interaction and also topredict soil behaviour (soil resistance, soil deformations) under variousoperations such as piling, ploughing, jetting etc. Thus understanding thefundamentals of soil classification is vital for pipeline designengineers. Generally, soil behaviour is categorised as " drained" or " undrained". Soilbehaviour depends on the rate of loading (i.e. the rate at which force isapplied to the soil). If the rate of loading is greater than the rate at whichpore water (water that is present in the inter-particle voids) is able to movein or out of soil inter-particle voids, then the soil is said to behave in anundrained manner. The volume change of the soil is zero, and the behaviour ofthe soil is independent of inter-particle forces. If the rate of loading isslower than the rate at which pore water is able to move in or out of soilinter-particle voids, the soil is said to behave in a drained manner. In summary, whether a soil (sand or clay) behaves in a drained manner orundrained manner, depends on the rate of loading with respect to thepermeability of the soil. CLAY behaviour is commonly considered to beundrained, because the rate of loading is usually much greater than the rate atwhich pore water can move in or out of inter-particle voids (i.e. thepermeability of CLAY is very low ~10–9m/s). Hence, the strength of CLAY isgiven as " undrained shear strength", denoted by symbol Su or Cu, and measuredin kilopascals (kPa). SAND behaviour is commonly considered drained, becausepore water can move in or out of inter-particle space at a greater rate thanthe rate of loading. Hence, the SAND strength is given in terms of frictionangle using the symbol f. It is to be noted that if CLAY is sheared at a veryslow rate (~ 0.001 mm/min), such that enough time is allowed for the pore waterto move in or out of the inter-particle voids, then it will not exhibitundrained shear strength. Instead, it will behave more like sand withapplicable clay friction angle. Similarly, if SAND is sheared at a very fastrate, such that the pore water does not have enough time to move around, thenSAND can exhibit undrained behaviour.

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The Heathrow expansion project proposals comprise major works around Heathrow airport to allow the construction of new terminal buildings and the third runway. Owing to the critical nature of uninterrupted operation of both runway and the M25 motorway, the preliminary design had to be developed to minimise maintenance operations. At over 140 m in the total length, the adoption of an integral bridge of this length is in excess of most integral bridges in the UK, particularly where full-height abutments are being utilised. The maximum predicted expansion length also lies outside the limit equilibrium method set out in PD6694-1. As such, it was agreed that a full soil–structure interaction study should be carried out to assess the impacts of the structural behaviour of the bridge and its abutments. This paper looks at the calibration work for that study, the derivation of the earth pressures behind the walls, the behaviour of the abutment and how this compares with the predictions set out in PD 6694-1.