Seabed Soil Classification, Soil Behaviour, and Pipeline Design

Abstract

Abstract 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.