Evaluation of the Remoulded Shear Strength of Offshore Clays and Application to Pipeline-Soil and Riser-Soil Interaction
Quantifying the sensitivity of fine grained sediments is an important aspect of site characterisation. The sensitivity gives an indication of the likely effects of pre-peak cyclic loading, while the remoulded undrained shear strength is directly relevant
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Centre for Offshore Foundation Systems, University of Western Australia, Australia
Introduction
Quantifying the sensitivity of fine grained sediments is an important aspect of site characterisation. The sensitivity gives an indication of the likely effects of pre-peak cyclic loading, while the remoulded undrained shear strength is directly relevant for various offshore applications (see examples in Fig. 1). It is customary to assume that the shaft friction acting during the installation of piles, suction caissons or anchors may be taken as the remoulded undrained shear strength (Semple and Gemeinhardt, 1981). Equally the cyclic bearing resistance of laterally loaded piles or pipelines and catenary riser systems may be expressed in terms of the remoulded shear strength of the seabed sediments at shallow depth. A further example is for the operative strength of material in debris resulting from a submarine slide, which may also be estimated directly from the remoulded shear strength. The implicit assumption is that the water content of the sediments are not changed during the particular design application (which may not be true in some cases), so that the remoulding merely destroys any bonding and other natural structure of the material. Natural soils show rate-dependent shear strength and also a reduction in strength as they are sheared and remoulded. Different design applications involve varying degrees of these factors, and it is useful to consider a schematic ‘map’ of soil tests and design applications (Fig. 2) in respect of strain rates and degree of soil remoulding involved in the application (Randolph et al., 2007). C. di Prisco et al. (eds.), Mechanical Behaviour of Soils under Environmentally Induced Cyclic Loads © CISM, Udine 2012
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M. Randolph
Figure 1. Example applications of remoulded shear strength.
The remoulded undrained shear strength (su,rem ) may be estimated from indirect measurements such as by means of field penetrometer tests, by direct measurement such as in a field vane test or laboratory element test on remoulded material, or by correlation with the intact shear strength (su ) by means of a sensitivity (St ). However, because of strain rates that may vary by several orders of magnitude between different forms of soil test and different applications, the use of a (fixed) sensitivity can be very misleading.
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Measurement of Remoulded Shear Strength
The most common methods of estimating the remoulded shear strength of fine-grained sediments (i.e. those where the water content will typically remain constant during the remoulding process) include: 1. unconsolidated undrained (UU) triaxial tests on remoulded material; 2. field or laboratory vane shear tests, carried out to sufficiently large rotations of the vane; 3. fall cone tests on remoulded material; and 4. sleeve friction (fs ) from cone penetration tests. More recently, full-flow (T-bar and ball) penetrometers have become to be used widely in the offshore industry, particularly at the shallow depths relevant for pipeline design and for shal
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