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Behavior of marine silty sand subjected to long term cyclic loading
Comportement du sable limoneux marin soumis à une charge cyclique de longue durée
Safdar M., Kim J.M.
Pusan National University, Busan, South Korea
ABSTRACT: The foundations for offshore wind turbines are demanding due to the dynamic nature of the offshore loading. A greater
understanding of the behavior of wind turbine foundation soil, will certainly lead to the stable construction of foundations which in
turn, will make offshore wind farms a more feasible part of the solution to the global energy problem. This paper presents the results
of cyclic direct simple shear test (CDSS) to explain the long term cyclic behavior of marine silty sand. Cyclic behavior of marine sand
are based on the number of loading cycles, cyclic shear strain amplitude, relative density, and cyclic stress ratio. These results are
modeled and can be applied to design offshore wind turbine foundations.
RÉSUMÉ : Les fondations pour les éoliennes offshore sont principalement exigeante en raison de la nature dynamique du chargement
offshore. Une meilleure compréhension du comportement de l'éolienne des sols de fondation, va certainement conduire à la
construction des fondations stables qui à leur tour, feront de parcs éoliens en mer un rôle plus possible de la solution au problème
mondial de l'énergie. Ce document présente les résultats d'essai de cisaillement cyclique directe simple (CDSS) pour expliquer le
comportement cyclique à long terme de sable limoneux marin. Comportement cyclique de sable marin sont basés sur le nombre de
cycles de charge, cyclique d'amplitude de contrainte au cisaillement, la densité relative et du taux de contrainte cyclique. Ces résultats
sont modélisés et peut être appliquée à la conception fondations d'éoliennes off-shore.
KEYWORDS:
Cyclic Loading, Offshore Wind Turbine, CDSS, Cyclic Stress Ratio
1 INTRODUCTION
Understanding the behavior of offshore marine sand subjected
to long term cyclic loading is very vital in solving several
offshore geotechnical problems. Several researchers have
studied behavior of clay and sand subjected to cyclic loading.
(Vucetic et al. 1988) studied the degradation of marine clays
under cyclic loading. (D. Wijewickreme et al. 2005) studied the
cyclic loading response of loose air-pluviated Fraser river sand.
(K.H. Andersen 2009) investigated in detail, the bearing
capacity of the soil under cyclic loading, and stated that the
cyclic shear strength and the failure mode under cyclic loading
depend on the stress path and the combination of average and
cyclic shear stresses. Safdar et al., 2013, studied the cyclic
behavior of marine silty sand subjected to symmetrical cyclic
loading. Different approaches have been made as an attempt to
include cyclic loading in the design procedure of offshore wind
turbine foundation (Soren et al. 2012).
1.1
Stress controlled CDSS test
Constant volume direct simple shear (DSS) test is a reliable
method for measuring undrained shear strength of undisturbed
or compacted soil samples. The DSS test is most similar to the
CU triaxial test in that samples are consolidated prior to
shearing. The simple shear is the test condition that only normal
and shear stress acting on top face of a specimen is defined,
whereas the displacement constraints exist for the other
boundaries: The bottom face of specimen is theoretically fixed,
and the radial strain on specimen is zero.
The CDSS test procedure is based on that of a constant-
volume direct simple shear testing of soils, which has been
studied extensively for half a century and is described in the
standard ASTM D6528-07. The sample is consolidated under a
normal load within a wire-reinforced membrane (in this study)
or a stack of thin rings that provide lateral confinement.
Once consolidation is complete, a horizontal shear force is
applied to one end of the sample. The sample height is
continuously maintained during shear to ensure constant
volume. Rather than measuring pore pressures, which would
require complete saturation of the sample, the pore pressure
response is inferred from the change in vertical stress which is
monitored throughout the test (Baxter et al 2010). In this way
changes in applied vertical stress (
Δ
σ
v
), which are required to
keep the sample height constant, are assumed to be equal to the
excess pore water pressure (
Δ
u) that would develop if the test
were truly undrained with pore pressure measurements (Finn,
1985, Dyvik et al. 1987).
Figure 1 Simple Shear Condition, (Dyvik et al 1987)
