Development of a combined VHM loading apparatus for a geotechnical drum centrifuge

Abstract

Foundations, especially those for offshore structures, are often subjected to combined vertical (V), horizontal (H) and moment (M) loading conditions. Experimental studies of foundation behaviour under complex combined loading are normally limited to laboratory testing at unit gravity. Only a few studies have been performed at enhanced gravities in a geotechnical centrifuge owing to difficulties in the design of actuators with independent control of movements in three degrees-of-freedom. In this paper, a combined VHM loading apparatus, with independent control of the in-plane vertical, horizontal and rotational movements is reported. Test results that measure the size and shape of the combined VHM yield surface of a spudcan footing of a mobile jack-up platform in soft clay illustrate the capabilities of the new apparatus.

Jack-up push-over analyses featuring a new force resultant model for spudcans in soft clay

“…Its ratio to the corresponding compressive capacity (V T /V 0 ) is denoted as χ. In this model, χ is assumed to be 0.6 based on centrifuge tests in kaolin clay (Zhang et al, 2013) when w p Z H c þ0.5D. This ratio is consistent with other experimental researches on extraction behavior of spudcan footing in soft clay (Purwana et al, 2005;Bienen et al, 2009a;Gaudin et al, 2011).…”

“…Employing a combined loading apparatus developed for a geotechnical drum centrifuge (Zhang et al, 2013), the VHM bearing capacity surface of a typical spudcan was investigated at various embedment depths (up to 1.45D) in normally consolidated kaolin clay in the enhanced gravity field of the centrifuge (Zhang et al, 2014a). To expand the experimental database for wider ranges of soil properties and embedment depths, a numerical approach with a large deformation finite element (LDFE) simulation of the spudcan installation and subsequent small strain finite element analyses to define the VHM bearing capacity surface was proposed (Zhang et al, 2014b).…”

A plasticity model for spudcan foundations in soft clay

“…Least squares regression analysis was performed for each complete set of compressive swipe tests at different depths. As the pure vertical tests indicate a tensile capacity ratio of 0.6, and indeed the best-fit values found by optimization are around 0.6 (Zhang et al 2013), the value is held at 0.6 for simplicity. Table 3 collates the optimized parameters for the three embedment depths and Fig.…”

“…The tests were performed with a combined VHM loading apparatus developed recently for the geotechnical drum centrifuge at The University of Western Australia (UWA) (Zhang et al 2013). The apparatus features independent displacement control of a model's in-plane vertical, horizontal, and rotational degrees of freedom.…”

“…The loading arm consisted of a strain-gauged inner arm (used to measure the V, H, and M forces acting solely on the spudcan and reported here at the LRP) and a stiff outer shield (that isolated the combined loads on the loading arm). A more detailed description of the spudcan, loading arm, and test set up are available in Zhang et al (2013).…”