Large Deformation Finite Element (LDFE) modelling is conducted to study the bearing capacity of large offshore foundations in limited clay depth. Complementary visualising centrifuge experiments are reported in clay with interbedded sand, correlating with the numerical study. The current squeezing methodology neglects the conical underpart of the spudcan and any possible deformation of the underlying layers and hence does not predict the measured resistance well. An alternate approach overcoming the limitations of the squeezing theory is presented and verified.

Introduction

Offshore jackup drilling rigs are often supported by a quasicircular or sometimes polygonal foundation with a conical underpart commonly referred to as a spudcan. The jackup generally operates in shallow-to-medium water depth (up to ~150 m). Seabed stratigraphies in medium water depths can be layered, consisting of several alternate layers of sand and clay (Baglioni et al., 1982; Kostelnik et al., 2007; Dutt and Ingram, 1984). Limited knowledge is currently available for foundation installation in more than two-layer soil stratigraphies. This could be due to the inherent difficulties in physically modelling more than two-layer stratigraphies directly in the geotechnical centrifuge due to possible boundary effects (Ullah et al., 2014; Ullah et al., 2016). Some centrifuge tests mimicking several offshore soil deposits were recently reported in Hossain (2014) for soil stratigraphies up to six layers.

In the absence of detailed analytical methods, solutions developed initially for two-layer stratigraphies are recommended for more general multi-layer stratigraphies (ISO, 2012). This extended application requires additional assumptions that are often not realistic and require further investigation. The International Organization of Standardization (ISO) guidelines suggest that the bearing capacity calculation for soft clay over a strong soil layer (such as sand or stiff clay) should proceed first by calculating the soil resistance from the available single-layer solutions, such as those of Skempton (1951) or Houlsby and Martin (2003), until the depth of transition (dt) is reached. (See Fig. 1 where ID is the relative density and φcv is the constant volume friction angle.) dt refers to the depth measured from the top of the sand layer to the transitional point on the load-penetration curve where the transition from a near linear uniform clay-type response occurs.

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