Abstract

In the field of offshore geotechnical engineering, the bearing capacity evaluation of the foundation in layered deposits is a critical issue, due to the larger dimensions than those of onshore foundations in general. The slip circle method and the numerical simulation software FLAC are utilized to investigate the foundation failure modes and the associated bearing capacity factor. The results indicate that the collapse load of the footing on a two-layered cohesive soils would vary with the relative strength and the normalized layer thickness. Furthermore, a significant reduction in bearing factor might occur and the situation should be of particular concerned.

INTRODUCTION

There are several offshore wind farms under constructions currently in eastern Taiwan Strait, and it is expected to complete 5.5 GW capacity by 2025. According to the offshore geotechnical investigation data from the Taiwan Power Company (2018), it shows that the in-situ soil conditions of the offshore wind farm in the proposed locations are extremely complex, and the soil under the seabed often includes layered of sand, silt and clay, as shown in Fig. 1. Because the excess pore water pressure is induced for silts during shear, they could be regarded as cohesive soils in the analysis of foundation bearing capacity (Chi et al., 2019).

Brown and Meyerhof (1969) and SNAME (2008) indicated that there are at least three different foundation failure mechanisms in layered soils: I) general shear failure; II) squeezing; III) punch-through shear failure. If the soil conditions of adjacent strata do not vary significantly, the failure mechanism tends to general shear failure. While the foundation is placed on a softover-strong cohesive soil system, the second failure mechanism might occur. However, if a footing is rested on a strong-over-soft cohesive soil system, punch-through shear failure should be paid more attention because it may cause catastrophic disasters which a small additional foundation penetration could lead to a significant reduction in bearing capacity. Additionally, Osbourne and Paisley (2002) defined punch-through as "an unexpected jack up footing rapid settlement resulting in consequential loss in drilling time".

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