Estimation of spudcan penetration resistance in stratified soils from field piezocone penetrometer data

Stefanus Safinus

    Research output: ThesisDoctoral Thesis

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    Abstract

    Over the past 20 years, emerging offshore exploration into new regions with complex soil conditions has driven the need for a quick and reliable site-specific assessment method for ‘mobile’ jack up installation and preloading. As a result, recently jack-up rigs integrated with piezocone penetrometers and on-board assessment systems have been considered. This integrated system allows in situ site investigation to be carried out at the precise location, and estimate spudcan penetration resistance profile just before preloading operation. However, to achieve this, an automated method that can interpret piezocone penetration test (CPTu) data and calculate a load penetration curve in complex multilayer soil is required.
    This thesis proposes a new unified analysis method for spudcan penetration in general multilayer soil. The method uses discrete soil layers to handle high soil variability. Components of penetration resistance are calculated from the discrete layers and then integrated to obtain the total resistance. In contrast to the conventional methods that use the ‘wished-in-place footing’ assumption, the proposed unified method considers the penetration process chronologically to capture the evolution of the soil profile. The analysis method consists of three main steps: (i) identification of the failure configuration, (ii) calculation of the penetration resistance, and (iii) updating of the soil layer.
    Accurate estimation of spudcan penetration resistance depends strongly on a proper consideration of the soil plug. In two-layer soils with a distinct strength variation, the bearing (or plug) base may be located either directly beneath the spudcan or at the surface of the underlying layer, whichever provides the lowest resistance. A similar principle is applicable to highly layered soils. The key to finding the ultimate failure configuration is a systematic comparison of the potential penetration resistances for different positions of the bearing base, starting from the spudcan base and incrementally progressing downward. A simplified model (with a cylindrical plug) is used for this purpose. Once the bearing base is identified, an accurate analysis model (i.e., the geometry of the plug) is employed to calculate the penetration resistance of the ultimate failure configuration.
    The input parameters (soil layering and soil parameters) for spudcan analysis are interpreted directly from the CPTu data. To identify the layer boundaries from the CPTu data, an improved multivariate statistical method is proposed. Soil types and relevant soil parameters for each identified layer are then estimated using the existing soil classification charts and (semi-) empirical correlations, respectively.
    As an alternative to calculating the spudcan penetration resistance from the soil strength parameters, a direct CPTu to spudcan correlation approach can be used. The possibility of combining a direct correlation approach with the unified method is briefly discussed.
    This unified method was calibrated and validated against a high-quality centrifuge test data and case histories of actual jack up installation. Penetration tests in layered soils with up to four layers of siliceous and calcareous sediments were conducted at the UWA geotechnical centrifuge facility. Existing experimental data from the literature were also collated. Field data consisting of CPTu data and spudcan penetration profiles were obtained from the InSafeJIP database and Keppel Offshore and Marine Ltd.
    The proposed unified method improves the current ISO guidelines 19905-1 that provides design methods for penetration into single- and two-layer sediments and suggests using a bottom-up approach for multilayer deposits. Subjective selection of the failure mechanism prior to analysis is no longer necessary, because the mechanism is identified automatically during an analysis and better estimation accuracy is achieved through the consideration of a soil plug.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2016

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