Finite element analysis of spudcan penetration in homogeneous and two layered soil deposits

Mobile jack-up units remain favourable in the offshore industry due to its capability to self-install and ability to work in moderate water depth. Prior to its mobilization to the site location, it is required to have a site-specific assessment (SSA). The SSA will identify any potential problems related to foundation conditions during installation. One of the important site specific assessments before deploying jack-up units is the preload check, which is carried out to predict the load-penetration response. The offshore industry has published the ‘Guidelines for the Site Specific Assessment of Mobile Jack-Up Units’ (SNAME 2008) and more recently, the International Standard Organization has published ISO 19905-1: 2012 in order to standardize jack-up assessment procedures. The guidelines adapt the framework used for onshore application following conventional bearing capacity theory to assess spudcan penetration depth. However, these guidelines are limited in discussing the approach in working in multi-layered soils. The conventional procedures described in the guidelines may not be sufficiently accurate since the methods cannot take a proper account of the nature of continuous spudcan penetration process. In practice, a layered system is commonly encountered and the installation process can be hazardous, with the potential of punch-through failure when the spudcan penetrates into strong over weak materials. A better understanding is therefore required. This thesis proposes that an analysis based on numerical modelling can be one possible alternative in evaluating spudcan bearing capacity in the layered system. This study presents the application of a finite element modelling approach, called Press-Replace (PR) Technique, which is based on a small strain geometry update procedure. This technique can be applied in any geotechnical software that is currently available for engineering practice. The PR Technique is employed to investigate the performance of penetrating spudcan foundations on homogeneous soil (sand and clay) and two layered soil deposits (sand overlying clay). The numerical method is firstly verified against previous experimental and numerical test data. A parametric study is also conducted to see the influence of normalized soil properties and geometry on the load penetration curves. Overall, the modelling approach used in the present study shows a good agreement, compared to other published results. The PR Technique shows its capability to simulate the penetrating spudcan foundations. In addition, several interesting findings are identified based on the parametric study: (i) the stress-level-effect and the prominent influence of dilatancy angle on the spudcan penetration in sand; (ii) the difference of soil flow mechanism in clay that leads to the attainment of the bearing capacity factor in deep penetration; and (iii) the onset of punch-through in double layered case that is highly determined by the sand thickness ratio. Lastly, some design charts are presented for all the investigated cases in the present study. These charts might be used to generate full spudcan bearing resistance-depth curves in sand, clay and sand overlying clay.

Subject

Spudcan
bearing resistance
load penetration response
layered soil deposits
punch-through
PR Technique
geometry update

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master thesis

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