Load-Penetration Behavior Prediction of Spudcan Foundations in Multi- Layered Clayey Soils

نوع: Type: thesis

مقطع: Segment: PHD

عنوان: Title: Load-Penetration Behavior Prediction of Spudcan Foundations in Multi- Layered Clayey Soils

ارائه دهنده: Provider: Hojat Mehralizadeh

اساتید راهنما: Supervisors: Dr. Masoud Makarchian

اساتید مشاور: Advisory Professors:

اساتید ممتحن یا داور: Examining professors or referees: ِDr. Abolfazl Eslami, Dr. Kazem Fakharian and Dr. Abbas Qadimi تاریخ و زمان ارائه

زمان و تاریخ ارائه: Time and date of presentation: 2021/10/10- 8:00 am

مکان ارائه: Place of presentation: Online

چکیده: Abstract: Using the jack-up drilling rigs is common for temporary and permanent drilling in the semi-deep seabed (up to 150 meters). The installation of these rigs into the seabed was performed by a single foundation called spudcan. The spudcan geometry has changed over the years, but today the spudcans have a conical shape and a tip to create a point for safer and faster penetration. New geometries, such as skirted spudcan, are under study and development. The soil profile is the most important factor affecting the bearing capacity of the spudcan during installation and operation. A soil layer on one or more weak layers affects the behavior of the spudcan in that layer. If the soil layer is located on a weaker layer (layers), during the punch-through failure, the penetration of the spudcan may experience hazards such as rapid and uncontrollable penetration, inclination, or failure of the legs, and finally, the collape of the jack-up at the installation site. In addition, a soil layer on a stronger layer may lead to no penetration of the legs, lateral slide, and finally damage to the jack-up platform, or adjacent structures. Because the jack-up rigs may experience different profiles in their lifetime, it is necessary to estimate the bearing capacity of the spudcan and determine the appropriate penetration depth for safe operation. Most of the previous research and standard codes related to the installation and operation of jack-up rigs is idealized the soil conditions and convert the multi-layered soils into double-layered which leads to a difference in the results in reality and the penetration plan. Also, there is little research on the different geometries of spudcan and the presentation of optimal geometry. In this thesis attempted to analyze the bearing capacity-penetration behavior of spudcan by neglecting the usual simplifications and using the results of physical and numerical modeling of spudcan penetration in single, double, and three-layered clay soils. In addition, considering the effects of layer's interaction and important parameters affecting the bearing capacity of spudcans. Finally, using the modeling results and assistance of mathematics is presented simple and practical prediction equations for the use of marine structural engineers. In addition to conventional geometry of spudcan, other geometries such as skirted and reverse spudcan have been studied in this research. Also, the penetration of CPT cone as one of the methods to determine the undrained bearing capacity of soil layers has been investigated in this research. The results of this thesis showed that cement-treated clay can be used as a stronger layer in the physical modeling of spudcan penetration in clay, but the reduction in bearing capacity factors in layers with cement should be considered in multi-layered soils in subsequent analyses. The results of the penetration of CPT cones in the present study showed that decreasing the value of PI due to the addition of cement decrease the cone factor. Also, there is a negligible difference between the results of cone resistance with different diameters. Therefore, cones with smaller diameters can be used in physical modeling with good reliability. The modeling results of spudcans with different geometries showed that reverse and skirted geometries had more bearing capacity than conventional spudcans. For example, in physical modeling, skirted and reverse spudcans had 7% and 14% more bearing capacity than conventional spudcan, respectively. This increase in bearing capacity was also observed in the numerical modeling results, which were 4% and 8% for skirted and reverse spudcans, respectively. Also, reverse and skirted spudcans had the most peak resistance in the stranger layer and the most resistance drop after the peak resistance (punch-through potential). The shape of the bearing capacity curve in the soil layer located on one or more other layers depends on the thickness and the shear strength parameters of that layer and the underlying layers. If the layer is located on a weaker layer, its bearing capacity curve is similar to a line, which can be estimated by determining two points (including the peak resistance and the resistance at the layer's interface). It is possible to estimate this curve with three points if the mentioned layer is located on a stronger layer (including initial resistance, resistance at the depth of 0.6H and the resistance at the layer's interface). In this study, correlations have been proposed to predict this curve in multi-layered clay soils. The results showed that the prediction correlations have been successful in estimating the bearing capacity curve of spudcan in various types of multi-layered clay profiles and single-layered clays with linearly increased undrained shear strength in depth.

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