Abstract
Prior to any drilling operations, spudcan foundations supporting jack-up legs are routinely preloaded through augmenting the weight of the rig by ballasting the hull. One of the major geohazards related to spudcan installation is the potential for punch-through failure, i.e. uncontrolled rapid leg penetration due to the reduction of soil bearing capacity. This is a general concern for sites where stratified seabed comprises a surface or interbedded strong layer overlying a soft layer, in particular with the move towards heavier rigs and deeper waters.
In order to avoid unexpected punch-through failure, accurate rather than conservative estimate of spudcan penetration resistance profile is required. However, current design guidelines ISO standard 19905-1 recommend assessing the spudcan penetration resistance by using a framework of conservative bearing capacity formulations, without taking into account the true soil failure mechanisms associated with spudcan penetration in multi-layer soils. The suggested ‘bottom-up approach’ combines the methods developed for wished-in-place footings in single layer and two-layer soils (i.e. squeezing for weak-over-strong layering system and punch-through for the reverse), neglecting the influence of continuous spudcan penetration and trapped soil plug; and in a strong-weak-strong layering system, the effect of the 3rd layer on the bearing capacity in the 1st layer cannot be captured appropriately. It is a two-step approach in which soil strength parameters are derived from the site specific soil investigation data for use in bearing capacity models. Alternatively, for deeper water sites with the difficulty in obtaining high-quality soil samples, the idea of correlating the spudcan penetration resistance directly with the results from the in-situ cone penetration test (CPT) is increasingly being considered. Thus far, correlations have been established only for single layer soils.
The motivation for this study emanated directly from the ‘future needs’ identified by the latest version of ISO standard 19905-1. The thesis presents the research on the bearing response of spudcan foundation in multi-layer soils with the potential for punch-through. Large deformation finite element (LDFE) methods were employed. The prime objective was to develop rational and accurate design approaches for assessing spudcan penetration resistance in multi-layer soils. The proposed design approaches can be divided into two categories: (i) mechanism-based design approach with spudcan penetration resistance calculated using soil parameters extracted from site investigation data; and (ii) CPT-based design approach with spudcan penetration resistance calculated directly from in-situ cone penetrometer tip resistance profile.
Four configurations of stratified deposit were considered, including (i) two-layer stiff-over-soft clay, (ii) three-layer non-uniform clay with an interbedded stiff clay layer, (iii) three-layer uniform stiff-soft-stiff clay, and (iv) clay-sand-clay deposits with and without a 4th layer stiff clay. Clay layer was simulated using an elastic-perfectly plastic Tresca soil model extended for strain softening and rate dependency of the undrained shear strength, while sand layer was modelled using a modified Mohr-Coulomb model. A number of analyses were performed with the aim of validating the numerical models against existing data from centrifuge tests and case histories. Overall, satisfactory agreement was obtained between the computed results and measured data, confirming the capability and accuracy of the numerical models.
Parametric studies were then performed for spudcan and cone penetration to create a database for the development of new mechanism-based and CPT-based design approaches that rectify the deficiencies of the existing design methods. The new mechanism-based design approaches account for the true soil failure mechanisms, and strain softening and rate dependency of the undrained shear strength. Design formulas were proposed to estimate the evolution of the soil plug height during spudcan penetration and the corresponding influence on punch-through and squeezing. Accordingly, CPT-based design approach was proposed by establishing direct correlations between the penetration resistances of spudcan and cone for each configuration of soil profile. In addition, adjustment factors were proposed for improving the ISO suggested design methods and the design methods recently proposed by other researchers.
The predicted profiles using the proposed design approaches were compared with the data from centrifuge tests and case histories. The ISO bottom-up approach was also adopted for comparison. Predictions using the new approaches were found to be in good agreement with measured load-penetration profiles, while under most circumstances the ISO bottom-up approach provided conservative estimation for the bearing capacity and overestimation for the depth of triggering squeezing.
In order to avoid unexpected punch-through failure, accurate rather than conservative estimate of spudcan penetration resistance profile is required. However, current design guidelines ISO standard 19905-1 recommend assessing the spudcan penetration resistance by using a framework of conservative bearing capacity formulations, without taking into account the true soil failure mechanisms associated with spudcan penetration in multi-layer soils. The suggested ‘bottom-up approach’ combines the methods developed for wished-in-place footings in single layer and two-layer soils (i.e. squeezing for weak-over-strong layering system and punch-through for the reverse), neglecting the influence of continuous spudcan penetration and trapped soil plug; and in a strong-weak-strong layering system, the effect of the 3rd layer on the bearing capacity in the 1st layer cannot be captured appropriately. It is a two-step approach in which soil strength parameters are derived from the site specific soil investigation data for use in bearing capacity models. Alternatively, for deeper water sites with the difficulty in obtaining high-quality soil samples, the idea of correlating the spudcan penetration resistance directly with the results from the in-situ cone penetration test (CPT) is increasingly being considered. Thus far, correlations have been established only for single layer soils.
The motivation for this study emanated directly from the ‘future needs’ identified by the latest version of ISO standard 19905-1. The thesis presents the research on the bearing response of spudcan foundation in multi-layer soils with the potential for punch-through. Large deformation finite element (LDFE) methods were employed. The prime objective was to develop rational and accurate design approaches for assessing spudcan penetration resistance in multi-layer soils. The proposed design approaches can be divided into two categories: (i) mechanism-based design approach with spudcan penetration resistance calculated using soil parameters extracted from site investigation data; and (ii) CPT-based design approach with spudcan penetration resistance calculated directly from in-situ cone penetrometer tip resistance profile.
Four configurations of stratified deposit were considered, including (i) two-layer stiff-over-soft clay, (ii) three-layer non-uniform clay with an interbedded stiff clay layer, (iii) three-layer uniform stiff-soft-stiff clay, and (iv) clay-sand-clay deposits with and without a 4th layer stiff clay. Clay layer was simulated using an elastic-perfectly plastic Tresca soil model extended for strain softening and rate dependency of the undrained shear strength, while sand layer was modelled using a modified Mohr-Coulomb model. A number of analyses were performed with the aim of validating the numerical models against existing data from centrifuge tests and case histories. Overall, satisfactory agreement was obtained between the computed results and measured data, confirming the capability and accuracy of the numerical models.
Parametric studies were then performed for spudcan and cone penetration to create a database for the development of new mechanism-based and CPT-based design approaches that rectify the deficiencies of the existing design methods. The new mechanism-based design approaches account for the true soil failure mechanisms, and strain softening and rate dependency of the undrained shear strength. Design formulas were proposed to estimate the evolution of the soil plug height during spudcan penetration and the corresponding influence on punch-through and squeezing. Accordingly, CPT-based design approach was proposed by establishing direct correlations between the penetration resistances of spudcan and cone for each configuration of soil profile. In addition, adjustment factors were proposed for improving the ISO suggested design methods and the design methods recently proposed by other researchers.
The predicted profiles using the proposed design approaches were compared with the data from centrifuge tests and case histories. The ISO bottom-up approach was also adopted for comparison. Predictions using the new approaches were found to be in good agreement with measured load-penetration profiles, while under most circumstances the ISO bottom-up approach provided conservative estimation for the bearing capacity and overestimation for the depth of triggering squeezing.
| Original language | English |
|---|---|
| Qualification | Doctor of Philosophy |
| Publication status | Unpublished - Jul 2015 |