Kevin J. L. Stone

This paper presents a physical model study of the feasibility and performance of a monopiled gravity base structure (MGBS) for offshore foundation applications. The research study builds on the current interest in hybrid foundation systems such as monopiled footings. The proposed MGBS is essentially a conventional gravity base structure (GBS) with a projecting monopile or caisson. The system relies on the self-weight of the GBS to drive the projecting monopile into the seabed. Once installed, additional ballast can be added to the GBS, and if necessary installation of the projecting monopile can be enhanced through the use of suction. The test programme has concentrated on the response of the system once installed and has investigated a range of geometries for circular monopiles. The results clearly demonstrate a significant enhancement to lateral resistance offered by the addition of a monopile to a GBS.

Offshore foundation systems are constantly evolving to meet the needs of developments in the energy sector. These developments may be induced by the requirements of moving into ever deeper water for hydrocarbon recovery or creating foundation systems for renewable energy sources such as offshore wind farms. One such approach is that foundation systems are developed that combine several foundation elements to create a hybrid system. In this way it may be possible to develop a foundation system that is more efficient for the combination of vertical and lateral loads associated with the offshore environment, and in particular wind-powered generators. This paper presents the results from a physical and numerical modeling program undertaken to investigate the performance of hybrid, monopiled footing foundations under combined monotonic loading conditions in sand.

Interaction of active seismic faults with oil well casings has been investigated to evaluate the interaction process and determine approaches to mitigate damage and deformation of the well casing. The methodology used was a 2-way approach consisting of (1) undertaking a series of unscaled physical model tests in a geotechnical centrifuge using on simplified geological analogues and (2) validation of 3D numerical models by verification and calibration to the experimental test results. Two type of geological scenarios are presented: an unconsolidated cohesionless deposit and a bedded rock mass deposit with 60° dip bedding. A basement discontinuities of displacement (normal faulting) and of slope and displacement (sloped base deformation) were introduced. The interaction with the propagating faults through the model was studied with post-experiment dissection and PIV analysis. These information were used to calibrate a numerical models and draw appropriate conclusions with regard to mitigation methods. Interaction of active seismic faults with oil well casings has been investigated to evaluate the interaction process and determine approaches to mitigate damage and deformation of the well casing. The methodology used was a 2-way approach consisting of undertaking a series of UN scaled physical model tests in a geotechnical centrifuge using on simplified geological analogues and validation of 3D numerical models by verification and calibration to the experimental test results. The oil well/fault interaction is typically a problem to be addressed over several hundred meters of the subsurface. The chapter covers the physical modelling aspect of this research project with minor highlights on the numerical simulation. Two criterions on the casing/fault interaction is investigated: the fault network density and the type of geological formation. Centrifuge modelling technique is used, providing more flexibility with the casing analogue material selection. The centrifuge experiment was performed at Schoffield Centre, University of Cambridge, using the 150 gT centrifuge.

This paper presents the results of a series of centrifuge model tests undertaken to evaluate the lateral response of circular piles fitted with fins. The geometry of the fins in terms of their length and orientation with regard to the direction of loading (i.e. projected area), and depth of embedment were investigated. The study was conducted at an enhanced acceleration level of 50 gravities in medium to dense uniformly graded dry sand. Only monotonic lateral loading was applied, and the results are presented through load versus displacement plots, from which lateral capacities are derived. The data set obtained from the tests is compared and appraised with data from other researchers and the influence of the fin geometry on the development of lateral capacity is discussed. This chapter presents a centrifuge model study on the effect of fin geometry on the performance of monopiles under monotonic lateral loading in dry sand. The geometry of the fins in terms of their length and orientation with regard to the direction of loading, and depth of embedment were investigated. All the tests reported were carried out on the balanced beam centrifuge at the University of Brighton. The model piles were driven into the soil approximately 55 mm from the edge of the container with light blows from a soft hammer. The model and actuator is set-up on the laboratory bench and then loaded onto the centrifuge. A simple approach to design is to consider the lateral load development as the sum of the lateral capacity of the pile plus an additional contribution from the fin. The extent of this contribution will be a function of the fin geometry and its location on the pile.

A long-term laboratory test programme of conventional compression and extension tests was carried out with test durations from 8 to 22-months, in a purpose built environmentally controlled facility, with specially designed loading frames and modified triaxial cells. In addition, Scanning Electron Microscope (SEM) techniques were employed in an effort to investigate the micro-mechanical response. Creep strains appeared to trigger an ageing process that produces elevated post-creep strength and stiffness irrespective of the applied stress path.

This paper reports the results of fall-cone tests and quasi-static cone penetration tests undertaken on mixtures of fine gravel and highly plastic clay. The influence of the gravel on the plasticity of the mixture is quantified through the derivation of liquid and plastic limits obtained by direct testing of the mixture. The plastic limit used in this study is defined as the moisture content corresponding to a hundredfold increase in the liquid limit undrained strength, and is denoted by PL₁₀₀. Liquid limits were determined from conventional fall-cone tests. The range of plasticity of the mixture is defined through the plasticity index, derived from the fall-cone liquid limit and the PL₁₀₀ index. The study demonstrates that the plasticity of mixtures of fine- and coarse-grained soil can be quantified without the requirement to remove the coarser fraction. The influence of the coarse fraction on both the undrained shear strength and the index properties was observed to be significant for gravel volume fractions greater than about 50%. This gravel volume fraction is associated with the maximum void ratio of the gravel, which suggests that interaction between the gravel particles at high gravel contents results in a frictional contribution to the mixture behaviour.

A simple conversion of a standard triaxial apparatus for stress path testing is presented and illustrated. The conversion is effected by incorporating a balanced ram arrangement into the top plate of a conventional triaxial cell, which, together with some additional modifications to the sample top cap and ram connection details, provides a cost-effective stress path testing cell. This modified triaxial cell, combined with a suitable external loading frame, allows the axial and radial stresses to be independently controlled. Consequently, any desired stress path can be applied to a specimen mounted in the modified cell. The functionality of the modified cell is demonstrated through the results of long-term stress-path tests performed as part of a creep study of intact medium porosity chalk. In this study, the modified cells were used with cantilever type loading frames, and subjected to continuous loading for periods in excess of 400 days. The performance of the modified cells was very satisfactory, and demonstrates that such an arrangement can provide a simple and a cost-effective system for conducting laboratory stress-path tests.

Centrifuge modeling of the 17th Street Canal and Orleans Canal North levees was performed in this study. During hurricane Katrina the levees on the 17th Street Canal failed, leading to breaches in the outfall canal in the city. Two mechanisms were observed in the centrifuge modeling that could cause a breach. First, a water-filled crack formed in front of the floodwall as the water in the canal rose above the top of the levee. The levees on the 17th Street Canal, which were supported on clay foundations, failed when this cracking led to a translational (sliding) failure in the clay layer commencing at the toe of the floodwall. The levees at Orleans Canal North, where failure did not occur, were also modeled to demonstrate that the model tests could successfully simulate failure and nonfailure conditions. The centrifuge model tests identified the importance of the crack formation in relation to the stability of the floodwall. These tests also confirmed that levee geometry, floodwall depth of penetration, a...

This paper describes the use of a soil minipenetrometer (SMP) to determine the strength and index properties of fine-grained soils. The SMP has been developed to allow both fall cone and quasi-static penetration tests to be performed. Displacement controlled quasi-static penetration tests can be used for the direct measurement of undrained shear strength, both for remolded and undisturbed samples. In addition the quasi-static penetration test can be used to define an additional lower plastic limit parameter, the PL100 , which represents the moisture content of a fine-grained soil with an undrained strength 100 times that at the liquid limit. This approach offers the advantage that removal of the coarse fraction is not required to estimate the PL100 .