Undrained sliding resistance of shallow foundations subject to torsion

While the behavior of shallow foundations under vertical load combinations has been the subject of numerous studies, the response of shallow foundations subjected to combined horizontal and torsional loading has received considerably less attention. New offshore applications of shallow foundations for LNG facilities and other subsea structures have underscored the importance of the behavior of shallow embedded foundations subjected to combined in-plane translation and torsion. This study investigates the undrained bearing capacity of rectangular and square shallow foundations under eccentric horizontal loads through comparisons of various limit equilibrium and plastic limit analysis solutions to 3-D finite element solutions. In general, the plastic limit approach considered in this paper agrees well with the finite element solutions, although it has some tendency to over-predict capacity at greater embedment depths. The studies revealed a general insensitivity in the shape of the yield envelope to variations in embedment depth, which permits a simplified analysis suitable for first order estimates of load capacity. The variables considered in this study include footing aspect ratio, embedment depth, and load direction in addition to eccentricity.This is the author's accepted manuscript. The final version can be found published by ASCE in the Journal of Geotechnical and Geoenvironmental Engineering here: http://ascelibrary.org/doi/abs/10.1061/(ASCE)GT.1943-5606.0001138

CAPACITY OF SHORT PILES AND CAISSONS IN SOFT CLAY FROM GEOTECHNICAL CENTRIFUGE TESTS

Geotechnical centrifuge tests were conducted to examine the behavior of low aspect ratio piles and caissons in clayey soils subjected to high moment loading. Model piles with aspect ratio of two were tested in the 150g-ton centrifuge at Rensselaer Polytechnic Institute. Results include moment-inclination and force-displacement response for different loading conditions. Numerical studies were also performed consisting of three dimensional finite element simulations in order to predict capacities. The comparisons are performed in terms of the total resistance that is exerted by the soil on the caisson. This paper focuses on presenting the ultimate bearing capacity factors including both experimental and numerical results. In addition, results are compared to a series of studies available in the literature, which include upper bound solutions and experimental results.US National Science Foundatio

Numerical prediction of undrained response of plate anchors under combined translation and torsion

The undrained pure translational and torsional capacity of anchors and the plate response under eccentric translational loading is investigated in this paper using three-dimensional finite element (3D-FE) analysis. Plastic limit analysis is adopted to establish a benchmark solution for ultimate translational resistance with satisfactory agreement with the FE values which confirms the developed numerical model. Although plate thickness has a marked impact on the maximum shear and torsion resistance, the shape of failure envelope is minimally affected by thickness. A simple three-degree-of-freedom interaction equation is curve-fitted to FE failure datapoints. Representative interaction relationships are introduced for square and rectangular plates

Exploring the lateral capacity of squat piles in soft clay through geotechnical centrifuge modelling

© 2017 IEEE. Many offshore structures currently in use are supported by piles with large length-to-diameter aspect ratios, because it is well known that such foundations can hold large forces and moments. In environments where long piles are not suitable, structures will use foundations with very low aspect ratios such as skirts and mats. Capacity of long piles has been studied for decades and is well documented, whilst more recent tests have also addressed the behaviour of skirts, mats, and other low-aspect ratio foundations. The vertical and lateral capacity of mid-size foundations, with aspect ratios between one and five, has generally been thought too low for the requirements of most offshore structures. However, in recent years, structures of increasingly different shapes and sizes have been used in offshore environments, such as water-based renewable energy sources or marginal oil and gas platforms. In many of these cases, the usage of a low aspect ratio foundation could significantly reduce installation and transportation costs. Limited studies have been performed on such foundations, and most of the existing work uses only analytical and numerical solutions. Geotechnical centrifuge tests and corresponding numerical analyses were started at Texas A&M University and were continued at the University of Cambridge on the lateral capacity of piles with an aspect ratio of two in normally consolidated clay. Piles were loaded under both pure rotation and a mix of rotation and translation. This work is relevant to offshore structures requiring foundations that are strong but easily installed and cost-efficient, specifically structures secured with piles that experience point loads either through or above the water. It is also of interest for structures in difficult environments, such as areas too shallow or sedimentary for long piles or too fragile for skirts and mats.National Science Foundation (USA), the National Secretary of Science and Technology (Panama

Centrifuge 2D gravity on a vertical rotational reference frame

With the advent of high-accuracy sensors and increased interest in geotechnical centrifuge testing simulating loading within serviceability limits, a stronger understanding of the magnitude and orientation of centrifuge gravity relative to the scale model is necessary. This paper presents a methodology for determining two-dimensional centrifuge gravity within a model independently of centrifuge type or geometry, which can be used to recompose the gravity field from the direct measurement of a single gravity vector, given angular velocity. Finally, the methodology is compared to the mechanics of drum and beam centrifuges to provide physical meaning to coordinate rotation variables. United States National Science Foundation, project "Capacity and Performance of Foundations for Offshore Wind Towers," Award Number: 505 104160

Use of a MEMS accelerometer to measure orientation in a geotechnical centrifuge

Microelectromechanical systems (MEMS) accelerometers are becoming more prevalent in geotechnical engineering and geotechnical centrifuge modelling. In centrifuge experiments these sensors have shown great promise, but still exhibit limitations. This paper proposes a new methodology for the use of single-axis, low-g, high-accuracy MEMS accelerometers to measure the orientation of an object on the vertical rotational plane of centrifugal acceleration and Earth's gravity in a geotechnical centrifuge. The method specifically compensates for the measured cross-axis acceleration by an MEMS accelerometer when in a high-g environment. This is done by determining the apparent internal misalignment of the MEMS sensing unit, relative to its packaging, from a high-g cross-axis calibration. The misalignment can then be used to correct the measured orientation of the sensor relative to a centrifuge gravity vector. When compared to simplified approaches, measurements of absolute orientation are improved by 0·89° and the standard deviation of measurements between multiple sensors is reduced by 0·71°. Overall, this new methodology significantly improves the accuracy of orientation measurements by MEMS accelerometers in the geotechnical centrifuge, opening the door to use these inexpensive sensors in more experiments. </jats:p

The Use of Sodium Pyrophosphate to Improve a Translucent Clay Simulate

In the ever expanding quest to understand the nature and behavior of soil, translucent and even transparent media have been developed to serve as soil simulates. These artificial soils can be used in experimental models to make visual measurement of phenomena such as geosystem kinematics, soil mass movement, soil particle motion, and pore fluid flow that would be nearly impossible to obtain in natural opaque soils without expensive equipment or boundary effects. One successful type of translucent clay simulate is lithium sodium magnesium silicate (LNM silicate); however, it’s low density/high void ratio results in higher than typical permeability, low undrained shear strength, and extremely long consolidation times. Until now, translucent soil simulates of only 4.5% by mass LNM silicate to total mass have been possible. This paper provides a method for creating mixtures of translucent LNM silicate gel/glass as high as 15% by mass with the additions of an emulsifier, sodium pyrophosphate decahydrate (SPP), which impedes gelation so additional silicate powder can be added. Further, digital image processing techniques are used to present a relationship between LNM silicate, SPP, and translucency and an analysis of the modified simulate’s permeability and consolidation properties, with comparisons to natural clays, is also included.The lead author would like to acknowledge the excellent work of the undergraduate researchers on this project: Elliese Shaughnessy for the laboratory work and MATLAB programming necessary to create the Laponite-SPP curve, Kristen Ewert for conducting and interpreting the consolidation experiments, Nicholas Boardman for mixing the testing specimens and conducting the permeability experiments, and Tom Anderson for assisting with the permeability experiments. We would also like to thank Dr. Cassandra Rutherford for her assistance on the project. This research was funded through the National Science Foundation, Award Number: 1041604.This is the author accepted manuscript. The final version is available from American Society of Civil Engineers via http://dx.doi.org/10.1061/9780784480151.00

Seafloor-riser interaction model

Fatigue stresses associated with extreme storms, vessel movements, and vortex-induced vibrations are critical to the performance of steel catenary risers. The critical location for fatigue damage often occurs within the touchdown zone, where cyclic interaction of the riser with the seabed occurs. Developing a model for seabed stiffness requires characterization of a number of complex nonlinear processes including trench formation, nonlinear soil stiffness, soil suction, and breakaway of the riser from the seafloor. The analytical framework utilized in this research considers the riser-seafloor interaction problem in terms of a pipe resting on a bed of springs, the stiffness characteristics of which are described by nonlinear load-deflection (P-y) curves. The P-y model allows for first penetration and uplift, as well as repenetration and small range motions within the bounding loop defined by extreme loading. The backbone curve is constructed from knowledge of the soil strength, the rate of strength increase with depth, trench width, and two additional parameters, while three parameters are necessary for the cyclic response. © ASCE 2009

Effect of combined translation and torsion on undrained uplift capacity of plate anchors: Plastic limit analysis (PLA) solution

Uplift capacity of plate anchors have been the focus of numerous studies, since anchor plates are designed for pull out in normal operating conditions. However, the response of plate anchors under six-degrees-of-freedom loading caused during extreme loading conditions is poorly understood. The purpose of this study is to propose a simple yet sufficiently accurate analytical solution to investigate the behavior of plate anchor under combined in-plane translation and torsion and to evaluate its effect on the plate uplift bearing capacity. To this end, a modified plastic limit analysis (PLA) approach is introduced and compared with limit equilibrium (LE) and simplified upper bound baseline solutions. The proposed method is verified with three dimensional finite element (3D-FE). The variables considered in this study include plate aspect ratio, plate thickness, as well as load direction and eccentricity. Results of analytical solutions indicate the insensitivity of the “shape” of the shear-torsion yield envelope to plate thickness. This finding facilitates the use of simplified yet reasonable yield envelope for infinitely thin plate obtained from simplified PLA approach for other plate thicknesses. The “size” of the failure envelope (controlled by pure torsional and translational capacity) could be predicted fairly accurately by PLA and LE methods. Combination of these analytical methods offers a simple yet reasonably accurate solution to describe shear torsion response of anchor plate. The obtained shear-torsion yield envelope is then fitted in the generalized six-degrees-of-freedom yield surface which describes the reducing effect of moment, torsion, and planar forces on the uplift capacity of plate