Theory on Measuring Orientation with MEMS Accelerometers in a Centrifuge

Microelectromechanical systems (MEMS) sensors have become a common part of everyday life and can be found in a number of consumer electronics. Specifically, MEMS accelerometers have become widespread because of their low cost, due to mass production techniques, and ability to sense constant acceleration. This ability allows devices, such as cellular phones, to measure their rotation relative to Earth's gravity. These properties also make MEMS accelerometers an option for measuring the rotation of geo-structures, such as foundations, in the field or in scale model geotechnical centrifuge tests. MEMS accelerometers appear to be especially beneficial for measuring orientation in centrifuge experiments because they are not limited by the design constraints of traditional tilt sensors: a single constant acceleration vector (Earth's gravity). This paper presents the theory behind using single-axis MEMS accelerometers to measure the orientation of an object on a plane of reactive centrifugal acceleration and Earth's gravity within a geotechnical centrifuge. The paper specifically addresses cross-axis sensitivity which can significantly impact measurements and is typically excluded from simpler theories.The authors acknowledge the National Science Foundation, the Network of Earthquake Engineering Simulations (NEES), and the project Capacity and Performance of Foundations for Offshore Wind Towers, Award Number: 1041604. Additionally, we would like to acknowledge the NEES site at Rensselaer Polytechnic Institute.This is the author accepted manuscript. The final version is available from ASCE via http://dx.doi.org/10.1061/9780784479087.24

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

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

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

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

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

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

Rotational Behavior of Squat Monopiles in Soft Clay from Centrifuge Experiments

his paper presents initial results from geotechnical centrifuge experiments on squat gravity caissons/monopiles under cyclic rotational loading in soft clay under undrained conditions. These experiments were conducted in the 150g-ton centrifuge at Rensselaer Polytechnic Institute. In total three monopiles with length to diameter aspect ratios of two were tested. Though this aspect ratio is a bit atypical it could be a reasonable option for offshore renewable hydrokinetic systems or a component in tripod or tetrapod wind tower foundation systems. Results are focused on behavior arising from an unrestricted vertical coordinate, allowing self-weight to contribute to combined loading, and the effect of load eccentricity and depth of rotation on rotational resistance.The authors would like to acknowledge the assistance provided by the Center for Earthquake Engineering Simulations at Rensselaer Polytechnic. This project was funded by National Science Foundation, Award Number: 1041604.This is the author accepted manuscript. It is currently embargoed pending publication

Response of short monopiles for offshore wind turbine foundations: virgin and post-cyclic capacity

Geotechnical centrifuge tests were conducted to examine monotonic behavior of low aspect ratio piles in soft clays. Monopiles with aspect ratio of two were tested in the 150g-ton centrifuge at Rensselaer Polytechnic Institute. Initial pile test results include force-displacement for displacement controlled loading. This paper focuses on ultimate capacity of short aspect ratio monopiles after cyclic loading. Post-cyclic monotonic capacity is compared to virgin monotonic capacity for pure rotational loading over a range of eccentricities.US National Science Foundation, Award Number: 104160

Centrifuge and Numerical Modeling of Monopiles for Offshore Wind Towers Installed in Clay

Offshore wind power has gained momentum as a means to diversify the world’s energy infrastructure; however, little is still known of the global stiffness behavior of the large diameter low aspect ratio monopiles which have become the foundation of choice for offshore wind towers. Traditionally, offshore foundations have been associated with gravity structures for the oil and gas industry, which in general need to resist large vertical loads with limited lateral and moment loading. However, wind towers are purposely designed to be subjected to large lateral and moment loads from the wind and waves in order to maximize power generation. Geotechnical centrifuge tests were conducted and numerical models are being developed to examine the behavior of low aspect ratio piles in clayey soils. Monopiles with aspect ratio of two are being tested in the the 150g-ton centrifuge at Rensselaer Polytechnic Institute. Initial results include momenttheta and force-displacement for various loading conditions. Numerical studies consist of finite element (FE) simulations in order to predict capacities and permanent deformations. The comparisons are to be performed in terms of the total resistance that is exerted by the soil on the caisson. FE studies allow to model capacity for different displacement fields and also to compute interactions between different loading modes. This paper outlines our progress to date including both numerical and experimental results.The authors acknowledge the assistance provided by the personnel at the NEES facility at Rensselaer Polytechnic Institute. The authors also acknowledge the National Science Foundation, NEES, and the project Capacity and Performance of Foundations for Offshore Wind Towers, Award Number: 1041604.This is a metadata record relating to an article that cannot be shared due to publisher copyright