Development of a laboratory testing methodology to represent small strain soil conditions for laterally loaded monopiles in sands

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Date

2018-05-04

Authors

Munson, James Michael

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Abstract

Offshore wind turbines present unique engineering challenges, many of which stemming from founding the superstructure in the shallow ocean floor rather than on dry land. Several foundation types are available for offshore wind turbines, one of which is relatively short, stiff monopiles. Current design standards for laterally loaded monopiles were developed in the early 1970s, based on tests conducted on long slender piles on Mustang Island, Texas. These lateral load tests serve as the basis for many design standards, which work well for piles subjected to large lateral deformations but are unverified for the small strain range. This thesis presents a laboratory testing methodology developed to find more relevant design standards for piles subjected to small strain lateral loading in sands. This methodology was developed using laboratory model tests of two different sand cell scales. Preliminary tests were conducted to determine the best method to measure small-strain in-situ properties of sands. Shear wave velocity was of greatest interest in these preliminary tests as it can be used to determine small-strain shear modulus, or G, of sand. Small soil cells were instrumented with 28Hz geophones, then seismic waves were propagated through the soil body to determine small-strain properties including small-strain shear modulus. Various receiver arrays and seismic sources were used in preliminary testing to find the most reliable test setup before final testing. In final testing, spherical penetrometers of various diameters, embedded at various depths, were used to subject a soil body to small strains under 1G monotonic and cyclic loading. Load-displacement curves were obtained from these final tests and compared to those found from numerical modeling conducted in a parallel study. This study concludes that the developed methodology for preparing a soil cell and subjecting a zone to small strains through the use of a spherical penetrometer can be reliably repeated. This study also concludes that the developed testing methodology is representative of those in-situ conditions felt by laterally loaded monopiles.

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