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Publication Evaluation of Helical Piles as Anchors for Floating Offshore Wind Turbine Foundation Systems(2019-01-01) Harris, Martha ShannonThe development of floating offshore wind energy is essential for growth of the renewable energy industry, and to decrease the reliance on fossil fuels and emission of greenhouse gasses. Advances in ease of construction and installation of turbines can make the floating offshore wind industry more marketable and feasible on a large scale; this can in part be achieved by innovation in foundation anchoring systems. This thesis presents an evaluation of helical pile foundations for offshore deep-water application as anchors for floating offshore wind systems with catenary mooring lines in clay and sandy soils. This research outlines the influence of helical geometry, angle of installation and pile group effects on helical pile capacity and required installation torque. Axial capacities were determined using Terzaghi’s general bearing capacity equations, lateral capacities were determined using American Petroleum Institute Recommended Practice and required installation torque was estimated using a Cone Penetration Test based system of equations. Models of semisubmersible and spar buoy floating offshore wind turbine platforms with single-line and multiline catenary mooring systems were used to generate expected anchor loads for design. Given the variance in functionality, both single piles and pile groups of four and nine were considered in 5 the analysis. Load conditions and pile configurations were compared to suction caissons and evaluated for efficiency and ease of construction. The use of a spar buoy platform and multiline system decreased the required helical pile size compared to a semisubmersible platform and a single-line mooring system. Helical piles designed for sand were smaller in size compared to helical piles in clay due to greater strength of the sand. Individual helical piles designed for pile groups were smaller in overall geometry than single piles, where battered helical pile groups were lighter and vertical helical pile groups were heavier than single piles in terms of net weight. However, smaller helical piles required a fraction of the torque needed to install the larger, single helical piles. For efficiency in terms of capacity per unit weight, single and groups of helical piles are more efficient than suction caissons, using less steel to produce the same capacity. For catenary line systems the helix of a vertically installed helical pile does not contribute to lateral load resistance but it does provide a means for potentially quieter installation compared to driven piles.Publication Influence of Reconsolidation Procedure on Small Strain Shear Modulus and Undrained Shear Behavior of Silts Subjected to Tube Sampling Disturbance(2018-01-01) Pandey, ShreyaThis thesis presents results of a laboratory testing program that studied the effects of laboratory simulated tube sampling disturbance on the undrained shear strength behavior of reconstituted low plasticity silts at overconsolidation ratios of 1.0 and 3.6. The three test soils consisted of two different mixtures of kaolin clay and silica silt and a reconstituted natural Dedham silt. Triaxial tests with bender elements were conducted on the reconstituted samples using the Ideal Sampling Approach (ISA) followed by post-ISA reconsolidation and undrained shear. The specimens were subjected to ± 1% and ± 3% ISA axial strain cycles. Changes in shear wave velocity and small strain shear modulus during simulation of tube sampling was used to develop a better understanding of the effects of sample disturbance on the undrained stress-strain-strength behavior of silts. Laboratory simulated tube sampling disturbance changed the undrained shear behavior of the low plasticity silts from contractive to dilative. The reconstituted silt samples experienced significant loss in effective stress due to ISA disturbance which decreased with an increase in overconsolidation ratio and plasticity. The normalized undrained shear behavior of the tested reconstituted silt samples did not show any dependency on the consolidation stress level, however, an increase in the consolidation stress level increased the tendency for the low plasticity silts to exhibit dilative behavior. The effect of Recompression and SHANSEP consolidation procedures on recovering the undisturbed behavior was found to be dependent on the plasticity of the soil, to some extend on the pre-ISA consolidation stress, level of ISA disturbance experienced by the specimen and the overconsolidation ratio. The measured shear wave velocity and small strain shear modulus at various stress states showed significant reduction during ISA disturbance which could be used as an indicator of sample disturbance in similar types of silts. However, after reconsolidating the specimens back to the initial effective stress state the shear wave velocity and small strain shear modulus values were mostly recovered indicating little to no influence of sample disturbance. The amount of reduction in shear wave velocity and small strain shear modulus was found to be dependent on plasticity of soil, level of disturbance and overconsolidation ratio.Publication Evaluation of New Methods for Determining the Minimum and Maximum Index Densities of Sand(2019-01-01) Ganji, HosseinThis thesis presents the results of a laboratory testing program that evaluated the applicability of a new-established method for determining minimum and maximum dry density values by Norwegian Geotechnical Institute (NGI). Seven fine to medium grained cohesionless sand batches with different inclusion of fines content were selected for a series of testing programs in compliance with American Society for Testing and Materials (ASTM) and NGI recommendations. All three methods of ASTM D4254 – 16 and method 1B of ASTM D4253 – 16 were applied for minimum and maximum dry density determination, respectively. The results compared with the NGI minimum and maximum dry density outcomes. The minimum density values obtained from the testing program revealed that the NGI procedure produces a lower minimum dry density value compared to the ASTM methods. It is also worth mentioning that the NGI mold dimensions likely play a role in generating lower minimum density values. Due to the lower ratio of diameter to height for the NGI mold than v the ASTM mold, already deposited sand particles tend to form an arch. This arching reduces extra movement of the particle which resulted in lower minimum density values. The maximum dry density results showed that the NGI method generates equal values to the ASTM method for soils with less than 2 %, by dry mass, fines content. For sands with composition of fine content more than 9 %, by dry mass, the ASTM method results were a little higher than the NGI ones. Additionally, grain size distribution analysis were performed in compliance with ASTM D60913 – 04 in order to investigate any evidence of particle crushing during each maximum density approach. The results indicated no evidence of particle degradation during each maximum dry density method test.Publication Aging and Property Changes of Clay Around Driven Piles(2016-01-01) Zapata, Hasian RThe purpose of this research was to determine how soil disturbance caused by the installation of piles (of differing types and geometries) in clay affect the short and long-term capacity of piles. Several types of piles were installed in lightly overconsolidated clay at three different test sites in Amherst, Massachusetts. Before and after pile installation, an in-situ testing program consisting of field vane shear tests was carried out around piles installed at one of the three testing sites. Undrained shear strength and water content profiles allowed for an approximate determination of changes in the behavior of the clay surrounding some of the piles installed at different aging periods. The excess pore pressures within the soil surrounding the piles was monitored during and after pile installation by means of collected representative samples located at various depths immediately adjacent to the pile. The changes in pore pressure during pile installation were indicators of the soil deformations caused by the pile installation. After allowing a recovery period following installation (at all sites), piles with differing geometries were loaded to failure under axial tensile loads. Load-settlement curves were generated for different piles at different aging times after installation. The Undrained Shear Strength of the clay adjacent to the pile was also monitored at different aging times after installation by performing field vane tests. Disturbed samples were collected after each test to monitor the water content. The determined water content at different aging times was used as an indicator of the distribution of excess pore pressures and distribution of soil deformations caused by pile displacement. The Undrained Shear Strengths and water content were used as principal parameters (controlling factors) for the correlation to the short and long-term capacity of the pile.Publication Study of Minimum Void Ratio for Soils with a Range of Grain-Size Distributions(2013-01-01) Yang, ZhenningMinimum void ratio or maximum packing density is an important soil property in geotechnical engineering. It apply to volume change tendency control, fluid conductivity control and particles movement. Previous researchers have attempted to predict maximum packing density by empirical/graphic method, rock correction method, alpha method. Based on the concepts of F. de Larrard in concrete mixture research, we have developed a mathematic model that can predict the minimum void ratio for soils with a wide range of particle size. Probability density function-lognormal distribution was tested and used to provide a reasonable representation for soils with a range of grain-size distribution. We incorporate the lognormal distribution in the mathematical model, and predict the minimum void ratio for various types of soil gradations. The validity of the model is evaluated. The evaluation of the model is also performed on several sets of data in the literature, which include binary packing system of steel balls, ternary packing system of spheres, mixtures of round and crushed aggregates, and soils containing gravelly sand with silt. Comparison of the results will be discussed.Publication FRACTURE TOUGHNESS ASSESSMENT OF SHALES BY NANOINDENTATION(2015-01-01) Liu, YunquiNanoindentation has been reported as an effective tool for realizing the strength and stiffness (modulus of elasticity) of different materials including thin film materials, cementitious composites and rock or clay mineral materials. However, to the best of the authors’ knowledge, nanoscale fracture of shale materials has rarely been examined. Fracture toughness property of shale has been typically studied by observing crack growth of notched macroscale specimens subjected to flexural or tension loads. This research discusses the possible characterization of fracture toughness of shale using contact mechanics at the nanoscale. The analysis method is based on two traditional fracture toughness measurement methods which are used for some different materials. One is the radial crack length measurement method and the other one is based on evaluating the energy absorbed by radial cracks propagating from the indentation imprint in brittle materials. Nanoindentation experiments performed using Berkovich and Cube corner nanoindenter are reported. The Young’s Modulus, Hardness and fracture toughness of shale are extracted from nanoindentation experiments. Fracture toughness for different types of shale are evaluated using both crack length measurement and an equivalent elastic crack energy approach and are compared to macroscale fracturetoughness of different types of shale from the literature. Mechanical properties for different types of shale are calculated from nanoindentation tests and mineralogy is found to play an important role in controlling mechanical properties of shales. An increase in organic content and clay minerals content decreases both Young’s Modulus and Hardness of shale. But the deviation of fracture toughness calculation results in this project cannot be ignored comparing with results from macroscale tests. There is some influence for fracture toughness measurement by size effect and shale heterogeneity. Shale heterogeneity and compositional variability should be considered as serious challenges in quantifying the mechanical properties of shale in the future work.Publication THE EFFECT OF IMMEDIATE SALINE WATER CURING ON THE STRENGTH, COMPOSITION, AND MICROSTRUCTURE OF GEOPOLYMER CEMENT(2015-01-01) Duran, LindsayThe low durability and low resistance of ordinary Portland cement led to an investigation into the viability of geopolymer cement as an alternative to well cement for soil improvement in saline environments and well cement in offshore drilling and carbon sequestration projects. This thesis presents the results of a laboratory investigation into the effect of immediate saline water curing on the strength, composition, and microstructure of geopolymer cement. The experimental program began with preliminary work in designing adequate geopolymer cement and developing a curing environment that simulates offshore or underground conditions and facilitates immediate curing of cement specimens. Pure geopolymer cement with a Si/Al ratio of 1.78 was synthesized from an admixture of Class C fly ash and metakaolin and immediately cured in saline water of 0, 15, and 35 ppt concentrations. After 28 days of water curing, the specimens were removed from their curing environments and characterized for strength, composition, and microstructure. Class G well cement specimens were simultaneously cured under similar conditions and analyzed as a source of comparison. Results indicatethat the strength of the geopolymer cement increases with increasing salinity, while the Class G cement exhibits opposite behavior. Mineralogical compositional analysis (X-ray diffraction) demonstrates the formation of a geopolymer through alkali activation of amorphous silica and alumina. Chemical compositional analyses (pH, X-ray fluorescence) revealed an increased rate of chemical exchange between the cement slurry and curing water occurred with decreasing salinity for both cement types. The microstructural characterization was carried out using scanning electron microscopy and energy dispersive X-ray spectroscopy. The geopolymer cements possessed nanoporosity and observed reacted product that was independent of curing salinity. All of the findings suggest that geopolymer cement derived from an admixture of metakaolin and Class C fly ash would be a viable alternative to ordinary Portland cement in well cementing applications.Publication HYDROGEOLOGIC CHARACTERIZATION OF A TILL MANTLED LEAKY FRACTURED BEDROCK AQUIFER(2013-01-01) Lukas, WilliamIn glaciated areas till can serve as a leaky aquitard for fractured bedrock aquifers. The characterization of a leaky till mantled fractured bedrock aquifer is presented. Multiple high resolution data sets were collected in response to unprecedented changes in the hydraulic head at a research site in eastern Massachusetts. Local scale aquifer and aquitard hydraulic conductivity, transmissivity and storativity values are determined through the analysis of purge recovery tests using Ostendorf and DeGroot (2010) and Cooper et al (1967) theories. The local scale parameters are in turn used to determine large scale aquifer transmissivity and storativity through an analysis of the drawdown due to irrigation pumping using Hantush (1960). It is confirmed that the till acts as a leaky aquitard to the fractured bedrock aquifer, with geometric mean hydraulic conductivities of 7.2E-9 m/s for the till and 1.6E-7 m/s for the bedrock. The storativity of the till is 2.7E-4 while that of the fractured bedrock is 6.7E-5. The transmissivity values determined through Cooper et al. (1967) overestimate the large-scale transmissivity of the aquifer by at most a factor of 3, as previously determined by Barker and Black (1983).Publication INVESTIGATION OF INSTALLATION TORQUE AND TORQUE-TO-CAPACITY RELATIONSHIP OF SCREW-PILES AND HELICAL ANCHORS(2015-01-01) Ruberti, MarkInstallation torque has been used in the design of helical anchors (Screw-Piles) since the late 1960s. KT factors released by the manufacturer relating ultimate capacity of Screw-Piles to installation torque allow engineers to calculate a design installation torque which is necessary to achieve the design capacity in the field. These KT factors have been based on shaft geometry alone (Hoyt and Clemence 1989). Recent full-scale uplift tests in both clay and sand have shown that the traditional methods of analysis for estimating uplift capacity based on microscale tests are not representative of macroscale behavior. A soil wedge does not fully develop in many cases and failure is a result of local bearing capacity in the soil immediately above the lead helix and side resistance along the pipe shaft. The relative contribution of these two components to the uplift capacity depends on the specific geometry of the Screw-Pile, not only the shaft geometry, but also the configuration of the helices, the soil type, and depth of embedment of the helical plate (Lutenegger 2015). Full-scale installations and load tests were performed on anchors of varying lead section geometry, shaft geometry, number of helices, soil type, and depth of embedment. Both direct (TORQ-PIN and Chance Digital Indicator) and indirect (hydraulic pressure) methods were used to monitor torque during installation. The direct methods were used to evaluate the reliability of hydraulic pressure readings and how different combinations of machine, torque head, and operator can affect the torque during installation. This paper will investigate how these factors affect KT and also determine the factors that affect the accuracy of torque measurement in the field.