A Study on Soil-Structure Interaction of Axially Loaded Sheet Piles
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Abstract
Sheet piles are geotechnical structural elements often used in canals and rivers for soil retention and scour protection. Short span bridge abutment design in North Carolina and the U.S., for locations near bodies of water, typically uses sheet piles for soil retention and scour protection, while piles installed behind the sheet piles are used for axial load bearing. This dissertation investigates the feasibility of extending the function of these sheet piles to also act as axial load bearing foundation elements. Neglecting any axial load bearing potential provided by the sheet piles is likely a conservative design approach as Europe has successfully utilized the axial load bearing capacity of sheet piles in bridge abutment constructions for over fifty years (Carle and Whitaker 1989; Rybak and Zyrek 2013; SACILOR ; Skyline Steel LLC 2009; Yandzio 1998). Incorporating the axial load bearing capacity of sheet piles has the potential to significantly reduce construction costs and installation times by reducing the number or length of piles required for bridge abutment designs. This design approach has not been adopted in the U.S. partly due to the scarcity of full-scale axial load tests on instrumented sheet piles. The main focus of this research is to help address this scarcity and assess the soil-structure interaction and axial load bearing capacity of axially loaded sheet piles. This research involves a series of full-scale axial load tests on well-instrumented sheet piles. The results are used to examine the soil-structure interaction behavior for this foundation system in detail and provide methods for predicting this behavior for design purposes. The first series of load tests are performed under controlled soil conditions at the University of North Carolina at Charlotte (UNCC) Energy Production and Infrastructure Center (EPIC) geotechnical test pit, and the second set of tests are carried out under field settings at the equipment yard of the International Construction Equipment (ICE) facility in Matthews, North Carolina. The testing performed at the field site additionally included load testing of an H-pile to permit for comparisons between the axial stiffness and load capacity of a sheet pile pair and a pile section conventionally used for axial load bearing in bridge abutments. The results are compared with capacity predictions made using static methods and load-settlement curves obtained using different load transfer analyses. Additionally, the nature of the soil-structure interaction for a foundation with a wall or plate geometry is investigated further and compared to a cylindrical geometry. Analytical methods are used to study this behavior and it is found that the axially loaded foundation wall exhibits a different response than the cylindrical pile. The results for the foundation wall are used to develop new theoretical load transfer curves for load-settlement predictions of axially loaded sheet piles. Load transfer analyses using the developed T-Z and Q-Z curves are compared with measured load-settlement and load transfer curves resulting from pile load testing.The load test results performed for this research, under the soil conditions at the laboratory and field sites, indicate that sheets piles have favorable axial load bearing characteristics and comparable performance to other driven pile types commonly used as axial load bearing foundations for short-span bridge abutments. Deep foundation methodologies for analysis and design of conventional driven piles are found to be applicable for assessing axial load capacity of sheet piles. The methods evaluated include static methods based on geotechnical in-situ tests, such as the standard penetration test (SPT) and cone penetration test (CPT), and methods based on dynamic measurements obtained during pile installation, such as Pile Driving Analysis (PDA) and Case Pile Wave Analysis Program (CAPWAP). The level of accuracy of the different capacity prediction methods are compared and return similar levels of uncertainty for sheet pile capacity estimates as obtained for H-pile capacity estimates used in the field test program. Plugging represents a key aspect when estimating the axial capacity of sheet piles. Plugging occurs when soil moves together with an axially loaded pile rather than shearing at the soil to pile interface. This behavior influences the areas involved in shaft and end-bearing. Plugging has the effect of increasing the load bearing surface near the toe of the pile, thereby increasing effective toe resistance, while shaft area and shaft resistance is typically reduced along the pile where plugging occurs. The change in end-bearing area for sheet piles due to plugging can be especially large due to the thin cross section of this foundation type. Plugging behavior for sheet piles can have significant implications for ultimate load capacity and is considered in greater detail as part of this study.The applicability of load transfer methods to predict load-settlement curves and axial load transfer mechanisms for sheet piles is also assessed using the results of the different axial load tests performed in this research. Load-settlement curves predicted using load transfer analysis show good agreement with the behavior measured during load tests. Empirical as well as theoretical load transfer curves are considered and compared to experimental estimates obtained as part of this study.