SCREW ANCHOR TEST PROGRAM (Part II): FIELD TEST RESULTS and DESIGN IMPLICATION D.J.Y. Zhang, R. Chalaturnyk, P.K. Robertson, D.C. Sego, and G. Cyre Department of Civil and Environmental Engineering, University of Alberta, Edmonton, AB ABSTRACT: This paper presents the test results obtained from a field experimental study conducted by the University of Alberta on screw anchor piles used in Alberta. A total of 18 pile load tests including compression, tension and lateral pile tests were performed on two sites underlain by soils typically found in Alberta. The axial and lateral loading behavior of the screw pile installed in lacustrine clay (University Farm site) and sand dune (Sand Pit site) were studied. Key Words: screw anchor, compression, tension, lateral, pile load test 1.0. INTRODUCTION Screw anchors have been extensively used in foundation applications, such as transmission towers, guyed towers, pipelines and braced excavations. From a brief review of current research and industry practices, design methods for predicting screw anchor capacity can be categorized into “cylindrical shear”, “individual plate bearing methods”, and “empirical methods”. The complexity and variability of Alberta soil, due to glacial history, create uncertainties to adapt these design methods. Thus, the University of Alberta has carried out an experimental study to investigate the load- displacement behavior of multiple screw anchors installed in soils typically found throughout Alberta. The field program consisted of a total of 18 full scale pile load tests including compression, tension and lateral loading, performed at two sites underlain by different material. The soil types at test sites were Lake Edmonton Clay at the University Farm site (cohesive material) and Sand dunes at the Sand Pit site (cohesionless material). The site investigation, instrumentation and installation of screw piles are presented in Zhang et al., (1998). This paper summarizes and discusses the test results obtained during the study including the ultimate pile load capacities achieved, load-settlement relationships, lateral test results and the axial stress distribution along the pile shaft under static load conditions. These full scale field test results will be used to develop a reliable design method for screw anchor piles installed in Alberta. 2.0. RESEARCH BACKGROUND Research on the analysis and design of individual plate anchors and shallow foundations was initiated with the development of the transmission line industry in the 1950’s (e.g. Meyerhof and Adams, 1968, Adams and Hayes, 1967). Because the anchors were mainly used for resisting tensile forces, experimental studies were focused on the ultimate anchor capacity in different soils under static uplift load conditions. It was not until the 1980’s that the design of multi-helix screw anchor piles became a research interest. From a brief review of previous work, two design methods, namely the cylindrical shear method and individual plate bearing method, are commonly used to predict the uplift capacity of multi-helix anchors. In addition, an empirical method called the installation torque method is commonly used in the industry. This method was developed based on empirical correlation, but lacks explicit definition related to traditional geotechnical concepts. However, it has been used successfully in the construction of thousands of anchors over the past twenty five years, as outlined by Hoyt and Clemence, (1989). The cylindrical shear method assumes that the uplift capacity is derived from shear resistance along a cylindrical failure surface and bearing resistance above the top or bottom helix, as shown in Figure 1. The individual plate bearing method assumes that bearing failure occurs above each individual helix (Figure 2). The total uplift resistance is the sum of the individual capacities. The installation torque method predicts the uplift capacity by correlating the installation torque and uplift capacity. This is an approach analogous to the relationship between pile driving effort and pile capacity. Previous research has shown that factors such as pile geometry, soil disturbance caused by pile installation, soil material properties, and ground water condition, can affect the anchor capacity significantly (Bradka, 1997). However, two major factors, the embedment ratio (H/D) and the inter-helix spacing or the spacing ratio (S/D) are the main contributors to the ultimate capacity. They are, thus, considered in more detail in this study.