Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
BACKGROUND AND HISTORY OF ANALYSIS OF LATERALLY LOADED DEEP
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| 1.2 BACKGROUND AND HISTORY OF ANALYSIS OF LATERALLY LOADED DEEP FOUNDATIONS Lateral loads in deep foundations occur under various conditions. In some applications, such as noise walls or landslide stabilization, lateral loads are the principal design load. For bridge foundation applications, lateral loads must be fully considered in the design. In many cases, lateral loads control the diameter or width of the foundations. Even relatively small lateral loads may influence the structural response of bridge foundations, depending on the load and structure characteristics. The magnitude, point (or area) of application, orientation, duration, and frequency of occurrence of lateral loads, and the response of the surrounding geomaterials will be different for each project application. Lateral loads on deep foundations may result from either the structure to be supported or from the surrounding ground. A concentrated horizontal load and/or an overturning moment applied at the top of the foundation element are typically encountered in many transportation applications including bridge foundations, signal and sign structures, and noise walls. In these applications, loads applied by the superstructure are relatively independent of the subsurface conditions. In other applications, such as deep foundations used for slope stabilization or earth retaining structures, or piles subjected to lateral spread, loads acting on foundation elements are heavily dependent on soil conditions and the magnitude of soil movement, which in turn is affected by the selection of the deep foundation. Numerous methods are available to analyze the response of deep foundations subjected to lateral loads. These methods have evolved from simple methods based on elasticity or plasticity theories to fully nonlinear methods. Simple analytical methods use geotechnical parameters estimated from conventional geotechnical tests. More advanced analyses methods rely not only on geotechnical parameters but also on other types of data, including pile load test results. Significant improvements in the analysis, design, and load testing of laterally loaded piles have been made (Reese and Van Impe 2001) and powerful computer programs have been developed for handling complex calculations of pile resistance under lateral loads. One of the first analysis methods available was based on the concept of the subgrade modulus (Terzaghi 1955). In this method, which considers an elastic beam resting on an elastic foundation (Hetenyi 1946), the lateral reaction of soil against the pile is assumed to be dependent on the soil deflection. Subsequently, other elastic-based methods were developed to provide preliminary estimates of the response of piles (e.g., Poulos and Davis 1980). Methods based on plasticity theory, including that proposed by Broms (a, were developed to estimate the ultimate lateral capacity of single piles and are based on the well-known concept of lateral passive earth pressure. The concept of an equivalent pile length was introduced in the late s to facilitate structural calculations. In this concept, the effects of flexibility of the soil embedding the pile are replaced by a fixed pile having an equivalent stiffness. The nonlinear response of deep foundations under lateral loads can be considered the “p-y” method (McClelland and Focht 1958). In this method, where pis the lateral soil reaction along the pile and y the lateral pile displacement. The soil is represented by a series of nonlinear springs, which are characterized according to the type of surrounding soil and other subsurface conditions (Reese et al., 1975). This methodology gained popularity in the sands after several studies were conducted to support the development of offshore platforms for oil exploration (e.g., Matlock and Reese 1961; Matlock, 1970; and Reese et al. 1975). 14 Computer-based analysis techniques incorporated the p-y concept to efficiently analyze the response of deep foundations under combined lateral loads and moments. One of the first computer programs to incorporate the p-y methodology was the program COMP (Wang and Reese 1993), which was developed under contract to the Federal Highway Administration (FWHA). This program analyzes the response of a single pile/shaft to the application of moments and lateral loads at the top of the pile. The p- y method is largely empirical. The strain wedge method was derived in the s to provide a more theoretical basis for correlating lateral soil resistance for laterally loaded deep foundations. In the strain wedge method, the soil resistance is correlated with mobilization of forces of a D passive soil wedge from a limit equilibrium solution of passive earth pressure resistance. Estimated strains in the passive soil wedge associated with the mobilization of the passive resistance are correlated to lateral displacements. The strain wedge method is performed using software in a similar fashion to the p-y method. Various commercial computer programs, which are based on either the finite element or finite difference methods, have been developed to handle more complex features including • three-dimensional (D) loading foundation geometry effects load nonlinearity simultaneous vertical and lateral loading nonlinear response of concrete pile sections pile groups These computer programs are often used in the design and analysis of deep foundations under combined vertical and lateral loads. The need for this document stems from the fact that, although various design procedures are currently used for the analysis of laterally loaded deep foundations, these procedures are not presented in a single reference. In addition, since the analysis requirements for laterally loaded deep foundations vary by locality and by published standards, there is a need for general guidelines to provide more consistent application of these procedures in the design of transportation facilities. Download 6.03 Mb. Share with your friends:
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