Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
Figure 12-5: A pair of sister bar strain gauges attached to a reinforcement cage
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| Figure 12-5: A pair of sister bar strain gauges attached to a reinforcement cage. An alternative to conventional inclinometer casings is to use in-place inclinometer arrays, or strings of in- place accelerometers. These instruments are attached to a continuously recording data logger system at ground level. These instruments enable deformations to be monitored during the load test, without having to stop the load test at intervals for manual, considerably reducing the time needed for testing. An alternative to inclinometer casings and in-place inclinometers are Shape Array Accelerometers (SAA). This instrument can be embedded in the shaft concrete, or grouted with cross hole sonic logging tubes. Boeckmann et al. (2014) describe the use of SAAs fora lateral load test setup for drilled shafts in shale. The SAA comprises a chain of sensors that measure tilt on a continuous basis during the test. This avoids the need to stop the testing at intervals to take manual readings of inclinometer casing. It also increases safety, by avoiding the need to make readings in close proximity to test elements under load. Further information on the selection and use of geotechnical instrumentation is available in Dunnicliff (1998) and Brown et alb DATA ANALYSIS A key objective of instrumented lateral load tests is to derive p-y relationships for use in design (also see Appendix C. In order to achieve these relationships, numerical methods are required to convert the strain gauge data into bending moment profiles and to derive p-y design curves. The procedure used is summarized in the following steps 190 Plot the profile of bending curvature (Ο ) with depth. The curvature is computed as the difference between the compression and tensile strains measured in each pair of strain gauges, divided by the horizontal distance between the two strain gauges in each pair. 1. The lateral deformation (y) with respect to depth is computed by double integration of the profile of bending curvature π¦π¦ = οΏ½ οΏ½οΏ½ β πππποΏ½ ππππ Equation 12-1) Where z = depth below the top of pile or drilled shaft. 2. The bending moment (M) profile with depth is computed by multiplying the profile of bending curvature by the flexural stiffness of the pile or drilled shaft ππ = πΈπΈπΌπΌβ Equation 12-2) Where E = Youngβs modulus of the reinforced concrete or steel element. I = Second moment of area of the section. 3. The soil resistance per unit length of pile/shaft (pis then obtained by double differentiation of the bending moment profile ππ = Equation 12-3) Double differentiation of the bending moment profile can amplify numerical errors, especially if the number of strain gauge pairs is limited or if inclinometer data is used for this purpose. It is therefore recommended that a numerical curve fitting procedure be applied to the raw instrumentation data, and then develop the p-y curves from a smoothed relationship. The numerical methods available generally involve polynomial curve fitting with varying degrees of numerical complexity. High order global polynomial curves (Reese and Welch 1975), piecewise polynomial curve fitting (Matlock and Ripperger 1956), cubic splines (Dou and Byrne 1996), weighted residuals (Wilson 1998; Yang et al. 2005), and B- splines (de Sousa Coutinho 2006) have been used to evaluate lateral load test instrumentation data. Yang and Liang (2006) provide a summary of the different mathematical approaches for evaluation of strain gauge data from lateral load tests. Of the various techniques described, they recommended the use of piecewise cubic polynomial curve fitting to achieve profiles of p with depth as it has been shown to accommodate the nonlinear behavior of the foundation materials and p-y responses in layered soil profiles. Download 6.03 Mb. Share with your friends:
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