Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
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- 7.4.4 Commentary on the Use of Computer Programs for Group Analysis
| 7.4.3 Finite Element Programs Comprehensive finite element programs are also capable of modeling the soil around a group of deep foundation elements as a continuum and for modeling its nonlinear behavior. Such programs also include the provisions for gapping and slippage that may occur at the foundation/soil interface. Therefore, these programs not only capture the nonlinear behavior of the soil and deep foundation element, but also model the pile-soil-pile interaction. The disadvantage of using general purpose FEM code programs is that they require a significant effort to establish a three-dimensional finite element mesh to model a foundation group. The constitutive modeling of the soil can pose technical challenges for most practitioners as well. The use of the general purpose FEM codes is generally not cost-effective for routine analysis and applications, especially when other programs are available. FEM maybe worth exploring on large or very complicated projects. 7.4.4 Commentary on the Use of Computer Programs for Group Analysis Care must be exercised when using software to perform analysis of groups of deep foundation elements. The available programs have robust capabilities for analyzing complex problems. However, these programs have limitations based on the inherent assumptions, methods, models, and default parameters used in their development, as is true of any software program. It is the responsibility of the designer to understand potential limitations or issues with the use of available software tools. Some examples of areas where care needs to be exercised with these programs are discussed below. • P-y Curves Software programs generally do not have the same number of p-y curves available as some of the programs for individual pile analysis. In some cases, with unusual, complex, or region- specific geomaterials, the available p-y curves in the program may not match well with the actual geomaterials. The user may need to make some simplifying assumptions regarding which P-y curve is most applicable for the subsurface materials at the project site. In such cases, it maybe appropriate to perform parametric studies using multiple p-y models to assess how significant an impact these simplifying assumptions may have on the results and how sensitive the results are to variations in the soil models. • P-multipliers: Rollins et al. (2000) performed analysis of a laterally load tested pile group and found that when the default p-multipliers in a software program were used, the computed pile-soil-pile resistance was about 35 percent higher compared to the resistance computed with p-multipliers based on the full-scale test results. Default values of p-multipliers should therefore be used with caution. The recommended approach is to use the p-multipliers in Table 7-1, or to use values based on local practice or testing programs, rather than default values in the software. 112 Passive resistance of the cap Numerous methods, including the log-spiral, Rankine and Coulomb methods, and p-y method, are available to estimate the passive resistance against the cap. Rollins et al. (2000) concluded that the log-spiral method provided the best agreement with the measured resistance. Estimates of passive pressure using the Rankine method significantly underestimated the passive soil resistance acting on the cap whereas the Coulomb method overestimated the passive resistance on the cap. Similar conclusions were made by Duncan and Mokwa (2001). Other aspects of analyses of groups of deep foundation elements not directly related to lateral foundation analysis, may need to be considered in the overall analysis of the group. Examples include – Settlement potential, as these programs may not adequately account for long-term settlement potential and its resulting impacts on foundation design, including downdrag loads, changes in subsurface soil properties, loss of contact with the cap, potential for unsupported pile/shaft length, and potential buckling of piles. – Elastic shortening of the piles/shafts under axial loads may not be addressed directly by the computer codes. For very long piles/shafts, elastic shortening of the piles may need to be evaluated. Download 6.03 Mb. Share with your friends:
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