Deformation analysis methods for drilled shaft foundations subjected to lateral and overturning moment loads
Creators: name: Foster, Byron Holth
Publisher: name: University of Utah
Year: 2016
Subjects: drilled shaft foundations
four spring model
p-y model
reliability based design
soil structure interaction
strain wedge model

description: Drilled shaft foundations are widely used in many civil engineering applications where deep foundations are required because they are relatively easy to construct and are suitable for resisting lateral, axial, and overturning moment loads.  While the analysis of drilled shafts subjected to axial loads is fairly straightforward, it is much more difficult to analyze drilled shafts subjected to lateral and overturning moment loads due to the complex nature of the soil-structure interaction.   It has been suggested that the p-y model, which is currently the most commonly used model for performing these types of analyses, considerably overestimates deformation of semi-rigid to rigid drilled shafts subjected to lateral and overturning moment loads.  While the p-y model has been shown to reasonably predict deformation of flexible steel pipe piles and drilled shafts, it has not been verified for rigid to semi-rigid drilled shafts.  The major objectives of this investigation were to identify other methods in current use that might be more appropriate for analyzing this type of drilled shaft and to assess the accuracy of each analysis method by comparing the results from each method to the results of large-scale load tests.  The literature review revealed several analysis methods, which range from simple analytical methods to complex numerical methods.  In addition to the p-y model, other commonly used analysis methods include the strain wedge model and the four-spring model.  All of these models are semi-empirical and rely to some extent on experimentally-observed data and simplifying assumptions about the soil-structure interaction.  The p-y model, the strain wedge model, and the four-spring model are implemented in the commercial software packages LPile, DFSAP, and MFAD, respectively.  Several large-scale load tests for rigid to semi-rigid drilled shafts were also identified in the literature review. The information from these load tests was used to perform analyses using the LPile, DFSAP, and MFAD program, and the results were compared to the experimentally-observed results. The results suggest that MFAD is the most accurate model for granular soils and that DFSAP is the most accurate model for cohesive soils.  For the foundations considered in this investigation, there was no apparent correlation between the accuracy of the DFSAP or MFAD results and the rigidity of the foundation; however, the accuracy of the LPile results tended to decrease as the foundation rigidity increased.  A parametric study was conducted to investigate how the soil input properties affect the results, and how sensitive the models are to variations in these properties.  The parametric study showed that LPile and DFSAP are most sensitive to input properties of the angle of internal friction (?) and undrained shear strength (Su) of the soil, while DFSAP is most sensitive to the modulus of deformation (Ep) of the soil.  A statistical analysis of the combined data from each analysis method resulted in design equations for estimating semi-rigid to rigid drilled shaft deflection using LPile, DFSAP, or MFAD for a target level of reliability.