|Abstract or Summary
- Drilled shaft foundations provide significant geotechnical resistance for support of structures, such as highway bridges, traffic sign, and signal pole structures, and are used widely to meet their structural foundation requirements. The amount of steel reinforcement in drilled shaft foundations has increased over the past several decades to account for anticipated seismic hazards. Increased reinforcement may lead to increased possibilities of anomalies within shafts due to the increased difficulty for concrete to flow through reduced clearance between the reinforcement. High-strength steel reinforcement and permanent steel casing may be used to mitigate the concreting concern. However, the comparison of axial and lateral load transfer between drilled shafts with and without permanent steel casing and high-strength reinforcement has not been previously investigated, raising questions regarding the suitability of existing analytical approaches for the evaluation of axial and lateral load transfer. In addition to axial and lateral loading, deep foundations may need to resist torsional loads, resulting from wind loading on traffic sign and signal pole structures, or seismic loading on curved or skewed bridges. However, the understanding of the actual resistance to torsion provided by deep foundation elements is not well established. The design methods for deep foundations in torsion at the ultimate limit states need to be evaluated and their accuracy needs to be quantified with loading test data. Furthermore, the accuracy of existing load transfer-based torsion-rotation methods to predict the full-scale, in-service rotation performance that considers state-dependence of the soil needs to be quantified.
Two uncased instrumented drilled shafts were constructed and used to evaluate the torsional capacity and load transfer at full-scale. The quasi-static monotonic and cyclic torsional loading tests were conducted. Based on the results of the torsional loading tests, design methods to predict ultimate resistance were proposed. To facilitate the serviceability and ultimate limit state design of geometrically-variable deep foundations constructed in multi-layered soils, a torsional load transfer method was presented using a finite difference model (FDM) framework. Simplified state-dependent spring models, relating the unit torsional resistance to the magnitude of relative displacement, were developed in consideration of soil-structure interface shear test results. Parametric studies illustrated the significant effect of nonlinear soil responses and nonlinear structural response on the torsional behavior of deep foundations.
The axial and lateral load transfer of drilled shaft foundations were studied using four instrumented drilled shafts at full-scale: two uncased and two cased drilled shafts, reinforced with either mild or high strength steel reinforcement. Based on the results of axial loading tests, selected axial load transfer models were evaluated and modified to produce region-specific axial load transfer models to aid the design of drilled shaft bridge foundations for similar soils in the Willamette Valley. The effects of permanent casing on axial load transfer were summarized to provide an up-to-date reference on the reductions expected based on construction sequencing and installation methods. The lateral responses of the test drilled shaft foundations indicated that the high-strength reinforcement could be used without detriment to the lateral performance of drilled shafts; and the cased shafts responded in a more resilient manner than uncased shafts at the same nominal diameter due to their significantly greater flexural rigidity. Based on the empirical soil reaction-displacement (p-y) curves, a region-specific p-y curve model was proposed with recommendations to account for pseudo-scale effects due to the increasing contribution of shaft resistance to lateral resistance with increased diameter.