Design Of Short Drilled Shaft Foundations In High-plasticity Clay Under Inclined Loading

Drilled shaft foundations are primarily used to support structures such as bridge piers, towers, buildings, transmission towers, and roadway cable barriers. Generally, the main characteristics of drilled shafts are to transfer the loads to the stronger layers in the vertical direction and also their ability to withstand lateral movements from lateral loads. Though the design of shafts are well established with respect to combinations of loads and moments, their behavior under pullout type inclined loading is not well established. The inclined load applied is the combination of lateral and uplift loads at the same time and structures such as ends of cable barriers and transmission towers that are supported by the drilled shafts. This dissertation research mainly focuses on the use of drilled shafts supporting at the ends of the cable median barrier systems which play an important role in protecting people's lives by reducing cross-over collisions on highways. Several foundation problems were detected when drilled shafts were used to connect all the cables of the cable barrier systems. During December 2006 to February 2007, several failures of three cable median barrier were observed in Kaufman County near Dallas without any traffic related vehicular impacts. Preliminary investigation of failures showed that failed drilled shafts were located in a high plasticity clay environment. Causes of failures are attributed to cold temperature induced shrinkage in the cables that significantly increased the tensions in them. Other factors consist of soil saturation from prolonged rainfall events and potential use of smaller size drilled shafts at the site. In order to understand the causes of failures, the present dissertation research is performed to study the inclined load capacities of the drilled shafts. As a part of the research, various sizes of drilled shafts were selected and constructed in a clayey soil environment similar to the one where foundation distress was observed. Geotechnical sampling and laboratory testing were performed and the test results showed different soil layer types including high plasticity clayey soil near the ground surface. Soil strength properties for unsaturated (field condition) and saturated conditions were measured and these are used in the analysis of field test results.A new load test setup for the application of an inclined tensile loading on the drilled shafts was designed to simulate the natural field loading condition. The capacities of different sizes of drilled shafts from field test were tested and measured with this setup and these results showed that the dimensions of reaction test piers and spacing used between reaction and test piers have provided results that are not influenced by the boundary effects or reaction shaft movements. All load tests were conducted until the foundations reached failure. Test results were analyzed with the uplift capacity and lateral capacity analyses models proposed by many researchers as well as Finite Element Method (FEM) based numerical modeling in understanding the behaviors of drilled shafts under this inclined loading. After obtaining good simulation results, the analytical models are further used for various hypothetical foundation dimensions and for various undrained shear strengths of soils. These results are used to develop foundation design charts for inclined loading conditions. An illustration of design chart selection and its use is explained. Additionally, drilled shaft construction guidelines for cable barrier systems and recommendation for periodic maintenance are provided.