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Improvement of the geotechnical axial design methodology for Colorado's drilled shafts socketed in weak rocks
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    Drilled shaft foundations embedded in weak rock formations (e.g., Denver blue claystone and sandstone) support a significant portion of bridges in Colorado. Since the 1960s, empirical methods and rules of thumb have been used to design drilled shafts in Colorado that entirely deviate from the AASHTO design methods. The margin of safety and expected shaft settlement are unknown in these methods, however, both are needed for the implementation of the new and more accurate AASHTO Load and Resistance Factor Design (LRFD) method in CDOT design guidelines. Load tests on drilled shafts provide the most accurate design information and research data for improvement of the design methods for drilled shafts. As a part of the construction requirements for the T-REX and I-25/Broadway projects along I-25 in Denver, Colorado, four Osterberg (O-Cell) load tests on drilled shafts were performed in 2002. The bedrock at the load test sites represents the range of typical claystone and sandstone (soft to very hard) encountered in the Denver metro area. To maximize the benefits of this work, the O-Cell load test results and information on the construction and materials of the test shafts were documented, and an extensive program of simple geotechnical tests was performed on the weak rock at the load test sites. This includes standard penetration tests, strength tests, and pressuremeter tests. The analysis of the all test data and information and experience gained in this study were employed to provide: 1) best correlation equations between results of various simple geotechnical tests, 2) best-fit design equations to predict the shaft ultimate unit base and side resistance values, and the load-settlement curve as a function of the results of simple geotechnical tests, and 3) assessment of the CDOT and AASHTO design methods. Three implementation products are recommended in this study. First, preliminary design methods as required in the LRFD method for drilled shafts with conditions close to those encountered at the four load test sites. All the qualifications and limitations for using these design methods are presented (e.g., construction procedure). Incorporation of these methods will lead to savings in CDOT future construction projects. Second, preliminary recommendations to improve and standardize geotechnical subsurface investigation for drilled shafts in Colorado. Third, a detailed strategic plan for CDOT to identify the most appropriate design methods per LRFD for Colorados drilled shafts embedded in various weak rock formations. This plan of six tasks requires: (1) acquiring the testing and site specific resistance parameters of the LRFD, and (2) the assembly and analysis of a database that contains the results of old and new load tests, and results of simple geotechnical tests at the load test sites. Detailed guidelines for planning and performing new load tests on drilled test shafts in CDOT future construction projects and data analysis are provided. References, appendices, figures, tables, 199 p.
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