The contents of this report reflect the views of the author(s), who is(are) responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official views of the Colorado Department of Transportation or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. The preliminary design recommendations should be considered for only conditions very close to those encountered at the load test sites and per the qualifications described in Chapter 6. Use of the information contained in the report is at the sole discretion of the designer.

As part of the I-15 Corridor Reconstruction Project through downtown Salt Lake City, nine sets of full-scale load tests were performed at locations selected as representative of the subsurface conditions along the corridor alignment. Static compression, uplift, and lateral load tests were conducted at each test site location. The test program included dynamic monitoring of pile installation and restrike events using high-strain testing and analysis methods. Dynamic test results from restrike events on companion piles compared well with the measured static axial compression load test capacities. Subsurface conditions along the alignment range from deep clays to dense alluvial sands above the groundwater table. The piles derive their support from shaft friction or a combination of shaft friction and endbearing, depending on the strata present. The test and subsequent production piles are consistently indicating large capacity gains with time (setup), regardless of the subsurface conditions. The setup in the soft to stiff lakebed clays is attributed to remolding of the clays during pile driving and subsequent reconsolidation. The setup in the dense sands is comparable with setup described by others in dense marine sands. Relationships were developed to predict long-term pile capacity based on dynamic test results from the end of installation and on data from the beginning of restrike. These relationships are being used during production pile installation and restrike events as an integral part of pile evaluation, troubleshooting, and quality control/quality assurance acceptance procedures.

Drilled shaft foundations embedded in weak rock formations support a large percentage of bridges in Colorado. Since the 1960s, empirical methods that entirely deviate from the AASHTO design methods have been used for the axial geotechnical design of these shafts. The margin of safety and expected shaft settlement are unknown in these empirical methods. Load tests on drilled shafts provide the most accurate design and research data for improvement of the design methods. Four Osterberg axial load tests were performed in Denver on drilled shafts embedded in soil-like claystone, very hard sandy claystone, and extremely hard clayey sandstone. An extensive program of simple geotechnical tests was performed at the load test sites, including standard penetration tests (SPT), unconfined compressive strength tests (UCT), and pressuremeter tests (PMT). Information on the construction and materials of the test shafts was documented, followed by thorough analysis of all test results. Conservative equations were suggested to predict the unconfined compressive strength and mass stiffness of weak rocks from SPT and PMT data. Colorado Department of Transportation (CDOT) and AASHTO–FHWA design methods for drilled shafts were thoroughly assessed. Design equations to predict the shaft ultimate unit base resistance ( qmax), side resistance ( fmax), and an approximate load–settlement curve as a function of the results of simple geotechnical tests were developed. The qualifications and limitations for using these design methods are presented (e.g., construction procedure, field conditions). Finally, a detailed strategic plan to identify the most appropriate design methods per LRFD for Colorado's drilled shafts was developed.

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