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Evaluation of Scale Effects in Load-Displacement Response of Drilled Shafts Based on Field Load Tests
Civil and Environmental Engineering
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Load testing such as standard top-down or bi-directional tests are widely accepted practices that provide foundation designers with site-specific information about the nominal resistance of drilled shaft foundations. The application of load testing in drilled shaft foundations provides insight into the load-displacement behavior which can be used to reduce risk, improve reliability and confirm design assumptions. Typically, these tests are justified for mid to large-size projects where foundation construction costs are substantial and can be offset by design improvements achieved from performing such tests. As an example, in a recent project in Northern Nevada, a bi-directional test on a 7-ft diameter, 115-ft length drilled shaft imposed an initial cost of $125K to the project, however resulted in a 15% reduction in the total foundation cost which was equivalent to $1.3 million in savings. The relative inexpensiveness of testing drilled shafts as compared to the potential savings on large projects is self-evident. However, the current practice presents challenges when considering the magnitude of loads required in fully mobilizing the capacity of large diameter drilled shafts. In some cases, the feasibility of these tests is impractical because the associated initial cost is not guaranteed to outweigh the final savings. As a result, load testing is sometimes left out of the design phase of projects although their use as deep foundation systems for buildings and large span bridges is widespread. Oftentimes, the magnitude of loads applied in such drilled shaft load tests are selected to limit the degree of mobilization, allowing the foundation to be utilized as a production shaft. This practice restricts our knowledge of the true capacity of the drilled shaft at failure; hence the nominal resistance remains unknown. As a result, cost effective and innovative solutions for performing load tests on smaller diameter drilled shafts and then being able to extrapolate the result to gain knowledge on the response of large diameter shafts is of key interest. In this research, we explore relationships of diameter dependency of unit side and base resistances between small and large diameter drilled shafts using available field load test data. A dataset of 145 load tests, conducted on drilled shaft foundations, were collected as part of an evaluation of scaling effects in load-displacement response of drilled shaft foundations. Drilled shafts ranging in diameter from 2 to 9.8 feet with lengths of embedment ranging from approximately 13 to 250 feet were collected as part of this study. The dataset represents drilled shafts installed in a broad spectrum of coarse and fine-grained soil types. Careful review of load test records established 612-unit side resistance curves for our evaluation with roughly 12% exhibiting a fully-mobilized load-displacement behavior. Comprehensive review of all available load test reports, subsurface investigation reports and geotechnical design reports were used to establish generalized subsurface profiles for each respective drilled shaft. Generalized soil material types were assigned to each unit side and base resistance curves for further analysis of scaling effects. Estimations of geotechnical properties (vertical effective stress and undrained shear strength) were developed and used as normalization parameters to reduce the measured unit side resistance to a standard confining stress. Estimation of the properties used as normalization parameters relied on in-situ, laboratory measurements as well as empirical correlations. In this study, two existing extrapolation methods were evaluated and a new one is being proposed for further evaluation to predict fully mobilized load-displacement behavior from partially mobilized records. Each proposed method of extrapolation will be presented and discussed, identifying potential drawbacks in their application to predict maximum unit side resistance. Using the data for predicted and measured fully-mobilized response, values of unit side resistance were compared against drilled shaft diameter. Comparisons were made for general soil type and grouped by the following: a) Predominant Soil Type, b) State, c) USCS and, d) Shaft construction method. Results using measured and predicted fully mobilized data generally showed considerable variability in non-normalized and normalized values of maximum resistance. The relationships observed in these plots indicates that shaft diameter dependency in unit side resistance is minimal or non-existent. Results of this study conclude that scaling effects in unit side resistance are insignificant, however these results support Federal Highway Administration (FHWA) guidance on the use of prototype drilled shafts to measure fully mobilized load-displacement response in shafts loaded to failure. Performing load tests on prototype drilled shaft foundations no less than ½ the diameter of the intended production shaft diameter should yield similar capacities in unit side resistance.