Evaluation of bearing capacity design for shallow foundation in cohesionless soil with API and ISO



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A database with 217 cases of load tests on shallow foundations, mostly in/on granular soils, was compiled. With this information, the comparison of predicted bearing capacity using different design methods, the American Petroleum Institute Recommended Practice 2A-LRFD (API RP2A-LRFD,1989), the American Petroleum Recommended Practice 2GEO (API RP2GEO, 2011), and International Standard Organization (ISO, 2003) is carried out.
The answer to which standard makes a good prediction varies with the way to define failure load from load-displacement curve. Overall, ISO has a higher prediction compared with API RP2A and API RP2GEO. For the cases with vertical concentric loading conditions, if the capacity at plastic region and is defined as failure load, then the prediction by API RP2A is closest to measured capacity. If the capacity corresponding with 10% of footing width is considered as failure load, then the API RP2A and API RP2GEO underestimate the capacity, while ISO is in a good agreement with capacity corresponding with 10% of footing width. The prediction by API RP2GEO generally has a good agreement with capacity at tangent intersection region. In most of the cases with vertical eccentric loading, all three standard underestimate the measured capacities no matter which interpreted capacities is considered as failure load, while ISO makes slightly overprediction at the lower range of eccentricities and underprediction at higher range of eccentricities for some cases. In the inclined loading condition, the prediction by API RP2GEO is minimum among the three standards, while ISO proposed the highest prediction. All three prediction underestimate the capacity under inclined loading condition.
From this study, it is found that small scale load laboratory test reveals a qualitative understanding with impact of vertical eccentric and concentric inclined loading on ultimate capacities. However, it is hard to detect a clear best prediction based on small scaled laboratory load test results. The well-controlled field test results, especially the field tests with vertical eccentric and concentric inclined loading condition, are valuable to evaluate the consistency of predicted capacity by three standards.