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Reference: Zekkos, D.P., Bray, J.D., Der Kiureghian, A. (2004), “Reliability of shallow foundation design using the standard penetration test”, 2nd International Conference on Site Characterization Proceedings, 19-22/9/2004, Porto, Portugal, Vol.2, pp. 1575-1582.
The sources of uncertainty, variability and bias in the performance of the Standard Penetration Test were identified using an extensive literature review and were classified in a manner that should assist the practice in recognizing and reducing them. These uncertainties and additional uncertainties introduced in the design process are then incorporated in a reliability-based evaluation of the design of shallow foundations. The results of the reliability analyses show that the factor of safety approach can provide an impression of degree of conservatism that is often unrealistic. At times,foundations with smaller factors of safety have smaller probabilities of failure than foundations with higher factors of safety.The reliability-based approach provides rational design criteria,accounting for all key sources of uncertainty in the foundation engineering process, and thus should be the basis of design.
An extensive literature review was made to evaluate sources of uncertainty in the performance of the SPT, based on the previous work of many researchers (Kulhawy and Mayne (1990), Schmertmann (1975), Barton (1990), ASTM D6066-96, Youd et al. (2001), Kulhawy and Trautmann (1996)).
Table 1 summarizes a total of 27 sources of uncertainty and bias, classified in five categories. This classification aims to assist the practicing engineer in taking the necessary measures to minimize the uncertainties in the performance of the test:
Figure: Note that for the same factor of safety you get different Probability of Failures, depending on the type of soil.
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