The Applications and Benefits of Dynamic Testing on Piled Foundations for Uplift Designs

David A. Lee, Robert Brady, Tong Liu

In the proceedings of: GeoMontréal 2013: 66th Canadian Geotechnical Conference; 11th joint with IAH-CNC

Session: Foundations VI

ABSTRACT: This paper will focus on dynamic testing vs blow counts for projects that cover a large area or distance with varying soil conditions where the design criteria is governed by the uplift resistance of the foundations. To support this, it will outline the minimum geotechnical assessments and engineering review requirements for the pre-construction phase and how these relate to the dynamic testing results during the construction phase. In addition this paper will suggest that there is a need for a minimum standard for dynamic testing of deep foundations in Canada and will provide supporting information for this argument with the finding of a recent case study. First this paper will explain the details of why the dynamic formula has fundamental limitations and the back ground theory to the wave equation analysis and PDA/CAPWAP software. Secondly it will discuss why, when the design is governed by uplift resistance, dynamic monitoring has demonstrated varying and possibly better results versus the dynamic formula and even minimum blow counts derived from a wave equation analysis. This point will be supported both by the information gained, such as pile integrity and resistance distribution, and the lower factor of safety that can be utilized with dynamic testing. To further facilitate this point, two situations where dynamic testing proved more valuable than blow counts determined by the wave equation analysis will be described with case studies. Situation 1 is where PDA/CAPWAP showed that a pile with a failing blow count actually met minimum capacity criteria. Situation 2 is when the minimum blow counts are met but the PDA/CAPWAP results determine that the minimum capacities are not met. For both of these situations this paper will describe how soil conditions, set-up factors and dynamic results can be used by engineers to make design validations or field changes to a design to promote design adequacy and economic benefits. Finally a summary of the described benefits of dynamic testing will be used to illustrate why minimum standards are needed for dynamic testing and education in Canada. With the introduction of dynamic pile monitoring and wave equation analysis software driven pile foundation design and construction could greatly benefit from these tools. By utilizing these tools engineering of deep foundations could be advanced to increase the safety of a design and reduce the number or length of piles installed, thus promoting both a safer and more sustainable design. The observed results of a blow count criteria alone can be misleading for both engineers and contractors and not accurately reflect the capacities for the uplift design. Using the focus of this article as one example, this paper would like to offer alternatives to the design engineers that a dynamically monitored pile is a tested pile. In addition to this the time has come for governing boards in Canada to set standards where a percentage of driven piles on all structural foundations must be tested, and those who are providing dynamic monitoring services should be educated to a specified minimum qualification. Whether it is steel, concrete or wood there is no replacement for a dynamic or static load test and due to the lengthy time and expense of a static test, dynamic testing should be a minimum standard on all deep foundation.

RÉSUMÉ: couvrant une grande région ou une longue distance et dont les conditions des sols varient et les critères de conception sont gouvernés par la résistance du pré-contrôle par analyse dynamique durant lstandard minimum quant aux essais dynamiques des fondations profondes au Canada et comportera des informations supplémentaires à cet égard en présentant des résultats d Premièrement, cet article expliquera le raisonnement derrière les limitations fondamentales de la formule dynamique de discuté des raisons pour lesquelles, lorsque la conception est gouvernée par la résistance en tension du pieu, le contrôle par analyse dynamique démontre des résultats qui varient et potentiellement meilleurs que la formule dynamique de battage et même du sé en utili

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