Closure to “Predicting End Bearing Capacity of Post-Grouted Drilled Shaft in Cohesionless Soils” by Gray Mullins, Danny Winters, and Steven Dapp

Abstract

The authors have presented a very interesting paper on postgrouted drilled shafts PGDS , a deep foundation system that is relatively new to North American geotechnical practice. A clear strength of the paper is the documentation of full-scale field test data for five test sites, particularly given the scarcity of these types of data on deep foundation systems. In the opinion of the discussers, however, the paper does not provide important information regarding procedures and data reduction of the field tests conducted. This lacking information is important so that readers can have a complete story. However, the discussers believe this paper is a very useful contribution toward establishing a reliable design rationale and methodology for PGDS in different types of soils. Review of the references cited in the paper shows that the case histories involved load tests carried out using the Statnamic device, which typically involves test durations of less than a second. This is important information that should be mentioned explicitly in the paper. Although Statnamic testing has been used for quite some time and its usefulness is not questioned, it is important that this aspect be mentioned in the paper and any associated assumptions involved in the field data reduction should be stated and discussed. Furthermore, some important details on the full-scale load tests were not found in the references cited for each case history, and it would be helpful to the geotechnical community’s understanding of the use of postgrouted drill shafts if some additional information is provided by the authors. This is particularly important in North America, where this deep foundation system is relatively new. Specifically, the following design issues should be specified for each case history presented: 1 whether grout pressure was locked in or not; 2 how end bearing load versus tip displacement was measured or backcalculated during each Statnamic test; and 3 whether the end bearing versus tip displacement results presented were corrected for possible damping and load rate effects that can be associated to the Statnamic testing procedure. In closing, the authors have provided a very valuable paper that illustrates the promise of this new deep foundation system and improves our knowledge of the design issues involved with postgrouted drilled shafts. As our knowledge and understanding of some of the pending technical and design issues related to the use of postgrouted drilled shafts increase, we will likely find that