Closure to “Shaft Resistance of Drilled Shafts in Clay” by Tanusree Chakraborty, Rodrigo Salgado, Prasenjit Basu, and Mônica Prezzi

The authors have derived relationships to determine the variation of shaft resistance with depth over the length of a drilled shaft pile during different stages of its construction and installation in clay soils through the use of critical-state soil parameters in a one-dimensional, axisymmetric, finite-element analysis (FEA). Their work was aimed at determining a rational approach to the calculation of shaft resistance of drilled shafts, which so far has been based on empiricism. The validity of the developed relationships was evaluated against available results of full-scale load tests and FEA predictions and found to be generally in good agreement. A particularly important and commendable aspect of the authors’ developmental work is the influence of the method of pile installation on the alpha factor (a, shaft friction coefficient) used in determining the unit shaft resistance based on the drained and undrained shear strength of the soil. Certain simplifying assumptions were imposed by the authors on the mechanisms of installation of a drilled shaft, which, for the most part, appear to be reasonable. The development of the authors’work covers normally consolidated, lightly overconsolidated, and highly overconsolidated clays, and incorporates both drained and undrained behavior of the soils in the determination of an alpha factor, which at present is used to determine shaft resistance from the undrained shear strength in a total stress design approach in pile design. In general, the process of removal and reinsertion of the auger during the installation of a drilled shaft pile results in severe shear straining of the side walls of the drilled shaft. This can be conceptualized to create a somewhat polished surface of the side wall, hence affording a soil-contact surface with the pile that derives its shaft resistance to movement of the pile on loading, based on its criticalstate or residual friction angle depending on the magnitude of the effective normal stress. However, in reality the friction angle influencing the shaft resistance also would depend on the nature/consistency of the clay soil (i.e., whether normally consolidated, lightly overconsolidated, or heavily overconsolidated). As a result, some aspects of the assumptions made by the authors require further elaboration for a clearer understanding of the behavior of the soil influencing the shaft frictional resistance resulting from the mechanism of installation of a drilled shaft pile, as outlined subsequently. The installation process of a drilled shaft pile in clay results in the pile wall undergoing several insertions and removal of the auger to effect a pile to the desired depth because the actual auger is normally a short length of the auger system of which the kellybar attachment is the longest component. The auger is extended from the kellybar as the augering process occurs to effect pile construction to a desired depth. The authors’work, however, appears to indicate that the drilled shaft installation results from a one-stage insertion and removal of the auger. For normally consolidated and lightly overconsolidated saturated clays, the augering method, while causing roughness at the drilled shaft wall by the leading end of the auger on its insertion, also causes removal of the rough interface when the auger progresses with depth, and further on auger removal by the soil that adheres to the auger during the removal phase. This soil is often in a softened state, and, in the process of removal of the auger, results in the interface of the drilled shaft wall being smeared with softened clay. Shaft resistance is afforded in this case by the friction between the soil surface of the drilled shaft wall and soil adhering to the pile shaft. This process would engage the residual friction angle of the soil through soilto-soil shearing contact, which can result in a minimum value of the residual friction angle. On the other hand, when the clay is highly overconsolidated, the drilled shaft wall becomes scored on insertion of the auger. This scoring is not readily filled with softened soil as the clay is not as pliable as the case of the normal or lightly overconsolidated soil; hence, there is much more roughness at the interface between the constructed shaft and the adjacent soil. This roughness ensures that failure occurs within the soil immediately in contact with the pile rather than through soil-to-soil shearing contact. In this case, the friction angle would likely trend toward the critical-state friction angle. However, it also often is noted that, for heavily overconsolidated clays (e.g., clay shales), an insert (back scratcher) is attached to the auger, which ensures that the shaft wall is scored or ribbed beyond the diameter of the auger to ensure that grooves thus formed during insertion and removal would allow for rough interface surfaces to occur when the concrete is poured. This situationwould result in failure taking place within the soil as the process does not contribute to the vertical shearing of the soil as in the case of augering. Thus, in such cases, the shaft resistance should be governed by the peak friction angle rather than the critical-state or residual friction angle of the clay. This discussion indicates that the proposed alpha (a) values in the derived relationships in Eqs. (4) and (5) of the authors’ paper should recognize the variation of possible friction angles resulting from the nature/consistency of the clay soils in which the drilled shafts are being constructed. The discusser would appreciate if the authors can comment on whether a computer program like FLAC3D also would be suitable to simulate the loading and unloading of the soil that occurs during the installation process of auger insertion and removal. Overall, the paper has provided a more rational approach to the derivation of a values that can be determined for both drained and undrained loading, which otherwise conventionally were associated with determining the shaft resistance based on the undrained shear strength of the soil.