Contribution of Vertical Skin Friction to the Lateral Resistance of Large-Diameter Shafts

Large-diameter drilled shafts embedded in stiff materials (e.g., clay and weak rock) experience larger lateral resistance compared to the resistance values obtained from available soil-pile models, which usually produce a conservative design. The axial skin friction (i.e., vertical side shear) developed on the side of the large-diameter drilled shafts in stiff clay and weak rock (i.e., cohesive intermediate geomaterials) improves shaft resistance to the lateral loads (i.e., the shaft-head lateral stiffness). This paper presents a model that calculates the vertical skin friction induced by the vertical-displacement component of the shaft deflection and section rotation through the use of the t-z relationship. The model determines the shaft-deflection resisting moment caused by the axial skin friction on the passive side of the drilled shaft. Up to a 40% increase in the shaft-head lateral stiffness (Kd) (i.e., stiffer foundations) could develop as a result of the consideration of the vertical side shear resistance. In addition, the lateral response of the superstructure would be influenced by the variation of Kd. This study also shows the degrading effect of vertical side shear on the shaft-head lateral stiffness with the increase in shaft deflection. A number of full-scale drilled-shaft load tests have been used in this comparison, and the results obtained from the presented model highlight the contribution of vertical skin friction caused by the shaft deflection/rotation to the lateral resistance of large-diameter shafts.