Research on post-grouted drilled shafts has focused primarily on post-grouted tips. Here, four full-scale shaft load tests were conducted to investigate the behaviors and performance of combined tip-and-side grouted superlong and large-diameter drilled shafts in extra-thick fine sand layers. The enhanced mechanism of the combined grouted drilled shafts is analyzed, and a rational approach for analyzing their load–displacement response is presented. The side and base resistance of the combined grouted drilled shafts exhibited significant strengthening, substantially increasing the bearing capacity and effectively controlling settlement. Under the ultimate load, >60% of the shaft head displacement was caused by shaft compression; a relatively small load proportion was carried by the shaft base. The superlong and large-diameter drilled shaft can be treated as a friction shaft, and the combined tip-and-side grouting cannot change the bearing characteristics. The hyperbolic model describes the relationship between the side resistance and relative shaft–soil displacement and captures the base resistance–displacement response. The proposed approach is verified with a case history, and the bearing behaviors of a large-diameter drilled shaft under an extra-thick fine sand layer are analyzed. These results clarify the bearing characteristics of combined grouted shafts and can help guide the design of post-grouted shafts.