Abstract: The influence of the slope effect on the horizontal bearing capacity of slop-adjacent digging piles cannot be overlooked. To investigate such influence, nine groups of full?scale horizontal static load tests were carried out on digging piles situated beside and upon slopes in in-situ clayey ground. The relationship curves between the ultimate horizontal bearing capacity, ultimate displacement and the pile-to-slope distance, as well as curves of the equivalent proportional coefficient against the horizontal displacement were acquired. Formulas correlating the horizontal resistance ratio coefficient and the pile-to-slope distance were also derived. The research demonstrates that both the equivalent proportional coefficient nh and proportional coefficient m decrease non?linearly as the slope-adjacent distance is reduced. The coefficients converge to constant values approximating those for level?ground conditions when the normalized slope-adjacent distance L/d≥9. The slope effect becomes prominent at L/d≤5. Every further reduction of 2.5 in L/d causes a 10%–15% drop in coefficient nh, a 15%–20% drop in coefficient m, and a 10%–15% reduction in the critical horizontal load Hcr. The established formulas for nh and m with respect to the slope-adjacent distance in clayey ground can serve as a reference for the design of slope-adjacent digging piles.