Abstract: In order to study the bearing behavior of a flexible single pile under combined horizontal and uplift loading, two groups of structural testing, comprised of 6 pile models in total have been carried out. The results show that for a flexible single pile, pre-applied uplift load with a magnitude smaller than half of pure uplifting capacity(Tu1) lead to significant improvement of ultimate horizontal bearing capacity, achieving greater horizontal resistance compared to pure ultimate horizontal bearing capacity (Hu1) Additionally, it has been observed that this horizontal bearing capacity increases initially but decreases subsequently under further increasing uplift load. This load increasing results in reduction of bending moment and soil resistance whilst degrades horizontal stiffness, and consequently, the horizontal displacement of pile increases initially but decreases subsequently under resultant effect. The ultimate uplift bearing capacity of a flexible single pile could be improved under pre-applied horizontal loads not exceeding 0.5Hu1, mainly attributed by increasing the average side friction of pile within the distance of 10 times the diameter of pile below the ground surface. This capacity is strengthened under larger horizontal load, and when this load reaches 0.5Hu1, the magnitude is increased by 17.1% compared with Tu1. This behavior could be taken into account with respect to pile design. The precision of test results has been verified by theoretical analysis, and a formula for calculating the ultimate uplift bearing capacity of flexible single pile under combined loads has been proposed. The corresponding analysis results have achieved good agreements with testing data and the error is less than 6%.

Key words: pile foundations, bearing behavior, model tests, combined loading, soil-pile interaction, loess subgrade