Abstract：The effect of pile length on the load transfer modes of single and multiple gravel piles is studied using a self-developed two-dimensional experimental apparatus with lever-weight loading systems and directive measurement of pile-bottom pressure by stress ring. In the tests，mechanical-crushed marble stone with an average diameter of 0.75 cm is chosen as the material of gravel piles；and the mixture of manual-disposal Lanzhou loess with certain water contents and Shapotou sand in the volume proportion of 3∶1 is used to be the soil around pile. The test results show that there exists a critical length(more specifically，a critical ratio of pile length to pile diameter). In detail，a majority of top load is borne on the bottom within a short gravel pile under the critical length，but on the top arch within a long one over the critical length，the different load transfer modes result in the different failure modes. The failure of the short gravel pile is induced by the pile plunging into the bottom soil，but the failure of the long one is caused by the shear failure in a triangle region of the lateral soil which induces the swelled failure of the top arch. Additionally，in the case of multiple gravel piles，the top load on each gravel pile can influence the pressure distribution at the bottom of the other piles；the interaction between the piles can accelerate the formation of the top arches，and effectively decrease the critical length consequently. This study indicates that different theoretical formulas should be proposed to calculate the bearing capacity of short and long gravel piles.
