Abstract: The cementation formed by free iron oxide and its special structural connections between particles are closely related to the mechanical properties of laterite. To investigate the effect of free iron oxide on the mechanical properties of laterite in situ, selective dissolution of free iron oxide in laterite was carried out using a Dithionite-Citrate-Bicarbonate solution. The shear strength indexes of laterite at various depths under different iron removal conditions were determined through borehole shear testing (BST). The results indicated a significant correlation between the content of free iron oxide and the duration of DCB solution immersion, with a rapid removal rate in the initial stages followed by a slower rate in the later stages. The stress-strain curve of laterite exhibits strain hardening and deformation in BST testing. At the same depth, the shear strength of laterite decreases as soaking time increases, with a reduction of up to approximately 79.0% within the initial four days of testing. For the same soaking time and depth, the shear strength of laterite increases with increasing normal stress. Similarly, at the same soaking time and normal stress, the shear strength of laterite increases with the increase of depth. As soaking time increases, free iron oxide in the forms of “envelope”, “bridge” or “single particle” is progressively eliminated, leading to damage in the microstructure of laterite. The original dense structure develops pores, cracks, and becomes loose, resulting in a macro-level reduction in the mechanical strength of laterite.

Key words: borehole shear test (BST), free iron oxide, lateritic, shear strength