Abstract: Soil organic matter (SOM) usually occurs in mineral-associated or particulate forms, with significant variations in the physical and chemical properties among different forms of organic matter. In soil mechanics, there has been focusing on the influence of SOM content on the macroscopic engineering properties of soil. To date, limited knowledge exists regarding the influence of SOM occurrence form on soil engineering properties. In this study, soil samples with different SOM contents w_u were manually prepared, and the contents of various occurrence forms of SOM were measured using Fu’s method. Direct shear tests were conducted under drained and undrained conditions to elucidate the variation in ultimate shear strength and shear strength parameters with SOM content w_u, while also examining the impact of SOM occurrence form on the shear strength of organic soil. The experimental outcomes are as follows. The internal friction angle undergoes a notable decrease with increasing w_u under undrained conditions, which can be categorized into three distinct stages: a significant decline (Stage I), a transition phase (Stage II), and a stable change (Stage III). w_u corresponding to the endpoint of stage I approximates the threshold w_u,2, suggesting that the pronounced reduction in internal friction angle with w_u augmentation primarily occurs in organic soils dominated by mineral-associated SOM. Stage III emerges approximately after w_u > 25%. Under drainage conditions, the internal friction angle diminishes with w_u augmentation, yet its variation is independent of the occurrence form of SOM. No discernible correlation exists between cohesion of organic soil and occurrence form of SOM under drained and undrained conditions. Mechanism analysis reveals that mineral-associated SOM facilitates lubrication and diminishes friction between soil particles under undrained conditions. When the content of particulate form SOM reaches a critical threshold, the mechanical properties of the soil transforms from a frictional material to a colloidal material. Nevertheless, under drainage conditions, SOM’s susceptibility to compression results in the soil skeleton ultimately comprising primarily mineral soil particles, regardless of SOM content or occurrence form.

Key words: organic soil, shear strength, soil organic matter occurrence forms, cohesive force, internal friction angle