In recent years, using carbon nanomaterials as soil amendments has increased owing to their unique properties. However, the effects of carbon nanomaterials on soil properties, particularly soil shear strength and penetration resistance, remain limited. Therefore, we evaluated the effects of 0, 50, and 100 mg kg⁻¹ of graphene, graphene oxide, reduced graphene oxide, multi-walled carbon nanotubes, and multi-walled carbon nanotubes-OH on the shear strength and penetration resistance of a sandy loam soil under 100, 75, and 50% field capacity (FC) moisture conditions in a six-month experiment. The effects of the different nanomaterials on shear strength and penetration resistance were different. Specifically, graphene and multi-walled carbon nanotubes-OH had no significant effect on shear strength, whereas 100 mg kg⁻¹ of them increased penetration resistance by 31.4% and 22.4%, respectively, compared to the control. However, graphene oxide, reduced-graphene oxide, and multi-walled carbon nanotubes increased both penetration resistance and shear strength, and this effect increased with increasing application level. Furthermore, the interaction effect of different moisture conditions and the application levels on shear strength was statistically significant only at 50% FC and in the graphene oxide treatments, resulting in a reduction in shear strength. In contrast, penetration resistance decreased by 17.7% and 15% in reduced graphene oxide and multi-walled carbon nanotubes treatments, respectively. Findings indicated that the type, application rate, and physicochemical properties of nanomaterials play decisive roles in the manner by which these nanomaterials modify soil, and that the targeted application of these materials can be an effective approach for improving soil quality.