Abstract Suffusion, one of the modes of internal erosion, has been widely detected in both natural deposits and filled structures. It is the phenomenon that the fine particles in soil gradually migrate through the voids between the coarse particles, leaving behind the soil skeleton. In this paper, the main focus is on the changes in soil strength due to internal erosion. A series of one-dimensional upward seepage tests at a constant water head is performed to cause internal erosion in a soil sample by controlling the three variable parameters, namely (a) the fine content, (b) the relative density of the soil, and (c) the maximum imposed hydraulic gradient on the specimen. The mechanical consequences of the internal erosion are examined by cone penetration tests. The internal erosion indicated by the loss of fine particles causes changes in the void ratio and a significant increase in hydraulic conductivity, resulting in a decrease in the soil strength from its initial value.