Abstract: A method and device for determining in-situ rheological properties of earth materials are disclosed. A low-angle cone penetrometer is pushed into a predrilled cylindrical pilot hole of smaller diameter, to cause enlargement of the pilot hole. In one embodiment, the load applied to the cone is held constant and the relationship between the cone penetration and the time is recorded. In another embodiment, either the load on the cone or the rate of penetration into the pilot hole is held constant and the relationship between the penetration or the penetration rate and the resistance of the material against the enlargement of the pilot hole is recorded. The rheological properties of the material, such as the creep and time or rate-dependent deformation and strength properties, are then deduced from the recorded data.