A numerical investigation on the effect of rotation on the Cone Penetration Test

The cone penetration test (CPT) is one of the most popular in-situ soil characterization tools. However, the test is often difficult to conduct in soils with high penetration resistance. To resolve the problem, a rotary CPT device has recently been adopted in practice by rotating the rod to increase the penetrability, particularly in deep dense sand. This study investigates the underlying mechanism of the rotation effects from a micromechanical perspective using models based on the discrete element method (DEM). With rotation, the cone penetration resistance (qc) decreases by up to 50%, while the cone torque resistance (tc) increases gradually. These results are also used to successfully assess existing theoretical solutions. The mechanical work required during penetration was observed to keep rising as the rotational velocity increased. Microscopic variables including particle displacement and velocity field show that rotation reduces the volume of disturbed soil during penetration and drives particles to rotate horizontally, while contact force chain and contact fabric indicate that rotation increases the number of radial and tangential contacts and the corresponding contact forces, forming a lateral stable structure around the shaft which can reduce the force transmitted to the particles below the cone, thus decreasing the vertical penetration resistance.