The FDM-DEM Coupled Method for Studying the Stress Reduction Characteristics of the Cone Penetration Test

This paper investigates the reduction in radial stress from the cone-shoulder region to the friction sleeve during cone penetration testing (CPT), with particular reference to the interpretation of sleeve friction using cylindrical cavity expansion theory. The limiting cavity-expansion stress does not, in general, represent the radial stress acting subsequently on the friction sleeve because the surrounding soil undergoes unloading and stress redistribution following passage of the cone shoulder. A coupled finite-difference–discrete-element (FDM–DEM) model is therefore employed to quantify this stress reduction and to examine its underlying mechanisms. A stress-reduction coefficient is introduced to relate the radial stress acting on the friction sleeve to the limiting cavity-expansion stress. The numerical results show that the reduction in radial stress is associated with pronounced loading–unloading behavior, changes in stress path, radial-stress evolution and the rotation of the principal stress axes. For the silty clay and CPT configuration considered, the radial stress at the middle of the friction sleeve is approximately 70% of its peak value near the cone shoulder. Incorporation of this stress reduction provides a more physically consistent basis for relating cavity-expansion stress to sleeve friction, although the resulting coefficient is case-specific and requires further validation for other soil and stress conditions.