Interpretation of Cone Penetration Testing in Silty Soils Conducted under Partially Drained Conditions

The standard penetration rate used in cone penetration tests (CPTs) is 20mm/s, regardless of soil type, which yields fully drained penetration in sand and fully undrained penetration in clay. However, for silty soils that represent an intermediate grain size composition and unique characteristics compared with sand and clay, the standard rate of penetration results in partially drained penetration, often leading to misinterpretation of this soil type. In this study, 15CPTs, with penetration rates varying from 0.5 to 60mm/s, were performed at a test site in northern Denmark, where the subsoil consisted primarily of sandy silt with clay bands. The results illustrated that when the penetration rate is reduced, the cone resistance increases, but the pore pressure decreases. The transition between undrained and fully drained penetration was determined by converting the results into a normalized penetration rate. Also evaluated and presented in this paper is how cone resistance obtained under partially drained conditions underestimates the interpreted relative density Dr and friction angle phi. Triaxial test results on undisturbed silt samples were applied for this analysis.