Experimental study of primary consolidation time for structured and destructured clays

• Investigating a change law of primary consolidation time with stress levels.
• Comparisons of primary consolidation time determined by different methods.
• Investigating remaining pore pressure at EOP determined by settlement–time curves.
• Investigating additional settlement induced by remaining pore pressure dissipation.

Incremental load one-dimensional consolidation tests were performed on undisturbed and reconstituted specimens of seven natural clays with a predominant clay mineral of illite. Both the settlement and the base pore pressure under step load increments were measured during the dissipation of pore pressure. The change law in the primary consolidation time determined by the pore pressure dissipation with increasing stress level is found to be contrary to that determined by the Taylor method and the Casagrande method which are often used in laboratory tests and engineering practice. The primary consolidation time determined based on the time-dependent settlement observations is smaller than that determined by the pore pressure dissipation for the natural clays investigated. Pore pressure is generally not completely dissipated at the primary consolidation time determined by the Taylor method and the Casagrande method. Such remaining pore pressure may reach a quite high percentage of the step load increment for the investigated specimens. The settlement determined by the Taylor method and the Casagrande method may be significantly underestimated due to the dissipation of remaining pore pressure. Attention should be paid to the possible distortion on consolidation behavior in laboratory tests and engineering practice due to the remaining pore pressure and the associated settlement.