Hello Duong Nguyen

As you know pre-consolidation pressure is the maximum effective vertical overburden stress that soil experienced in its past, but adding NaCl for pore fluid if it is more than a particular amount will weaken the formation of soil and deforms the microstructure with time in consolidation period especially if the soil is expansive, so I believe the reason is excessive NaCl in water for saturating the soil in oedometer test

Thank you Jamhiri.

The materials used are kaolinite and mixture of kaolinite and bentonite. Using NaCl as the pore fluid will change the diffuse double layer thickness and result in decreasing the repulsive forces. Consequently, the modified effective stress will increase. So I believe that it will lead to increase Pc. I think the Pc, in this case relates to the repulsive and attractive forces.

I used the Casagrande method to find the limits of plasticity and liquidity.

In order to determine the soil preconsolidation pressure, I used a device called a consolidometer.

Watch article on the subject: Load support capacity of an Oxisol cultivated with sugarcane and mechanization effects on the soil

Available in: https://www.researchgate.net/publication/276205174_Capacidade_de_suporte_de_carga_de_Latossolo_Vermelho_cultivado_com_cana-de-acucar_e_efeitos_da_mecanizacao_no_solo

Data Capacidade de suporte de carga de Latossolo Vermelho cultiva...