Determination of the red bed soft rock’s apparent preconsolidation pressure

Apparent preconsolidation pressure is an important parameter for soft rock. In this paper, investigated red bed soft rock was taken from Dingxi region, Gansu province, China. In order to obtain the apparent preconsolidation pressure of the soft rock, firstly, a new YS-1 high pressure oedometer was redesigned and used in the step loading confined compression experiments. Due to the soft rock’s low compressibility, the compressive curve was flat. Numerical mapping method was proposed considering curvature variation of the curve was not obvious. Secondly, the actual boundary condition which is the void ratio should monotonically decrease with the vertical pressure in the given domain was introduced. The existing 7 mathematical models were compared using the new defined conditions. The results showed that the Harris model was in good agreement with the compressive experimental results. Finally, the apparent preconsolidation pressure of the soft rock was calculated using the equations established in this research.