How to simulate the seepage field

[LiaoMk]

Dear all，

We want to simulate the process of soil particles being carried away by seepage in a soil layer, and there exists a rectangular sandy soil specimen with a length of 0.48 m and a height of 0.1 m in a two-dimensional plane. The left boundary of the specimen is used as the upstream waterward side by applying water pressure, and there is an outlet with a diameter of 0.02 m at the top boundary, and the center of the outlet is 0.35 m away from the left boundary, and the rest of the boundary is an impermeable layer.

However, our simulations show that it is difficult for the water pressure to be transferred to the set seepage outlets, where the water pressure setting of 5kPa、50kPa or -5kPa gives the same result.

Why does this happen? And how to simulate the seepage field in soil layer?