Unexpected initial effective stress distribution after geostatic initialization in CPT model

[theaerls17-star]

Dear all,

I am having an issue with the initial stress field for a simple geometry.

After the initial/geostatic stress initialization, the contour of effective stress does not look as I expected. I expected a relatively smooth increase of vertical effective stress with depth due to gravity and the surcharge layer on top. However, as shown in the attached image, I get a strong horizontal band of very negative stress near the top/interface region, and the top surcharge area also shows a striped/oscillating pattern.

In ParaView I am plotting:

My model has a surcharge layer at the top and soil below it. The surcharge thickness is 0.1 m and the soil thickness is 0.9 m.

Also here is the GID Stress initialization configuration of the model