UCSD models Material update for static analysis

[oscar]

I have a 1-D soil column (quad elements) with dashpots at the bottom as in Lysmer Boundary case. Lateral nodes are constrained in both directions. Im trying to find the reactions at the bottom and apply them to solve the static gravity analysis. However, Im confused with the material update stage.

When I only use elastic material stage as in;
CASE1:
analysis Static
updateMaterialStage -material 1 -stage 0
updateMaterialStage -material 2 -stage 0
analyze 1

Im getting the reaction forces at the bottom as 129.496 196.2 in x and y directions respectively for one of the nodes at the bottom.

I get the same results when I use two analysis by udating from elastic to plastic as in;
CASE2:
analysis Static
updateMaterialStage -material 1 -stage 0
updateMaterialStage -material 2 -stage 0
analyze 1

updateMaterialStage -material 1 -stage 1
updateMaterialStage -material 2 -stage 1
analyze 1

However if I only use the plastic case as in:
CASE 3:
analysis Static
updateMaterialStage -material 1 -stage 1
updateMaterialStage -material 2 -stage 1
analyze 1

The reaction force at the bottom node changes in x direction (184.501 196.2). It changed from 129.496 kN to 184.5 kN.

Can someone explain me the reason for this? Note that I'm only using the gravity forces. So I thought the plastic stage isnt reached for the 1st and 2nd cases so I got the same answers. However, when I only use the plastic stage the results changed.
Any help is appreciated.

Oscar