Excessive movement on spring due to the tank settlemenet

[suminmoon]

Hi,
I met unknowable situation at the site.
First of all, civil engineer has designed tank foundation with consideration of short-term and long-term settlement.
During the hydro-test, settlement and rebound occurred as 34mm for fully filled tank and 27mm for empty tank after hydro-test.
We tried to adjust spring hangers connected with the tank few months after the hydro-test, and we checked the indicator on spring when the tank is fully filled and lowly filled points 74mm and 41mm.
Basically, spring moves according to the tank level because the fluid of tank makes elastic settlement. Which means the differential of spring should be around the differential between settlement and rebound when its hydro-test stage. However, in this case, the differential of spring is over 20mm. That phenomenon doesn't make sense for me.
Who can explain that phenomenon? or somebody has similar experience on site?
I need your help.

Thanks.

 

[TGS4]

Settlement has two types: short-term and long-term. The short-term settlement is spring-like - basically what you described. Long-term settlement is more akin to creep - typically not recoverable, but takes time.