In preloading process, we apply a certain weight of surcharge on soil to consolidate and dissipated water comes out to surface which offers a foundation soil of less susceptible to settlement.
In preloading, we can also allow horizontal discharge of water by providing vertical drains may be of sand or geosynthetic drains. Now come to the point, secondary consolidation.
Secondary consolidation offers a very small settlement which even cannot be measured. This is that portion of settlement which occur after fully dissipation of excess hydrostatic pore pressure. We can explain as the result of creep in soil ‘skeleton’. We know creep is a function of time i.e. it takes time.
When soil is preloaded for sufficient time before structures are built on that soil, the major part of settlement has already been taken place and left mainly second type of consolidation stated above.
So in preloaded soil, in prediction, total settlement, secondary consolidation is very important and significant too. Thus long term settlement of such structures will be due to secondary consolidation. We can provide an expression for secondary consolidation as below:
Where H = Thickness of layer of our interest
to and t are times in between secondary consolidation is to be measured.
Cα= rate of secondary consolidation which can be explained as vertical strain for each increase in log cycle in between to and t (i.e. during secondary consolidating).
The soil type like peats and organic soils are greatly affected by secondary consolidation; the extreme settlement encountered in such soils cannot be mitigated by preloading. Thus vertical drains also have actually no influence on secondary settlement.
Rate of this consolidation in precompressed clays are lower than that of normally consolidated clays and when a clay layer is subjected to surcharge, it becomes in overconsolidated state. Thus the parameter Cα is decreased to (say) 50% or sometimes even 10% of original value.
Let’s take an example, if Cα value for normally consolidated clay is 0.01, when it reached preloaded state Cα might be decreased to 0.005 and even less say 0.001.