When calculating consolidation settlement under structure loads, it is recommended to use the net foundation stress (net foundation stress= stress of the structure - the stress of the excavated soil). For a normally consolidated soil, how is this assumption correct?
For example, if the structure loads are around 300 kPa, considering the excavated soils, the net stress is around 40 kPa. The settlement caused by 40 kPa is not significant.
My concern is, the excavated area consisted of soil materials, the newly introduced elements are different from soil materials and thus, settlement should be calculated by ignoring the excavated portions. Unless, the soil profile changes, which in that case, settlement will be evaluated considering the existing condition.
#consolidation #geotechnical #settlement
I did not quite get the whole of your question.
In the first part of the question you are asking how,
Net Foundation Stress = Stress of structure - Stress of the excavated soil, assumption holds in normally consolidated soil. The answer is quite clear when we are calculating the NET stress (it basically is part of gross pressure which is in excess to initial overburden stress) on the ground due to structure or so. It means that we are talking about the stress at foundation level. We excavate the portion of the soil upto the foundation level before going into construction part, this portion of soil when not excavated by virtue of its unit weight and density puts some stress onto the soil at foundation level, now when it is excavated this additional stress is not there on the soil at foundation level hence we deduct it from gross stress to get net foundation stress on the soil.
Further when calculating settlements, there are different formulae for normally consolidated soils and over consolidated soils.
Thank you Muneeb Farooq .
Exactly, my question was, when we excavate the soil, indeed there is reduction of stress, but instead of the excavated soil we are placing a structure on the normally consolidated soil. By doing this, sometimes the excavated stress is more than the additional stress and hence in those situations, additional settlement or settlement is not expected. Indeed the structure will exert its normal loads (Dead+live) when the construction is completed the stresses from the foundation will definitely cause additional settlements. Hence my question, the excavated soil is not equal to the placed structure.
Dear Ahmed Hussein Olat
1. You said "Exactly, my question was, when we excavate the soil, indeed there is reduction of stress, but instead of the excavated soil we are placing a structure on the normally consolidated soil."
Reply: When you excavate, the soil tends to become over-consolidated depending on the amount of excavation. We may or we may not be placing the structure on NC soil. It could be OC soil. We have to work that out. After this when you apply structural loads, the settlement could happen which depends on the applied load intensity.
2. You said "By doing this, sometimes the excavated stress is more than the additional stress and hence in those situations, additional settlement or settlement is not expected."
Reply: This may not always be true. If it is, it could be because the settlement of OC soils is lesser than that of NC soils.
3. You said "Indeed the structure will exert its normal loads (Dead+live) when the construction is completed the stresses from the foundation will definitely cause additional settlements."
Reply: This is true depending upon type of soil, load intensity etc.
However, in general practice, conservatively the soil is considered as NC.
Thank you B.R. Madhusudhan you have ansewred my questions clearly.
In addition, lets say even when you remove the excavated soil the soil is still normally consolidated. the base pressure-the excavated soil= pressure that causes less settlement compared to the settlements calculated from only the base pressure without removing the excavated soils.
In this situation, will the excavated soil be neglected?
Dear Ahmed Hussein Olat
Consider the original ground condition without any structure. The 'excavated soil' which you are referring to, was already present as overburden soil. Do you expect any settlement without any structural load and only by overburden? Generally, we assume the answer as 'No'.
You are choosing to consider the soil as NC even after excavation. Fine.
Now you are applying structural load. So any settlement that happens is predominantly because of the structural load alone even though you back fill it with excavated soil.
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