Analysis According to NEN (Buismann, Ladd)
This method computes both the primary and secondary settlement. When computing the method accounts for overconsolidated soils and differentiates between two possible cases:
- Sum of the current vertical effective stress in the soil and stress due to external surcharge is less than the pre-consolidation pressure so that only additional surcharge is considered.
- Sum of the current vertical effective stress in the soil and stress due to external surcharge is greater than the pre-consolidation pressure so that the primary consolidation is set on again. The primary settlement is then larger when compared to the first case.
Primary settlement of the ith layer of overconsolidated soil (OCR> 1) is provided by:
for: σor+ σz ≤ σp (sum of the current vertical stress and its increment is less than the preconsolidation pressure):
for: σor+ σz > σp (sum of the current vertical stress and its increment is greater than the preconsolidation pressure):
where: | σor,i | - | vertical component of geostatic stress in the middle of ith layer |
σz,i | - | vertical component of incremental stress (e.g. stress due to structure surcharge) inducing layer compression | |
σp,i | - | pre-consolidation pressure in the ith layer | |
e0 | - | initial void ratio | |
hi | - | thickness of the ith layer | |
Cc,i | - | compression index in the ith layer | |
Cr,i | - | recompression index in the ith layer |
Primary settlement of the ith layer of normally consolidated soil (OCR= 1) reads:
where: | σor,i | - | vertical component of geostatic stress in the middle of ith layer |
σz,i | - | vertical component of incremental stress (e.g. stress due to structure surcharge) inducing layer compression | |
eo | - | initial void ratio | |
hi | - | thickness of the ith layer | |
Cc,i | - | compression index the ithlayer |
Secondary settlement of the ith layer assumes the form:
for: σor+ σz ≤ σp (sum of the current vertical stress and its increment is less than the preconsolidation pressure):
for: σor+ σz > σp (sum of the current vertical stress and its increment is greater than the preconsolidation pressure):
where: | hi | - | thickness of the ith layer |
Cαr,i | - | secondary compression index below preconsolidation pressure in the ith layer | |
Cα | - | index of secondary compression in the ith layer | |
tp | - | time to terminate primary consolidation | |
ts | - | time required for secondary settlement |
If we specify the value of pre-consolidation index of secondary compression the same as for the index of secondary compression, the program does not take into account in the computation of secondary settlement the effect of pre-consolidation pressure.
Literature:
Netherlandish standard NEN6740, 1991, Geotechniek TGB1990 Basisen en belastingen, Nederlands normalisatie-Institut.