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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 preconsolidation 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 preconsolidation pressure so that the primary consolidation is set on again. The primary settlement is then larger when compared to the first case.

Primary settlement

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

-

preconsolidation pressure in the ith layer

eo

-

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

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 preconsolidation 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 preconsolidation pressure.

Literature:

Netherlandish standard NEN6740, 1991, Geotechniek TGB1990 Basisen en belastingen, Nederlands normalisatie-Institut.