Post-Seismic settlements

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The cause of reconsolidation settlements registered after a seismic event in a soil is due to dissipation of the pore water pressure as the water is expelled from the concerned area.

To estimate the magnitude of this settlement is necessary to characterize the various soil layers from a geotechnical point of view through in situ and laboratory tests.

The number of investigated verticals will be more larger the more important is the work to be carried out and how extensive is the area of investigation.

It is necessary to prepare with appropriate surveys the extent of fluctuations in groundwater levels and consider in the analysis the less precaution condition.

For each of the investigated verticals will be assessed the post cyclic reconsolidation settlements.

The reconsolidation settlement, for granular saturated liquefiable soils and for cohesive soils, can be calculated using the following expression:

 

where H is the height of the generic layer and εvr (%) represents the post-cyclic volumetric strain defined by:

 

where:

α experimental constant between 1 and 1.5

e0 the initial void ratio

Cr=0.225 Cc the post-cyclic reconsolidation ratio

Cc compression ratio

 

Note: There are some empirical relations that allow to evaluate, in an approximate way, the compression ratio. In the case of granular soils are functions of the relative density, in the case of cohesive soils are functions of the plasticity index.

 

In the case of cohesive soils, Loadcap calculates the ratio of pore water pressure as:

 

where

σ'0        is the initial value of the effective mean pressure at the considered depth

 

 

σ'v0 is the effective vertical pressure and k0 the thrust coefficient at rest

 

 

γmax        is the maximum shearing strain reached during the earthquake

β        is taken equal to 0.45 (experimental coefficient)

γv        is the volumetric deformation threshold, determined by cyclic laboratory tests

Ma        can also be evaluated, in first approximation, with the relationship that follows:

 

OCR is the overconsolidation ratio, A and B are the experimental coefficients that can be calculated by linear interpolation from the table below:

 

Values suggested for the coefficients A and B

 

The ratio of pore water pressure, in the case of loose liquefiable soils, is determined by linear interpolation from the values ​​reported in table below, depending on the amplitude of the maximum deformation induced by the ground.

 

Pore water pressure ratio ru as a function of γmax

 

The amplitude of the maximum shearing strain γmax is calculated from the following relationship:

 

where

amax,s peak acceleration at the ground level of the design earthquake

g        acceleration of gravity

sv        total vertical pressure

rd        reduction factor of the seismic action that puts into account the deformation of the subsoil determined by the

 relation rd=1-0.015z;

G        shear modulus corresponding to the strain level γmax

 

The shear modulus can be determined from laboratory tests or using the table below by applying a reduction factor to the shear modulus G0 (shear modulus at low strains).

 

Reduction factor of the shear modulus in the first 20 m as a function of the acceleration amax,s

 

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