Once the design seismic action is defined, it becomes necessary to determine the effect of the local seismic response through specific analysis.
In the absence of this analysis, in order to define the seismic action we can refer to a simplified approach, that is based on the determination of the reference subsoil categories. (Tab. 3.2.II and 3.2.III).
Table 3.2.II – Subsoil categories
Category |
Description |
|---|---|
A |
Outcrop rocky mass or very rigid soils characterized by Vs,30 superior to 800 m/s, eventually including in the surface an alteration layer, with a maximum thickness of 3 m |
B |
Soft rocks and deposits of very dense coarse grain soils or very dense fine grain soils with thickness superior to 30 m, characterized by gradual improvement of the mechanical properties with depth and Vs,30 values between 360 m/s and 800 m/s, ( or NSPT,30 > 50 in coarse grain soils and cu,30 > 250 kPa in fine grain soils). |
C |
Deposits of medium dense coarse grain soils or medium dense fine grain soils with thickness superior to 30 m, characterized by gradual improvement of the mechanical properties with depth and Vs,30 values between 180 m/s and 360 m/s, ( or 15 < NSPT,30 < 50 in coarse grain soils and 70 < cu,30 < 250 kPa in fine grain soils). |
D |
Deposits of poor consistency coarse grain soils or poor consistency fine grain soils with thickness superior to 30 m, characterized by gradual improvement of the mechanical properties with depth and Vs,30 inferior to 180 m/s ( or NSPT,30 < 15 in coarse grain soils and cu,30 < 70 kPa in fine grain soils) |
E |
Subsoil terrains of C or D type, with thickness not superior to 20 m, laying on the reference substratum (with Vs > 800 m/s) |
The classification is based on the values of the equivalent propagation velocity Vs,30 of the shear waves in the first 30 m of depth.
For shallow foundations and retaining walls made of embankments this depth is considered beginning from the footing of the foundation..
The direct measurement of the propagation velocity of the shear waves is highly recommended. In case this determination is not possible, the classification can be done in base of the values of the equivalent number of blows of the dynamic penetrometric test (Standard Penetration Test) NSPT,30 in soils consisting mostly in coarse grain and in base of the equivalent undrained strength cu,30 in soils consisting mostly in fine grain.
For the soils in the S1 and S2 categories in Tab. 3.2.III specific analysis are required to define the seismic actions, especially when the presence of soils susceptible to liquefaction and/or very sensitive clays can produce soil collapse.
Table 3.2.III – Additional subsoil categories
Category |
Description |
|---|---|
S1 |
Deposits of soils characterized by values of Vs,30 inferior to 100 m/s (or 10 < cu,30 < 20 kPa), that include a layer of at least 8 m of poor consistency fine grain soils, or include at least 3 m of peat or clays with high organic content. |
S2 |
Deposits of soils susceptible to liquefaction, of sensitive clays or any other subsoil type that can not be classified in the previous types. |
The equivalent velocity of the shear waves Vs,30 is defined by:
The equivalent dynamic penetrometric strength NSPT,30 is defined by:
The equivalent undrained strength cu,30 is defined by:
In the previous expressions we have indicated with:
hi= thickness (in m) of the i layer comprised in the first 30 m of depth;
VS,i= velocity of the shear waves in the i layer;
NSPT,i= number of NSPT blows in the i layer;
cu,i= undrained strength in the i layer;
N= number of layers comprised in the first 30 m of depth;
M= number of layers of coarse grain terrains comprised in the first 30 m of depth;
K= number of layers of fine grain terrains comprised in the first 30 m of depth;
In case of subsoils formed by stratifications of coarse grain terrains and fine grain terrains, with similar thickness in the first 30 m of depth, included in one of the categories from A to E, and when direct measurements of the shear waves velocity are not available, one can proceed as follows:
- determine NSPT,30 in the coarse grain terrain layers comprised in the first 30 m of depth;
- determine cu,30 in the fine grain terrain layers comprised in the first 30 m of depth;
- determine the categories corresponding to the NSPT,30 and cu,30 parameters;
- refer the subsoil to the worse category.
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