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Groung types according to Eurocode 8

According to Eurocode 8 (§3.1.2), there are five typical ground types (A, B, C, D, E) and 2 special ground types (S1, S2)  that may be used to account for the influence of local ground conditions on the seismic action. The average shear wave velocity in the top  30 m from the surface is computed according to the following equation: (EC8 § 3.1.2 (3))

where

hi and vi denote the thickness (in meters) and the shear wave velocity (at a shear strain level of 10-5 or less) for the i-th formation or layer, in a total of N.

If the value of vs,30 is not available, the number of block outs per 0.3 m in ΝSPT test can be used. If this number is not available either, the undrained cohesion cu can be used. The following table presents the description of each ground type, and the definition parameters.

Table 1: Ground types according to Eurocode 8(EC8 § 3.1.2 Table 3.1)

Ground type and description

vs,30

ΝSPT

cu

A: Rock or other rock-like geological formation, including at most 5 m of weaker material at the surface.

>800

-

-

B: Deposits of very dense sand, gravel, or very stiff clay, at least several tens of meters in thickness, characterized by a gradual increase of mechanical properties with depth.

360-800

>50

>250

C: Deep deposits of dense or medium dense sand, gravel or stiff clay with thickness from several tens to many hundreds of meters.

180-360

15-50

70-250

D: Deposits of loose-to-medium cohesionless soil (with or without some soft cohesive layers), or of predominantly soft-to-firm cohesive soil.

<180

<15

<70

E: A soil profile consisting of a surface alluvium layer with vs values of type C or D and thickness varying between about 5 m and 20 m, underlain by stiffer material with vs > 800 m/s.

 

 

 

S1: Deposits consisting, or containing a layer at least 10 m thick, of soft clays/silts with a high plasticity index (PI > 40) and high water content

<100

-

10-20

S2: Deposits of liquefiable soils, of sensitive clays, or any other soil profile not included in types A – E or S1

 

 

 

 

The following figures present the elastic response spectrums defined by Eurocode 8 for each ground type. As mentioned, Eurocode 8(§3.2.2.2(2)) defines 2 spectrum types: Type 1 for regions with high seismic activity (defined as Μ > 5,5), and Type 2 for regions with average seismic activity (Μ < 5,5). Spectrums for each ground type are presented that include ground types: Α - rock , Β – very dense sand, gravel or very stiff clay, C – dense or medium dense sand, gravel or stiff clay, D – loose-to-medium cohesionless soil (with or without some soft cohesive layers), or of predominantly soft-to-firm cohesive soil, Ε – soil profiles consisting of a surface alluvium layer with vs values of type C or D and thickness varying between about 5 m and 20 m, underlain by stiffer material. The vertical axis is the spectral acceleration of an elastic structure normalized to the ag.

Figure 1: Elastic response spectrum Type 1 according to EC8 for damping  5%           (EC8 § 3.2.2.2)

Figure 2: Elastic response spectrum Type 2 according to EC8 for damping  5%              (ΕC8 § 3.2.2.2)

Next, Tables 1.4 and 1.5 present the effect of the local ground factors on the seismic actions because of the spectral ground acceleration. The factors that are depending on the ground type are the ground, S, and the periods TB, TC and TD, of which the definition is given right under the following tables. Eurocode 8 predicts different values for these factors depending on the elastic response spectrum that has been adopted in each region (Type 1 and Type 2).

Table 2: Values for the elastic response spectrum Type 1 according to Eurocode 8

(EC8§ 3.2.2.2 Table 3.2)

Ground type

S

TB

TC

TD

A

1.0

0.15

0.4

2.0

B

1.2

0.15

0.5

2.0

C

1.15

0.20

0.6

2.0

D

1.35

0.20

0.8

2.0

E

1.4

0.15

0.5

2.0

 

Table 3 : Values for the elastic response spectrum Type 2 according to Eurocode 8

(EC8 § 3.2.2.2 Table 3.3)

Ground type

S

TB

TC

TD

A

1.0

0.05

0.25

1.2

B

1.35

0.05

0.25

1.2

C

1.5

0.10

0.25

1.2

D

1.8

0.10

0.30

1.2

E

1.6

0.05

0.25

1.2

where:

TB is the lower limit of the period of the constant spectral acceleration branch

TC is the upper limit of the period of the constant spectral acceleration branch

TD is the value defining the beginning of the constant displacement response range

of the spectrum

S     is the soil factor

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