Identification of ground types

Clause 3.1.2(1) The influence of the local soil condition on the seismic response of structures can be quantified by defining ground types with different mechanical properties. Five ground types have been selected to identify the soil profiles. Alphabetical capital letters (A, B, C, D and E) are used for such profiles. Table 3.1 of EN 1998-1 ('ground types1) provides for each ground type a description of the stratigraphic profile and the parameters used to classify the soil. Three parameters have been used in Table 3.1 of EN 1998-1. They are ranked as follows:

• average shear wave velocity (vs 30)

• number of blows evaluated with the standard penetration test (Nspr)

• undrained cohesive resistance (cj.

It should be noted that the site classification can be based on the values of vs 30 where available. Otherwise, values of NSVT are used. Values of NSPJ and cu are typically used in geotechnical problems to define the mechanical properties of soil. For soils A and E, values of A^spt and cu have not been provided in Table 3.1 of clause 3.1.2 of EN 1998-1. Ground type B exhibits values of iVSPT greater than 50, and ca greater than 250 kPa, while ground type D has iVSPT less than 15 and cu less than 70. Intermediate values of ,/VSPT and cu are used to classify ground type C.

The ground characterization in Table 3.1 of EN 1998-1 ranges from rock or other rock-like geological formations (ground type A) to surface alluvium with a thickness varying between about 5 and 20 m (ground type E). Ground type A is characterized by a shear wave velocity vs 30 of greater than 800 m/s. For smaller values of vs 30 the stratigraphic profile comprises very dense sand, gravel or very stiff clay (ground type B, 360 < v 30 < 800 m/s). Ground type C includes deep deposits of dense or medium-dense sand, gravel or stiff clay; the shear wave velocity is 180 < vs 30 < 360 m/s. If vs 30 < 180 m/s, the deposit consists of loose-to-medium cohesionless soil or of predominantly soft-to-firm cohesive soil. Clauses 3.1.2(2), The shear wave velocity (vs) is relatively straightforward to measure, either in situ or in the 3.1.2(3) laboratory. The shear modulus (G) is directly related to vs by the relationship

in which p is the mass density of the soil, which is often easily evaluated. Therefore, reliable estimates of G depend on accurate measurements of vs. Equation (D3.1) provides upper bounds for shear moduli (Gmax): as the strain increases, the soil stiffness decreases. The modulus reduction factor, i.e. the ratio G/Gmax, depends on several environmental and loading conditions. In addition, soil profiles are frequently heterogeneous; drill boreholes maybe essential to identify different strata and their thickness. The latter can also be utilized to estimate shear wave velocities (up-hole, down-hole, cross-hole, bottom-hole and in-hole). Geotechnical characterization of the heterogeneous soil profile is performed by means of the average value of the velocity vs.

The average shear wave velocity vs 30 is defined inequation (3.1) of clause, in which hi and v; are the thickness (in metres) and the shear wave velocity (evaluated at shear strain level not greater than 10"6) of the zth formation or layer, in a total of N layers, of the top 30 m. Recent comprehensive studies have shown that, during strong shaking, soil strains can be as high as 7 = 5 x 10"3 or even more, leading to shear modulus ratio G/Gmax s 1/10.5

Two special ground types, namely S! and S2, have also been listed in Table 3.1 of EN 1998-1. The former includes 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. Ground type S2 accounts for all the other soil profiles, and includes deposits of liquefiable soils and sensitive clays. Soil type S2 is thus likely to fail under earthquake ground motion, which may cause severe structural damage. Clause 3.1.2(4) requires special studies for ground type S2.

Similarly, soil type S5 can generally produce anomalous seismic site amplification and soil-structure interaction effects (see Section 6 of EN 1998-5) which significantly influence the earthquake characteristics and hence seismic action at the construction site. Soil Sj exhibits, in fact, very low values of shear wave velocity, low internal damping and an abnormal range of linear behaviour. Liquefaction of soils leads to catastrophic failure. On level ground, it causes loss of bearing capacity of foundation systems, and on sloping ground it gives rise to flow conditions, although on very gentle slopes lateral spreading occurs. There may be some delay for the effects of liquefaction to appear on the surface. This circumstance generally occurs when the liquefied deposits are at some depth overlain by a relatively impermeable clay layer. It takes time for the water under high pressure at that depth to flow out through the clay layer thus affecting the pore pressures in the top layer and causing damage. The upper layer will first swell and then consolidate, while the liquefied layer will consolidate. The time and the pressure gradient will depend on the relative consolidation and swelling characteristics of the two layers.6 Detailed geotechnical studies should assess the effects of the thickness of soil layer and shear wave velocity of the soft clay/silt layer and the variation of stiffness between layers of soil Sj and the underlying materials on the response spectrum.

Using only the ground types included in Table 3.1 of EN 1998-1 to assess the stratigraphy at a given construction site may result in extreme oversimplification. Consequently, further classification of the ground conditions can be made to conform more closely to the stratigraphy of the site and its deeper geology.

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