17 1. Shear velocity measurement in boreholes Table 1.2 Method suitability according to objective. Requirement Modeling the ground response for the purposes of sensitive construction design Designing a construction to conform to the Eurocode 8 soil classification Level of knowledge about the site Overall knowledge of the site is insufficient to establish velocity logging of the ground under the proposed building Detailed knowledge (velocity logging) available at several points around the site, but not in the ground below the proposed building No special knowledge The Vs 30 is known in several points around the site, but not for the ground below the proposed building Recommended measurement Crosshole coupled with downhole or PSSL coupled with crosshole in the top 10 meters Downhole or PSSL coupled with MASW and AMV Downhole coupled with MASW and AMV MASW and AMV if space is sufficient and unaffected by multiple underground networks This chapter addresses invasive geophysical methods to obtain a Vs log, i.e. the following methods: downhole, uphole, crosshole and logging. The case studies presented are from EDF feedback from numerous soil surveys conducted over the last 5 years, as well as from the Inter Pacific research project whose objective was to compare geophysical methods for site seismic characterization (Garofalo, 2016). Non-invasive methods are not covered in this document. 1.1.2 Applications The most common application concerns the design and sizing of civil engineering structures. Indeed, in the Eurocode 8 soil classification, one of the three main parameters for classifying soils is based on a average of shear wave propagation velocity within the top 30 meters. This parameter is conventionally called “Vs 30”. Table 1.3 Mechanical parameters according to Vp, Vs and ρ (from Bourbié, 1986). Mechanical parameter Equation with Vp, Vs and r Young’s modulus, Edmax (Pa): ρV V V V V S P S P S 2 2 2 2 2 3 4− − Lamé constant, l (Pa) ρ V V P S 2 2 2− ( ) Bulk modulus, K (Pa) ρ V V P S 2 2 4 3 − ( ) The shear modulus, rigidity or Coulomb’s coefficient (Gdmax or µ in Pa) ρVS 2 Poisson’s ratio, g (without dimension) V V V V P S P S 2 2 2 2 2 2 − − ( )
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