FEATURE ARTICLE Applied Seismic Tools for Geotechnical Site Characterization In geotechnical engineering, there is considerable interest in applied seismic techniques that provide accurate estimates -5 P S of low strain (<10 ) in-situ compression (V or P-wave) and shear wave (V or S- wave) veloci t ies. Accuracy in the estimation of V and V is of paramount P S importance because these values are squared during the calculation of various elastic constants (e.g., shear modulus, Poisson’s ratio and Young’s modulus) that are used in dynamic soil analyses and liquefaction assessments. Dynamic analysis in geotechnical practice deals with the soil response under dynamic loading (e.g., earthquakes, machine vibrations, blasting, compaction, pile driving, wave loading and ice loading). A fundamental assumption made in most of the dynamic analysis techniques is that soil deformations are a result of vertically propagating shear waves. Liquefaction is a phenomenon in which the dynamic loading of saturated soil results in the material properties to change suddenly from a metastable solid state to a liquefied state. Liquefaction eliminates the bearing capacity of the soil, or, to use the words of Seed and Idriss (1982), “if a saturated sand is subjected to ground vibrations, it tends to compact and decrease in volume; if drainage is unable to occur, the tendency to decrease in volume results in an increase in pore water pressure, and if the pore water pressure builds up to the point at which it is equal to the overburden pressure, the effective stress becomes zero, the sand loses its strength completely, and it develops a liquefied state.” The shear wave velocity is an important parameter for evaluating liquefaction potential because it is influenced by many of the variables that influence liquefaction (e.g., void ratio, soil density, confining stress, stress history and geologic age). It should be noted that near surface in-situ site characterization is very important for lique- faction assessment: extensive geotechnical analyses of the catastrophic liquefaction that occurred in Christchurch, New Zealand, in 2010 and 2011 clearly showed that near surface rather than deep liquefaction resulted in extensive foundation damage. where d is the thickness of layer i, V is the interval V at layer I, and i S Si . The site classification schemes are designated SA or A (Hard Rock) through SF or F (Soft Soil Profile). Elastic constants relationships V or, more accurately, the average shear S wave velocity for the top 30 m (100 ft) of soil (V ) also forms the basis of site hazard S30 classification under the National Earth- quake Hazards Reduction Program (NEHRP) Uniform Building Code (UBC), the International Building Code (IBC) and Eurocode 8. V S (unit of m/s) is calculated 30 from the following formula: Applied Seismic Techniques for In-Situ Characterization Downhole seismic testing (DST), crosshole seismic testing (CST) and spectral analysis of surface waves (SASW) are widely used to measure in-situ V and V . DST and CST P S S rely upon the generation of the acoustic V P and V source waves. The P-wave is a longitudinal wave where the particle motions are in the same direction as the ray path with the particle motions going through alternating periods of dilation and Site classes in NEHRP UBC Site Class SA or A SB or B Description Hard Rock Firm to Hard Rock SC or C Very Dense Soil and Soft Rock SD or D SE or E SF or F Stiff Soil Profile Soft Soil (Clays) Profile Special Study Soils (e.g., liquefiable soils, sensitive clays, soft clays > 36 m (118 ft) thick) AUTHORS Erick Baziw and Gerald Verbeek, Baziw Consulting Engineers Ltd., Vancouver, Canada DEEP FOUNDATIONS • MAY/JUNE 2018 • 103 Mean Shear Wave Velocity to 30 m (100 ft), m/s (ft/s) Vs > 1500 (4920) 760 to 1500 (2490 to 4920) 360 to 760 (1180 to 2490) 180 to 360 (590 to 1180) < 180 (< 590) --- 30
