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Lecture 12

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Dynamic soil properties
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  NPTEL- GEOTECHNICAL EARTHQUAKE ENGINEERING   Dept. of Civil Engg. Indian Institute of Technology, Kanpur 1  Module 3 DYNAMIC SOIL PROPERTIES (Lectures 10 to 16)   Lecture 12 Topics 3.3.9 Spectral analysis of source waves test 3.3.10 Seismic cross-hole test 3.3.11 Seismic down-hole [Up-hole test] 3.3.12 Seismic cone test 3.3.13 High strain tests 3.3.14 Standard penetration test 3.3.15 Cone penetration test 3.3.16 Dilatometer test 3.3.17 Pressure meter test 3.3.9 Spectral analysis of source waves test The shape of a dispersion curve [Rayleigh wave velocity versus frequency (or wavelength)] at a particular site is related to the variation of body wave velocities with depth. The preceding steady-state test can be used opt generate a dispersion curve by repeating the test at different loading frequencies. This process, however, tends to the quite time consuming in the field. With the use of digital data acquisition and signal processing equipment, a dispersion curve can be obtained from an impulsive or random noise load. The measurement and interpretation of dispersion curves obtained in this way, known as  spectral analysis of surface waves (SASW) (Heisey et al., 1982; Nazarian and Stokoe, 1983; Stokoe et al., 1994), is one of the most significant recent advance in shallow seismic exploration. The SASW test is performed by placing two vertical receivers on the ground surface in line with an impulsive or random noise source, as illustrated in  (figure 3.22) . The output of both receivers is recorded and transformed to the frequency domain using the fast Fourier transform. After transformation, the phase difference,  , can be computed for each frequency. The corresponding travel time between receivers can  be calculated for each frequency from  NPTEL- GEOTECHNICAL EARTHQUAKE ENGINEERING   Dept. of Civil Engg. Indian Institute of Technology, Kanpur 2      (3.27) Figure 3.22:  Typical configurations of source and receivers in a SASW test. Receiver spacing is changed in such a way that      remains constant Since the distance between receivers,     , is known the Rayleigh wave  phase velocity and wavelength can be calculated as functions of frequency:       (3.28)         (3.29) With modern electronic instrumentation, these calculations can be performed in the field virtually in real time. The results can be used to plot the experimental dispersion curve ( figure 3.23 ). While the test should, in theory, yield good results for a single receiver spacing, practical considerations dictate that several different receiver spacing be used. At each spacing, the midpoint between the two receivers is kept at the same distance from the source. Identification of the thickness and shear wave velocity of subsurface layers involves the iterative matching of a theoretical dispersion curve to the experimental dispersion curve. The Haskell-Thomson solution (Thomson, 1950; Haskell, 1953), for a series of uniform elastic layers of infinite horizontal extent is used to predict the theoretical dispersion curve. Initial estimates of the thickness and shear wave velocity of each layer are then adjusted until the values that produce the best fit to the experimental dispersion curve are identified. This identification procedure is usually referred to as inversion (Nazarian, 1984). For profiles in which the shear wave velocity varies irregularly with depth, the dispersion curve may be influenced  by higher-mode Rayleigh waves (Gucunski and Woods, 1991; Takimatsu et al., 1992).  NPTEL- GEOTECHNICAL EARTHQUAKE ENGINEERING   Dept. of Civil Engg. Indian Institute of Technology, Kanpur 3  Figure 3.23:  Experimental dispersion curve from SASW test. (After Gucunski and Woods, 1991) SASW tests have a number of important advantages over other field tests. They can  be performed quickly, they require no borehole, they can detect low-velocity layers, and they can be used to considerable depth (>100m). SASW testing is particularly useful at sites where drilling and sampling are difficult. 3.3.10 Seismic cross-hole test Seismic cross-hole tests use two or more boreholes to measure wave propagation velocities along horizontal paths. The simplest cross-hole test configuration (figure 3.24.a)  consists of two boreholes, one of which contains an impulse energy source and the other a receiver. By fixing both the source and the receiver at the same depth in each borehole, the wave propagation velocity of the material between the  boreholes at that depth is measured. By testing at various depths, a velocity profile can be obtained. When possible, use of more than two boreholes is desirable (figure 3.24.b)  to minimize possible inaccuracies resulting from trigger time measurement, casing and backfill effects, and site anisotropy.  NPTEL- GEOTECHNICAL EARTHQUAKE ENGINEERING   Dept. of Civil Engg. Indian Institute of Technology, Kanpur 4  Figure 3.24:  Seismic cross-hole test: (a)  direct measurement using two-hole configuration; (b)  interval measurement using three-hole configuration Since the impulse sources must be located in the borehole, variation of the p-waves/s-wave content is more difficult than for methods in which it is at the surface. When explosive sources are used, the wave content is shifted toward higher p-wave content when larger charges are used, particularly when detonated above the ground surface (Woods, 1978). A number of mechanical impulse sources have been used, including the driving of a standard penetration test sampler, vertical impact loading of rods connected to boreholes packers or jack, torsional impact loading of a torque foot at the bottom of the borehole (Stokoe and Hoar, 1978), and other techniques (Applegate, 1974; Stokoe and Abdel-razzak, 1975; Auld, 1977). The best results are generally obtained when the polarity of the impulse source in reversible, hence the frequent preference for mechanical sources over explosive sources. Example 5 Determine the SV-waves velocity from the cross-hole test trigger and geophone records shown in (figure 3.25) . The trigger and geophone are located 5 m apart. The solid line represents the response from a downward impact on a mechanical source and the dotted line represents the response from an upward impact. figure 3.25 Solution There is an obvious wave arrival at the geophone at about 2   after impact at the
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