Guidelines for Estimation of Shear Wave Velocity-4

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1 Study Overview
Characterization of the stress-strain behavior of soils is an integral component of many seismic

analyses, including site classification, hazard analysis, site response analysis, and soil–structure
interaction. The shear modulus (G) of geomaterials is highly dependent upon strain level. The
small-strain shear modulus (Gmax or G0) is typically associated with strains on the order of 10–3%
or less. With knowledge of Gmax, the shear response at various levels of strain can be estimated
using published modulus reduction (G/Gmax) curves.
Shear wave velocity (VS) is a valuable indicator of the dynamic properties of soil and
rock because of its relationship with Gmax, given by Equation (1.1):
Gmax = ρ · VS2 (1.1)
where soil density (ρ) is the total unit weight of the soil divided by gravity (9.81 m/sec2 or 32.2
ft/sec2). Gmax has units of force per length squared (i.e., kPa or psf).
Gmax and VS are primarily functions of soil density, void ratio, and effective stress, with
secondary influences including soil type, age, depositional environment, cementation, and stress
history [Hardin and Drnevich 1972a, b]. Table 1.1 summarizes the effect of increasing various
parameters on VS.
Gmax can be measured in the laboratory using a resonant column device or bender
elements. While the void ratio and stress conditions can be recreated in a reconstituted specimen,
other factors—such as soil fabric and cementation—cannot [Kramer 1996]. Laboratory testing
requires very high-quality, undisturbed samples. High-quality sampling and testing is quite
expensive and is often not possible for cohesionless soils. Additionally, laboratory tests only
measure Gmax at discrete sample locations, which may not be representative of the entire soil
profile.
1
Table 1.1 Effect of increase of various factors on Gmax and VS.*
*Dobry and Vucetic [1987] as reported by EPRI [1991].

Unlike laboratory testing, geophysical tests do not require undisturbed sampling,
maintain in situ stresses during testing, and measure the response of a large volume of soil. EPRI
[1991)] and Kramer [1996] discuss various geophysical methods for measuring the shear wave
velocities of geomaterials. Geophysical methods can be divided into two categories: invasive and
non-invasive.
Invasive methods require drilling into the ground. Common invasive methods include:
downhole logging, crosshole logging, suspension logging, and the seismic cone penetration test
(SCPT). The SCPT is a modified downhole measurement in conjunction with the conventional
cone penetration test (CPT). The SCPT has become more common in recent years because it is a
relatively rapid and cost-effective method of measuring VS. Site characterization can be achieved
using the SCPT for approximately $4000 to $5000 (one day of testing).
Non-invasive geophysical methods include: spectral analysis of surface waves (SASW),
seismic refraction, and seismic reflection. Table 1.2 presents a comparison and summary of each
of these methods [Andrus et al. 2004].
In situ measurement of VS has become the preferred method for estimating the small-
strain shear properties and has been incorporated into site classifications systems and ground
motion prediction equations, as discussed in the following two sections.
2
Table 1.2 Comparison of various in situ VS measurement methods.
1.1 SITE CLASSIFICATION
The Caltrans Seismic Design Criteria classifies sites based on VS of the top 30 m of the soil
profile (VS30). Sites are divided into the six categories (Soil Profile Types A through F) presented
in Table 1.3. The Caltrans site classes are consistent with those used by other codes and
standards, including the National Earthquake Hazard Reduction Program [BSSC 2003],
American Society of Civil Engineers [ASCE 2006, 2010], and the California Building Code
[CBSC 2010].
For site classification, VS30 is calculated as the time for a shear wave to travel from a
depth of 30 m to the ground surface, not the arithmetic average of VS to a depth of 30 m. As
shown in Equation (1.2), the time-averaged VS30 is calculated as 30 m divided by the sum of the
travel times for shear waves to travel through each layer. The travel time for each layer is
calculated as the layer thickness (d) divided by VS.
VS30 = 30 / Σ (d/VS) (1.2)
For example, the VS30 for a soil profile containing 18 m of soft clay (VS = 90 m/sec) over
12 m of stiff clay (VS = 260 m/sec) would be calculated: 30 / (18 / 90 + 12 / 260) = 122 m/sec
[Dobry et al. 2000]. The time-average method typically results in a lower VS30 than the weighted
average of velocities of the individual layers: (90 · 18 + 260 · 12) / 30 = 158 m/sec.
3
Table 1.3 Caltrans/NEHRP soil profile types.
1
Site Class E also includes any profile with more than 10 ft (3 m) of soft clay, defined as soil with Plasticity Index >
20, water content > 40%, and undrained shear strength < 500 psf (25 kPa).
2
Site Class F includes: (1) Soils vulnerable to failure or collapse under seismic loading (i.e., liquefiable soils, quick
and highly sensitive clays, and collapsible weakly-cemented soils). (2) Peat and/or highly organic clay layers more
than 10 ft (3 m) thick. (3) Very high plasticity clay (PI > 75) layers more than 25 ft (8 m) thick. (4) Soft to medium
clay layers more than 120 ft (36 m) thick.
For cases where measured VS data is not available, alternative site class definitions are
provided in terms of standard penetration test (SPT) resistance for cohesionless soils and
undrained shear strength for cohesive soils. Additional criteria, such as plasticity index, water
content, organic content, collapse potential, and liquefaction potential, must also be considered
when assigning a soil profile type.
The Caltrans Seismic Design Criteria specifies using uncorrected SPT N-values for site
classification [Caltrans 2006]. It is common geotechnical practice to correct field SPT N-values
for variations from standard practice (i.e., hammer energy, sampler type, borehole diameter, and
rod length). For some applications, it is also common practice to normalize N-values to a
reference overburden stress (typically, 1 atmosphere). For the purpose of site classification, it is
appropriate to apply correction factors intended to account for deviations from the standard test
method, such as hammer energy or non-standard samplers, but not appropriate to normalize N-
values by the overburden pressure. In addition to site classification, VS may be required for site-
specific seismic evaluation or dynamic analysis when required by the seismic design criteria.
1.2 NEXT GENERATION ATTENUTATION PROJECT
The Next Generation Attenuation (NGA) project is a multidisciplinary research program

coordinated by the Pacific Earthquake Engineering Research Center (PEER) Lifelines Program
[Power et al. 2008]. Most previous ground motion prediction equations used broad site
categories, such as “deep soil,” “soft rock,” and “hard rock,” to describe site conditions
[Abrahamson et al. 2008]. The NGA ground motion relationships use VS30 as the primary
parameter for characterizing the effects of sediment stiffness on ground motions.

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Characterization of the stress-strain behavior of soils is an integral component of many seismic

Gmax = ρ · VS2 (1.1)

*Dobry and Vucetic [1987] as reported by EPRI [1991].

1.1 SITE CLASSIFICATION

VS30 = 30 / Σ (d/VS) (1.2)

1.2 NEXT GENERATION ATTENUTATION PROJECT

The Next Generation Attenuation (NGA) project is a multidisciplinary research program

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