Measurement of Properties - Geotechnical Engineering Investigation - page 252

Estimating Values of E s
Moduli for strain levels in the order of 10 6 can be estimated from shear-wave velocities
from crosshole or uphole seismic surveys (Swiger, 1974). Shear-wave velocities are used
because they can be measured above and below the groundwater level, whereas com-
pression-wave velocity can be measured only above groundwater level, since it is
obscured by the compression-wave velocity for water.
Under loads of the order of 2 to 3 tsf, strains in dense sands are small, approximately
10 3 , but higher than the strains occurring during seismic testing that require adjustment
for analysis. In granular soils, E d and G d have been found to decrease with increasing
strain levels (Hardin and Drnevich, 1972). A relationship between shear strain and axial
strain as a function of strain level is given in Figure 3.91 . The ratios given on the abscissa
are used to reduce the field shear and compression modulus for use in analysis.

Case Study
In a study reported by Swiger (1974), good agreement was found between settlements com-
puted from seismic direct surveys and large-scale in situ load tests and the actual settlements
measured on the structure for which the study was made. For a Poisson's ratio of 0.3, the val-
ues of E s were of the order of 4

10 6 psf (2000 tsf). The primary settlements occurred within
about 1 h of load application, but the magnitude of the secondary settlement appeared to
approximate that of the primary and to continue over a period of some years. Approximately
25% occurred in the first year after load application (about 4-8 tsf foundation pressure).

Steady-State Vibration Methods
Purpose
Steady-state vibration methods are performed to obtain in situ values of E d and G d .

Principles
Ground oscillations are induced from the surface causing Rayleigh waves. The Rayleigh wave
velocity V r is used directly as the shear-wave velocity because, for Poisson's ratios of 0.35 to
0.45, V r 0.935 to 0.95 V s , a difference which is of little engineering significance (Richart, 1975).

E d and G d are then computed from V r (for V s ) using equations given in Table 3.27.

1.0

0.8

Shear strain δ

0.6

0.4

FIGURE 3.91
Strain modulus relations for sands. (After
Seed, H. B., Proceedings of the 7th International
Conference on Soil Mechanics and Foundation
Engineering , Mexico City, 1969; from Swiger,
W. F. , Proceedings of ASCE, Conference on
Analysis and Design in Geotechnical Engineering ,
University of Texas, Austin, Vol. II, 1974, pp.
79-92. With permission.)

Axial strain ε

0.2

0

10 − 2

10 − 6 10 − 5 10 − 4 10 − 3 10 − 1

Strain