EARTHQUAKE-INDUCED SETTLEMENT - Geotechnical Earthquake Engineering - page 193

volumetric compression: the looseness of the soil based on the standard penetration test and
the amount of shear strain based on the peak ground acceleration a p .
Note in Fig. 7.7 that the curves are labeled in terms of the uncorrected N values. As a
practical matter, the curves should be in terms of the standard penetration test ( N 1 ) 60 values
[i.e., Eq. (5.2), Sec. 5.4.3]. This is because the ( N 1 ) 60 value more accurately represents the
density condition of the sand. For example, given two sand layers having the same uncor-
rected N value, the near-surface sand layer will be in a much denser state than the sand layer
located at a great depth.
To use Fig. 7.7, both the ( N 1 ) 60 value of the sand and the peak ground acceleration a p
must be known. Then by entering the chart with the a p / g value and intersecting the desired
( N 1 ) 60 curve, the volumetric strain ( H / H, expressed as a percentage) can be determined.
The volumetric compression (i.e., settlement) is then calculated by multiplying the volu-
metric strain, expressed as a decimal, by the thickness of the soil layer H.

7.4.3 Method by Tokimatsu and Seed

A much more complicated method for estimating the settlement of dry sand has been pro-
posed by Tokimatsu and Seed (1987), based on the prior work by Seed and Silver (1972)
and Pyke et al. (1975). The steps in using this method are as follows:

1. Determine the earthquake-induced effective shear strain eff . The first step is to
determine the shear stress induced by the earthquake and then to convert this shear stress
to an effective shear strain eff . Using Eq. (6.6) and deleting the vertical effective stress v 0

from both sides of the equation gives

cyc 0.65 r d v 0 ( a max / g ) (7.1)

FIGURE 7.7 Simple chart that can be used to determine the settlement of dry sand. In this figure, use the
peak ground acceleration a p and assume that N refers to ( N 1 ) 60 values from Eq. (5.2). ( Reproduced from
Krinitzsky et al. 1993, with permission from John Wiley & Sons. )