Report - Oregon State Library: State Employee Information Center ...

4.6 EVALUATION OF GROUND SETTLEMENTS FOLLOWING
CYCLIC LOADING
As excess pore pressure generated by cyclic loading dissipates due to drainage, the soil consolidates,
which results in ground settlement. Similarly, non-saturated cohesionless soils will contract during
cyclic shearing resulting in surface settlements. The magnitude of the settlements will reflect the
density of the soil, intensity of the ground motions, the factor of safety against liquefaction, and the
thickness of the loose soil deposit. Field observations document post-earthquake settlements of soils
adjacent to bridges of over 1 m (Hamada et al. 1995; Yasuda et al. 1996). These include flat sites in
the absence of lateral spreading. Damage modes include pavement damage, uneven grades at the
transition from soil to pile-supported approach structures, and abutment damage (Seed et al. 1990;
Yasuda et al. 1996).
Several methods have been developed for estimating the magnitude of earthquake-induced
settlements in sandy soils. The most widely adopted methods have been developed by Ishihara
and Yoshimine (1992) and Tokimatsu and Seed (1987). The method proposed by Ishihara and
Yoshimine has been produced in the form of a design chart relating volumetric strain in sandy
soils to soil density and the factor of safety against liquefaction (FS L ; Figure 4.12). This analysis
requires that FS L be computed for the sandy deposit, or each sub-layer within the deposit. The
methods outlined in Chapter 3 for estimating the triggering of liquefaction are used. The percent
compression of each sub-layer can then be easily estimated by using Figure 4.12.
Although the procedure of Ishihara and Yoshimine was developed for saturated sandy soils it can be
applied for unsaturated soils in an approximate manner. This assumes that volumetric behavior of a
dry or partially saturated sand during drained cyclic loading is similar to the volumetric behavior of
the soil following the application of undrained cyclic loading on a saturated specimen (i.e., postloading
consolidation due to the dissipation of excess pore pressure). In both scenarios the
volumetric strain that is developed is a function of the initial void ratio of the soil, the effective
confining stress prior to cyclic loading, and the intensity and duration of the cyclic loading.
As illustrated in Figure 4.12, in order to estimate the volumetric strain the factor of safety against
liquefaction must be obtained. This calculation accounts for the influence of the four factors (e, ′ c ,
CSR, MSF) previously listed on the estimated volumetric strain. Clearly, an unsaturated soil is not
prone to liquefaction regardless of its density; therefore the concept of developing a factor of safety
against liquefaction does not seem appropriate for this scenario. However, loose to medium dense
sandy soils will experience volumetric strains during loading. A possible approach for applying the
method to unsaturated soils is to first compute the FS L as if the soil were saturated, then enter the
chart at the appropriate FS L and density. The results of this approximation should be tempered by
calculations using the Tokimatsu and Seed method as follows. Because the Ishihara procedure was
developed for clean sands, a correction is required for silty sands and silts. The (N 1 ) 60 values should
be modified using correction factors for fines content (Youd and Idriss 1997) prior to using Figure
4.12. Also, the N 1 -values shown in Figure 4.12 correspond to typical Japanese equipment and
procedures, and are thus representative of an SPT energy ratio of approximately ER m = 55%. The
corrected and standardized SPT (N 1 ) 60 values used to develop estimates of FS L should be increased
by about 10% when using Figure 4.12 to estimate the resulting volumetric compression.
95