Soil-Structure Interaction Analysis

DOE-STD-1022-94

The strain dependent shear modulus and damping curves should be developed based on site-specific testing

results and supplemented as appropriate by published data for similar soils. Effects of sampling disturbance

and machine characteristics must be carefully evaluated in developing these relationships. The effects of

confining pressures (that reflect the depths of the soil) on these strain dependent soil dynamic characteristics

should be assessed and considered in site response analysis.

5.5.3 Soil-Structure Interaction Analysis

Soil-structure interaction (SSI) analyses shall be carried out when required to ascertain the influence of the

interaction of the structure and the surrounding soil on the response of the structure to the defined site free

field ground motions. Soil-structure interaction effects are more significant for heavy and/or embedded

structures.

The effect of soil-structure interaction should be considered for SSCs in Performance Category 3 and shall

be performed for SSCs in Performance Category 4. SSI analyses shall use the design free-field ground

motion as input. The same soil parameters specified in Paragraph 5.5.2. shall be obtained for SSI analyses.

Due to the uncertainty in the input ground motion as well as soil parameters and structural properties used

in the SSI analysis a relatively wide range of soil shear moduli as required by ASCE Standard 4-98 (ASCE,

1998a) and DOE-STD-1020-2002 is recommended so that a conservative structure response calculation may

be expected.

Dynamic soils properties can vary significantly depending on whether soil layers are saturated. For SSI

analysis, unsaturated soil properties should be used for soil layers above the normal water table unless the

site conditions indicate that additional soil saturation occurs frequently or for long durations.

5.5.4 Ground Failure Evaluations

5.5.4.1 Seismic liquefaction of Soils

Liquefaction is a soil behavior phenomenon in which cohesionless soils (sand, silt, or gravel) under saturated

conditions lose a substantial amount of strength due to high pore water pressures generated in the soils by

strong ground motions induced by NPH, such as earthquakes or wave actions. Potential effects of

liquefaction include reduction in foundation bearing capacity, settlements, land sliding and lateral movements,

flotation of lightweight structures (such as tanks) embedded in liquefied soil, and increased lateral pressures

on walls retaining liquefied soil. The general procedure for evaluations of liquefaction potential is given in

the Appendix C of the EPRI Report "A Methodology for Assessment of Nuclear Power Plant Seismic

Margin" (EPRI, 1988). Detailed procedures to be used for liquefaction evaluation are presented in NCEER,

1997.

Investigations of liquefaction potential typically involve both geological and geotechnical engineering

assessments. The parameters controlling liquefaction phenomena are: the lithology of the soil at the site, the

ground water conditions, the behavior of the soil under dynamic loading and potential severity of the

vibratory ground motion. The following site-specific data shall be acquired and utilized along with

state-of-the-art evaluation procedures (e.g., Seed and Idriss, 1982; Seed et al.,1985):

Soil grain size distribution, density, static and dynamic strength, stress history and geologic age of

the sediments,