DESCRIPTION OF THE “GROUND” SECTION

In order to establish the amplification factors for each relevant oscillation frequency, the user can select one of the two alternative approaches available in the system, namely the base method or the advanced method. In case the base method is chosen, the system applies the  Sabetta and Pugliese (1996) attenuation relationship and the amplification factors are estimated by comparing the attenuation law for generic alluvial deposits (i.e., deep alluvium with soil depth larger than 30m, or shallow alluvium with soil depth less than 30m) with the relationship applicable for rock. The computation is based on magnitude and epicentral distance as defined by means of a numerical disaggregation procedure, and with reference to the fundamental period of the analyzed structure. On the other hand, if the advanced method is selected, the system applies advanced amplification functions and computes the seismic hazard at the ground surface by means of a convolution procedure. 

The advanced amplification functions currently used by the system, which were obtained by running a large number of non linear numerical analyses on a suite of pilot soil columns, are still preliminary and in some instances could provide unreliable results. Furthermore such functions are best suited for relatively homogeneous soil conditions, whereas relationships applicable to more complex layered sites are currently being tested. 

The effects of specific topographic features on local site amplification are also introduced in the analysis by a simplified approach, adopting the factors proposed in the Eurocode 8.
In case the base method is chosen, the online system performs a liquefaction analysis in non-plastic saturated soils by applying the simplified procedure summarized by Youd et al. (2001), and keeping into account both the key soil properties (i.e., relative density and fines content) and the relevant earthquake characteristics (i.e., PGA and magnitude). The earthquake magnitude is obtained by the disaggregation procedure mentioned above. In the base method the amplification and liquefaction analyses are mutually  uncoupled, and therefore potential pore presssure build-up in the soil deposit does not affect the resulting accelerations at the ground surface. 

For this reason, if liquefaction is foreseen both the estimated hazard at the ground surface and the estimated structural damage could be affected by a considerable error. If soil liquefaction is deemed likely, the system displays a warning message recommending more refined studies to be carried out off-line. Soil liquefaction could cause lateral spreading, very significant vertical settlements, slope instability, and could also affect considerably site amplification (e.g., Youd and Carter, 2005). On the other hand, if the advanced method is selected, the effects of pore pressure build up on site amplification can be taken into account by selecting suitable amplification functions among those available in the system (see below). If desired, a simplified liquefaction analysis can also be carried out by switching from the advanced to the base method option.

Using the system interface, the user can provide information on the local soil characteristics in a simple and effective way, with no need of entering any digits or textual descriptions. The available tools (e.g., drop-down menus, sliders and selection boxes) allow the user to select the desired options among multiple possible alternatives. In some instances, if the required information is not available (i.e., I don’t know option) the system applies a default value as depicted in the menu.

The different pieces of information that need to be entered in the Ground section are described in the following. 

Metodology
In this field the user can choose between the base method or, as an alternative, the advanced method of analysis. If case the base method is chosen, the system computes the amplification factors by comparing the  Sabetta and Pugliese (1996) attenuation relationship for generic alluvial deposits with the attenuation relationship for rock. On the other hand, if the advanced method is selected, the system applies advanced amplification functions and computes the seismic hazard at the ground surface by means of a convolution procedure.
Geology
This field contains information required by AESP for the on-line-simplified estimate of the amplification factors for each relevant oscillation frequency. 

If case the base method is chosen, the following possible alternatives can be selected:  

If the structure is founded directly on a rock outcrop, the amplification analysis is not carried out and the acceleration values are obtained directly from the system’s database.

If this information is not available (i.e., I don’t know) the system will adopt the default option (i.e., Option b).

If the advanced method is selected, the following possible alternatives can be selected:  

If the structure is founded directly on a rock outcrop the amplification analysis is not carried out and the acceleration values are obtained directly from the system’s database.

Surface Topography
This field contains information required by AESP for the on-line-simplified estimate of the effects of specific topographic features on local site amplification. To this purpose the factors proposed in the Eurocode 8 are adopted. 

The following possible alternatives can be selected:  

Option d is the basic condition where the ground is represented as an infinite half-space with a horizontal ground surface.
If this information is not available (i.e., I don’t know) the system will adopt the default option (i.e., Option d).

Soil Type
The information provided in this field is relevant for the estimate of site amplification (advanced method only), for the estimate the non-linear response of the building in terms of displacements (both methods), and for the simplified soil liquefaction analysis in sands and gravels (base method only).

If the structure is founded directly on a rock outcrop (Option a of the Geology field), or if the base method has been selected and the ground water surface is deeper than 20m (Option e of the Groundwater depth field), this field will be inactive.

If case the base method is chosen, the following possible alternatives shall be selected:  

If this information is not available (i.e., I don’t know) the system will adopt the default option (i.e., Option a).
If case the advanced method is chosen, the following possible alternatives can be selected:  

Groundwater Depth
The information provided in this field is relevant for site amplification assessment (advanced method only) and for the simplified soil liquefaction analysis (base method only) in non plastic soil.

If the structure is founded directly on a rock outcrop (Option a of the Geology field) or the soil deposit is characterized by clay or plastic silt (Options b or c of the Soil type field) this field will be inactive.

If the base method is selected, the following possible alternatives can be selected for the ground water depth:  

The ground-water depth adopted by AESP in the calculations is obtained by adding 2m to the minimum depth value within the selected range (e.g., if the selected range is from 0 to 5m, the value adopted in the calculations is: 0+2=2m).
If the advanced method is selected, the following possible alternatives can be selected for the ground water depth:  

If this information is not available (i.e., I don’t know) the system will adopt the default option (i.e., Option a).

Relative Density / Consystency
The information provided in this field (applicable to sands and gravels only) is required to estimate, by means of a simplified approach, the non-linear response of the building in terms of displacement (both base and advanced methods), and to carry out a simplified soil liquefaction analysis (base method only). On the other hand, it is not used to define the parameters affecting site amplification.

If the structure is founded directly on a rock outcrop (Option a of the Geology field) or the soil deposit is characterized by clay or plastic silt (Options b or c of the Soil type field), or the base method has been selected and the ground water table is deeper than 20m (Option e of the Groundwater depth field) this field will be inactive.

The following possible alternatives can be selected:  

If this information is not available  (i.e., I don’t know) the system will adopt the default option (i.e., Option c).

Fines Content (base method only)
The information provided in this field (applicable to sands and gravels only) is required to carry out a simplified soil liquefaction analysis. On the other hand, it is not used to define the parameters affecting site amplification.

If the structure is founded directly on a rock outcrop (Option a of the Geology field) or the ground-water depth exceeds 20m (Option e of the Groundwater depth field) or the soil deposit is characterized by clay or plastic silt (Options b or c of the Soil type  field) this field will be inactive.

The following possible alternatives can be selected:  

If this information is not available  (i.e., I don’t know) the system will adopt the default option (i.e., Option a).

Bedrock depth (advanced method only)
This field is relevant for the estimate of site amplification.
The user will select among the following alternatives:

If this information is not available (i.e., I don’t know) the system will adopt the default option (i.e., Option f).

If the structure is founded directly on rock (Option a of the Geology field) this field will be inactive.

Susceptibility to Cyclic Mobility (advanced method only)
The information provided in this field (applicable to sands and gravels only) is relevant for the estimate of site amplification.

The user will select among the following alternatives:
 

If this information is not available (i.e., I don’t know) the system will adopt the default option (i.e., Option b).

If the structure is founded directly on a rock outcrop (Option a of the Geology field) or the ground-water depth exceeds 20m (Option e of the Groundwater depth field) or the soil deposit is characterized by clay or plastic silt (Options b or c of the Soil type field) this field will be inactive.

Soil VS30 (advanced method only)
In case the elastic fundamental frequency of the soil deposit (see the Fundamental frequency field) is not available, the information provided in this field is used to estimate site amplification.
The user will choose a suitable VS30 value among several different alternatives available in the menu. Such values are selected by the system keeping into account the input soil properties and bedrock depth.

For instance:
 

The selected value may be obtained from direct VS measurements carried out at the site, or may be estimated based on correlations with available geotechnical parameters.
If the structure is founded directly on a rock outcrop (Option a of the Geology field) this field will be inactive.

Geotechnical Investigation (advanced method only)
The information provided in this field is used to estimate site amplification. In particular, it allows to establish a proper standard deviation value to be adopted in the convolution procedure, depending on the quality of the available geotechnical investigation.
The user will select among the following alternatives:
 

If this information is not available (i.e., I don’t know) the system will adopt the default option (i.e., Option b).
If the structure is founded directly on a rock outcrop (Option a of the Geology field) this field will be inactive.
This field is currently inactive and proper parameters are being developed.

Soil Plasticity  (advanced method only)
The information provided in this field is used to estimate site amplification. In particular, it allows to characterize the behavior of plastic soils (i.e., silt and clay) in the non-linear range.
The user will select among the following alternatives:
 

If this information is not available (i.e., I don’t know) the system will adopt the default option (i.e., Option a).

If the structure is founded directly on a rock outcrop (Option a of the Geology field) or the soil deposit is composed of non plastic soil (Options a of the Soil type field) this field will be inactive.

Bedrock VS-ROCK (advanced method only)
The information provided in this field is used to estimate site amplification.
The user will select among the following alternatives:
 

If this information is not available (i.e., I don’t know) the system will adopt the default option (i.e., Option b).

If the structure is founded directly on a rock outcrop (Option a of the Geology field) this field will be inactive.

Soil Fundamental Frequency (advanced method only)
The elastic fundamental frequency provided in this field is used to estimate site amplification.
The user will choose a suitable fFUND value among several different alternatives available in the menu. Such values are determined by the system keeping into account the input soil properties and the bedrock depth.

For instance:
 

The selected value may be obtained from direct VS measurements or microtremor records collected at the site, or may be estimated based on correlations with available geotechnical parameters.
In case the elastic fundamental frequency of the soil deposit is not available (Option m), the VS30 value selected in the Soil VS30  field is used to estimate site amplification.
 If the structure is founded directly on a rock outcrop (Option a of the Geology field) this field will be inactive.

References
Eurocode 8 – ENV 1998-5. Design of structures for earthquake resistance – Part.5, foundations, retaining structures and geotechnical aspects.

Sabetta F. and A. Pugliese, 1996, Estimation of response spectra and simulation of non-stationary earthquake ground motions, Bull. Seism. Soc. Am., 86(2), pp. 337-352.

Youd T.L. et al., 2001, Liquefaction Resistance of Soils: Summary Report from 1996 NCEER and 1998 NCEER/NSF Workshops on Evaluation of Liquefaction Resistance of Soils, Journal of Geotechnical and Geoenviromental Engineering, ASCE, Vol. 127 N°10, pp.817.

Youd T.L. and B.L. Carter, 2005, Influence of soil softening and liquefaction on spectral acceleration, Journal of Geotechnical and Geoenviromental Engineering, ASCE, Vol. 131, N°7, pp.811-825.