1616
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
2 THE EFFECT OF LOCAL SITE AND SOIL PROFILE
STRATIFICATION ON AMPLIFICATION
The amplitude of seismic waves increase during they pass
through soft soil layers near surface. This phenomenon is
known as site amplification. That is the major factor that affects
the earthquake damages.
Sub-layers worked like filters that damping off some seismic
waves and amplify some others, so it may cause destruction of
earthquake increased in small area. Sub-layers depth and their
stiffness as well as their shear wave velocity can be effective on
amplification ratio.
3 EARTHQUAKE STANDARDS & BUILDING CODES
Standard seismic codes like Euro Code and Iranian Earthquake
Code (2800) use the average shear wave velocity in top 30
meters of soil profile to identify the soil category. In this regard
soil profile is classified into 6 groups (A-F) in Euro Code as
shown in Table1.
Table 1. Ground type based on Euro Code8.
Ground
type
Description of stratigraphic profile
v
s,30 (m/s)
A
Rock or other rock-like geological
formation, including at most 5 m of weaker
material at the surface
> 800
B
Deposits of very dense sand, gravel, or
very stiff clay, at least several tens of metres
in thickness, characterised by a gradual
increase of mechanical properties with
depth.
360 - 800
C
Deep deposits of dense or medium- dense
sand, gravel or stiff clay with thickness from
several tens to many
hundreds of metres.
180 - 360
D
Deposits of loose-to-medium cohesionless
soil (with or without some soft cohesive
layers), or of predominantly soft-to-firm
cohesive soil.
< 180
E
A soil profile consisting of a surface
alluvium layer with
v
s values of type C or D
and thickness varying between
about 5 m and 20 m, underlain by stiffer
material with
v
s > 800 m/s.
S
1
Deposits consisting, or containing a layer at
least 10 m thick, of soft clays/silts with a
high plasticity index (PI
= 40) and high water
content
< 100
(indicative)
S
2
Deposits of liquefiable soils, of sensitive
clays, or any other soil profile not included
in types A - E or
S
1
Average shear wave velocity from surface to depth 30
meters is calculated from Equation1. Each category of soils has
a specified effect on seismic amplification and seismic force
that induced to structures.
(1)
(1)
In this Equation V
s
is average shear wave velocity, d
i
is
thickness of each layer and V
si
is shear wave velocity for each
layer.
This is the criteria for identify soil category in Earthquake
standards and building codes. As noted above based on results
of V
s
equation the soil category identified and amplification is
predicted.
E soil type is one of soil categories in Eurocode and Iranian
Earthquake Code (2800) that include soft soil layers laid on stiff
soil layers and show more amplification compared to other
categories in Earthquake Codes.
4 CLASSIFICATION OF BOREHOLES IN BAM SITE
Observations after field boring, sampling and primary analysis
of boreholes shows some contradictions between what expected
according to Earthquake standards and Building codes and what
happened during earthquake. Figure1 shows boreholes on Bam
site and E soil type range (that specified by solid lines).
Based on Eurocode8 predictions amplification is expected
only in boreholes of this area- E soil type- but significant
amplification ratio was seen out of this area.
Figure 1. Bam site map and boreholes.
Also based on these contradictions two group of boreholes
for more investigation specified:
1. Boreholes are not on E soil type area of Bam and results
of analysis do not confirm Earthquake Codes
predictions for amplification ratio (BH1, BH7,
BH16).
2. Boreholes are not on E soil type area of Bam and results
of analysis confirm Earthquake Codes predictions for
amplification ratio (BH3, BH8, BH14).
All of the analysis have done with EERA software based on
one dimensionally shear wave propagation theory. Detail of
boreholes and soil layering include soil type and shear wave
velocity along depth of each group of boreholes is shown on
figure 2 &3.
In each borehole soil layers basin density, hardness, soil
type and shear wave velocity are divided and detailed analysis
was done for 14 time histories. These group of time histories
based on earthquake magnitude, distance between station and
earthquake epicenter, techtonic, PGA and fault type were
selected. The list of seven couple time histories that selected for
this study is shown in Table2.
Table 2. Analysis time histories.
PGA(T)
PGA(L)
Country
Earthquake
1.039
1.497
United States Cape Mendocino
0.152
0.220
Turkey
Kocaeli
0.721
0.758
United States
Landers
0.473
0.411
United States
Loma Prita
0.565
0.871
Iran
Ardal
0.636
0.798
Iran
Bam
0.378
0.332
Iran
Tabas
