Civil Engineering Reference
In-Depth Information
TABLE 14.11
Site Class Definitions
Average properties in top 100 ft
a
Standard
Soil undrained
Site
Soil
Shear wave
penetration
shear strength,
s
u
,
class
profile name
velocity,
v
s
, (ft/s)
resistance, (
N
1
)
60
(psf)
A
Hard rock
v
s
5000
N/A
N/A
B
Rock
2500
v
s
5000
N/A
N/A
C
Very dense soil and
1200
v
s
2500
(
N
1
)
60
50
s
u
2000
soft rock
D
Stiff soil
600
v
s
1200
15
(
N
1
)
60
50
1000
s
u
2000
E
Soft soil
v
s
600
(
N
1
)
60
15
s
u
1000
E
Any profile with more than 10 feet of soil having the following characteristics:
plasticity index PI
20, moisture content
w
40%, and undrained shear strength
s
u
500 psf
F
Any profile containing soils having one or more of the following characteristics:
1. Soils vulnerable to potential failure or collapse under seismic loading such as lique-
fiable soils, quick and highly sensitive clays, collapsible weakly cemented soils.
2. Peats and/or highly organic clays (
H
10 feet of peat and/or highly organic clay
where
H
thickness of soil).
3. Very high plasticity clays (
H
25 feet with plasticity index PI
75).
4. Very thick soft/medium stiff clays (
H
120 feet) with
s
u
1000 psf.
Notes:
a - see Chap. 20 of ASCE Standard 7-10 (2010) for further details.
For SI: 1 foot
0.0929 m
2
, 1 pound per square foot
304.8 mm, 1 square foot
0.0479 kPa. N/A
Not
applicable. ASCE Standard 7-10 recommends use of uncorrected
N
-values instead of (
N
1
)
60
.
Sources:
Table 20.3-1 and Chap. 20 of ASCE Standard 7-10
Minimum Design Loads for Buildings and Other
Structures
(2010).
sufficient detail to determine the site class, site class D shall be used unless the building
official or geotechnical data determines that site class E or F soils are present at the site.”
As indicated in Table 14.11, the site class is based on the average condition of the
material that exists at the site from ground surface to a depth of 100 ft (30 m). The best is
site class A that consists of hard rock. The worst is site class F, where there are soil profiles
that may liquefy during the earthquake or there are soft soils that can increase the peak
ground acceleration. The site class categories listed in Table 14.11 were apparently derived
from the soil profile types in Table 11.1.
The site class should be determined for the expected site conditions. For example, if the
ground surface will be raised or lowered by grading operations, then the site class analysis
should be based on the final as-built conditions. Likewise, if the site has a layer of potentially
liquefiable soil that will be treated by a site improvement method, then the anticipated condi-
tions after treatment should be used in the determination of the site class.
The type and depth of the foundation could also affect the site class. For example, if a site
has an upper layer of potentially liquefiable soil, then in accordance with Table 14.11, it is
site class F. However, if a structure is built with piers that penetrate the potentially liquefiable
layer and are embedded in solid rock, then the dynamic forces exerted on the structure during
the earthquake are probably closer to those values based on site class A rather than site class F.
Hence, it takes considerable experience and judgment in the determination of the site class.
