Civil Engineering Reference
In-Depth Information
As indicated in Table 14.11, the selection of the site class is based on the material
type and engineering properties, such as the shear wave velocity, standard penetration test
values, and the undrained shear strength. Average properties in the upper 100 ft (30 m) are
determined by using equations, such as the following (Equation 20.4-4, ASCE Standard
7-10, 2010):
d
c
[
(
d
i
s
ui
)]
s
u
(14.1)
where
s
u
average undrained shear strength to be used for Table 14.11
d
c
total thickness of cohesive soil layers in the top 100 ft (30 m)
d
i
thickness of an individual cohesive soil layer having test data, where the
summation of all the layers
d
c
s
ui
undrained shear strength corresponding to the layer
d
i
The procedure for determining the site class is as follows:
1.
Site class F:
Start with the four categories listed under site class F (see Table
14.11). If the site meets any one of these four categories, then the site is designated
as site class F.
2.
Site class E:
If a site is not a site class F, then check to see if it meets the criteria for
the definition of site class E in Table 14.11, that is, a soft clay layer of more than 10 ft
(3 m) in thickness meeting the plasticity index, moisture content, and undrained
shear strength criteria.
3.
Site class C, D, or E:
If a site does not conform to the two previous items, then
determine the average shear wave velocity, standard penetration resistance, and/or
the undrained shear strength. As indicated in Table 14.11, site class C has the best
soil properties (i.e., very dense soil), while site class E has poor soil properties (i.e.,
soft clay soil). Engineering properties of the soil are used to evaluate the site class
as follows:
a. Shear wave velocity: The shear wave velocity can be measured in situ by using
several different geophysical techniques, such as the uphole, down-hole, or cross-
hole methods.
b. Standard penetration test (SPT): ASCE Standard 7-10 (2010) states that the
standard penetration resistance (ASTM D 1586), as directly measured in the field
without corrections, should be used for Table 14.11. However, it is best to use
the SPT values that are corrected for both sampling procedures and overburden
pressure, that is, (
N
1
)
60
values (see Eqs. 5.1 and 5.2). This is because the (
N
1
)
60
value is a much more reliable indicator of the density of granular soil than uncor-
rected SPT values. If an uncorrected
N
value exceeds 100 blows/ft, then reduce
this value to 100 blows/ft for the site class analysis.
c. Undrained shear strength: ASCE Standard 7-10 (2010) states that the undrained
shear strength is to be determined in accordance with ASTM D 2166 (unconfined
compression test) or ASTM D 2850 (unconsolidated-undrained triaxial compres-
sion test). If a test indicates a value of undrained shear strength in excess of 5000
psf, then reduce this value to 5000 psf (240 kPa) for the site class analysis.
Two other considerations in selecting the site class C, D, and E are as follows:
(1)
Using more than one method:
In some cases, the soil properties may be
determined by using more than one method, such as for example by using
both geophysical techniques and the SPT. If the methods provide different site
classifications, then the site should be assigned to the category that corresponds
