Environmental Engineering Reference
In-Depth Information
Figure 6.33.
Peak effective friction angle of sands as a function of relative density and grain size
(Schmertmann, 1978).
Method (c) is commonly used for estimating the strength of filter zones and is sufficiently
accurate in most cases.
6.1.9
Estimation of the effective friction angle of granular soils
6.1.9.1 In-situ
tests
6.1.9.1.1 Standard Penetration Test (SPT)
The effective friction angle,
can be estimated directly from the SPT “N” values (e.g. using
the methods of Mitchell and Kalti, 1981; Peck et al., 1974) or by using the SPT to estimate
the relative density and then estimating the strength from the relative density and general
grain size characteristics. Schmertmann (1978) developed such a method which is shown in
Figure 6.33. Lunne et al. (1997) indicate the laboratory tests on which Figure 6.33 is based
were at a confining stress of 150 kPa, and that lower values of
should be used at high con-
fining stresses.
The relative density is best estimated using the method suggested by Skempton (1986).
He reviewed the available information and proposed a method where the SPT value is
corrected for the energy of the hammer, and to an effective overburden pressure of
100 kPa by:
N
E
m
(6.23)
60
where Nm
measured SPT “N” value; N
60
“N” value corrected to a hammer of 60%
efficiency and ERm
Energy Rating of the SPT hammer.
and
(N )
C
N
(6.24)
160
N
60
