TM 5-818-1 / AFM 88-3, Chap. 7
su = cu φu = 0
Terms descriptive of sensitivity are listed in table 3-5.
(3-11)
Generalized relationships among sensitivity, liquidity
(σ1 - σ3) =
index, and effective overburden pressure are shown in
su =
1
qu
(3-12)
figure 3-17. The preconsolidation pressure, rather than
2
2
the effective overburden pressure, should be used for
overconsolidated soils when entering this figure.
and is essentially independent of total normal stress.
Cementation and aging cause higher values of sensitivity
The undrained cohesion intercept of the Mohr-Cou-lomb
than given in figure 3-17.
failure envelope is cu.
e. Effective strength parameters, cohesive
(1) The undrained shear strength, s, , of
soils. As indicated in figure 3-14, the peak and residual
normally consolidated cohesive soils is proportional to
strengths may be shown as failure and postfailure
the effective overburden pressure, po. An approximate
envelopes. Values of the peak drained friction angle for
correlation is as follows:
normally consolidated clays may be estimated from
figure 3-18. After reaching the peak shear strength,
su = 0.11 + 0.0037 PI
(3-13)
overconsolidated clays strain-soften to a residual value
p0
of strength corresponding to the resistance to sliding on
an established shear plane. Large displacements are
(2) A correlation between the remolded,
necessary to achieve this minimum ultimate strength
undrained shear strength of clays and the liquidity index
requiring an annular shear apparatus or multiple
is shown in figure 3-15.
reversals in the direct shear box. Typical values of
(3) A correlation between the normalized
residual angles of friction are shown in figure 3-19.
su/p0 ratio of overconsolidated soils and the
f.
Shear strength, cohesionless soils.
overconsolidation ratio (OCR) is presented in figure 3-16.
(1) In sandy soils,
the cohesion is
The value of p0 in (su/p0)OC is the effective present
negligible. Because of the relatively high permeability of
overburden pressure.
Values of (su/p0) may be
sands, the angle of internal friction is usually based
estimated from this figure when (su/po)NC and the OCR
solely on drained tests. The angle of internal friction of
are known (NC signifies normally consolidated soils).
sand is primarily affected by the density of the sand and
d. Sensitivity, cohesive soils. The sensitivity
normally varies within the limits of about 28 to 46 de-
of a clay soil, St, is defined as follows:
St =
Undisturbed compressive strength
Remolded compressive strength
(at
same
water
content)
Table 3-5. Sensitivity of Clays
Sensitivity
Descriptive Term
0-1
Insensitive
1-2
Low sensitivity
2-4
Medium sensitivity
4-8
Sensitive
8-16
Extra sensitive
>16
Quick
U. S. Army Corps of Engineers
3-20
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