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10-14 CIVIL ENGINEERING REFERENCE MANUAL sss
T„ is a dimensionless number known as the time Jactor.
T„ depends on the degree of consolidation. (!,. U, is the
percent of the rota] consolidation (settlement) expected.
For U, less than 0.60 (60%•). T„ is given by equation
10.44.
Tv = 9rU? 10.49
Secondary consolidation can be identiSed by a plot of
void ratio (or settlement) versus time on a logazithmic
scale. The region of secondary consolidation is chazac-
terized by a slope reduction on the plot. The plot. can
be used to obtain important parameters necessary to
calculate the magnitude and progression of secondary
consolidation.
Table 10.7 should be used to find T„ Cur larger values of
U, .
Table 10.7
.Approximate Time Factors
U: T
V.nJ V.34
0.70 0.90
0.75 0.48
0.80 0.55
o.ss o.70
0.90 0.85
U.95 1.3
1.0 x
Following the end of primary consolidation (approxi-
mately T„ = 1 in equation 10.44), the rate of consolida-
tion will decrease considerably. The continued consoli-
dation is known as secondary consolidation.
21 SECOI~rDARY CO1~rSOLIDATIOIV
Secondary consolidation is a gradual consolidation
which continues long after the majorit}' of the initial
consolidation has occurred. Secondan• consolidation
ma}' not occur at all, as in the case of granular soils.
However, secondary consolidation may be a major fac-
tor for inorganic clays azld silts. as well as for highly-
organic soils.
The final void ratio, ef, at the end of the primary con-
solidation period is read from the intersection of the
projections of the primary and secondary curves. The
logarithmic slope of the secondaz}' compression line is
known as the secondary compression index, o.'s
n = -(logla t2 - loglo tt) 10.45
es - et
The coeffltient of secondary compression, Ca, can be
derived from this slope. The initial voids ratio, a°, can
be estimated from equation 10.39.
CQ = a 10.96
l+ea
The secondary consolidation during any period t~ - tl
is
Sseconda:y = CaHlog I t? I 10.9"!
tl
22 SLOPE STABILITY
A. HOMOGENEOUS, SOFT CLAY (~ = 0°)
For homogeneous. soft clay, the Taylor chart can be
used to determine tkle factor of safety against slope fail-
ure.ll .Alternatively. if the factor of safety is itnown.
the maximum depth o! cut or the maximum cut angle
can be determined.
step 1: Calculate the depth factor. d. from the slope
height and the depth from the slope toe to
the lowest point on the slip circle.
d = H 10.98
void ratio,
e

tt
time, log t
Figure 10.12 Primary and Secondar}~ Consolidation
The slope height. H. is essentially the depth
of the cut. D is the vertical distance from
the toe of the slope to the firm base below
the clay.
step 2: Based on the depth factor; d. and the angle
of the slope, determine the stability number,
N°.
t6 The Secondan' compression index generally ranges from 0 to
0.03, and seldom exceeds 0.04.
t 7 The nrnJar arc method and mcfhod of adcea can also be used to
analyze a parti[ular failure surface. However, if the failure surface
is unknown, these methods may require trial and error solutions
to lo[ate the critical failure plane.
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secondary consolltlarion I `~