Engineering Geology

E n g i n e e r i n g G e o l o g y
in the void ratio and porosity. In the case of some Chinese loess, the void ratio varies from
0.81 to 0.89 and the porosity from 45–47%. Lutenegger and Hallberg (1988) observed that
the bulk densities of unstable loess (such as the Peorian Loess, United States), tend to range
between 1.34 and 1.55 Mg m -3 . If this material is wetted or consolidated (or reworked), the
density increases, sometimes to as high as 1.6 Mg m -3 (Clevenger, 1958).
The liquid limit of loess averages about 30% (exceptionally, liquid limits as high as 45% have
been recorded), and their plasticity index ranges from about 4 to 9%, but averages 6%. As
far as their angle of shearing resistance is concerned, this usually varies from 30 to 34∞.
Loess deposits are better drained (their permeability ranges from 10 -5 to 10 -7 m s -1 ) than are
true silts because of the fossil root-holes. As would be expected, their permeability is appreciably
higher in the vertical than in the horizontal direction.
Normally, loess possesses a high shearing resistance and can carry high loadings without
significant settlement when natural moisture contents are low. For instance, moisture contents
of undisturbed loess are generally around 10%, and the supporting capacity of loess at
this moisture content is high. However, the density of loess is the most important factor controlling
its shear strength and settlement. On wetting, large settlements and low shearing
resistance are encountered when the density of loess is below 1.30 Mg m -3 , whereas if the
density exceeds 1.45 Mg m -3 , settlement is small and shearing resistance fairly high.
Unlike silt, loess does not appear to be frost susceptible, this being due to its more permeable
character, but it can exhibit quick conditions as with silt and it is difficult, if not impossible,
to compact. Because of its porous structure, a “shrinkage” factor must be taken into
account when estimating earthwork.
Several collapse criteria have been proposed that depend on the void ratios at the liquid limit,
e 1 , and the plastic limit, e p , and the natural void ratio, e o . Fookes and Best (1969) proposed
a collapse index, i c , which involved these void ratios, which is as follows:
i
c
e - e
o
=
e - e
l
p
p
(5.2)
Previously, Feda (1966) had proposed the following collapse index:
i
c
mS / - PL
r
=
PI
(5.3)
in which m is the natural moisture content, S r is the degree of saturation, PL is the plastic limit
and PI is the plasticity index. Feda also proposed that the soil must have a critical porosity of
40% or above and that an imposed load must be sufficiently high to cause structural collapse
when the soil is wetted. He suggested that if the collapse index was greater than 0.85, then
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