Environmental Engineering Reference
In-Depth Information
Frost susceptibility
: Many thin ice layers can form in cold climates, resulting in ground
heave.
Liquefaction susceptibility
: Nonsusceptible.
Piping
: It occurs in dispersive clays.
Rupture Strength
Consistency
provides a general description of strength identified by the relationship
between the natural moisture content and the liquid and plastic limits and by the uncon-
fined compressive strength. Parameters include the peak drained strength (
c
' and
φ
), the
φ
r
), and the resid-
ual undrained strength (
s
r
). Sensitivity is defined as
s
u
/
s
r
, and is a measure of the loss in
strength upon remolding.
Failure occurs by general shear, local shear, or punching shear. Collapse upon saturation
or under a particular stress level occurs in certain clays (see
Section 10.5.2
for porous clays)
from which minerals have been leached, leaving an open, porous structure.
peak undrained strength (
s
u
,
φ
0), the residual drained shear strength (
Deformability
Compression
, by plastic deformation, occurs in clays during the process of consolidation.
Clay soils retain their ”stress history” as overconsolidated, normally consolidated, or
underconsolidated (see
Section 3.4.1).
During consolidation there is substantial time delay
caused by low permeabilities slowing the neutralization of pore-water pressures.
Overconsolidated, fissured clays, however, deform in a manner similar to
in situ
rock, i.e.,
displacements occur at the fissures, possibly combined with consolidation.
Expansion
is a characteristic of partially saturated clays in the presence of moisture. The
amount of expansion varies with mineral type and swelling pressures, and volume
changes can reach substantial magnitudes. Not all clays or clay mixtures are susceptible.
Organic Soils
Hydraulic Properties
Permeability of peat and rootmat, primarily fibrous matter, is usually very high and, for
organic silts and clays, is usually low. In the latter cases, systems of root tunnels can result
in
k
values substantially higher than for inorganic clays.
Rupture Strength
Peat and rootmat tend to crush under applied load, but shallow cuts will stand open
indefinitely because of their low unit weight, as long as surcharges are not imposed.
Organic silts and clays have very low strengths, and generally the parameters for clay
soils pertain. Embankments less than 2 m in height placed over these soils often undergo
failure.
Deformability
Organic materials are highly compressible, even under relatively low loads. Fibers and gas
pockets cause laboratory testing to be unreliable in the measurement of compressibility,
which is best determined by full-scale instrumented load tests. Compression in peat and
rootmat tends to be extremely rapid, whereas in organic silts and clays there is a substan-
tial time delay, although significantly less than for inorganic clays. Rootmat undergoes
substantial shrinkage upon drying. The shrinkage can reach 50% or more within a few
weeks when excavations are open and dewatered by pumping.
