Environmental Engineering Reference
In-Depth Information
direction from the source. In the United States, the deposits are thicker and more clayey
along the Missouri and Mississippi Valleys, and thinner and more sandy to the west in the
high plains area where they grade into the sand hills of Nebraska.
Compressibility
Compressibility is most significantly related to collapse upon saturation, the potential for
which is a function of the natural density and moisture content of the deposit as shown in
The potential for settlement has been related to the natural dry density and moisture
content in terms of the Proctor density and moisture and to the natural dry density in
relation to the liquid limit (USBR, 1974), as given in
Figure 7.72.
Case I (Figure 7.72b) indi-
cates that a low natural density is associated with void ratios larger than those required
to contain the liquid limit moisture. Thus the soil, if wetted to saturation, can exist at a
consistency wet enough to permit settlement. Case III indicates that the natural densities
are high enough and the void spaces are too small to contain the liquid limit moisture
content. The soil will not collapse upon saturation, but will reach a plastic state in which
there will always be particle-to-particle strength. Because of the uniformity of the loessial
soils in Kansas and Nebraska (LL
30-40%), the criteria for settlement upon saturation
vs. natural densities and surface loadings have been developed as given in
Table 7.11.
Note that when the natural density is 80 pcf or less, the deposit is highly susceptible to
settlement.
Shear Strength
At natural moisture contents loess has relatively high strength, as well as low compress-
ibility, because of its slight cementation. Some typical strength envelopes are given in
Figure 7.73;
it is seen that wetting has a severe effect.
Unconfined compressive strength in the dry condition may be several tsf. At natural
moisture contents (usually less than 10%), loess has an apparent cohesion which may be
as high as 15 psi (1 tsf) and generally ranges from 5 to 10 psi (0.3-0.6 tsf) for the
Kansas-Nebraska loess; tan
. Effective stress parameters
of these magnitudes provide the strength that permits loess to stand vertically in slopes 50
to 80 ft (16 to 24 m) high, even with its characteristic low densities (Gibbs et al., 1960).
When loess is wetted, cohesion is reduced to less than 1 psi (0.07 tsf) and even for initially
ϕ
ranges from about 0.60 to 0.65
°
84
86
Low-density loess
Low natural moisture
Low-density loess
prewetted
High-density loess
Low natural moisture
High-density loess
prewetted
88
90
92
94
96
98
Wetted
FIGURE 7.71
Typical consolidation curves for Missouri
River basin loess. (From Clevenger, W. A.,
Proc. ASCE, J. Soil Mech. Found. Eng. Div
., 82,
1958. With permission.)
100
102
0 1020304050607080
90
100110
Load (psi)