Environmental Engineering Reference
In-Depth Information
Collapse Phenomenon
Loess is one of the deposits referred to as collapsing soil (see
Section 10.5).
When sub-
merged in water, its physicochemical structure may be destroyed, and the soil mass may
immediately densify to cause ground subsidence, often of the order of 12 to 24 in. This
phenomenon does not appear to occur naturally, since its peculiar structure provides loess
with a high rate of vertical percolation adequate for rainfall, and a blanket of topsoil and
vegetation affords protection against wetting.
Terrain Features
The typical landform that develops in thick loess deposits is illustrated in
Figure 7.23.
Distinguishing characteristics are the crests of the hills at a uniform elevation (remnants of
the old loess plain); the drainage pattern which is pinnate dendritic
(Table 7.4);
and the
eroded slopes on both sides of the ridges that are uniform. Concentrated runoff on flat
slopes does not cause severe erosion, but when erosion does begin, it proceeds rapidly, and
because of the light cementation, vertical slopes develop in streams and gullies. This is par-
ticularly true in moist climates. Where loess is thin, erosion proceeds, creating a typical pin-
nate dendritic pattern, until the underlying formation, usually more resistant, is exposed.
Thereafter, erosion patterns reflect the characteristics of the underlying materials.
Engineering Properties
Index Properties
Loess deposits have distinct physical features that are strikingly similar from location to loca-
tion. Three types are generally identified: clayey, silty, and sandy. The trends in gradation and
plasticity characteristics of loess from the Kansas-Nebraska area are given in
Figure 7.70.
Clayey loess is usually in the ML-CL range and porosity of all loess is high: from 50 to 60%.
Loesses are generally uniform in texture, consisting of 50 to 90% silt-size particles exhibit-
ing plasticity, but the fineness increases and the thickness decreases in the downwind
Hydrometer analysis
Sieve analysis
Timing reading
U.S. Standard series
Clear square openings
7Hr.15Min.
3"
3"
1"
1
#
200
#
100
#
50
#
40
#
30
#
16
#
10
#
8
#
4
25HR.45Min.
60Min.
19Min.
4Min.
1Min.
3"
5" 6"
8"
100
0
90
10
80
20
70
30
30
60
40
20
50
50
Sil
ty loess
Sandy loess
CL
40
60
10
CL
30
70
CL
ML
ML
and
OL
ML
20
80
0
0
10
20
30
40
50
60
Liquid limit
10
90
0
100
.002
.008
.009
.019
.037
.074
.149
.297 0.42.590
1.19
2.
2.38
4.76
9.52
19.1
38.1
76.2
127
152
Diameter of particle in millimeters
Sand
Gravel
Clay (Plastic) to silt (Non-Plastic)
Cobbles
Fine
Medium
Coarse
Fine
Coarse
Gradation test
FIGURE 7.70
Trends of gradation and plasticity for loess from Kansas-Nebraska. (From Gibbs, H. J. et al.,
Proc. ASCE, Research
Conference on Shear Strength of Cohesive Soils
, Boulder, Colorado, June 1960, p. 331 B2. With permission.)
