Environmental Engineering Reference
In-Depth Information
Depth (m)
"N"
Soil type
Laboratory test data
0
LL
P I
w (%)
γ
e
1
3
Very soft to soft
45
21
32
1.25
1.94
red silty clay
2
2
46
19
31
1.19
2.01
3
5
( CL )
48
24
34
1.51
1.53
4
5
43
16
32
1.39
1.39
5
10
Stiff tan to brown
Note
:
6
12
Very silty clay
Block sample
7
16
gm/cm
3
γ
t
8
14
N
Blows per foot (SPT)
9
11
GWL
Groundwater table
not encountered
10
14
11
14
12
36
Becoming hard
50
13
31
14
5/1 cm (refusal)
15
FIGURE 10.29
Test boring log and laboratory test data for a porous clay derived from basalt (Araras, Sao Paulo, Brazil).
the aforementioned hard zone, which often contains limonite nodules. The effect of satu-
ration on a consolidation test specimen is given in
Figure 10.30,
a plot of void ratio vs.
pressure. The curve is typical of collapsing soils. In the dry condition strengths are high,
and excavation walls will stand vertical for heights greater than 4 or 5 m without support
in the same manner as loess.
The recognition of porous clays can often be accomplished by terrain analysis. Three fac-
tors appear to govern the development of the weak, open structure: a long relatively dry
period followed by heavy summer rains, a relatively high ground elevation in rolling, hilly
terrain with a moderately deep water table, and readily leachable materials. An examina-
tion of aerial photos, such as
Figure 10.31,
reveals unusual features for a clay soil: charac-
teristically thin vegetation and lack of any surface drainage system, both indicative of the
open porous structure. Here and there, where terrain is relatively level, bowl-shaped
areas, often 3 to 4 m deep and 20 m across, with no apparent existing drainage, seem to
indicate areas of possible natural collapse (Figure 10.31). These may have occurred during
periods of very heavy rains, which either created ponds or fell on zones that had been very
much weakened by leaching.
