Geology Reference
In-Depth Information
These results, together with the initial data, are summarized in the
form on pages 279 and 280. At the end of the chapter, two blank copies
of this form are included for the reader's use.
GRAPH
As explained in Chapter 18, a graph, on semilogarithmic paper, of
permeability (logarithmic scale) versus void ratio (arithmetic scale) is
normally included in the report of the results of a permeability test. See
the section “Graph” of Chapter 18 for a complete explanation.
As indicated previously, the falling-head method for determining soil
permeability may be used for both fine-grained and coarse-grained soils.
Table 19-1 gives some general information with regard to permeability
and drainage characteristics of soils. A permeability of 10
-
4
cm/s may be
considered borderline between pervious and impervious soils. A soil
with permeability less than 10
-
4
cm/s might thus be considered for a
dam core or impervious blanket, whereas one with permeability greater
than 10
-
4
cm/s might be used for a dam shell or pervious backfill [2].
SUMMARY
Table 19-1
Permeability and Drainage Characteristics of Soils
a
[3]
Coefficient of Permeability k (cm/s) (log scale)
10
2
10
1
10
1
10
2
10
3
10
4
10
5
10
6
10
7
10
8
10
9
1.0
Drainage
Good
Poor
Practically Impervious
Clean gravel
Clean sands, clean
sand and gravel
mixtures
Very fine sands, organic
and inorganic silts, mix-
tures of sand, silt, and
clay, glacial till, strati-
fied clay deposits, etc.
“Impervious” soils,
e.g., homoge-
neous clays
below zone of
weathering
Soil Types
“Impervious” soils modified by
effects of vegetation and
weathering
Direct testing of soil in its original
position—pumping tests; reliable if
properly conducted; considerable
experience required
Direct Determination
of k
Constant-head permeameter; little
experience required
Falling-head
permeameter;
reliable; little
experience
required
Falling-head
permeameter;
unreliable;
much experience
required
Falling-head permeameter;
fairly reliable;
considerable experience
necessary
Indirect Determination
of k
Computation from grain-size dis-
tribution; applicable only to
clean cohesionless sands and
gravels
Computation
based on results
of consolidation
tests; reliable;
considerable ex-
perience required
a
After Casagrande and Fadum (1940).
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