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Technical Committee 101 - Session II /
Comité technique 101 - Session II
prepared according to the parallel test procedure, with no
chemical dispersant and mechanical agitation.
5 RESULTS
The one hour readings as well as the calculated dispersion
ratios for the two dispersing agents are summarized in Table 2.
Table 2: Results obtained from 1 hour hydrometer readings and
ispersion ratios
d
Sample
Control
Na silicate &
oxalate
Dispersion
ratio %
Na
Hexameta-
phosphate
Dispersion
ratio%
ND309
1.9
7.9
17.7
13.9
10.1
UM108
2.6
11.6
22.4
16.6
15.7
ZT114
19.9
15.9
125.2
24.9
79.9
The test results show that there is significant variation in the
apparent clay fraction between the two dispersants. The samples
that were dispersed with sodium hexametaphosphate produced
more realistic dispersion ratios than those of the other samples.
This could mean that the samples were not completely
dispersed with this dispersing agent (sodium silicate + oxalate)
or that there was some variation in sample preparation.
However, as the samples were prepared together with utmost
care to ensure ideal representatives, the latter is unlikely.
The maximum dispersion should occur when a chemical
dispersant is added to the sample ensuring a total disaggregation
of all flocs of soil. A high dispersion ratio indicates that the
sample breaks down significantly without the use of a chemical
dispersant and is thus dispersive. In theory, the 0.005 mm
fraction for the parallel test cannot be higher than that of the
standard test with the use of a chemical dispersant. This is only
likely to occur if there are inconsistencies in the testing
procedure or if the chemical dispersant does not act fully on the
material. The dispersion ratio of 125.2% shown for the sodium
silicate/oxalate dispersant (Table 2) indicates that the dispersant
in the standard test did not completely disperse the particles or
possibly caused some flocculation to occur.
An additional observation is that the classification obtained
for the two dispersants do not correlate. Sample ND309 falls
into two different categories of dispersivity based on the
classification by Elges (1985). It is classified as being slightly
dispersive using sodium silicate/oxalate and non-dispersive
using sodium hexametaphosphate, which can lead to uncertainty
regarding treatment requirements, should it be used for
construction.
6 DISCUSSION
Although the results of only limited testing is reported in this
paper, it is clear that the incorrect classification of dispersive
soils as a result of test variations appears to be common.
Inconsistencies noted in the available literature include the
variations in test methods. Observations have found that
different authors indicate different particle sizes for the clay
fraction. TMH1 and ASTM use the 0.005 mm fraction as the
boundary for the clay fraction, whereas BSI uses the 0.002 mm.
Many authors quote the 0.005 mm fraction as the clay fraction
when determining dispersivity of a soil. A Dictionary of
Geology (1972) defines the clay fraction as a mineral particle
having a diameter less than 0.004 mm (1/256 mm). According
to Reeves et al (2006), the ASTM standards define the clay
fraction as being less that 0.005 mm and Japan defines the
fraction as less than 0.006 mm. However, the majority of the
countries listed define the clay fraction as particle sizes less than
0.002 mm. Once again there is no standard definition with
regards to the unit size for clay particles although a scan of the
literature shows that 0.002 mm is used more widely. As the
0.002 mm fraction is also the basis for classification of South
African soils according to Brink and Bruin (2002), this size
fraction should be taken as the upper limit of clay-sized
particles for future studies in South Africa.
The literature also indicates that during studies of dispersive
soils the initial indicator of dispersivity of the material is
generally classified on the basis of the double hydrometer test
by means of various indicator graphs/plots. Many workers
(Gerber and Harmse, 1987; Bell and Maud, 1994; Walker,
1997) have then proceeded to indicate that no single test
(including the double hydrometer test) can be used to identify
dispersive soils, and then propose classification rating systems
using a number of tests. It is postulated that many of the
ambiguities (i.e., the inconsistencies of results among workers)
are the result of the incorrect initial classification of the
dispersivity of materials as a result of variations introduced in
the double hydrometer test.
Most of the rating systems used currently in South Africa
seem to have been based on the initial classification of
dispersiveness by the double hydrometer test. Gerber and
Harmse (1987) used the test as a primary parameter when
developing the ESP-CEC chart. Walker (1997) included the
ESP-CEC chart as a parameter in the rating system and studies
carried out by Bell and Walker (2000) also make use of the
double hydrometer test when initially classifying the dispersive
soils.
This has resulted in the overlap of results within single
classification bands. Although it is assumed that in these
investigations, the materials have been tested following uniform
and standard procedures, preliminary testing has indicated
spurious results when sodium silicate/oxalate (the South African
road standard) is used as the dispersant (NITRR, 1986). It is
also noted that the dispersant standard in South Africa has
changed over time, possibly affecting the results, if they were
obtained from different laboratories over a prolonged period of
time. It can thus be assumed that this would be particularly
more so in projects carried out over short periods at various
times related to the general use of different dispersants with
time.
7 CONCLUSIONS
An investigation into the double hydrometer test method used
for the identification of dispersive soils has highlighted
differences that can be obtained on a single soil as a function of
the variation in test procedures. This is due to the relatively
ambiguous state of the test procedures resulting in different
interpretations of the test methods, and consequently misleading
results.
The double hydrometer test is a good example of
misinterpretations due to ambiguities. Despite the test being an
ASTM standard (ASTM D4221-99), many laboratories just
duplicate the standard hydrometer analysis procedure (TMH1
and ASTM D422-63), which invariably produces incorrect
results. The variability of the results obtained from the double
hydrometer test appears to be the cause of many of the
ambiguities and discrepancies in the classification systems
studied during this research. The incorrect classification of the
dispersiveness in the early stages of the investigations would
influence the entire analysis process negatively.
