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The SCS Double Hydrometer Test in dispersive soil identification
Essai SCS de double hydrométrie pour l'identification des sols dispersifs
Maharaj A., Paige-Green P.
CSIR Built Environment, Pretoria, South Africa
ABSTRACT: The standard testing procedures for the Soil Conservation Service (SCS) Double Hydrometer test, the Pinhole Test,
Crumb test and chemical analyses for the identification of potentially dispersive soils have recently been studied and
problems/anomalies identified. Recent testing suggests that many of the shortcomings related to testing of dispersive soils may have
been overlooked during past routine investigations. A comparative study involving the testing of three samples using one standard
laboratory test, namely the SCS double hydrometer test was carried out and some potential means of overcoming the problems have
been identified. The investigation has highlighted the differences that can be obtained on a single soil as a function of the variation in
test procedures. The variability of the results obtained from the double hydrometer test in particular appears to be the cause of many
of the ambiguities and discrepancies in the classification systems studied during this research. Problems related to the double
hydrometer tests pose the potential for misleading results since the test has been used as the basis for identifying the potential
dispersiveness of soils during the development of rating systems.
RÉSUMÉ : Les procédures de test standard pour le Soil Conservation Service (SCS) test hydromètre double, le test sténopé, le Crumb
test et des analyses chimiques pour l'identification des sols potentiellement dispersifs ont récemment été étudiés et des problèmes ou
anomalies identifiés. Des tests récents indiquent qu’un bon nombre de défauts liés à l’analyse des sols dispersifs peut avoir été négligé
au cours des enquêtes de routine conduites dans le passé. Une étude comparative entre l'essai de trois échantillons à l’aide d’un test de
laboratoire standard, à savoir le test SCS hydromètre double, a été effectué et des moyens possibles de surmonter les problèmes ont
été identifiés. L’enquête a mis en évidence les différences qui peuvent être obtenues pour un sol unique en fonction de la variation
dans les procédures de test. La variabilité des résultats, obtenus à partir du test SCS hydromètre double en particulier, semble être la
cause de bien des ambiguïtés et des incohérences dans les systèmes de classification étudiés au cours de cette recherche. Les
problèmes liés aux tests hydromètre double présentent le potentiel pour des résultats erronés, puisque le test a été utilisé comme base
pour déterminer la dispersivité potentielle des sols lors de l’élaboration des systèmes de notation.
KEYWORDS: Dispersive soils ; hydrometer ; soil testing
1 INTRODUCTION
The failure of dams and embankments as a result of the use of
dispersive clays has been recognized by engineers and
geologists in South Africa and internationally for many years. A
serious problem, however, still lies in the early identification of
dispersive soils (Paige-Green, 2008). Despite all the information
gained over the years, there is still no quick, simple and reliable
means of conclusively identifying dispersive soils. Many
methods have been proposed including the pinhole, double
hydrometer, crumb and chemical tests, either individually or in
combination. These, however, have not always been entirely
reliable and it was considered possible that the reason lay in the
actual testing procedures.
The standard testing procedures for the Soil Conservation
Service (SCS) Double Hydrometer test, the Pinhole Test,
Crumb test and chemical analyses have recently been studied
and problems/anomalies identified. Although no discussion
regarding these anomalies has been found in the literature, the
recent testing suggests that many of these shortcomings may
have been overlooked during past routine investigations. This
paper summarizes a comparative study involving the testing of
three samples using one standard laboratory test, namely the
SCS double hydrometer test and discusses some potential
means of overcoming the problems identified. The other tests
have been discussed separately (Maharaj, 2010a: 2010b: 2011:
2012).
2 HISTORY OF THE SCS DOUBLE HYDROMETER TEST
The Soil Conservation Service (SCS) double hydrometer or
dispersion test has been identified as one of the most
appropriate tests for classifying dispersive soils. The test
evaluates the dispersibility of a soil by measuring the natural
tendency of the clay fraction to go into suspension in water. The
procedure involves the determination of the percentage of
particles in the soil that are finer than 0.005 mm using the
standard hydrometer test. A parallel test is also carried out, in
which no chemical dispersant is added and the solution is not
mechanically agitated. The quantity of particles finer than 0.005
mm in the parallel test is expressed as a percentage of this
fraction determined in the standard test, which is defined as the
dispersion ratio or dispersivity of the soil (Walker, 1997).
Dispersion ratios greater than 50% are considered highly
dispersive, between 30 and 50% are moderately dispersive,
between 15 and 30% are slightly dispersive and less than 15%
are non-dispersive (Elges, 1985). Similar systems with different
limits were utilized by Gerber & Harmse (1987) and Walker
(1997).
The dispersion test was first described by Volk (1937) as a
means of determining the potential dispersiveness of soils. The
test has since been used extensively in this regard with minor
modifications. Volk’s test compared the weight of soil grains,
0.005 mm or smaller that slaked free when air-dried lumps of
soil were soaked in quiet, distilled water, with that of the entire
soil. This was expressed as the percentage dispersion.
Measurements of the clay in the soil-water suspension were
made by the pipette method (Volk, 1937).
