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Residual Soils and the Teaching of Soil Mechanics
Les sols résiduels et l’enseignement de la mécanique des sols
Wesley L.D.
University of Auckland, Auckland, New Zealand
ABSTRACT: There is a serious gap in the teaching of soil mechanics because of its failure to include coverage of residual soils as an
integral part of such teaching. A rough estimate suggests that at least half of the earth’s surface is covered by residual soils, and in
today’s world the most rapid growth and development is occurring in countries that contain a very high proportion of these soils. Civil
engineering students are graduating from universities around the world having studied soil mechanics to varying levels, but without
even being aware of the existence of residual soils, let alone having any understanding of their properties. The purpose of this paper is
to highlight the fact that while much of what is taught in soil mechanics courses is common to both soil groups, there are significant
and important areas where concepts applicable to sedimentary soils are completely irrelevant to residual soils.
RÉSUMÉ: L’omission de l’enseignement des sols résiduels est une lacune grave dans l'enseignement de la mécanique des sols. Une
estimation approximative laisse à penser qu'au moins la moitié de la surface terrestre est recouverte par des sols résiduels, et la
croissance et le développement les plus rapides dans le monde actuel a lieu dans des pays qui contiennent une proportion très élevée
de ces sols. Les étudiants en génie civil terminent leurs études à travers le monde en ayant étudié la mécanique des sols à des niveaux
variables, sans même être au courant de l'existence des sols résiduels ou de leurs propriétés. Cet article vise à mettre en évidence le
fait qu'alors qu'une grande partie de ce qui est enseigné dans les cours de mécanique des sols est également valable pour les sols
résiduels, il y a des chapitres significatifs et importants de l’enseignement où les concepts applicables aux sols sédimentaires sont
complètement hors sujet pour les sols résiduels.
KEYWORDS: Residual soils, soil mechanics teaching, stress history, formation, compressibility, slope stability
1 INTRODUCTION
Although residual soils are found on the earth’s surface almost
as commonly as sedimentary soils, their existence and
properties are rarely mentioned in soil mechanics courses and
text books. The result is that certain concepts developed from
sedimentary soil behaviour are routinely applied to residual
soils and routinely result in a mistaken understanding of their
behaviour. This is surely an indictment on those who teach soil
mechanics in our universities. It is well past the time when
residual soil behaviour should be an integral part of mainstream
soil mechanics, especially of its syllabus in university courses.
This paper is an attempt to highlight some significant aspects of
residual soil behaviour that should be essential material in basic
soil mechanics courses.
2 FORMATION
Figure 1 illustrates residual and sedimentary soil formation.
Residual soils are formed directly from their parent rock by
physical and chemical weathering, while. sedimentary soils
undergo further processes including transportation by streams
and rivers, sedimentation in lakes or in the sea,.followed by
consolidation.
Their formation method has some obvious influences on the
properties and behaviour of these two soil groups, the main
ones being the following:
(a) sedimentary soils undergo a sorting process during
erosion and re-deposition that give them a degree of
homogeneity that is not present in residual soils.
(b) residual soils do not undergo a consolidation process, and
their properties cannot be related to stress history. The terms
normally and over-consolidated have no relevance to residual
soils. Strictly speaking the parameters C
c
and C
s
are not
applicable to residual soils. The parameter C
c
is defined as the
(log) slope of the virgin consolidation line. It is readily apparent
from their formation process that there is no such thing as a
virgin consolidation line for a residual soil.
Rock
Residual soil
Sedimentary soil
Sea or lake level
Delta
deposits
Transport by stream
and river
Erosion by rainfall
and runoff
Figure 1. Soil formation (after Wesley, 2012)
(c) Some residual soils, especially those derived from
volcanic parent material consist of unusual clay minerals not
found in sedimentary soils
(d) Residual soils generally have much higher permeability
than sedimentary soils, which has important implications for
behaviour in oedometer tests and in estimates of short term and
long term stability of cut slopes.
3 CONSOLIDATION BEHAVIOUR
Figure 2 shows results of oedometer tests on samples of a
residual soil derived from the weathering of Peidmont
formation in southeastern USA. Figure 2(a) shows the results
plotted using the conventional log scale for pressure. This
convention arises from the behaviour of sedimentary clays
when deposited and consolidated under water. Values of pre-
consolidation pressure and over-consolidation ratio have been
determined from these graphs and are listed in the figure. As
noted earlier, stress history has no significant relevance to
residual soils, and assumptions that they should display pre-
consolidation pressures are erroneous.
