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th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
needed in the way the pore pressure is included in the analysis.
The example in Figure 8 illustrates this point.
Time
Time
Time
Pore pressure
Effective stress
Safety factor
End of construction
Long term
Long term steady state
- typical of low permeability
(sedimentary) clays
Fluctuating water table
- typical of medium to high
permeability (residual) clays
Sedimentary clays
Residual clays
Potential failure
surface
Storm
events
Seasonal
influence
Figure 7 Short and long term stability of cut slopes (after
Wesley, 2010).
This figure shows a steep cut slope subject to variable weather
patterns. One way in which the worst case pore pressure state
can be determined analytically is to assume that rainfall
continues long enough for the water table to rise to the surface
and create a stable seepage state. This may be excessively
conservative, but does at least put a lower limit on the
theoretical safety factor.
Analysis based on flow net
Analysis based on phreatic
surface inserted at ground level
SF = 1.15
SF = 0.74
Flow net for continuous
rainfall at surface
Assumed impermeable boundary
= 16.5 kN/m c = 50 kPa, = 40
3,
o
25m
40m
Figure 8 Influence of pore pressure assumptions on the estimate
of safety factor.
There are then two ways of including the pore pressures from
this state in a slip circle analysis. The first, and normal, method
is to determine the pore pressure directly from the vertical
intercept between the phreatic surface and the slip surface – the
“vertical intercept” assumption. In this case it will be the
vertical distance from the ground surface to the slip surface.
Almost all computer programmes make this assumption, which
may be reasonable in gentle slopes but can give very
misleading results in steep slopes, which is what the example in
Figure 8 illustrates.
The second method is to consider the practical situation
realistically and determine a flow net compatible with the
boundary conditions. The pore pressures can then be
determined from this flow net. It is evident from Figure 8 that
the vertical intercept assumption, which implies that
equipotential lines are vertical is physically impossible. The
short section of level ground at the top of the slope is an
equipotential line and flow lines will begin perpendicular to
this. The flow net shows that most of the equipotentials along
the slip surfaces are far from vertical.
The safety factors determined by the two methods, using the
computer programmes SeepW and SlopeW, are the following:
Vertical intercept assumption SF = 0.74
From the correct flow net: Safety Factor = 1.15
The difference is very large, and although many slopes in
residual soils may not be as steep as that in Figure 8, there are
many, especially in places like Hong Kong that are
considerably steeper Thus the error in the safety factor could be
even greater than that indicated in the Figure 8 analysis.
6. CONCLUSIONS
Although residual soils occupy about half the world’s surface
very few universities cover them in their soil mechanics
courses. This includes many universities surrounded on all
sides by residual soils. The result is that geotechnical engineers
routinely apply concepts valid only for sedimentary soils to
residual soils and gain a mistaken understanding of their
behaviour.
It is long past the time when residual soil behaviour should
be part of mainstream soil mechanics and an integral part of
university courses. The importance of this cannot be
overemphasised. Education today is globalised in a way it
hasn’t been in the past and large numbers of students from
Asia, Africa and Latin America are obtaining their education in
the universities of Western countries. Residual soils tend to be
predominant in the former counties, but only sedimentary soil
behaviour is covered by degree courses in the latter. Students
thus return to their home countries unaware that significant
parts of the soil mechanics they have been taught do not apply
to the residual soils they are highly likely to encounter in their
own countries.
7. REFERENCES
Janbu, N. 1998. Sediment deformation.
Bulletin 35, Norwegian
University of Science and Technology,
Trondheim, Norway.
Pender, M.J., Wesley, L.D. Twose, G., Duske, G.C., and
Satyawan Pranjoto 2000. Compressibility of Auckland
residual soil. Proc.
GeoEng2000 Conf
, Melbourne.
Wesley, L.D. 2000. Discussion on paper: Influence of in situ
factors on dynamic response of Piedmont residual soils.
ASCE Journal of Geotechnical and Geoenvironmental
Engineering
. 126 (4), 384-385.
Wesley, L.D. 2010a, Fundamentals of Soil Mechanics for
Sedimentary and Residual Soils. John Wiley & Sons Ltd,
New York.
Wesley, L.D, 2010b. Geotechnical Engineering in Residual
Soils. John Wiley & Sons Ltd New York.
Wesley, L.D. and Pender, M.J. 2012. Aspects of soil mechanics
teaching.
Proceedings, 11
th
Australia New Zealand
Conference on Geomechanics,
Melbourne.
