slope safety: factors and common misconceptions · generated between two ... only the common...
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SLOPE SAFETY:
FACTORS AND COMMON MISCONCEPTIONS By Ir. Dr. Gue See Sew* & Fong Chew Chung**
*Managing Director **Geotechnical Engineer, Gue and Partners Sdn Bhd
1.0 INTRODUCTION The collapse of Block 1 of Highland Towers and the
recent tragic landslide at Taman Hillview had
prompted many “experts” to put forward their
hypothesis or likely causes of the landslide in the area.
Some of the hypothesis are quite factual while some
are misleading or without proper basis. In addition, the
general public who are not familiar with slope
stability, are concerned about hill slope developments,
especially if they live near hill slope areas.
This article aims to explain to those who are not
familiar with slope stability and intend to highlight the
main factors affecting slope stability. It also presents
some common misconceptions on landslides. How
does landslide occur? What are the important factors
affecting it? What are some of the common
misconceptions about landslides? What should we do
with abandoned projects near hill slopes? These are
some of the questions this article will answer along
with illustrations to simplify the complex
phenomenon.
2.0 ANATOMY OF A SLOPE Figure 1 shows a typical slope consisting of (i) ground
profile with some vegetation, (ii) ground water table,
(iii) partially saturated soil above ground water table,
(iv) saturated soil below ground water table and (v)
weathered and/or competent rock.
In the analysis of slope stability to determine whether
a slope is safe, potential slip surfaces (Figure 2) are
postulate on a slope cross-section. These slip surfaces
are analysed in terms of the total driving forces and
total resisting forces. The factor of safety (FOS) is
determined from the ratio of resisting forces to driving
forces. The lowest FOS is the critical stability of the
slope.
Figure 1: Anatomy of a Typical Slope
Figure 2: Potential Slip Surfaces
With the above features of a typical slope, this article
introduces several fundamental concepts found in
slope stability. The first concept is friction. Friction is
generated between two bodies when the bodies are
moving against each other as shown in Figure 3. From
the illustration, there is a normal force (N) causing the
two bodies to come in contact, a driving force (T) and
frictional resistance (F). Two important events arise:
(1) If T increases, F also increases until a limit in
Partially Saturated Soil
Saturated Soil Water Table
WEATHERED ROCK
ROCK
ROCK
POTENTIAL SLIP SURFACES
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which the two bodies will slide against each other; (2)
As N increases, F increases as well. F is a function of
soil properties and the weight of the two bodies in
contact.
Figure 3: Concept of Friction
In slope stability, the main properties of soil for slope
analysis are soil unit weight (γ), apparent cohesion (c’)
and friction angle (φ). Relating the earlier concept of
friction to slope stability, the forces N and T can be
replaced by the force components in slope; N is
analogous to the self weight of the soil, F is the shear
resistance at the potential slip surface and T is the
driving forces caused by soil self weight and/or
surcharge (Figure 4). The governing equation for the
resistance of the potential slip surface to shearing is
based on the Mohr-Coulomb equation:
( ) ( ) ''tan cun +−= φστ
Figure 4: Friction Concepts in Slope
Where τ is shear stress, σn is the normal vertical
stress, u is the pore water pressure, φ’ and c’ are the
friction angle and apparent cohesion of soil
respectively.
Therefore, in a slope stability analysis, a slope is
unstable when the summation of shear forces or
resistance along the potential slip surface is less than
the driving forces.
The second concept is the role of water pressure in
slope stability analysis. In soil, water pressure exists if
the soil is below the ground water table (saturated
soil). The main effect of water pressure on a sliding
plane is the reduction of normal pressure or forces on
soil particle to soil particle at contact. Thus the shear
stress is reduced and correspondingly the shear
resistance is also reduced.
The third concept is suction. Suction occurs in
partially saturated soils where water is drawn out of
the voids between soil particles mainly through
evaporation. This creates a vacuum effect pulling the
soil particle together, which increases normal
pressure, or forces on the soil particles thereby
increase the shear resistance. However, the suction
effect in slopes is temporary and is easily diminished
when water re-enters into the voids (for example,
infiltration during prolonged rainfall).
3.0 IMPORTANT SLOPE STABILITY FACTORS
There are many factors influencing the stability of
slopes. Here, only the common important factors are
covered and explained. Firstly, the properties of the
soil such as friction angle, apparent cohesion and unit
weight are important in slope stability. As an
illustration, consider these two extremes: The first is a
near vertical rockface with a building on top and is
able to do so without much stability concerns (Figure
5). The second is gentle beach at a seaside where the
gradient is very gentle and yet is not stable to build a
structure directly on it (Figure 6). These two examples
F T
TSoil
N
W
F
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illustrates that stronger soil or rock can support a
building/load compared to weaker soil or rock.
Figure 5: Building on Steep Rockface
Figure 6: Gentle Beach
Secondly, slope geometry is important as illustrated in
Figure 7. Low and gentle slope is safer than high and
steep slope for similar soil. It is because the latter has
more mass on the upslope acting as driving forces (F)
compared to that of a gentle slope.
Figure 7: Effect of Slope Geometry
Thirdly, ground water table profile is an influencing
factor in slope stability. The ground water table for
hillslopes is generally low and fluctuates with time
and rainfall events. Figure 8 shows two general types
of ground water table profile which may be found in a
slope. High ground water table increases the risk of
failure as the shear resistance in the potential failure
plane decreases due to increased water pressure
between soil particles as explained earlier. In addition,
the ground water table on the upslope acts as
additional driving forces. All these factors decrease
the FOS of a slope.
Figure 8: Effect of Ground Water Table
Fourthly, slope maintenance is also an important
factor. Poorly maintained slopes can lead to slope
failure. These may include, amongst others,
damaged/cracked drains, inadequate surface erosion
control and clogged drains. Eventually, erosion of the
slopes allow the formation of gullies (Figure 9) or
cause localised landslips (Figure 10) which will
propagate with time into bigger landslides if erosion
control is ignored.
Figure 9: Gullies on Slopes
Finally, excavation or unengineered activities at the
toe of the slope can cause slope instability. These
activities disturb the stabilising soil mass at the toe of
hill and hence reducing the FOS of the slope. In
addition, activities such as stockpiling earth which
imposes surcharge loads at the top/crest of the slope
Steep Slope
Gentle Slope
Low Groundwater Table
High Groundwater Table
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also decreases the FOS of a slope as this surcharge
increases the driving forces.
Figure 10: Localised Erosion on Slopes
4.0 COMMON MISCONCEPTIONS Here we attempt to debunk some of the common
misconceptions often appear in our media about slope
safety and explain why they are misconceptions.
(1) The first misconception is “Soil tests showed that
the slope is safe”. Soil tests are factual reports of the
soil properties at the location in which the test is
carried out. Soil tests alone do not tell us whether a
slope is safe. Rather, an engineer needs to study the
overall slope and carry out engineering analyses of the
slope using the soil tests results and slope geometry to
determine the FOS of a slope. As iterated earlier,
slopes are complex and they are not man made
materials, hence its geology and composition can vary
significantly over a short distance. Geological
features, soil types and properties have significant
influence on slope stability. Hence detailed
investigations and analyses should be carried out to
ensure safety. Soil tests only provide the parameters
for analyses and designs of slopes.
(2) “Heavy rain causes slope failure”. This is not
correct, although it triggers landslips. Increased
rainfall raises the ground water table and decreases the
FOS of the slope. The minimum FOS generally ranges
from 1.2 to 1.4 depending on the risk to life and
economical ramifications. The threshold value at
failure is unity. A simple analogy of FOS can be
illustrated using the example of weight lifting.
Suppose the maximum weight a person could lift is 50
kg, and when the person is given 50 kg, then the FOS
at failure or threshold is 1.0 (50 divided by 50). If the
person is given 40 kg, then the FOS is 1.25 (50
divided by 40).
However, properly engineered slopes should not fail
as the slopes should have been designed for the most
probable water table during heavy rainfall. The
exception is when the actual rainfall is greater than the
designed return period of rainfall.
(3) “Erosion will not cause slope failure”. This
statement is also not entirely correct. Erosion can
propagate a slip further and cause a bigger landslide.
There are two types of slope failures due to erosion.
One type is an erosion that starts at the toe of the
slope, propagates upslope and eventually trigger the
slope to fail. The other type is a propagation of
erosion from slope crest towards downslope. In both
cases, the small and localised erosion is further eroded
by rainfall and surfacial water flow, causing more soil
mass to fail. This is repeated until the whole slope is
not stable and slides. Uncontrolled erosion can lead to
slope failure.
(4) “Retaining walls always prevent slope failure”.
The public may think that structural solutions like
retaining wall is very strong and hence can retain soil
mass of the slope without problems. However, this
may not be the case. Un-engineered walls can cause
slope failure as shown in Figures 11 and 12. A
properly designed retaining wall by a professional
engineer should not fail as the retaining wall has been
properly designed to our codes of practice to retain the
soil mass and ground water table.
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Figure 11: Collapsed Rubble Wall
Figure 12: Failed RC Wall
(5) “Slopes are maintenance free”. Slopes are not
always maintenance free. The maintenance such as
clearing of clogged drains and patching up localised
erosion spots are required. Poorly maintained slopes
will lead to slope failures. Clogging increases water
pressure build-up through seepage and localised
erosion can propagate landslides. Slopes should be
regularly maintained following a maintenance manual.
(6) “The slope has been standing for more than 10
years! So it is safe!”. This is not necessarily true as
Figure 13 shows that natural slopes can fail suddenly
without warning even though it’s been standing for
years. Natural slopes may be currently standing up
without signs of failure but the factor of safety could
be low and near the threshold. Hence it is not safe to
assume that natural slopes are usually safe. It has to be
investigated and analysed.
Figure 13: Failure of Natural Slope
(7) “EIA report ensures slope stability”. An EIA
report is a study of the environmental impact for a
proposed development will have in the area and
surroundings. It is used as a planning tool for
development. However, it does not examine the
engineering of the slopes in detail to determine
whether a slope is safe and the required stabilisation
measures, if any. Detailed investigation, analysis and
design would only be carried out after the approval of
EIA report but before the approval of earthwork plans.
(8) “Geological report shows that the slope is safe”.
Geological report covers the history of the soil and the
underlying bedrock to explain the geological
formation of the site and highlight its geological
features, types of rock present, soil stratification,
weathering grade and minerals present. It does not
cover the engineering and design of slopes.
In the face of the public perceptions of these reports,
only an engineer’s report or a geotechnical report with
interpretation of field and laboratory tests and detailed
analyses for slopes, will show whether a slope is safe.
If the natural slopes with its proposed platforms do not
have adequate factor of safety, then strengthening
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measures such as regrading of slope, retaining walls
and soil nails should be recommended. Construction
drawings and specifications would then be prepared
for implementation. Site supervision by the team from
the design consultant is a prerequisite component to
ensure slope safety.
5.0 ABANDONED HILLSLOPE PROJECTS Hillslope projects may be abandoned due to financial
difficulties or for any other reasons. However, partly
developed hillslope is usually left as it is. This poses
many risks to public safety and some of the risks are
presented here.
Incomplete Earthworks Hillslope developments mostly involve substantial
earthworks to prepare the necessary platforms for
building construction. These earthworks involve
regrading the existing slopes and transporting its fill to
form the required slopes. However, in an abandoned
hill slope project such as in Figure 14, the earthworks
are not complete and the cut and fill slopes are not
fully graded to the design and safe gradient. In
addition, soil erosion takes place and gullies formed
could de-stabilise the slopes. Hence slopes in
abandoned projects are often not stable in the long
term and are susceptible to continued erosion and
ingression from rainfall.
Figure 14: Abandoned Hillslopes
Incomplete Slope Strengthening Works
In addition to earthworks, there are some earth
retaining structures or soil reinforcement which were
originally designed to stabilise and retain the slopes.
However, if these slope strengthening works are not
completed, they may not fully retain or strengthen the
soil slopes originally designed for. Hence, the stability
of the slopes is in doubt.
Incomplete Drainage Works
Similarly, incomplete drainage works reduces the
stability of the slopes as it affects the ground water
table. These incomplete drainage works may cause
build up of water pressure by the incomplete
channelling of water flow to the main drainage outlets.
Subsequently, the build up water will seep into the hill
slopes, raising the ground water table profile and
therefore increasing the risk of slope instability.
No Slope Maintenance Most abandoned projects would be left as it is without
further maintenance. As a result, drainage paths gets
blocked or silted by the accumulation of decayed
vegetation and soil. In addition, ponding on several
locations of the slope can occur, which may trigger
progressive failures such as mudflow.
6.0 CONCLUSION Stability of slopes is affected by various factors but
the important factors are soil properties, slope
geometry, ground water table, slope maintenance and
unengineered activities at toe and loading on top of a
slope.
Slopes must be properly planned, investigated,
analysed and designed to ensure safety. Strengthening
measures such as retaining walls and soil nails are
usually needed with regrading to achieve the safe
construction platform. Proper and adequate site
supervision by the design consultant team is critical to
ensure the slope safety.
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With adequate measures taken, environmental and
safety conscious hillslopes developments such as in
Figures 15 and 16 can be safely constructed for living
close to the nature.
Figure 15: Properly Designed Slope
Figure 16: Proper Hillslope Development