Page 1 of 1

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

Page 2 of 7

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

Page 3 of 7

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

Page 4 of 7

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.

Page 5 of 7

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

Page 6 of 7

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.

Page 7 of 7

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