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1.0 INTROCDUCTION
1.1 Slope Failure
Slope failure, also referred to as mass wasting, is the down slope movement of rock
debris and soil in response to gravitational stresses. Three major types of mass wasting
are classified by the type of down slope movement: falls, slides, and flows and also
represents one of the most active processes in modifying the landscape in areas of
significant relief. Mass wasting involves material other than weathered debris (notably in
rock slides) but most mass wasting phenomena occur in a thick mantle of regolith, the
rock and mineral fragments produced by weathering. The general term landslide is used
to describe all rapid forms of mass wasting. Some of the Slope failure factors are :
a) Slope Gradient
Slope gradient is probably the major cause of mass wasting. Generally speaking,
the steeper the slope, the less stable it is. Therefore, steep slopes are more likely
to experience mass wasting than gentle ones. A number of processes can
oversteepen a slope. One of the most common is undercutting by stream or
wave action. This removes the slope's base, increases the slope angle, and
thereby increases the gravitational force acting parallel to the slope. Wave
action, especially during storms, often result in mass movements along the
shores of oceans or large lakes. Excavations for road cuts and hillside buildingsites are another major cause of slope failure. Grading the slope too steeply, or
cutting into its side, increases the stress in rock or soil until it is no longer strong
enough to remain at the steeper angle and mass wasting ensues. Such action is
analogous to undercutting by streams or waves and has the same result, thus
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explaining why so many mountain roads are plagued by frequent mass
movements.
b) Water Content
The amount of water in rock or soil influences slope stability. Large quantities of
water from melting snow or heavy storms greatly increase the likelihood of slope
failure. The additional weight that water adds to a slope can be enough to cause
mass movement. Furthermore, water percolating through a slope's material
helps to decrease friction between grains, contributing to a loss of cohesion. For
example, slopes composed of dry clay are usually quite stable, but when wet,
they can quickly lose cohesiveness and internal friction and become an unstable
slurry. This occurs because clay, which can hold large quantities of water,
consists of platy particles that easily slide over each other when wet. For this
reason, clay beds are frequently the slippery layer along which overlying rock
units slide down slope.
c) Overloading
Overloading is almost always the result of human activity and typically results
from dumping, filling, or piling up of material. Under natural conditions, a
material's load is carried by its grain-to-grain contacts, and a slope is thus
maintained by the friction between grains. The additional weight created by
overloading increases the water pressure within the material, which in turn
decreases its shear strength, thereby weakening the slope material. If enough
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material is added, the slope will eventually fail, sometimes with tragic
consequences.
1.2 Slope stability
Slope stability is the potential of soil covered slopes to withstand and undergo
movement. Stability is determined by the balance of shear stress and shear strength. A
previously stable slope may be initially affected by preparatory factors, making
the slope conditionally unstable. Slope stability is based on the interplay between two
types of forces which is driving forces and resisting forces. The driving forces promote
downslope movement of material while the resisting forces deter movement. When
driving forces overcome resisting forces, the slope is unstable and results in mass
wasting. The main driving force in most land movements is gravity and the main for
resisting force is the material's shear strength.
Safety Factor (SF) = The ratio of resisting forces to driving forces:
SF =
If SF > 1 then safe
If SF < 1 then unsafe
1.3 Retaining wall
Retaining structures are built for the purpose of retaining or holding back a soil mass
(Cheng and Jack, 2005). A simple retaining wall simply depends on its weight to achieve
stability hence as call as the gravity wall. In case of taller walls, large lateral pressure
tends to overturn the wall, and for economical reasons, cantilever walls are more
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preferable. As for a cantilever wall, it has a part of its base extending underneath the
backfill and the weight of the soil above this part of the base to help prevent overturning
(Craig, 1993). The material placed behind the retaining wall is highly desirable to be free
draining and granular material. Clayey soils make extremely objectionable backfill
material because of the excessive lateral pressure they create.
Figure 1.3.1 : Simple retaining wall
Figure 1.3.2 : Cantilever retaining wall
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2.0 LITERATURE REVIEW
Slope stability is a way to protect any slope from sliding or side collapse, or against
weather conditions and erosions. So it is very important for the engineer to analyses the
slope stability before any construction are being carried out to ensure the slope is safe
and does not risk any human life.
According to (Chandler, 1991), during site investigation, which is in the relation to the
slope stability, the main aims of site investigation are : a) to obtain an understanding of
the development and nature of natural slopes, and of the processes which have
contributed to the formation of different natural features; b) to assess the stability of
various forms of slopes under given conditions; c) to assess the risk of instability in
natural or artificial slopes, and to quantify the influence of engineering works or other
modifications to the stability of an existing slope; d) to facilitate the redesign of failed
slopes, and the planning and design of prevention and remedial measures; and e) to
analyze slope failures which have occurred and to define the causes of failure.
Site investigations can be consider under 3 main headings which are: a) desk study; b)
field study; and c) laboratory work. For desk study, the aim here is to obtain all available
information with regard to the site and its geological environments. It will involve a
search through records, maps, (topographical and geological), and any other
information which is relevant to the geology, history and present condition of the site.
As for field study, it is to record accurately the topography of the site, to determine the
precise nature of the geological deposits underlying the site and to determine their
engineering properties, either by the collection of good quality samples which can be
tested subsequently in the laboratory, or by performing tests in-situ. And lastly is the
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laboratory works which is done to obtain information, additional to that obtained
from in situ tests, on the composition and properties of the materials encountered on
any site. Laboratory tests can be grouped under three main headings: a) tests
for classification and identification; and b) tests for engineering properties (Bromhead,
1992). The first group include tests to determine the particle-size distribution of the
material, index property tests (Liquid and Plastic Limits), specific gravity tests, and tests
to determine the bulk density and water content of the soils. While the second group of
tests includes those to determine the engineering properties of the soils such as
permeability, compressibility and shear strength.
According to (Leventhal, 1987) the accurate measurement of the shearing resistance or
shear strength of a material is essential in attempting to predict future instability or to
assess the present or past stability condition. As stated previously, shear strength tests
must be performed on samples of the highest quality if reliable information is to be
obtained. Even when this condition is satisfied, however, there may still be cases where
the shear strength measured in the laboratory differs from that mobilised in situ. Shear
strength properties of soils are defined by two parameters, apparent cohesion c and the
angle of shearing resistance υ .
The shear box was probably the first type of apparatus used for the measurement of the
shearing resistance of soils. The apparatus consists essentially of a square brass box
split horizontally at the level of the centre of the soil specimen which is held between
metal grills and porous stones. The horizontal force acting on the upper part of the box
is gradually increased until the specimen fails in shear. The shear force at failure s f is
divided by the cross-sectional area A to give the shearing stress t f at failure. The
vertical stress s n is provided by a vertical load on the sample, normally by dead-weights
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and a lever system. The horizontal load is applied by pushing the lower part of the box
by means of an electric motor and gearbox. Volume changes are monitored by a dial
gauge mounted to show the vertical movement of the top loading platen.
Figure 2.0.1 : Shear test example
Figure 2.0.2 : Results of shear box test
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The factor of safety of a slope in soil possessing cohesion and friction can be written as
Where the factor safety for retaining wall is if it is more than 2, it is considered unsafe
and if it is less than 2, it is considered safe.
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3.0 Problem details
3.1 Problem Statement
Prelude:
Kinabalu Times, 18 Jan 2013
Kota Kinabalu: Villagers of a small kampong at suburban of Kota Kinabalu city
have alarmed the authority of a possible slope failure located next to their housing
area.
The slope was cut during the construction of a road two years ago but the
contractor failed to provide adequate measures to ensure the safety of the
villagers.
It‟s village chief, Mr. Ali Rahman said they feared the safety of schoolchildren going
to and back from nearby secondary school. “During rainy seasons, the soil becomes
wet and soggy, and tragedy can happen in any minute” he added.
Upon contacted, the Public Work Department (JKR) representative, Ir. Lim
confirmed of receiving the public complain. Ir. Lim said a geotechnical company
has been commissioned to assess the slope stability, to determine the soil
properties in that area and to purpose the retaining wall structure design. “Upon
receiving their technical report, the department will ensure swift suitable measures
are taken to solve this problem” he assured.
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3.2 Objective
a) To assess the slope stability
b) To determine the soil properties in that area
c) To propose the retaining wall structure design
3.3 Slope details
Location : Jalan Bantayan Minintod, Kota Kinabalu, Sabah.
Slope physical properties :
a) Steep slope
b) Clayey looks
c) Place is currently under construction to improve the safety of factor
14m
7m
=63.435
Figure 3.2 .1 : Slope of location
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Based from the sketching of the slope;
15.653sin 90 = 14sin
x = 63.435 m
3.4 Work Flow Chart
Site Visit
1) Sieve analysis
Soil Sampling
3) Atterberg Limit Test2) Compaction Test
Soil Classification
4) Direct Shear Test
Soil Engineering Properties
Cullman Method GeoStudioSlope Stability Analysis
Retaining Wall DesignRankine Method QuickRWall 4.0
Recommendation of Retaining Wall
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4.0 Methodology, Results & Discussion
4.1 Experiment 1: Sieve Analysis
Objective:
To classify soil sample according to USCS standard based on its grain size distribution.
Results:
Mass of oven-dry specimen, = 2500
Sieve
Opening
(mm)
Sieve
weight
before
sieving (g)
Sieve weight
+ weight of
soil retained
after sieving
(g)
Mass
retained
(g)
% Mass
retained
Cumulative
% mass
retained
% Finer
3.35 1017 1563 546
27.43718
593
27.437185
93
72.562
814
2.36 1039 1253 214
10.75376
884
38.190954
77
61.809
045
2 1077 1121 44
2.211055
276
40.402010
05
59.597
99
1.4 979 1114 135
6.783919
598
47.185929
65
52.814
07
0.6 928 1147 219
11.00502
513
58.190954
77
41.809
045
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0.425 793 872 79
3.969849
246
62.160804
02
37.839
196
0.3 827 1192 365
18.34170
854
80.502512
56
19.497
487
0.15 790 1007 217
10.90452
261
91.407035
18
8.5929
648
0.075 795 936 141
7.085427
136
98.492462
31
1.5075
377
Pan 1004 1034 30
1.507537
688 100 0
= –
= 2500 – 1990
= 510
Since the mass loss of soil after sieving is less than 2% of total weight of soil before, the
data is acceptable.
Analysis of Data:
The soil will be classified according to USCS standard step by step.
Table 4.1.2: Data from Sieve Analysis
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a) The soil is a „coarse -grained soils‟ because more than 50% of the soils is retaine d
on No. 200 sieve. (The percentage retained of soils on No. 200 sieve is, 100% −
1.5075% = 98.4925% )
b) Thus, in accordance to plasticity chart, the fraction of soil is low plasticity clay.
(The analysis using the plasticity chart is explained further in the next
experiment; The Atterberg Limits)
Conclusions:
Based on the classification of soils using USCS standard, the type of soils that was
experimented is low plasticity clay. Thus, the type of soil is coarse with a fraction of low
plasticity clay. Since t he soil if of coarse type, the experiment to determine the soil‟s
angle of internal friction and its cohesion is by shear box testing.
4.2 Experiment 2: Atterberg Limits Test
Objective
To determine the liquid limit and plastic limit of sample
Results
The equations used to obtain the result are:
Mass of water = Mass of container + wet soil - (Mass of container + dry soil)
Mass of dry soil = (Mass of container + dry soil) - (Mass of container)
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Moisture content (%) =Mass of water
Mass of dry soil × 100%
Test Number 1 2 3 4 5
Penetration (mm)
28
.9
2
7
28
.5
21
.6
20
.2
23
.3
20
.1
21
.7
20
.2
27
.7
25
.7
26
.7
33
.8
32
.7
34
.9
Average
Penetration (mm) 28.13 21.7 20.667 26.7 33.8
Mass of Container(g) 8.119 7.928 22.501 21.058 19.575
Mass of container
+ Wet Soil (g) 10.9113 9.735 24.222 26.132 24.410
Mass of container
+ Dry Soil (g) 10.261 9.348 23.856 24.974 23.216
Mass of Water (g) 0.649 0.387 0.3663 1.158 1.194
Mass of Dry Soil
(g) 2.141 1.419 1.354 3.916 3.640
Moisture Content
(%) 30.342 27.265 27.051 29.573 32.797
Container Number 1 2 3 4
Mass of Container (g) 8.059 87.7212 7.6516 87.2409
Mass of Container + Wet Soil 8.8985 89.2477 9.4797 89.6503
Table 4.2.1: Data for Casagrande Method
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(g)
Mass of Container + Dry Soil
(g) 8.7561 89.0148 9.1579 89.2081
Mass of water (g) 0.1424 0.2329 0.3218 0.4422
Mass of Dry Soil (g) 0.6971 1.2936 1.5063 1.9672
Moisture Content (%) 20.427 18.004 21.364 22.4786
Analysis
a) To determine the liquid limit of the sample by using Casagrande method, the
graph of moisture content against number of blows is plotted on the semi-log
graph. The value of liquid limit (LL) is the correspondent value of moisture
content when the number of blows is 25.
0
5
10
15
20
25
30
35
40
0 5 10 15 20 25 30 35
P e n e t r a t i o n
( m m
)
Moisture Content (%)
Table 4.2.2: Data for Plastic Limit
Figure 4.2.1: Graph of Moisture Content Vs Number of Blows
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Therefore, the value of liquid limit for the sample is 26.80%
b) The average value of plastic limit:
PL = 20.427+18.004+21.364+22.4794
= 20.568%
c) The value of Plastic Index:
PI = LL - PL
= 26.80-20.568
= 6.232%
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d) Based on the plasticity chart above, the value of Liquid Limit and Plastic Index
fall at the region of CL or OL. Since the sample does not contain organic matter,
the sample can be classified as CL which means low plasticity clay or lean clay.
Conclusion
Therefore, the sample taken has a liquid limit of 26.80%, plastic limit of 20.0568% and
plastic index of 6.232%. This sample is classified as low plasticity clay or lean clay.
4.3 Experiment 3: Shear Box Test
Objective:
Figure 4.2.3: Plasticity Chart
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To determine the cohesion and angle of internal friction of a dry granular soil.
Results:
a) Loose States
Table for Shear Load versus time (Loose States) with corresponding weights
Times
(s)
Shear Load (N)
5.5kg 15.5kg 25.5kg
20 0.6 0 0
40 2.9 0 0
60 6.7 4.3 0
80 10.8 12.4 0
100 5.1
120 13.9
Table for Vertical Displacement versus time (Loose States) with corresponding weights
Times
(s)
Shear Load (N)
5.5kg 15.5kg 25.5kg
20 7.2 5 2.1
40 50.8 27.5 6.8
60 91.1 61.9 8.1
80 86.2 9.5
100 35.2
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120 68.8
Table for Horizontal Displacement versus time (Loose States) with corresponding
weights
Times
(s)
Shear Load (N)
5.5kg 15.5kg 25.5kg
20 0 1.6 0
40 0 6.9 29.1
60 69.2 12.5 24.8
80 34.1 21.2
100 19.3
120 17.1
140 -28.1
b) Dense States
Table for Shear Load versus time (Dense States) with corresponding weights
Times
(s)
Shear Load (N)
5.5kg 15.5kg 25.5kg
20 3.2 4 0
40 4.6 5.8 2.8
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60 6.8 8.4 5.8
80 8.8 8.4 9.8
100 10.2 11.8
Table for Vertical Displacement versus time (Dense States) with corresponding weights
Times
(s)
Shear Load (N)
5.5kg 15.5kg 25.5kg
20 34.2 0 17
40 84.4 0 20.2
60 94 8 33.2
80 94 48.4 70
100 89.6 110
120 134.2 17
Table for Horizontal Displacement versus time (Dense States) with corresponding
weights
Times Shear Load (N)
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(s) 5.5kg 15.5kg 25.5kg
20 0 0 4.2
40 10 0 8
60 24.4 0 9.8
80 33.4 1 11
100 1 13.2
120 1
Calculation:
a) Loose States
• For case no. 1 where the mass of the hanger is 5.5kg:
The value of Normal stress, σ =
=
= 14987.5 Nm -2
The value of Shear stress, τ = .. .
= 3000 Nm -2
• For case no. 2 where the mass of the hanger is 15.5kg:
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The value of Normal stress, σ =
=
= 42237.5 Nm -2
The value of Shear stress, τ = .. .
= 3444.444 Nm -2
• For case no. 3 where the mass of the hanger is 25.5kg:
The value of Normal stress, σ =
=
= 69487.5 Nm -2
The value of Shear stress, τ = .. .
= 3861.111Nm -2
Data for shear stress and normal stress (loose states)
Shear stress, τ (kN/m 2) Normal stress, σ (kN/m 2)
3.000 14.9875
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3.444 42.2375
3.861 69.4875
b) Dense States
• For case no. 1 where the mass of the hanger is 5.5kg:
The value of Normal stress, σ =
=
= 14987.5 Nm -2
The value of Shear stress, τ = .. .
= 2444.444 Nm-2
• For case no. 2 where the mass of the hanger is 15.5kg:
The value of Normal stress, σ =
=
= 42237.5 Nm -2
The value of Shear stress, τ = .. .
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= 2833.333 Nm -2
• For case no. 3 where the mass of the hanger is 25.5kg:
The value of Normal stress, σ =
=
= 69487.5 Nm -2
The value of Shear stress, τ =.
. .
= 3277.780 Nm -2
Data for shear stress and normal stress (dense states)
Shear stress, τ (kN/m 2) Normal stress, σ (kN/m 2)
2.444 14.9875
2.833 42.2375
3.278 69.4875
a) Loose Soil
τ = c + σ tan ø
From graph, the apparent cohesion, c = 2000 N/m 2
tan ø = (τ – c)/ σ
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= (3861.111 - 2000) Nm -2 / 69487.5 Nm -2
= 0.0268
Angle of internal friction, Ø = 1.534 ˚
b) Dense Soil
τ = c + σ tan ø
From graph, the apparent cohesion, c = 1875 N/m 2
tan ø = (τ – c)/ σ
= (3277.78 – 1875) Nm -2 / 69487.5 Nm -2
= 0.020
Angle of internal friction, Ø = 1.160 ˚
0
10000
20000
30000
40000
50000
60000
70000
80000
0 10000 20000 30000 40000 50000 60000 70000 80000
Figure 4.3 .1 : Loose ( Shear stress vs normal stress )
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Discussion
a) Based on the result obtained, the angle of internal friction, ø for loose state is
1.534° and for dense state is 1.160°. Conclusion:
The cohesion for loose is 2000 N/m 2 and dense state is 1875 N/m 2 while angle of
internal friction for loose and dense state is 1.534˚and 1.160˚respectively.
4.4 Experiment 4: PROCTOR COMPACTION TEST
Objective :
0
10000
20000
30000
40000
50000
60000
70000
0 10000 20000 30000 40000 50000 60000 70000
Figure 4.3 .1 : Dense ( Shear stress vs normal stress )
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To determine the maximum dry unit weight of compaction of soils.
Results :
Mass of dry soil used : 2.5 kg
Classification of soil : SW (USCS Classification)
Measurement of mould :d=100 mm, h=100 mm
Volume of mould :7.85x10 -4 m3
Mass of base : 3.242 kg
Trial No 1 2 3 4 5
Mass of wet soil + mould +
base (kg)
5.22 5.312 5.242 5.192 5.169
Mass of wet soil (kg) = W 1.378 1.47 1.381 1.342 1.327
Buld density of soil, ρ (kg/m 3) 1869.74 1994.57 1873.81 1820.9 1800.54
Container No 1 2 3 4 5
Mass of container (g) 84 8 8 7 7
Mass of wet soil + container
(g)
104 28 28 26 27
Mass of dry soil + container (g) 10.62 24.02 23.53 21.52 22.07
Mass of water (g) 3.38 3.98 4.47 4.48 4.93
Mass of dry soil (g) 16.62 16.02 15.53 14.52 15.07
Water content (%), w 0.169 0.199 0.224 0.236 0.247
Dry density of soil, ρd(kg/m 3) 1599.44 1663.53 1530.89 1473.22 1443.9
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=ρ/(1+w)
Table 4.4.1: Results of the Standard Proctor Compaction Test.
Graph 4.4.1: Graph of Proctor Curve for the soil sample.
From graph 4.4.1,
, = 1663.53 3
, = 0.19 %
, = × 9.81
= 1663.53 × 9.81
= 16.32 / 3
Conclusion :
1400
1450
1500
1550
1600
1650
1700
0 0.05 0.1 0.15 0.2 0.25 0.3
D r y
D
e n s
i t y o
f s o
i l
Water Content (%)
Proctor Curve
1663.53 kg/m^3
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The maximum dry density is 1663.53 kg / m 3. Therefore, the unit weight is 16.32 kN/m 3
.
4.5 Determination of Factor of Safety (FOS) for Slope
To determine the factor of safety for the slope, the group are using manual calculation
and software.
I. Manual Calculation (Culmann Method)
For the manual calculation, the equation used is;
=4 sin β cos ∅
(1 − cos( −∅ )
cd =c
F.Sc
tan ∅ =tan ∅
. ∅
From the shear box testing result, ∅= 1.160 °
From graph, the apparent cohesion, c = 1875 N/m 2
Safe depth of cut = 14m
Unit weight of the soil, = 16.32 kN / m 2
Angle of horizontal to cut surface, β = 63.435
(The angle of friction, ∅ used is the result from the dense soil because it has lower value
of ∅ compared to loose sample and this will result in lowest possible value of the factor
of safety for the slope.)
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Calculation
F.S ф фd cd F.Sc
1 1.16 16.59 0.113
2 0.58 16.87 0.111
3 0.38 16.96 0.11
= 0.111
0
1
2
3
4
0 1 2 3 4F.S ф
F.Sc
Figure 4.4.2 : F.Sc vs F.S ф
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Significance Factor of Safety for Design,
Safety Factor Significance
Less than 1.0 Unsafe
1.0 - 1.2 Questionable safety
1.3 – 1.4 Satisfactory for cuts, fills; questionable for dams
1.5 – 1.75 Safe for dams
From the above table, since the = 0.111 which is less than 1.0,
therefore the slope is unsafety and a retaining wall is needed to avoid slope failure.
II. Software Calculation
For software calculation, the group is using software called Geoslope Design. By
inputting the necessary parameter such as the slope‟s height, angle, angle of internal
friction, unit weight and cohesion. The result is as follows.
Source: Liu C. & Evett J. B., (2005). Soils and Foundations. Singapore: Pearson Prentice Hall
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The green coloured area is the critical area where the slope failure may occur. The
factor of safety for the slope is 0.109 which is less than 1.0. Thus, the slope is unsafe.
There is also other possible slope failure but since the slope failure is the most critical.
Comparison for manual calculation and software calculation
Manual Software
Factor of safety = 0.111 Factor of safety = 0.109
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5.0 RETAINING WALL DESIGN
5.1 Proposed Wall Retaining Structure (Manual)
Kh = 1 – sin υ 1 + sin υ
Kp = 1+ sin υ 1− sin υ
= 0.960 = 1.041
Ph = 1/2 Kh H 2 ɣ Pp = 1/2 Kp H 2 ɣ
= 1/2 (0.960) (16.5) 2(16.32) = 1/2 (1.041) (12.5) 2(16.32)
123
4
5
6
7
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= 2132.698 kN/m = 1327.275 kN/m
Component Weight Component (kN/m) Moment arm (m) Righting moment
kN.m/m
1 14x15x23.5x0.5 = 822.5 7.83 6440.175
2 2x14x23.5 = 658 10.5 6909
3 14x15x23.5x0.5 = 822.5 13.167 10829.583
4 21x2.5x23.5 = 1233.75 10.5 12954.375
5 2x10x23.5 = 470 10.5 4935
6 14x5x16.32x0.5 = 571.2 14.83 8472.8
7 4.5x16.32x14 = 1028.16 18.75 19278
Total ΣMv = 5606.11 ΣMr = 69818.928
Analyse the factor of safety for Sliding.
F.S sliding = (μ)( ΣV) + Pp = 0.55 5606 .11 +1327.2752132 .698
= 2.068 > 1.50 Safe against sliding
Analyse the factor of safety for Overturning.
Mo = Ph (H/3) = (2132.698) (16.5/ 3)
= 11729.9367 kN.m/m
Noted that for the calculation of active and passive pressure, the cohesion isconsidered cohesion less.
Noted that the passive pressure at toe is not considered in the manualcalculation.
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F.S overturning = ΣMrΣMo =69818.928
11729.9367
= 5.95 > 1.50 Safe against overturning
Analyse the factor of safety for Bearing Capacity Failure.
x = ΣMAΣV =ΣMr − ΣMo
ΣV = 69818.928 − 11729.937
5606.11
= 10.362 m
e = Base2
– x = 212
− 10.362 = 0.138< L/6 (i.e. 21/6 = 3.5)
My = Qe = 5606.11 (0.138) = 775.064 kN.m
X = Base2
= 212
= 10.5 m
Iy =3
12=
1(21) 3
12= 771.75 m 4
A = bh = (1)(21) = 21 m 2
q =
± ±
qL = 5606 .1121
+ 775.064(10.5)771.75
= 277.503 / ^2
qR = 5606.1121
− 775.064(10.5)771.75
= 256.413 / ^2
F.S bearing capacity failure 277.503 / ^2 < 620 / ^2 (Assumed allowable
pressure bearing)
620277.503
= 2.234 > 1.5 Safe against bearing capacity
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5.2 Proposed Wall Retaining Structure (Software)
For the design of the retaining wall using software, QuickRWall is used. The following
picture is the recommended retaining walls.
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5.3 CONLUSION & RECOMMENDATION
For conclusion, the slope is unsafe since the factor of safety (FOS) is considered
in manual calculation or from using software is less than 1. The slope is considered fail
and unsafe. The soil has a liquid limit of 26.80%, plastic limit of 20.0568% and plastic
index of 6.232%. This sample is classified as low plasticity clay or lean clay known from
the previous experiment. Thus, a retaining wall is recommended to be built to avoid
slope failure in the future. Based from all the data obtained, a retaining wall for the
slope is assumed. It is assumed by using the Rankine method including using the
software known as QuickRWall 4.0 to get the most suitable retaining wall for the slope.
Since The allowable pressure bearing for the structure is 620 kPa which for the design
assumed is only 277.530 kPa thus making the structure safe against failure. The friction
coefficient assumed in the manual calculation and software is 0.55. As for the calculation
of active and passive pressure, the cohesion is considered cohesion less soil, thus using
the equation of a cohesion less soil of Rankine Theory while the software's calculation
includes the cohesion. Also, the active pressure's height is from top structure to bottom
of foundation while the passive pressure is from bottom structure to top of foundation.
Noted that the passive pressure at toe is not considered in the manual calculation as it is
calculated separately from the manual. That is why the answer for factor of safety of
overturning is different from software's calculation. The selection criteria is not based on
the cost, difficulty in building the retaining wall and other factors. Since the most
important factor in this project is only to avoid slope failure, other factor such as the
cost of building and the difficulty in building the retaining wall will not be discussed.
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6.0 REFERENCE
Bromhead, E.N. 1992. The stability of slopes . Blackie, London.
Chandler, R.J. 1991. Slope stability engineering . Thomas Telford, London.
Craig, R. F. Mekanik Tanah . Johor Darul Ta'zim: Universiti Teknologi Malaysia.
Leventhal, A.R. and Mostyn, G.R. 1987, Slope stabilisation techniques and their
application in Slope Instability and Stabilisation , ed. by B. Walker and R. Fell,
Balkema, Rotterdam.
Liu, C. and Evett, J. B. (2005). Soils and Foundations. Singapore: Pearson Prentice Hall.
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