Nghiên cứu sự thay đổi tính chất cơ lý của các loại đất tàn – sườn tích ở tây nguyên khi mưa lũ kéo dài có ảnh hưởng đến sự ổn định của sườn dốc cạnh đường ô tô

-1INTRODUCTION
1. Urgency of the topic
Vietnam’s Central Highlands is the Southwest mountainous area of the country,
including the provinces: Lam Dong, Dak Nong, Dak Lak, Gia Lai, KonTum.
The Central Highlands is a land with enormous potentials for development, has
a key strategic location for politics, economy, culture and national defense of the
whole country. The industrialization - modernization of the country in general and
of the Central Highlands in particular requires construction of more roads through
the provinces, such as:
- National Highway no.14 running from KonTum to Gia Lai, Dak Lak, Dak
Nong, Binh Phuoc to Ho Chi Minh City.
- National Highway no. 24 connecting KonTum with Ba To (Quang Ngai
Province).
- National Highway no. 25 connecting from Pleiku (Gia Lai) to Tuy Hoa
(Phu Yen Province).
- National Highway no. 26 connecting Dak Lak (Buon Me Thuot Province)
with Nha Trang City (Khanh Hoa Province).
- National Highway no. 27 connecting Dalat City (Lam Dong Province) with
DakLak (Buon Me Thuot Province).
- National Highway no.28 connecting Dalat (LamDong Province) with
DakNong.
- National Highway no.19 connecting Pleiku (GiaLai Province) with
QuyNhon City.
- National Highway no. 40 connecting with Xayden-Antoum (Laos) Po Y
Border gate with the National Highway no.14.
- Especially, the Ho Chi Minh Trail passing the provinces in the Central
Highlands, this is a key route which not only bears the strategic meaning in the
cause of industrialization – modernization, socio-economic development and
assurance of national security for the Central area and the Central Highlands but
also is historic route, associated with the liberation of the country (Truong Son
road).

- Moreover, many routes connecting townships with districts and remote
areas where many ethnic people are living, there are a lot of traffic routes serving
the construction of hydraulic works, hydroelectric power plants and tourist
operation in the Central Highlands provinces. The motorways running along hill
foots or high mountainous passes are formed by various types of soils with different
originations.


-2In the rainy season, after long periods of heavy rains, it often occurs the
creep of earth mounds on roadside, causing traffic jams and requiring long period
and cost for remedy.
One of the reason causing the above mentioned incident is mainly due to
long periods of rain resulting in the variation of durability of roadside earth mass,
causing large displacement leading to landslide. Therefore, the selected topic is:
STUDY ON THE CHANGE OF PHYSICAL PROPERTIES OF SOILS BUTTS BODY IN THE CENTRAL HIGHLANDS AREA AFTER PROLONGED
FLOODING THAT AFFECTS THE STABILITY OF THE SLOPE NEXT TO
THE MOTORWAY.
2. Purpose, object and scope of the study
Purpose of the study: Study on variation features of durability of residual soil
– deluvial deposit in the Central Highlands in dry condition (in dry season) and in
water absorption saturation condition (in rainy season); those are the basics for
evaluation of the stability of roadside hill soil and provision of necessary data for
reference of the readers in case of building traffic routes in the Central Highlands.
- Object of the study: The variation of physico-mechanical properties of
residual soil – deluvial soil mainly found in the Central Highlands relates to the
stability of earth slope. The stability of the slopes closing to the traffic roads is also
affected by vehicles on the roads. Within the scope of this thesis, only the reduction
of durability of soil due to long periods of rain affecting the factor of stability safety
of the slope is studied without taking into consideration of the impact of vibration
caused by the vehicles on the roads.

3. Scientific meanings and realities of the topic
a) Testing research determines variation features of natural density
and shearing parameters (, C) according to humidity (W) from the dry
(W
season to the rainy season of four types of residual soil – deluvial soil commonly
found in the Central Highlands. They are types of residual soil – deluvial soil
belonging to weathering crust in Basalt rocks, Granite intrusion rocks, terrigenous
sedimentary rocks and metamorphic rocks.
b) Calculate, compare and identify stability factor against sliding for the
same slope calculated by Bishop circular method (via Geo software - Slope
International Ltd. Canada) and by enhanced circular method of M.Н. Голbдштейн
and Г.Ц. Тер-cтепанян giving approximately same values. The research student has
selected the enhanced circular method of M.N.Gônxtên to calculate the limited
elevation of the slope (h) according to the falling gradient (1:m) of the slope basing
on an estimated stability factor K.


-3c) Using the data studied in item a, apply the calculation method in item b,
with the safety factor according to regulated safety factor of k=1.40, the research
student has calculated the limited elevation (h) according to the falling gradient
(1:m) and different humidity (W) of soil in the slope for four types of residual soil –
deluvial soil studied in the Central Highlands.
d) The study results provide necessary data for the reference of the readers in
designing or reviewing the stability of actual slopes with various elevations (h) and
the falling gradients (1:m) according to the dry and rainy seasons of four types of
soils commonly found in the Central Highlands.
4. Methods of study
- Study theories relating to the calculation of the stability of the slope and the
testing method identifying physico-mechanical characteristics of soil.
- Experimental study: Select survey locations for different types of soil in dry

and rainy seasons in many years, perform taking samples of undistributed soil for
testing to identify the physico-mechanical characteristics of soil in different seasons.
In addition, collect factual data for supplement.
- Report the studying results on scientific and seminar magazines; liaise with
various Surveyors, Designers and Contractors in the Central Highlands for
obtaining facts; discuss with the management agencies, such as the Department of
Science and Technology, Department of Transport, Department of Environment and
Natural Resources, Department of Agriculture and Rural Development in the
Central Highlands provinces to identify requirements to be studied as well as actual
experience of the local.
5. Structure of the thesis
The thesis comprises 2 sections: Explanation and Appendices.
The Explanation section includes 103 pages; beside the introduction, the
thesis comprises 04 chapters and the conclusion at the end of the thesis. At the end
of the explanation section, there are 5 pages listing the reference documents of local
and international authors and 1 page listing the articles of the research student
relating to the contents of the thesis.
Appendices: 28 pages, including:
Appendix to Chapter III: 17 pages
Appendix to Chapter IV: 11 pages


-4CHAPTER I
NATURAL CONDITIONS, ENGINEERING GEOLOGICAL FEATURES IN
THE CENTRAL HIGHLANDS AREA, LANDSIDE OF SLOPES ALONG
THE MOTORWAYS IN THE CENTRAL HIGHLANDS
1.1 OVERVIEW OF NATURAL CONDITIONS OF THE STUDIED AREA
1.1.1 Topographic and geomorphological features
The studied area comprises the provinces KonTum, Gia Lai, DakLak, Lam
Dong, a part of Quang Nam Province, Binh Phuoc and is mainly distributed at the

West of Truong Son. The topography comprises the following types [9]:
- Block mountains (Ngoc Linh, Mon Ray, Kon Ka Kinh, Dong Con Cho Ro,
Chu Yang Sin, Dong Don Duong, Tay Bao Lam, Nam Di Linh, etc.).
- Binh Son Nguyen erosion (Chu Pong – Chu Gau Ngo, Chu Ro Rang,
Xnaro, Dalat, etc.).
- Plateau basalts (Kon Ha Nung, Pleiku, Buon Ma Thuot, Dak Rlap, Bao
Loc, Dinh Van).
- Accumulated denudation valleys (Po Ko, KonTum, Dak To, Song Ba,
Krong Ana, ect.).
1.1.2 Meteorological features
1.1.2.1 Characteristics of rivers and streams:
The studied area gets the crest line of Truong Son Range as the datum line,
dividing the area into two main basins, i.e. the basin of rivers flowing into the East
sea, including the rivers Ba, Da Rang, Dong Nai, Be, Saigon, Vam Co, etc.
And the basin of rivers flowing into the Mekong River (in the West),
including the rivers SeRePok, PoCo, Se San, etc.
Basic characteristics of river and stream system in the region are short,
narrow, falling and there are many water falls. Rivers and streams in this region
commonly have 3 sections with specific characteristics, i.e. a section crossing hills
and mountains, a section crossing the highlands and the other crossing the plains.
In reality, the section crossing the hills and mountains has very little of
sediment. Only when flowing into the highlands, the plains or the valleys, can the
rivers expand and create large but not thick sediment.
1.1.2.2 Characteristics of rain:
Rainy season in the Central Highlands often lasts from May to October, the
rainfall during this period occupies about 75% of the annual rainfall. The average
annual rainfall in the region is about 1200mm to 3000mm.


-5In which:

Medium high mountainous area – Ngoc Linh: from 2500mm to 3000mm
Pleiku plateau: from 2600mm to 2800mm.
PoCo valley, Mandrak plateau: from 2000mm to 2500mm
Cheo Reo, An Khe, Krong Buk valleys: from 1200mm to 1400mm
In the South Central Coast region, the rainy season lasts from September to
December. The average annual rainfall is from 1100mm to 1300mm.
In the Southeast region, the rainy season lasts from August to November.
The average annual rainfall is from 1400mm to 2000mm.
1.1.2.3 Characteristics of wind:
In the Central Highlands, the Southwest monsoon prevails from May to
September, the average wind speed is from 4.1 to 5.2m/s. From November to April
of the following year, there is mainly Northeast monsoon.
The Central Highlands is less directly affected by storm from the East sea but
the storm can cause heavy rains on wide region, leading to floods, affecting the
production and daily activities of the people; especially, the floods can cause
damages to hydraulic works and traffic routes, etc.
In the South Central Coast, the Southwest monsoon prevails from May to
September, the Northeast monsoon prevails from October to April of the following
year. In addition, this region is also affected by storms and sweeping floods in the
period from August to October annually.
In the Southeast region, the Southwest and Southeast winds are equable
throughout the year.
1.1.3 Characteristics of weather and climate
The studied area is located in the monsoon tropical region with two specific
seasons, the rainy season and the dry season; the dry season starts from January to
May and the rainy season starts from June to December.
The annual average temperature in the Central Highlands (Cheo Reo) is
25.5oC, in the Southern Central Coast (Nha Trang) is 26.4oC, in the Southeast
region (Binh Duong) is 26.5oC.
The annual average humidity in the Central Highlands is from 74% to 90%,

in the Southern Central Coast and the Southeast region is from 75% to 80%.
The amount of radiation is abundant (on an average of about
140Kcal/cm2/year) but there are differences according to seasons. In the dry season,
the solar radiation is high, the period with high radiation is in April and May


-6(reaching 400 - 500 Kcal/cm2/day). In the rainy season, the solar radiation is lower,
the highest radiation intensity reaches 300-400 cal/cm2/day.
In months of dry season, because the evaporation exceeds the rainfall, such
as in Pleiku plateau, Cheo Reo – Phu Tuc region, making the soil exhaustedly dried,
grass weathered, the weather hot, and the underground water level deeply dropped,
etc.
The characteristics of weather, climate, hydrographic at the studied area are very
severe, the dry season is much different from the rainy season, seriously affecting
the construction conditions and the quality of construction works.

Figure 1.1 Map of the studied area


-71.2 ENGINEERING GEOLOGICAL FEATURES IN THE REGION
In the document [27] – an overview about the Engineering geological
conditions in the regions from Quang Nam – Da Nang to the Southeast region
introduces “the Map of engineering geology in the Central Highlands” (Figure 1.2).
Basing on tectonic regime, there are different geodetic formations and geological
complexes noted on the map.

Figure 1.2 Map of engineering geology in the Central Highlands


-81.2.1 Characteristics of geological structure

According to the studying results of Nguyen Viet Ky and Nguyen Van Tuan
[9] on geological strata, this region is popular with seven groups of rocks, i.e.:
1. Kainozoi unconsolidated deposit group originated from rivers, ponds,
marshes of Neogene period, distributing mainly along river valleys creating river
terraces, flat plains or filling fault-blocks under the form of weak consolidation.
2. Sedimentary rock group is mainly distributed in the Southern Central Coast,
including sedimentary rocks of the early – middle Jara period, some of them
belonging to Permi period with the strata system Chu Minh (Permi period); Ban
Don type (the early – middle age of Jara period) with 4 strata systems Dak Bung,
Dray Linh, La Nga, Ea Sup; the strata system Dak Rium (the late Creta period).
3. Metamorphic rock group with the age from the pre-Cambri period to the
early Paleozi period, distributing mainly in the Northwest, the North and the East of
the Central Highlands and including the following strata systems: Kon Cot,
Xalamco, Dak Lo, Ki Son, Re River, Tak Co, Vu Mountain, Tien An, Dak Ui, Dak
Long and Chu Se which are distributed under the form of high, sharp and strong
cleavage mountain.
4. Intrusive acid - neutral rock group includes rocks with age from Paleozoi and
Mezozoi periods, belonging to the complex Dien Binh, Ben Giang – Que Son, Hai
Van, Van Canh, Dinh Quan, Ca Mountain Pass, Ankroet, Ba Na, etc., creating high
mountain ranges.
5. Volcanic acid and neutral rock group includes rocks from Andezit (Dak Lin
strata system with the age from Cacbon – Permi period and Bao Loc mountain pass
system with the age from late Jara period – early Creta) to Ryolit, Felsit (Mang
Yang, Chu Prong, Nha Trang, Don Duong strata systems); these rocks create high
and sharp mountains with strong differentiation.
6. Mafic and super Mafic intrusion rock group occupies a very small area of the
studied region; they exit under the form of small blocks.
7. Mafic volcanic rock group includes Basalt, the types with the age from
Neogen to the Troskysit period with the strata systems Tuc Trung, Dai Nga and
Xuan Loc. This is the rock group with large distribution area, occupying up to ¼ of

the Central Highland area.
About the tectonic features, the Central Highlands is totally located in 2 large
tectonic belts, i.e. KonTum and Dalat belts (Nguyen Xuan Ba and nnk, 2000). The
boundary between these two belts is the fault system Ea Sup - Krong Pach. Each
tectonic belt has different characteristics of components, structure and their


-9geological features are really different. On each tectonic belt, many fault systems
are developed, such as Po Co, Ho sea – Chu Ho Drong, Mang Yang – An Trung
Mountain pass, Dak Min - Madagui, Đắk Min - Krong Bong, Ba River, the fault
systems Batơ - Kontum, Bien Hoa – Tuy Hoa, Da Nhim – Tanh Linh. At the studied
area, there’s the sign of new tectonic activities, this place develops horizontal and
vertical movements. The forms of geological catastrophes with endogenous origin
are often associated with these activities.
1.2.2 Weathering crust in the Central Highlands
There are different types of weathering, such as: chemical weathering,
physical weathering, biological weathering, etc. In the Central Highlands, due to
favorable conditions of the climate, the chemical weathering mainly occurs in this
region.
The agents of the chemical weathering mainly are water, oxide, carbonic
acid, organic acid and other acids dissolved in water.
The chemical weathering has very complicate features. Different processes
can be happen at the same time, such as dissolution, oxidation, ion exchange and
hydrolysis. The dominant of any process depends on the compositions and
properties of rock itself, ambient conditions, weathering time, depth, laying status of
rock.
1.2.2.1 Weathering crust in intrusion rocks:
Distributed into two large strips: one strip at the edge of the East, lasting
continuously from Tu Mo Rong to Krong Pa, Chu Yang Sin; the other strip locating
at the West of Truong Son, from DakGlie to Chu Prong, turning to Krong Pa in

Southeast direction. This region is popular with the weathering crust in intrusive
acid rocks with thickness form 5 to 10m, the largest weathering crust of 50m – 80m
in Granite – Migmatite rock locates at ManDen region, belonging to Chu Lai
geological complex, the smallest weathering crust of 0.5m-2.5m locates at the slope.
The top crust is totally weathered becoming clay and clay loam.
1.2.2.2 Weathering crust in volcanic rocks:
a) Weathering crust in Basalt volcanic rocks:
Distribute widely and cover most of 5 large Basalt plateaus: Kon Ha Nung,
Pleiku, Buon Ma Thuot, Dak Nong and Di Linh. They include two following
groups:
Weathering crust in Basalt Pliocen – early Pleistocen (βN2-QI1):
 Distribution: occupy most of the area of 5 large plateaus, except for the central
parts of Pleiku, Buon Ma Thuot, Dak Nong.


-10 Its thickness is from 10-20cm, the thickest part is at the plateau arc Kon Ha
Nung, Dak Nong with thickness of 32 - 82.5m on Granite-migmatit rocks, Chu Lai
geological complex, the thinnest part is at the edge of the plateau with thickness of
only 3m- 5m.
 The specific characteristic of this type of weathering crust in Basalt volcanic
rocks is laterite crust, the cross section from the top to the bottom includes 4 zones:
pedology, laterite, clay and weak metamorphic zones.
 Pedology zone is from 0.1-1m thick, mainly of clay mixing with tree roots and
some pieces of laterite.
 Laterite zone is from 0.5-12.3m thick under the form of gravels, grits, sticks,
bones, and pores with rather rigid structure.
 Clay zone is from 2-70.2m thick. This is argillaceous alteration under the form
of spherical remnant; this zone still remains the structure of mother rocks.
 Weak metamorphic zone of 1-5m thick is Basalt fracture forming crushed
stones, block stones; the minerals are mainly primary.

b) Weathering crust in Basalt Pleistocene volcanic rocks (βQ12):
 Distribution: develop at the centre of the plateau arcs Pleiku, Buon Ho, Krong
Ana, Dak Min, Duc Trong.
 Its thickness is from 15 - 20m, the thickest part at the plateau arcs Kon Ha
Nung, Dak Nong reaches 50 – 70m at Pleiku plateau arc, the thinnest part at Krong
Ana area is only 3m – 10m.
 The specific characteristic of this type of weathering crust in Basalt volcanic
rocks is the crust of argillaceous alteration, the cross section from the top to the
bottom includes 3 zones: pedology, clay and weak metamorphic zones.
 The pedological zone of 0 - 0.5m: mainly comprises clay slurry with tree roots.
 Clay zone of 5 - 10m is red brown clay transferred to spotted gray brown color;
it still remains the structure of mother rocks.
 The weak metamorphic zone of 1-3m is Basalt fracture forming crushed
stones, block stones; the minerals are mainly primary.
c) Weathering crust in neutral volcanic rocks:
 Distribution: develop in Andesite volcanic rocks in Ban Don, Bao Loc
mountainous pass, at the southeast of Di Linh, Da Dang.
 Its thickness is from 2 to 5m; the thickest part of 10-12m is at Dak Lin, in
Pleiku plateau arc; the thinnest part at Bao Loc mountainous pass is only 05 – 1m.
 The thickest zone on the top is the clay zone.
d) Weathering crust in acid volcanic rocks:


-11 Distribute in Sa Thay, Mang Yang, To No mountainous pass, Chu Prong, Tay
Krong Pa, Don Duong, Duc Trong, etc.
 Its thickness is from 5 to 10m, the thickest part of 20-25m is located at Mang
Yang mountainous pass, Pren, Mo Ray; the thinnest part on the slope, in the
cleavage valley is only 1 - 3m.
 The top zone is the weak metamorphic zone with thickness of 1 - 5m,
including clods, blocks of volcanic rocks covered with an argillaceous layer; the

inner layer is quite rigid.
1.2.2.3 Weathering crust in metamorphic rocks:
 Distribute in KonTum province, at the east and northeast of Gia Lai Province,
Iabang, MĐrăk (Đăk Lăk).
 Its thickness is from 10-20m, the thickest part of 50m-60m is located on Ho
Chi Minh Trail, Dak Lak section – Lo Xo mountainous pass; the thinnest part of
only 3-5m is on the slope, in the cleavage valley.
 The top zone is pedological zone of (0.2-1.5)m thick.
 The second zone is thick argillaceous zone of (10-15)m thick.
 The third zone is weak metamorphic zone of (3m-10)m thick.
1.2.2.4. Weathering crust in sedimentary rocks:
They are mainly sedimentary rocks at the age of Jara period.
 Distribute mainly from EaSup – Ban Don toward Dalat – Duc Trong
 Its thickness is from 10-15m, the thickest part of over 40m is located in Dalat;
the thinnest part is (1-2)m.
 The top zone is pedological zone of (0.3-1)m thick.
 The second zone is thick argillaceous zone of (2-18)m thick.
 The third zone is weak metamorphic zone of (2-4)m thick.
The catastrophes (external geodynamic process) on the weathering crust in different
types of rocks are different. On the weathering crust in intrusion rocks, the
phenomenon of gravity crash, landslide, erosion, etc may be found. On basalt
weathering, it’s possible to find the phenomenon of soil crack, landslide, erosion,
etc. On the weathering crust in acid volcanic rocks, it’s also possible to see the
phenomena as those happening on basalt weathering but the scale is small and the
concentration degree is better. The geodynamic processes commonly found in
metamorphic rocks are landslide, erosion of ditches, erosion of channels; in rainy
season, these processes develop strongly at places where the vegetational cover is
damaged, when the scarp cut into the weathering crust (DakGlei along Ho Chi Minh
Trail).

-12(a)

(b)

Figure 1.3 a-b Weathering crust in Basalt rocks - Weathering crust in Granite
intrusive rocks
(a)
(b)

Figure 1.4 a-b Weathering crust in terrigenous sedimentary rocks – metamorphic rocks
1.2.3 Physico-mechanical criteria, mineral and chemical components of
typical types of soils in natural conditions in the region
The document [17] of the authors Nguyen Van Tho, Nguyen Tai
summarizes and introduces the average values and physico-mechanical
characteristics the types of rocks on the weathering crust with natural structure in
the Central Highlands (table 1.1) and the scope of variations of main criteria
(table 1.2). For the type of soil in the Central Highlands, the author Nguyen
Thanh [25] has collected and amended the data on mineral components of soil
under different geological complexes; the development origins on the types of
mother rocks are different in each province (table 1.3). The authors Nguyen Viet
Ky and Nguyen Van Tuan [9] have studied and summarized the main mineral
and chemical components on the weathering crust in the Central Highlands
presented in table 1.4.


-13From the documents studied above, the research shows that the studied
area has the weathering crust developed in all types of rocks available in the
region. The degree of weathering on the types of rocks is really different,
depending on original petrographic nature of rocks. The physico-mechanical

criteria and the mineral components of soil in weathering crust are different, they
will affect the stability of the slope on traffic routes in the studied area.
Table 1.1 The average values of physico-mechanical criteria of soils with natural
structure

Geodetic
formation of
source rocks

Soil layer

Granular
composition by
percentage (%) of
weight

Type of soil
(1)

Alluvial
swamp

Soil on ancient
basalt rock
foundation
(bN2-Ql)

Soil on
terrigenousroc
k foundation

(siltstone,
sandstone)

Soil on
volcanic rock
foundation
(Dacite,

>2
%

%

0,50,005
%

(2)

(3)

(4)

(5)

2-0,5

<0,005

Natural
Hu

weight Densit Pore
mid
y index
ity Natu Dry
ral
W

o



c
3

eo
(11)

3

%

%

g/cm

g/cm

(6)

(7)

(8)

(9)

(10)

0,75

2,55

1,55

2,70

0,74

1,20

2,90

1,42

1,35

3,01

1,33

1,15

2,89

1,55

1,80

2,93

0,65

1,50

2,77

0,70

1,50

2,75

0,75

1,45

2,66

0,83

Clay mud, clay loam

20,0 35,0 45,0 97,0 1,45
mud ehQ
Clay soil, terrace clay 5,0 35,0 30,0 30,0 25,0 1,94
loam grades I, II-aQ
Layer 1: reddish
5,0 23,0 30,0 42,0 37,0 1,64
brown clay
Layer 2: spotted
color clay containing 20,0 25,0 25,0 30,0 30,0 1,75
array clots
Layer 3: Yellow3,0 33,0 33,0 31,0 31,0 1,51
brown, light purple,
reddish brown clay
Layer 1: Reddish
brown clay
containing 15-50%
35,0 27,0 13,0 25,0 10,0 1,98
laterite aggregation
(at some places,
aggregation occupies
50-70%)
Layer 2: motleycolored clay
20,0 21,0 21,0 38,0 15,0 1,73
containing 20-25% of
Laterite aggregation.
Layer 3: Yellowbrown clay
3,0 30,0 30,0 37,0 26,0 1,89
containing little
crushed gravel
Layer 1: Reddish

brown clay
6,0 12,0 50,0 32,0 18,0 1,71
containing 10-15% of
laterite aggregation


-14Riolite)

Layer 2: motleycolored clay, bright
8,0 8,0 41,0 39,0 22,0 1,85 1,52 2,74
gray with 10% of
crushed gravel
Layer 1: Reddish
brown clay with little 2,0 26,0 24,0 38,0 25,0 1,89 1,51 2,73
Soil on
laterite aggregation
metamorphic
rocks (gneiss) Layer 2: motley8,0 27,0 33,0 22,0 26,0 1,82 1,44 2,74
colored clay with
little laterite clay
Layer1: Clay, clay
loam in yellow1,0 39,0 21,0 39,0 23,0 1,66 1,35 2,70
Soil on deep brown, reddish
intrusion rock brown colors
foundation
Layer 2: motley(Granite,
3,0 34,0 35,0 28,0 24,0 1,61 1,30 2,71
Granodiorite) colored clay loam
with little crushed
gravel

Table 1.1 The average values of physico-mechanical criteria of soils with natural
structure (next)
Satura
ted
degree
G
%
(12)

Atterberg limit
Liqui
d

Plast
ic

WL
%
(13)

WP
%
(14)

Ductil
ity
index
Ip
%
(15)

Visco
sity

Shearing strength
Nature
Saturated
friction Cohesive friction Cohesive
angle strength angle strength

(16)

100,0



0,81
0,90

0,95

0,96

Hydraulic
conductivity

C
C

2

degree kG/cm degree kG/cm2
(17)
(18)
(19)
(20)

K
cm/s
(21)

03o30'

B

0,80

0,06

03o30'

0,05

10-6
10-5

88,0

40,0

23,0

17,0

0,12

21o00'

0,30

19o00'

0,20

70,0

58,0

40,0

18,0

-0,17

20o00'

0,30

18o00'

0,25

76,0

62,0

44,0

18,0

-0,78

22o00'

0,40

19o00'

0,30

80,0

63,0

45,0

18,0

-0,78

21o00'

0,40

19o00'

0,30

45,0

50,0

30,0

20,0

-1,00

25o00'

0,45

23o00'

0,45

60,0

51,0

30,0

21,0

-0,71

23o00'

0,50

21o00'

0,40

95,0

49,0

28,0

21,0

-0,10

21o00'

0,50

19o00'

0,40

58,0

38,0

20,0

18,0

-0,11

23o00'

0,53

20o00'

0,34

75,0

47,0

29,0

18,0

-0,39

24o00'

0,53

21o00'

0,34

2,0x10-4(6,0x10-51,0x10-3)
3,0x10-4(6,0x10-51,0x10-3)
3,0x10-4(6,0x10-51,0x10-3)
3,5x10-4(1,0x10-55,0x10-3)
1,0x10-4(3,0x10-51,0x10-3)
6,0x10-4(1,0x10-51,0x10-3)
1,0x10-4(5,0x10-55,0x10-3)
1,0x10-4(5,0x10-55,0x10-3)


-1584,0

53,0

32,0

21,0

-0,33

27o00'

0,60

21o00'

0,42

80,0

46,0

27,0

19,0

-0,05

23o00'

0,70

20o00'

0,47

70,0

50,0

32,0

18,0

-0,50

27o00'

0,42

24o00'

0,31

75,0

52,0

38,0

14,0

-1,00

25o00'

0,41

22o00'

0,30

1,0x10-4(5,0x10-55,0x10-3)

1,0x10-4(5,0x10-55,0x10-3)
1,0x10-4(5,0x10-55,0x10-3)
1,0x10-4(5,0x10-55,0x10-3)

Table 1.2 Variation range of main criteria

W, %

NAME OF SOIL

- Clays, terrace rocks at
grades I, II (aQ)
- Soil on ancient basalt
rock foundation (N2-Q1)
- Soil on terrigenous rock
foundation (siltstone,
sandstone)
- Soil on volcanic rock
foundation (Dacite, Riolite,
Andesite)
- Soil on metamorphic
rocks (gneiss)
- Soil on deep intrusion
rock foundation (Granite,
Granodiorite)

c,
g/cm3

Water

Angle of
Pore
Adhesive
saturation interior
index
force
index
friction
C,
,
G, %
e0
kG/cm2
degree

±5

±0.05

±0.05

±5

±30

±0.05

±10

±0.08

±0.10

±8

±30

±0.10

±8

±0.06

±0.06

±8

±30

±0.10

±10

±0.08

±0.08

±10

±30

±0.08

±10

±0.07

±0.07

±10

±30

±0.12

±10

±0.08

±0.08

±10

±40

±0.07

Natural
Dry
humidity density

Table 1.3 Mineral components of clay type soil in the Central Highlands
Province

Geological
composition
(source rock)

Gia Lai
Kontum

Acid intrusion rocks
Metamorphic rocks
Basalt volcanic
rocks
Deposit aQIV

Đăk Lăk

Acid intrusion rocks
Basalt volcanic
rocks
Terrigenous
sedimentary rocks
Deposit aQIV

Mineral composition
(in descending order of amount of substance in
the soil)
Kaolinite, less hydromica, gibsite, quartz

Kaolinite, hydromica, less quartz
Hydrogotite, hydrohematite, gotite, gibsite,
kaolinite, Gibait, hydrogotite, gotite, kaolinite, fewer
hydrogotite, hydrohematite
Kaolinite, montmorillonite mixed-hydromica,
hydromica
Kaolinite, vecmiculite, mixed-vecmiculite
hydromica Hydrogotite, kaolinite, gibsite, bomite,
Gibsite, bomite, kaolinite, hydrogotite, gotit,
hematiteKaolinite, hydromica, montmorillonite,
vecmiculite, hydrogotiteKaolinite, montmorillonite,
hydromica, mixed-vecmiculite hydromica


-16Kaolinite, hydrogotite, gotite, gibsite Gibsite,
kaolinite, hydrogotite
Kaolinite, gibsite, hydrogotite
Lam Dong
Kaolinite, less hydromica, hydrogotite, gibsite
Reservoir NdL
Hydromica, montmorillonite, kaolinite
Deposit aQIV
Kaolinite, hydromica, montmorillonite
Table 1.4 Main mineral and chemical compositions in weathering crusts in the
Basalt volcanic
rocks

Central Highlands
No.

1

2

3
4
5
6
7
8

Weathering crust

Main mineral
composition

Weathering crust in acid
Quartz, Kaolinit, geotit,
intrusion rocks in totally
hydromica, haluazitweathering zone
felspat
Weathering crust in Basalt
Pliocene – early
Kaolinit, gibsit, geotit
Pleistocene (N2- Q11) in
laterization zone
Weathering crust in Pliocene
Kaolinit, gibsit, geotit
– early Pleistocene (N21
Q1 ) basalt in clay zone

Weathering crust in middle
Kaolinit, gibsit,
Pleistocene basalt in clay
Monmorilonit
zone
Weathering crust in medium
Kaolinit, geotit,
volcanic rocks in clay zone
hydromica
Quartz, Kaolinit, gibsit,
Weathering crust in acid
haluarit, felspát,
volcanic rocks in clay zone
hydromica, geotit
Weathering crust in
Quartz, Kaolinit,
metamorphic rocks in clay
hydromica, geotit
zone
Weathering crust in
Quartz, Kaolinit,
Sedimentary rocks in clay
hydromica, geotit
zone

Main chemical
composition
SiO2
%

Al2O3
%

Fe2O3
%

70-80 10-20

0,37,0

10-15 15-50

20-45

30-42 24-27

12-25

30-50 15-20

13-20

30-40 10-20

20-30

65-75 10-20

1-10

50-70 20-25

4-10

50-60 20-25

5-10

1.3 LANDSLIDE CONDITIONS ON ROUTES IN THE STUDIED AREA
On the map (figure 1.1), the Central Highlands area has the main traffic routes,
i.e. – Ho Chi Minh Trail passing Quang Nam, KonTum
- National Highway no. 14 running from KonTum to Gia Lai, Dak Lak, Dak
Nong, etc.
- National Highway no. 24 connecting from KonTum to Ba To (Quang Ngai)


-17- National Highway no. 25 connecting from Pleiku (Gia Lai) to Tuy Hoa.
- National Highway no. 26 connecting from Dak Lak to Nha Trang.
- National Highway no. 27 connecting from Dalat (Lam Dong) to DakLak
- National Highway no. 28 connecting Dalat (Lam Dong) with Dak Nong.
- National Highway no. 19 connecting from Le Thanh border gate to Pleiku
(Gia Lai) and Quy Nhon.
- National Highway no. 40 connecting Xayden-Antoum (Laos), Po Y border
gate with the National Highway no.14.
The above mentioned routes, especially the Ho Chi Minh Trail, extend over
many complex terrains with different engineering features. In rainy season, due to
the impacts of rains and storms in long periods, rugged terrains, high mountains,
deep abyss, and fragile talus slope system, landslides often happen on the routes.
1.3.1 Common forms of landslides
Depending on the topographical and geological features, each section has

different types of landslides
1.3.1.1 Road sections passing almost vertical cliff foots:
Many giant block stones falling on to and barricading the roads are often
found. For example: figure 1.6 presents the landslide incident happening on Ho Chi
Minh Trail, section from Km67 to Km68 on Vi O Lac mountainous pass – National
Highway 24, Dakrong, Ta Rut and the terrible rock, landslide burying overall Ho
Chi Minh Trail at Lo Xo section, in Mang Khenh Village, Dak Mang Commune,
Dak Glei district – KonTum province.

a) Mountainous erosion happening on 20th January at Km67-Km68 on Violac
mountainous pass on the National Highway no. 24A


-18-

b) Terrible rock, landslide burying overall Ho Chi Minh Trail at Lo Xo section, in
Mang Khenh Village, Dak Mang Commune, Dak Glei District, Kon Tum province.
Figure 1.5 (a, b)

Figure 1.6 Image of collapse of cliff on Ho Chi Minh Trail
1.3.1.2 Road sections passing mountainous foots with different weathering crusts:
Depending on terrain and thickness of the weathering crust, there are following
forms of landslides:
a) Corrosion on slope and erosion on surface:
Commonly occur on gentle slope. In rainy season, dry soil surface will be
easily disintegrated and corroded, especially at earth slope on basalt soil. The
erosion phenomenon happens repeatedly in the rainy season, creating deep water
gullies, forming sharper earth slope and resulting in the landslide (figure 1.7).
b) Block landslide with almost circular cylinder sliding surface:
Occur on the slope with large thickness of weathering crust. For example:

some images of block landslides in the figure 1.8. The phenomenon may happen on
positive talus slope and negative talus slope.
c) Mixed landslide with combined sliding surface:
A part of block landslide occurs on relatively thick weathering layer, the top of the
slope and the flat sliding part occur on thin weathering layer at the foot of the slope
(figure 1.9)
d) Flat landslide: occurs on the surface of source rocks with thin weathering crust.
For example, some images of flat landslides in figure 1.10.


-19-

a) Slope without thrust cutting and grading b) Slope with thrust cutting and grading

Figure 1.7 Image of cross section of the slope eroded by storm water

a) Block landslide on possitive talus slope

b) Block landslide on negative talus slope. The landslide creates vaulted cleft deeply
corroding asphalt edge of Ho Chi Minh Trail
Figure 1.8 Images of the slopes with block landslides


-20-

Figure 1.9 Images of slope surfaces with combined landslides

Figure 1.10 Images of slope surfaces with flat landslides
1.3.2 Reasons causing landslides
The reasons causing landslides can be due to the minimization of durability

of rocks, variation of stress state in unfavorable direction happening on the slope, or
due to both mentioned reasons. According to the Russian scientist В. Д. Ломтадзе
(V.D.Lomtadze) [49], the reasons causing landslides are often because of increase
of elevation of the slope during the course of cutting, excavation or erosion,
construction of the slope; reduction of durability of rocks due to change of physical
state in case of water absorption, expansion, reduction of soil density, weathering,
damage of natural structure; creeping phenomenon in rocks, impacts of hydrostatic
pressure and hydrodynamic pressure on rocks causing penetration deformation
(groundwater erosion, flowing, transferring into shifting-sand condition, etc.),
variation of stress state of rocks on slope formation zone and construction of slope;
other external impacts, such as load on the slope, geological action and earthquake,
etc.
On the basis of surveying the phenomenon at the eroded, corroded locations
of the motorways in the studied area and collecting various comments from


-21Surveyors, Designers, Contractors and the Traffic project management units, the
research student has learnt the following reasons of landslides:
1.3.2.1 Due to characteristics of soil mass:
Due to cracked weathering, possibly due to bombing during the war time
making the soil mass cracked and less stable.
1.3.2.2 Due to rugged terrain, high mountain, deep abyss, too steep slope of
positive talus:
Some sections of roads passing the cliff foots are nearly vertical. If rocks are
cracked much, they can be collapsed due to the vibration of construction equipment,
movements of automobiles or due to impacts of storms on the cliff. The sections of
roads passing weathering hillside foots have quite large steep of 30-35o, the
phenomenon of landslides may occur at the hillside foots. At curving road sections,
such as at Lo Xo Mountainous pass in KonTum, it’s required to cut into the
mountain to create the pavement. When cutting into the mountain, the natural slope

is excavated, cut into the natural slope with depth from 5m to more than 30-40m.
The natural talus slope often has large steep, varying from 40o to 70o amd is graded
with the height of each grade from 7 – 10m in order to reduce the weight of the
talus slope.
1.3.2.3 Due to harsh weather:
- In sunny season: High temperature causes rocks dry, cracked, reduces the
adhesive strength of soil, and creates conditions for rocks to be easily weathered.
- In rainy season: The humidity of soil increases, causing the soil to be
disintegrated, and eroded, reducing the durability of soil. Stilling water in joints
causes pressure on the slope. If prolong rains happen, under the actions of water
flow, rocks are eroded and washed away.
1.3.2.4 Due to human impact:
The deforestation in the watershed and on the top of the mountain makes quick
concentration of water flow in case of happening rains and floods. The construction
of houses on the slopes, road repairs result in the destruction of the vegetational
covers, etc.
Among the reasons mentioned above, the most noticeable reason is the
impact of water on soil. In case of happing prolong rains, soil is being in dry
form turning into water-saturated condition, decreasing the durability of soil
and causing slope failure. However, how does the occurrence of variation of


-22physico-mechanical features of some residual soil – deluvial deposit in the
Central Highlands? Whether their variation level by year-round weather
causing less or much impact on the stability of the side slopes and the slopes
closing to the motorways in the Central Highlands has not been fully studied.
Studying the existing matters mentioned above is also the task and content of
this thesis.
1.4 RESEARCHING RESULTS OF INTERNATIONAL AND LOCAL
SCIENTISTS ON STABILITY OF TALUS SLOPE, SIDE SLOPE

Side slope is a soil mass surrounded by a vertical plane (vertical slope) or
inclined plane (slope) connecting two different levels of elevation (top of slope and
slope foot). The side slopes can be the natural ones as the natural inclined planes of
the hillsides, the mountain slopes, etc. or the artificial ones as the side slopes of the
embankments on the mountain slopes, the causeways, the earth dams , etc.
Under the impacts of the own weight of the soil mass and due to impacts of
natural elements or activities of people, rocks on the slope may be displaced
downward with different regimes and speeds resulting in the displacement and slope
slide.
1.4.1 Some recommendations about classification of slope displacements
Classification of slope displacements has been studied by the scientists for a
long time. However, so far there is no uniform classification of slope displacement
[13]. In this thesis, the research student would like to introduce some common
classifications often mentioned in the country in recent years:
1.4.1.1 Classifying according to D. J. Varnes:
In 1958, D. J. Varnes classified the displacements of the slopes according to
the forms of displacements and the types of materials in the sliding mass. According
to the forms of displacements, it’s possible to divide the displacements into
landslides, flow landslides and collapse. While classifying according to the types of
materials in the sliding mass, it’s possible to divide the displacements into
landslides, rock landslides and soil-rock-mixture landslides.
- Landslides are the displacements due to cutting destruction (sliding) of the
rock mass in one or more of the sliding surface. The sliding surface can be straight,
flat (in loose soil or stratified rock) or curve (in cohesive, homogeneous soil).
- Flow landslides are the displacements of the rock mass due to impacts of
water flow, normally occurring in water-saturated soft soil, reducing the shearing


-23strength of soil. Depending on the composition of the soil and the speed of the water
flow, the speed of the flow landslides can vary from fast to very fast or extremely

fast.
- Collapse is the displacements of the rock mass from very fast to extremely
fast speed. From the tope of the slope, the blocks and masses of rocks fall freely,
drop down, are hopped or rolled along the slope and then accumulate at the slope
foot. D. J. Varnes has made almost accurate classifications of the displacements of
rocks on the slope as follows:
Extremely slow, when the displacement speed is < 0.3m/ 5 years
Very slow: 0.3m/ 5 years – 1.5m/year
Slow: 1.5m/year – 1.5m/month.
Medium: 1.5m/month – 1.5m/day.
Fast: 1.5m/day – 0.3m/minute.
Very fast: 0.3m/minute – 3m/second
Extremely fast: >3m/second
1.4.1.2 Classifying according to A. Nemcok, J. Pasek and J. Rybar:
In 1974, A. Nemcok, J. Pasek and J. Rybar (Czechoslovakia) has divided the
displacements of the slope according to the displacement regimes and speeds of the
rocks on the slopes into 4 types: slow landslides, landslides, flow landslides and
rock and soil collapse.
- Slow landslide (creep sliding) is the phenomenon of slow displacement in a
long period of the block mass from the slope top to the slope foot. The displacement
speed is very slow, about some millimeters to some centimeters in 10 years. This is
the basic displacement – the initial stage of different types of soil and rock
displacement on the side slope.
- Sliding is a rather fast displacement of a rock mass in one or more sliding
surfaces, a division surface of the sliding mass and the foundation is not displaced.
The displacement speed of rocks and soils may reach some meters per night.
- Flow landslide is a fast displacement of a rock mass along the side slope
due to the water saturation of rocks and soils. The displacement speed of the flow
sliding may reach approximately some meters per minute and this type of sliding
often occurs in rainy season, especially when there are prolong rains on a large area.

Depending on the material composition of rocks and soils on the side slope in which
the flow landslide creates soil-mud flow or rock-mud flow.


-24- Collapse of rocks and soils is a very fast displacement of rock mass from a
vertical slope or along a sliding surface with high steepness. Because the
displacement speed is very fast (about some meters per second), it often causes
unexpected accidents on mountainous traffic routes. This way of classification has
been recognized by the Council for Mutual Economic Assistance as a uniform
classification of slope displacement and applied for the Socialist countries.
1.4.1.3 Classifying according to Ho Chat and Doan Minh Tam
In 1985, Ho Chat and Doan Minh Tam (the Institute of Transportation
Engineering Science – currently known as the Institute of Transport Science and
Technology) divided the landslides into four basic types of soil and rock
displacement on the slope: soil landslides, dropped landslides, erosions and soil and
rock collapse.
- Soil landslide is the displacement of soil mass along a definite sliding
surface, it is often in the form of rotating cylinder (when soils in the sliding mass
are relatively uniform) and sometimes the sliding surface cuts deeply into a rock
block lying beneath or slides along a source rock surface.
- Dropped landslide is the final stage of the erosion. Actually, it’s difficult to
identity the eroded wall, the sliding surface but sometimes, it is often found in
circular arch. Products in the sliding mass are totally displaced towards the slope
foot or the talus.
- Erosion is a local deformation; at the beginning, pieces of rocks at the foot
or top of the slope are peeled off and then developed toward the upper part.
- Rock and soil collapse is the phenomenon in which each piece of soil or
rock mass is thrown out from the slope top or the side slope toward the slope foot.
This way of classification is used in standards of the Transport industry as in
the standard 22.TCN.171.87 “Engineering geological survey process and design of

roadbed stabilization in regions with activities of landslide and erosion”.
1.4.1.4 Classifying according to the recommendation of Nguyen Ngoc Si
According to Nguyen Ngoc Si [13], no matter which way of classification is
applied, the difference among the classifications is not much, because actually there
are only such phenomena. Therefore, Nguyen Ngoc Si suggests that it’s necessary
to divide the slope displacement into 04 types: flow landslides, erosions, rock
collapse and rock falling.


-25- The phenomenon in which rocks and soils are displaced according to any
sliding surface, it’s possible a flat surface or round cylindrical surface or a
combination of those two surfaces.
- In the phenomenon of flow landslide, the displacement of rocks and soils
occurs mainly due to the impact of surface water flow and depending on appropriate
geological condition, it’s possible to create mud flow, rock drifting.
- In the phenomenon of slip (or erosion), pieces and masses of rocks at the
end of the slope foot are dropped on the slope foot in almost vertical direction. Such
slip may occur continuously in the subsequent mass forms.
- In the phenomenon of rock collapse and rock fall, the rock blocks, rock
masses from the slope top or the side slope drop or collapse and then fall downward
to the slope foot under the impact of their own weights.
In item 1.3, introduction about different forms of erosion commonly found in
the Central Highlands, there are forms of slope displacements and erosions as
presented in item 1.4.1.4. As the limitation of the research task within in this thesis
is to evaluate the stability of slopes closing to the motorways, the research student
has selected the method of calculating the stability of earth slope.
1.4.2 Methods of calculating stability of side slope sliding, slope sliding
The problem of stability of a soil mass is a particular problem of the general
theory on ultimate stressed state; however, it has some important features due to
special movement of the soil mass when it losses the stability.

The item 1.4.1 introduces some recommendations on classification of slope
displacement. However, the main reasons causing the instability of the soil mass
are:

1. Erosion process.
2. Destroy of the balance.
The erosion process often takes place very slowly, is hardly visible, it

depends on the external meteorological and geophysical conditions impacting on
the surface of the soil mass and these conditions are usually not taken into account
in soil mechanics [53].
The study of the stability of the soil mass and damaging condition of such
stability are the direct tasks of the soil mechanics problem.
The methods of calculating the stability of slope can be divided into two
groups: