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

Luận văn

-1- INTRODUCTION

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

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In 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 (W and shearing parameters (, C) according to humidity (W) from the dry 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

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c) 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

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

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CHAPTER 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

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In 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

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(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

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1.2 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

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1.2.1 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

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geological 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

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 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 (βQ 1 ):

 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:

 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)

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

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From 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

Hu mid ity

Natural weight Densit

y index Pore Natu

ral Dry Type of soil > 2 2-0,5 0,005 <0,005 W  0,5- o c  e o

loam grades I, II-aQ 5,0 35,0 30,0 30,0 25,0 1,94 1,55 2,70 0,74

color clay containing array clots 20,0 25,0 25,0 30,0 30,0 1,75 1,35 3,01 1,33Layer 3: Yellow-

brown, light purple, reddish brown clay 3,0 33,0 33,0 31,0 31,0 1,51 1,15 2,89 1,55

laterite aggregation (at some places, aggregation occupies 50-70%)

35,0 27,0 13,0 25,0 10,0 1,98 1,80 2,93 0,65

Layer 2: colored clay containing 20-25% of Laterite aggregation

motley-20,0 21,0 21,0 38,0 15,0 1,73 1,50 2,77 0,70

Layer 3: brown clay containing little crushed gravel

6,0 12,0 50,0 32,0 18,0 1,71 1,45 2,66 0,83

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Riolite) Layer 2:

motley-colored clay, bright gray with 10% of crushed gravel

colored clay with little laterite clay 8,0 27,0 33,0 22,0 26,0 1,82 1,44 2,74 0,90

1,0 39,0 21,0 39,0 23,0 1,66 1,35 2,70 0,95

Layer 2: colored clay loam with little crushed gravel

Shearing strength

Hydraulic conductivity

Nature Saturated Liqui

d

Plast ic

Ductil ity index

friction angle

Cohesivestrength

friction angle

Cohesivestrength

% % % % degree kG/cm2 degree kG/cm2 cm/s (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) 100,0 03o30' 0,06 03o30' 0,05 10-6

88,0 40,0 23,0 17,0 0,12 21o00' 0,30 19o00' 0,20 10-5

70,0 58,0 40,0 18,0 -0,17 20o00' 0,30 18o00' 0,25 2,0x101,0x10-4(6,0x10-3) -576,0 62,0 44,0 18,0 -0,78 22o00' 0,40 19o00' 0,30 3,0x101,0x10-4(6,0x10-3) -5-80,0 63,0 45,0 18,0 -0,78 21o00' 0,40 19o00' 0,30 3,0x101,0x10-4(6,0x10-3) -5-45,0 50,0 30,0 20,0 -1,00 25o00' 0,45 23o00' 0,45 3,5x10-4(1,0x10-5-

-5,0x10-3) 60,0 51,0 30,0 21,0 -0,71 23o00' 0,50 21o00' 0,40 1,0x101,0x10-4(3,0x10-3) -5-95,0 49,0 28,0 21,0 -0,10 21o00' 0,50 19o00' 0,40 6,0x101,0x10-4(1,0x10-3) -5-58,0 38,0 20,0 18,0 -0,11 23o00' 0,53 20o00' 0,34 1,0x10-4(5,0x10-5-

5,0x10-3) 75,0 47,0 29,0 18,0 -0,39 24o00' 0,53 21o00' 0,34 1,0x10-4(5,0x10-5-

5,0x10-3)

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84,0 53,0 32,0 21,0 -0,33 27o00' 0,60 21o00' 0,42 1,0x105,0x10-4(5,0x10-3) -580,0 46,0 27,0 19,0 -0,05 23o00' 0,70 20o00' 0,47 1,0x105,0x10-4(5,0x10-3) -5-70,0 50,0 32,0 18,0 -0,50 27o00' 0,42 24o00' 0,31 1,0x10-4(5,0x10-5-

-5,0x10-3) 75,0 52,0 38,0 14,0 -1,00 25o00' 0,41 22o00' 0,30 1,0x10-4(5,0x10-5-

5,0x10-3)

Table 1.2 Variation range of main criteria

NAME OF SOIL

Natural humidity

Dry density

Pore index

Water saturation index

Angle of interior friction

Adhesive force

W, % c,

g/cm3 e0 G, % ,

degree

C, kG/cm2

- Clays, terrace rocks at

- Soil on volcanic rock

foundation (Dacite, Riolite,

Andesite)

- Soil on metamorphic

rocks (gneiss)

- Soil on deep intrusion

rock foundation (Granite,

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

Province composition Geological

(source rock)

Mineral composition (in descending order of amount of substance in

Kaolinite, montmorillonite mixed-hydromica, hydromica

Đăk Lăk

Acid intrusion rocks Basalt volcanic rocks

Terrigenous sedimentary rocks Deposit aQIV

Kaolinite, vecmiculite, mixed-vecmiculite hydromica Hydrogotite, kaolinite, gibsite, bomite, Gibsite, bomite, kaolinite, hydrogotite, gotit, hematiteKaolinite, hydromica, montmorillonite, vecmiculite, hydrogotiteKaolinite, montmorillonite, hydromica, mixed-vecmiculite hydromica

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Lam Dong

Basalt volcanic rocks

Reservoir NdL Deposit aQIV

Kaolinite, hydrogotite, gotite, gibsite Gibsite, kaolinite, hydrogotite

Kaolinite, gibsite, hydrogotite Kaolinite, less hydromica, hydrogotite, gibsite Hydromica, montmorillonite, kaolinite

Kaolinite, hydromica, montmorillonite

Table 1.4 Main mineral and chemical compositions in weathering crusts in the

Central Highlands

No Weathering crust Main mineral

composition

Main chemical composition

Q1 ) basalt in clay zone

Kaolinit, gibsit, geotit 30-42 24-27 12-25

4

Weathering crust in middle

Pleistocene basalt in clay

zone

Kaolinit, gibsit, Monmorilonit 30-50 15-20 13-20

5 Weathering crust in medium volcanic rocks in clay zone Kaolinit, geotit, hydromica 30-40 10-20 20-30

6 Weathering crust in acid

volcanic rocks in clay zone

Quartz, Kaolinit, gibsit, haluarit, felspát, hydromica, geotit

65-75 10-20 1-10

7 metamorphic rocks in clay Weathering crust in

zone

Quartz, Kaolinit, hydromica, geotit 50-70 20-25 4-10

8

Weathering crust in Sedimentary rocks in clay

zone

Quartz, Kaolinit, hydromica, geotit 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)

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- 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 20 th January at Km67-Km68 on Violac

mountainous pass on the National Highway no 24A

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

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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

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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

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Surveyors, 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

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physico-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

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strength 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

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- 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

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- 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

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Theory method group: belonging to this group is the research work of the

academician from the Soviet Unit Academy of Science (formerly), the professor B.B.Соколовский (V.V.Xô-cô-lôp-ski-1942,1954) By solving the differential equation of limit equilibrium, , B.B Соколовский created a chart defining a curve line of stable slope for the condition of soil with the friction angle of φ ≠ 0 and the cohesive strength of c ≠ 0; this method is presented in the document [53] The outline of the slope defined according to this method has larger gradient in comparison with that in the calculation performed by other methods because it mobilizes overall limit capacity of soil [53]

Approximate method group: similar to the sliding circular arc method of the

authors Tsugaev, Terzaghi, А.A Ничипорович, Bishop and the stable equilibrium method Fb of Н.Н.Macлob These methods are presented in chapter 2 “Theoretical basis for calculating the stability of slopes closing to motorways in the Central Highlands”

1.4.3 Some solutions for prevention of slope sliding in the Central Highlands

1.4.3.1 Some methods for prevention of landslides in case the actual slope does not satisfy the requirements on sliding prevention:

1) Proper reallocation of soil and rock masses on the slope:

Depending on the topographic condition in thrust cutting and grading area, remove some soils at the active part causing the erosion of the slope; in addition, it’s possible to provide additional cover of cap concrete at the slope foot to increase pressure and moment of stability; in some necessary cases, it’s possible to build gravity retaining wall of reinforced concrete, stone, gabions at the slope foot to prevent scattering of soils and rocks on the roadbed, remain soils and rocks at the back of the retaining wall, and create more moment of stability This solution is simple but very effective

2) Paying special attention to drainage for the slope:

When designing slope, although the stability is calculated, it’s impossible to ignore the drainage for the slope; water impacting on the slope can be underground water or surface water For the slopes closing to the motorways in the Central Highlands, the survey documents [9], [33] show that very rare cases of underground water are found in the slope In the cases where the motorways run along the riversides, it’s possible to find underground water To drain underground water, it’s possible to use centralized water well, use pump to drain water or use tilting borehole to drain water from the slope as presented in the document [33] For the

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slopes closing to the motorways in the Central Highlands, the main solution is to adjust the surface water flow This solution is to reduce the impact of water in rainy season, reduce the absorption of water in soil resulting in the increase of soil density, reduction of soil durability, increase of sliding pressure, and reduction of anti-sliding strength On the other hand, adjustment of surface water flow to prevent the erosion of trench on the surface of the slope, increase the steep of the slope, resulting in minimizing anti-sliding safety coefficient The surface water drainage solution is often combined with the solution of thrust cutting and grading area creating drainage ditches along the thrusts on the slope; in case there are joints on the slope top, the joints shall be covered by pieces of soils and rocks or fully filled

to prevent water absorption

3) Protective covering for slope surface:

It’s possible to use vegetation covers, slope protection layer of stone, wire concrete, etc This method is to keep the mechanical characteristic of soil on the slope not to be reduced due to crack weathering and to prevent erosion for the slope

4) Strengthening soils and rocks:

For crack rock mass with many pores, it’s required to apply jet drilling by jointing compounds, creating artificial connections among the rock masses, increasing anti-sliding strength of the slope

5) Construction of anti-sliding works:

Currently, together with new advances in the field of rock mechanics and foundation, various new technical methods with high efficiency have been applied

in prevention of landslide for slope, such as retaining walls, piling or anchoring systems, etc [4], [8], [10], [12], [27]

- The bearing walls and retaining walls are used to prevent landslide thanks

to their own weights The bearing wall can be in the form of a column or a block and is normally built of reinforced concrete The retaining wall can be built by bricks, stones, common concrete, or reinforced concrete or pre-cast concrete, gabions and shall be design so as to ensure that the retaining wall is stable, not overturned; soils under the retaining wall are not settled, the retaining wall is not damaged due to external force, etc

- Different types of piles are driven into the ground to ensure slope stability

- Different types of anchors are often applied to prevent landslides

6) Special methods:

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In case of prevention of landslides in areas with difficult and costly conditions, special methods are applied, such as realigning the routes far from the slide area, building flyovers or tunnels to cross over or go through the slide area However, in case of applying one of these special methods, it’s required to consider carefully the economic-technical and aesthetic aspects in the region The prevention

of landslides for the motorways in the Central Highlands have not faced with this situation

1.4.3.2 Some main methods for prevention of landslides applied on Ho Chi Minh Trail and some traffic routes in the Central Highlands

- Water surface drainage system: Providing master drain, mechanical ditch, vertical ditch and side ditch along the talus foot to prevent erosion caused by the impact of surface water

- Thrust cutting and grading: grading on an average of 1:1.5 at sections of talus which are excavated deeply and soils and rocks are heavily weathered to minimize the weight of thrust cutting slope with width from 2 to 3 meters, the thrust surface is covered with cement concrete grade M150 of 10cm thick, 20% inclining toward the mountain and providing V-shaped surface water drainage on the thrust surface

- Strengthening talus slope: planting local grass or Vetiver grass, paving surface with cement concrete or coating with stones, cement concrete frame in combination with plantation of grass, or cement concrete mesh in combination with anchoring

- Gravity wall for retaining positive talus: building reinforced concrete wall

at weak geology, anchored gabion walls, PVC-coated rocks

- Tường chờ tạo lưu không for provision of slide settlement applied at locations with orphaned rocks, fissured rocks – weathered rocks which are easy to

be slipped away

Currently, there’s no optimal solution to prevent erosion of the slopes along the traffic routes in the Central Highlands Madam Le Minh Chau – Deputy General Director of Ho Chi Minh Trail Project Management Unit said: “Anti-sliding is a long-term matter on all roads going through complex geological areas or storm and flood-prone areas Therefore, the study of treatment to give out methods for protection of the works is essential, especially for Ho Chi Minh Trail” and recently

on 12th April 2011, Vietnam Institute of Geosciences and Mineral Resources

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together with Norwegian geological experts performed survey for Ho Chi Minh route Dr Tran Tan Van – Director said: “The most important matter in treatment of landslide and mitigation of landslide catastrophe, firstly, it’s required to perform well geological survey, geotechnical survey, and build drainage systems, this matter has not been totally resolved There are many locations facing with risks of great and serious landslides, blocking the roads and interrupting traffic on those routes; however, we have never had any methods for assurance of traffic in rainy and flood seasons”

Therefore, after finding the main reasons causing erosion of the slopes, it’s required to continuously monitor, perform testing of the solutions applied in reality and new methods applied abroad to draw out some appropriate methods for each topographic, hydrological conditions in the Central Highlands Figure 1.11 presents some images of anti-sliding methods

a) Anchored gabion walls for prevention of landslide on Ho Chi Minh Trail

b) Concreting at some sections on Ho Chi Minh Trail to prevent landslides

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c) Solidification – one of the criteria applied for Ho Chi Minh route

Figure 1.11: Some images of anti-sliding works on Ho Chi Minh Trail

1.5 CONCLUSION OF CHAPTER 1:

About geological conditions: The Central Highlands is located in two large

tectonic zones, i.e KonTum and Dalat zones Each tectonic zone has different features on compositions, structures and geological characteristics On each zone, there is development of many different fault systems

In the region, there are many different strata, geologies; the weathering crusts

on various types of rocks are different on their mineral compositions and mechanical properties

Weather features: Climate and hydrological conditions in the Central Highlands are very harsh, the dry season is completely different from the rainy season, affecting seriously to the construction conditions and the stability of the project

Traffic routes in the Central Highlands: The traffic routes stretch over

many complex and abrupt terrains, high mountains, deep abysses, and are affected

by prolonged rains; therefore, landslides on these routes often occur in the rainy season

Types of landslides: include collapse of cliffs, slide of side slopes in

weathering crusts under the form of block landslides, combined landslides, and flat landslides

Reasons causing landslides: There are many reasons causing landslides but

the main reason is the effects of water on soils in the rainy season, reducing the endurance of soils, increasing the weight of soils on the slope, water erosion, etc resulting in landslides Therefore, it’s necessary to study the variation of soil physico-mechanical properties of soils in the weathering crusts in case of water absorption to have basis for evaluation of the stability and proposal of methods for prevention of landslides on the traffic routes in the studied area

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CHAPTER I THEORETICAL BASIS FOR CALCULATION OF STABILITY OF SLOPES CLOSING TO MOTORWAYS IN THE CENTRAL HIGHLANDS

Observing the phenomenon of instability of slopes along the motorways in the Central Highlands presented in Chapter 1 shows that there are 3 main forms of landslides as follows:

* Flat landslide: occurs on source rock surface with thin weathering crust; it often has fracture form

* Block landslide with almost cylindrical sliding surface: occurs on slopes with thick and relatively homogenous weathering crust

* Combined landslide with combined sliding surface: A part of sliding block occurs on relatively thick weathering crust on the upper part of the slope and flat landslide occurs on thin weathering crust at the foot of the slope Depending on the sliding form, select the appropriate method for calculation of slope stability

As presented in item 1.4.2 in Chapter 1, the methods of calculation of slope stability can be divided into two groups

Theory method group: belonging to this group is the research work of the

academician from the Soviet Unit Academy of Science (formerly), the professor B.B.Соколовский (V.V.Xô-cô-lôp-ski-1942, 1954) By solving the differential equation of limit equilibrium, , B.B Соколовский created a chart defining a curve line of stable slope for the condition of soil with the friction angle of φ ≠ 0 and the cohesive strength of C ≠ 0; this method is presented in the document [53] The outline of the slope defined according to this method has larger gradient in comparison with that in the calculation performed by other methods because it mobilizes overall limit capacity of soil [53]

Approximate method group: similar to the sliding circular arc method and

the stable equilibrium method Fb

Application of theories of many scientists introduced in the monographic books or curriculum books about soil quality and soil mechanics [47], [51], [52] and [53]; in this part, the research student selects to introduce some appropriate methods

to calculate the stability of the slopes, the side slopes closing to the motorways in terrain conditions of the Central Highlands

2.1 FRACTURE FLAT SLIDING SURFACE

This is the form of sliding block commonly found on thin weathering earth bank locating on fracture source rocks (figure 2.1a) The method of calculation of slope stability according to this form is recommended by Professor Г.М.Шахуньянц and introduced by Professor Β.Ф.Бабкоa in the document [54] The sliding soil mass is divided into vertical slices as shown in the figure The

i i

i i i i

F  1cos(  1   )  cos     sin    , (2.1)

In which: Fi-1 : pressure transmitted on studied block from the upper block

Gi : weight of the block

Li : length of sliding arc surface

: internal friction angle of the sliding soil mass C: soil cohesive strength

If the member mass above the pressure Fi-1 has negative value, it’s not calculated The stability coefficient for each mass is:

i i i i i

i i

i i

G F

L C tg G

cos(

cos1

The method of calculation of slope stability presented above may be applied for cases subject to the effects of more complex loads

b) Figure 2.1 Diagram to determine the stability of sliding mass along the plane

2.2 CIRCULAR CYLINDRICAL SLIDING SURFACE METHOD

Many actual observations show that in the homogenous soil mass, the sliding soil mass displaces according to curve surface, it may be approximately considered

as circular cylindrical surface

Assuming that the centre of the slip steering cylindrical surface is at point O (figure 2.2a) The equation of equilibrium is Mo = 0 To establish an equation for momentums at the rotating point O, divide the slip steering ABC into some strips through some vertical cross section and consider the weight of each assigned strip locating at the cross point of the weight of ground strip Pi with respective sliding

i

i+1 Li+1i+1

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section while interacting forces on vertical surfaces of the strips are ignored (considered as pressure of adjacent strips with equal values while the direction is contrary) Then, distribute pressures of the weight Pi in the direction of turning radius and the direction perpendicular to it, establish the equation of equilibrium with a given value of zero, the total momentum of the forces for the rotating point is:

Ti.R - Nitg.R - cL.R = 0 (2.3) Reduce this expression for R, we obtain: Ti -  Nitg - cL = 0 (2.4)

In which:

L : length of sliding arc AC

, C: internal friction angle and soil cohesive strength

Ti và Ni: Component pressure of strips defined by graphic solution or calculation according to measurement of the angle i:

R L C tg N M

M K

i

r

i i

tr g

1

tr

g

T

L C tg N M

M K



1 (2.5)

However, solving the proposed problem by determining the stability coefficient for optional sliding arc has not ended because it’s required to select among the sliding arcs, sliding may occur at the most dangerous sliding arc

When designing motorways [54], to determine the allocation area of the most dangerous shear centers, apply the recommendation of the Swedish scientist – Professor Phenleniux Accordingly, the dangerous centers allocate on a straight line

AB (figure 2.2b) The values of the angles  and  for determination of the location

of such straight line are presented in table 2.1

According to the comment of Professor Н.А Цытович (N.A.Xư tô Vich) [53], the circular cylindrical surface method is widely applied in reality because it gives a certain stable reserve and is based on experimental data on the form of sliding surface in case of occurring rotating slip and based on site measurements The respective calculations show clearly that in some cases, the circular cylindrical method provides quite high degree of safety

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a Diagram on effects of forces b Central locations of dangerous sliding arcs

Figure 2.2 Diagram to determine the stability of slope according to

circular cylindrical sliding surface Table 2.1 Values of angles  and :

Slope coefficient Angle of slope

Values of angle, (degree)

  1:0.6

1:1 1:1.5 1:2 1:3 1:4 1:5

is only correct for the cases when the sliding arc surfaces with their sections fall towards possible displacement direction of the slope or the side slope in case of sliding according to circular cylindrical surface, when the strips in the sliding curves are distributed to one side from the direction of the vertical radius OA (figure 2.3)

If coding the sliding forces towards the sliding side (shearing) is Titr, while coding the forces in the direction contrary to the displacement direction and preventing the slope from slipping is Tig, the formula (2.5) will have the following form:

r i ig i

r i i r

i i

T

T L C tg

N K

1

1 1

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T4



 A

C B O

Figure 2.3: Diagram on forces impacting on sliding surface

Calculation method according to the recommendation of Professors M.Н Голbдштейн and Г.Ц Тер-cтепанян (M.N Gônxtên and G.I.Ter- Xtêpanion):

To improve and simplify the calculation according to the circular cylindrical sliding surface, the Professors M.Н Голbдштейн and Г.Ц Тер-cтепанян recommend to calculate the stability coefficient according to the expression (2.7) This method was presented by Professor Н.А Цытович (N.A Xư Tô vich) in the document [53] K = f.A + B

h

C

 , (2.7)

In which: f : friction coefficient of soil f = tg; C: soil cohesive strength;

h: height of slope, : natural density of soil in the slope

A & B : coefficients subject to geometrical dimension of falling wedge, expressed according to height distribution of the slope h, stated in table 2.2

The improved circular arc of M.Н Голbдштейн has many advantages in calculation but in actual materials, we have never found any author performing calculation, comparison of and test results by applying the circular arc method Therefore, in item 4.1 in Chapter 4, the research student calculates and compares the safety coefficient of slope stability according to the improved circular arc method of M.Н Голbдштейн and the circular arc method of Bishop

Table 2.2 Values of coefficients A and B for approximate calculation of slope

the side slopes

Mặt trượt đi qua nền và có tiếp tuyến nằm ngang tại độ sâu

 = 1/4h  = 1/2h  = h  = 1.5h

A B A B A B A B A B

2.3 CIRCULAR CYLINDRICAL SLIDING SURFACE WITH

CONSIDERATION OF SEEPAGE PRESSURE OR PORE WATER

2.3.1 Weight pressure method of Tsugaev (figure 2.4)

Figure 2.4 Diagram on calculation of sliding arc according to Tsugaev

Slope stability coefficient is calculated according to the formula:

.x

n 2 H

n μ.dS

b.γ

R tb tg n x

n 2 H

) n u

n 1

ε(H R K

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In which: R: radius of sliding arc; b: width of vertical earth pillar (b= const)

Xn : X-axis of earth pillar no n, xn > 0 when the earth pillar locating on the left of

OY axis

dSn : length of the sliding arc section locating within the earth pillar

, C : internal friction angle and cohesive strength of the soil layer where the studied sliding arc section goes through

tb, Ctb : internal friction angle and soil cohesive strength

b : width of vertical earth pillar

 ; k : dry soil density ; đn : uplift density of soil

Z1 ,Z2 : average height of earth locating above and under the saturation line of the earth pillar no n

2.3.2 Terzaghi method (figure 2.5)

Figure 2.5 Diagram on calculation of sliding arc according to Terzaghi

The slope stability is calculated according to the formula:

R

r n sinα n G

n L n C n tg n cosα n G K

J: Average gradient of seepage flow

 : Seepage force, calculated as follows = .J., : area of seepage zone

r : Distance from the seepage force  to the sliding arc, R: radius of sliding arc

n: Angle created by vertical line crossing shear center and normal of the sliding arc section center;

n , Cn: Angle of internal friction and soil cohesive strength of the earth pillar no n

2.3.3 Method of А.A Ничипорович (figure 2.6)

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According to this method, seepage force is converted into external force affecting on the external surface of the sliding arc and in the direction perpendicular

to the sliding arc

Figure 2.6 Diagram on calculation of sliding arc according to А.A

Ничипорович Stability coefficient K is calculated according to the formula:

 

n sinα n G

R

r o W n C.L tg

n ) n φ n (B n cosα n G K

In which: weight of earth pillar

k : density of dry soil; bh : density of saturated soil; w : density of water

Z1 , Z2 : average height of earth locating above and under the saturation line of the earth pillar no n

H2: height of water column from the saturation line to the bottom of the earth pillar

in the sliding arc section

un: pore water pressure affecting on the sliding arc section within the bottom range

of the earth pillar no n

Bn, n : Uplift force, seepage force

r : distance from the force Wo to the center of the sliding arc; R: radius of the sliding arc

n: angle created by the vertical line crossing shear center and normal of the sliding arc section center ;

Wo: hydrostatic pressure affecting on slope creating retaining force at down stream

Ln: length of each section of sliding arc at the bottom of the earth pillar in the sliding arc

(G.sinα

.LCu).tg

.G.sinαd

k(G.cosαK

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In which: kd : earthquake coefficient; G: weight of earth pillar

, C: internal friction angle and effective cohesive strength of soil

: angle between the vertical line crossing shear center and normal of the sliding arc section center and L : length of sliding arc at the earth pillar bottom

2.3.5 Software for calculation of stability

The most popular software for calculation of slope stability is GEO-SLOPE International Ltd (Canada) Slope/W is the graphical user interface software which can run in Wins 95/98/NT/2000 and XP environment; it is used for analysis of the stability of slope – saturated and unsaturated rocks with 9 different methods; Slope/W can analyze and solve the problems on heterogeneity on rock foundation with condition that sliding surface is predetermined by each mass, the slope bears external load and is strengthened

Slope/W applies the theory of equilibrium of forces and momentum for calculation of safety coefficient to prevent against damages The theory of General Limit Equilibrium - GLE is presented and used as a matter relating to safety coefficient of all methods generally for the problem on slope stability

Applying Slope/W, it’s possible to analyze both simple and complex matters for a series of sliding surface forms, condition of pore water pressure, soil characteristics, analysis method and loading conditions By applying the theory of limit equilibrium, Slope/W can model different types of heterogeneous soils, complex surface geometry of stratum and slide; pore water pressure conditions by using a large selection of soil models Analysis of slope stability can be implemented by using definite or probabilistic input parameters Emphasis on calculation and analysis of finite element tension can be used in calculation of limit equilibrium to fully analyze the stability of available slope; with full of these functions, Slope/W can be used in analyzing almost every problem on slope stability

2.4 “STABLE EQUILIBRIUM F P ” METHOD OF PROFESSOR Н.Н MACЛOB

Professor Н.Н Macлob (1943) recommends a formula to determine slope angle  of the slope on stable foundation, which is called “stable equilibrium method Fp”

According to this method, the angle of slope  is determined according to the formula:

h

C tg tg F

w

p     , (2.12)

 : slope angle with height h;

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; ; C: in order of density, friction angle, cohesive strength of soil mass on the side slope

a) a) If the density on the side slope is distributed evenly Po (figure 2.7b) the formula (2.12) will be:

o w p

P h

C tg

tg F

Figure 2.7 Diagram on calculation of slope according to method F p

For slopes composed of multiple layers of different soil thickness (hi), with different physico-mechanical characteristics i, i,Ci, the slope angle  is determined for each layer from the slope foot upward, such as in figure 2.8, we determine the slope angle i for each layer as follows:

3 3 2 2 1 1

1 1

1

h h h

C tg

2 2

2

h h

C tg

Figure 2.8 Diagram on calculation of multiple-layer slope according to the

method F p

Slope coefficient m = cotg = 1/tgΨ (2.15)

- When designing slope, the required slope coefficient [m] is determined via the safety coefficient K, i.e.: [m] = K.m (2.16a)

In case the roadside slope comprises a homogeneous type of soils but has great height, the slope is divided into many layers with hi = 5m, apply the formula (2.14) to calculate the slope angle i for the slope foot upward, and through

h m

Po

h1

hm1